Heat recovery ventilation (MHRV / ERV) again

[wakey_wake]

I will want to talk about generators, and no I don't want to annoy neighbours or anyone else.

 

However we're getting off MHRVs & ventilation so I'll reply on another thread in a bit.

 

Thanks all for the thoughtful comments!

[Tony Brooks]

1 hour ago, wakey_wake said:

All true, except probably not running a generator while on EHU unless I'm testing something.

Burning fuel to make electricity will also make heat and I would like to catch more of that if I can. Another of those stories for another day.

 

 

Again you seem to miss the mark here. Lets say you manage to reduce your electricity consumption to 25% of that you quoted for the domestic implementation so 25% of 200 watts = 50 watts. 50 watts from a 12V inverter equals roughly 5 amps allowing for inverter inefficiency. As you need to ventilate the boat 24 hours a day that equals 120 Amp hours needed.

 

As we should try not to discharge below 50% for any LA battery that equates to a 240 Ah battery bank just for the ventilation. If you go lithium it seems you can have a slightly smaller bank but as experience here suggest for long life you will be operating between about 80% and 20% charged it might not make as much difference to bank size as you might expect. Then you need to consider how you will put that much back into the batteries. I think this implies a generator running for long periods every day. If  you do decide that adequate generation will take place every day you can reduce the electrical requirements  by the time the generator is running. All this is predicated on you getting the units consumption down to 25% of the domestic one and I have my doubts because I would expect the heat exchanger to present a fair resistance to air flow so need powerful fans.

 

If you want electricity and usable heat then you might like to look for a second-hand Whispergen. A Sterling engined boiler that also produces electricity with less noise than a diesel generator. Unfortunately they are no longer in production.

 

Edited February 1, 2021 by Tony Brooks

[IanD]

1 hour ago, wakey_wake said:

I will want to talk about generators, and no I don't want to annoy neighbours or anyone else.

 

However we're getting off MHRVs & ventilation so I'll reply on another thread in a bit.

 

Thanks all for the thoughtful comments!

I think the problem is that you're worrying about just what colour the flame is when the real problem is that your a*se is on fire... ?

 

The savings of using heat recovery from exhaust air to warm incoming air are small compared to the problem of where the heat comes from in the first place -- yes if you're going to burn diesel (or LPG or whatever) to generate the heat then heat recovery is worthwhile, but the real problem is burning fuel for heating in the first place as the economy moves more and more towards renewable energy. Houses are facing exactly the same problem, hence the government drive in future to remove gas boilers and replace them with heat-pumps powered by electricity from renewables.

 

Given that a narrowboat is sitting in a lot of nice cool water, a water-source heat pump should be able to deliver around 5kW of heat (same as a decent stove/boiler/diesel heater) for 1kW of power, and is an even more obvious answer than a ground/air source one for houses -- apart from the small problem that nobody seems to make suitable ones because there's no demand...

 

The "where does the power come from?" is exactly the same question as for electric propulsion and onboard mains power -- onboard batteries powered by solar cells, plug-in charging when available, and (in the shorter term) an onboard generator until then -- even if this is powered by diesel or LPG (or biodiesel?) it would use about 40% less fuel than burning the fuel directly to warm the boat.

 

[stove/heater/boiler is maybe 75% efficient for heating, generator is 25% x COP=5 = 125% efficient -- where "efficiency" means heat delivered divided by energy in, so heat pump on its own is 500% efficient when powered by electricity]

[TheBiscuits]

2 minutes ago, IanD said:

Given that a narrowboat is sitting in a lot of nice cool water, a water-source heat pump should be able to deliver around 5kW of heat (same as a decent stove/boiler/diesel heater) for 1kW of power

 

You keep telling us that Ian, but the only one I can find real detail on is the CRT install on Sabrina 5 in Gloucester Docks.

 

That's 3.84 CoP theoretical and between 2.97 and 3.45 average according to the sales literature.

 

https://www.kensaheatpumps.com/ground-source-review-sabrina-5-canal-barge/

 

https://uh8ex3jph2xqg0pb4bs7if12-wpengine.netdna-ssl.com/wp-content/uploads/2014/03/TI-Shoebox-heat-pump-v6.3.pdf

 

 

Do you have any links or information for a boat-sized system that gives a CoP of 5?

[IanD]

1 minute ago, TheBiscuits said:

 

You keep telling us that Ian, but the only one I can find real detail on is the CRT install on Sabrina 5 in Gloucester Docks.

 

That's 3.84 CoP theoretical and between 2.97 and 3.45 average according to the sales literature.

 

https://www.kensaheatpumps.com/ground-source-review-sabrina-5-canal-barge/

 

https://uh8ex3jph2xqg0pb4bs7if12-wpengine.netdna-ssl.com/wp-content/uploads/2014/03/TI-Shoebox-heat-pump-v6.3.pdf

 

 

Do you have any links or information for a boat-sized system that gives a CoP of 5?

I did find some links (not boat-sized though...) which I posted in an earlier thread, I'll see if I can dig them out tonight...

Edited February 1, 2021 by IanD

[TheBiscuits]

2 minutes ago, IanD said:

I did find some links (not boat-sized though...) which I posted in an earlier thread, I'll see if I can dig them out tonight...

 

Thanks Ian.  

 

I like the idea of heat pumps, I just can't find one that gets anywhere near 5:1, and 3:1 is a much less compelling argument.

[IanD]

1 hour ago, TheBiscuits said:

 

Thanks Ian.  

 

I like the idea of heat pumps, I just can't find one that gets anywhere near 5:1, and 3:1 is a much less compelling argument.

Not an available off-the-shelf shoebox, but saying what *should* be possible... ?

 

Depending on what conditions are and which model, the Kensa "shoebox" looks to have a COP of between ~3 and ~4. Unfortunately they don't give enough details of exactly what the conditions are...

 

If COP is 3 then there's no fuel saving compared to burning diesel. But the energy source can be solar (or renewable if plugged in), which is the way the world is going.

 

Same for houses -- the driver is the need to switch from fossil fuels to renewable energy, which basically means electricity, and a heat-pump is at least 3x more efficient than electric heating (but a *lot* more expensive to install...). Running costs are not really what matters, nothing is as cheap as burning fossil fuels...

Edited February 1, 2021 by IanD

[David Mack]

52 minutes ago, TheBiscuits said:

 

Thanks Ian.  

 

I like the idea of heat pumps, I just can't find one that gets anywhere near 5:1, and 3:1 is a much less compelling argument.

I attended a talk some years ago about heat pumps for domestic (house) heating. The chap stated that a heat pump system required only about 1/3 of the energy input compared with a conventional gas or oil fired system. But he also noted that on a unit energy basis, domestic electricity was about three times the price of gas or oil, meaning that there would be no running cost saving to justify the significantly higher capital cost. 

I guess the same argument would be apply to boats, except the cost of boat-generated electricity (other than solar) is much higher than the mains, meaning the system may be more expensive to run (and that still assumes a boat system could match the CoP of a house system).

[IanD]

1 hour ago, David Mack said:

I attended a talk some years ago about heat pumps for domestic (house) heating. The chap stated that a heat pump system required only about 1/3 of the energy input compared with a conventional gas or oil fired system. But he also noted that on a unit energy basis, domestic electricity was about three times the price of gas or oil, meaning that there would be no running cost saving to justify the significantly higher capital cost. 

I guess the same argument would be apply to boats, except the cost of boat-generated electricity (other than solar) is much higher than the mains, meaning the system may be more expensive to run (and that still assumes a boat system could match the CoP of a house system).

It depends on where the energy comes from in the first place, and what the COP of the heat-pump is, which depends strongly on input (canal) and output (hot water) temperatures.

 

If you're burning diesel and the COP=5, then there's about a 40% fuel saving (not build cost!) using a diesel generator to drive a heat pump instead of burning the diesel in a heater/boiler/stove, with COP=3 there's no cost saving.

 

If the energy source is renewables (solar or plug-in recharging from renewables) then how do you run an onboard diesel heating system -- biodiesel? Where does this come from?

 

Either way it's the same case as comparing diesel and electric power (with generator) for a boat -- the lower running costs are swamped by the higher installation costs, you do it for other reasons like silent cruising or wanting to be green.

 

This all needs to be looked at together with propulsion, not separately. Burning gas or oil is cheap, but we can't carry on doing it everywhere...

Edited February 1, 2021 by IanD

[Detling]

Regarding using acoustic absorbing foam in the air ducts of the sealed generator, how well does it work when wet? Condensation, spray, rain etc are going to ensure it will have a high moisture content. 

 

If all the boats in an area of canal or marina used a water based heat pump, the ice would get very thick, they work best in flowing water.

Except for the Llangollen not many area of canal have that.

 

Using a heat pump to cool the boat in summer makes more sense, the sun shines on hot days providing plenty of solar power. 

[wakey_wake]

22 minutes ago, IanD said:

I think the problem is that you're worrying about just what colour the flame is when the real problem is that your a*se is on fire... ?

Is it a pretty blue flame? Then I know what'll be causing it...

 

I can't solve all the problems at once, so I was planning to start with the ones I can make a difference to.

I also can't discuss them all at once... I'm willing to try, but I doubt I can be productive.

 

22 minutes ago, IanD said:

The savings of using heat recovery from exhaust air to warm incoming air are small compared to the problem of where the heat comes from in the first place [...]

a water-source heat pump should be able to deliver around 5kW of heat (same as a decent stove/boiler/diesel heater) for 1kW of power, [...]

This property I'm in (with the oversized MHRV) also has an ASHP, and it is a noise nuisance. I think the noise is mostly conducted vibration from the fan, because I hear it spooling up and down. I'm told the compressor is constant speed.

Nobody is looking at its actual efficiency, but lots of people are crowing about how efficient it is. (Somebody did look at efficiency of a related system nearby and it's bad enough to suggest a major fault. From here it's minor local politics and I'm bailing out.)

 

My thoughts on this line had gone as far as wondering whether it's better to source heat from a skin tank or the bow thruster tube. I don't know, it might depend whether the bow thruster can run slow and quiet. And whether it will damage the bank/pilings while doing so.

 

 

22 minutes ago, IanD said:

[...] apart from the small problem that nobody seems to make suitable ones because there's no demand...

 

The "where does the power come from?" is exactly the same question as for electric propulsion and onboard mains power [...]

 

[stove/heater/boiler is maybe 75% efficient for heating, [...] heat pump on its own is 500% efficient when powered by electricity]

For me the need for 24kW hr per day (no cruising) looks too ambitious, unless I have EHU, so I wasn't expecting to have WSHP in my boat any time soon. However I would expect to be able to retro-fit one when circumstances change: all useful heat should go into water at some point.

There is also the interaction of "running the engine for the heat pump" and "waste heat from the engine", which is beyond what I can juggle in my head. I don't have data (yet) on the thermal inertia of an engine system but it seems relevant while the engine is warming up

 

When considering efficiency there is the question of driving the compressor off the engine (like a car aircon unit) ? vs electric motor drive from the generator being spun by the engine ?, and worse a battery charge/discharge cycle and inverter between. Why would you do the latter? Because you don't want two heat pumps ? unless you've done the maths first.

 

Would a car aircon unit interest you, size-wise? I've no idea what temperature ranges they can work in, but if it will cool that cabin to 15C when it's 45C outside then it'll work in some weathers. I've seen a car aircon under test take the cabin down to 9C & suspect it was power-limited at that point so I don't know if it would reach lower.

 

Another thing about heat pumps is the entire plumbing speciality that goes with them. Once it has been installed I have to call somebody in to service it or make changes.

 

It's a cool topic but that's about all the thought I've given it so far...

...for the future, when diesel engines are obsolete, an all-electric boat will run just fine on fusion power! ?

[dave moore]

The way things are going, by the time Diesel engines are obsolete, the system will be fragmented, if navigable at all. I could be wrong....

[Mike Adams]

On 30/01/2021 at 23:59, wakey_wake said:

Solid fuel burning will be needed, but I want a balanced flue not a front-feed stove. That's a story for another day, but I also dislike having a potential carbon monoxide source in my box.

Let us know when you find or make one I would be interested to see that on a narrowboat. Start by working out the volume of air a solid fuel stove of 6KW needs to function properly. Solution - find a residential mooring with shorepower  first and get electric heating. The cost of the electricity will be minimal compared to the cost of a proper res mooring. In London say 12K for the mooring I bet it won't be more than £500 for the electricity and less than solid fuel.

[TheBiscuits]

1 hour ago, Detling said:

If all the boats in an area of canal or marina used a water based heat pump, the ice would get very thick

 

Are you sure?  It's very unlikely when you look at the physics ...

[wakey_wake]

1 hour ago, Mike Adams said:

Let us know when you find or make one I would be interested to see that on a narrowboat.

It's (now) on the list I stuck in my profile here, so I don't forget. I have a promise of mooring but no boat yet.

 

1 hour ago, Mike Adams said:

Let us know when you find or make one I would be interested to see that on a narrowboat. Start by working out the volume of air a solid fuel stove of 6KW needs to function properly. Solution - find a residential mooring with shorepower  first and get electric heating. The cost of the electricity will be minimal compared to the cost of a proper res mooring. In London say 12K for the mooring I bet it won't be more than £500 for the electricity and less than solid fuel.

I'm far enough away from London ? and the EHU rate is quite reasonable - only a smidge over Octopus residential tariff.

That is basically my plan. The currently promised mooring has some issue with me doing welding (?), but until I have a definite boat (hence length) then it's open to change.

[Alan de Enfield]

28 minutes ago, wakey_wake said:

The currently promised mooring has some issue with me doing welding (?)

 

 

You may well find that most marinas will have limitations on 'hot-work' and you will need to provide proof of sufficient 3rd party insurance.

Example :

 

Work to Vessel

8.1 Minor running repairs and minor maintenance to your Vessel of a routine nature (including, but not limited to light sanding, staining and minor painting) which does not cause noise, disturbance or pollution to the Marina or other Marina users, may be carried out to your Vessel at its Berth.

8.2 Extensive/major work to your Vessel, including but not limited to external sanding, angle grinding, welding, internal refit, spraypainting etc. may not be carried out within the Marina except with WMM’s prior written consent. Where WMM consents to such work, consent is subject to you complying with WMM requirements in relation to the location where the work is to be carried out and all other provisions of this clause 8.

8.3 You may allow a Contractor to work on your Vessel in or via access through the Marina providing they have registered in the Marina Office and they are present within Office working hours. Before a Contractor may work on your Vessel in the Marina, you must ensure that a risk assessment and method statement has been completed by the Contractor and that a copy of the Contractor’s third-party liability insurance policy for a sum of not less than £5,000,000 has been provided to the Office. WMM reserves the right to withhold or revoke its consent. You are responsible for ensuring that any Contractor working on your Vessel registers with the Office upon their arrival at the Marina before any work is started. WMM may require a Contractor to leave the Marina if you or such Contractor fails to comply with any part of this Mooring Contract relevant to Contractors. WMM is entitled to levy a charge for allowing any external Contractor to work on your Vessel at or via access through the Marina. The amount charged is on display in the White Mills Marina office and is currently £7.00.

8.4 Whenever work is carried out on your Vessel, all WMM’s facilities, bollards, pontoons, service points etc. must be suitably protected, kept clean and tidy at all times, and appropriate health and safety measures adopted and followed. WMM may reclaim from you all costs incurred for remedial works required as a result of any damage caused by you, your crew, Contractors and visitors, to the Marina.

8.5 All work on your Vessel in the Marina must be carried out in strict accordance with the Marina’s health and safety and general management policies which are available from the Office and all relevant health and safety legislation and codes of practice. Where work is not carried out in accordance with this legislation and these policies and codes of practice, WMM may require that such work cease forthwith.

8.6 When work is carried out on your Vessel, whether by you, your agent or a Contractor, you must use your best endeavours to ensure that as little disturbance and inconvenience as possible is caused to WMM, to other users of the Marina and to persons residing in the vicinity. If WMM receives any complaint regarding the work from any person using the Marina or residing in the vicinity of the Marina or if WMM otherwise considers it necessary, WMM may require a review of the methods used so as to minimise any disturbance or inconvenience and/or require that the work is carried out in a different berth or hard standing area within the Marina (for which WMM is entitled to levy a charge) or require that the work cease forthwith.

8.7 Any Contractor undertaking work to your Vessel must leave the Marina forthwith upon completion or cessation of the work.

8.8 All work on your Vessel of whatever nature may be carried out daily only between 09.00 and 18.00 hours. All work on your Vessel must be completed within a reasonable period of time, taking into account the nature and extent of the work involved. Upon completion or cessation (whether at the requirement of WWM or otherwise) of any work on your Vessel you shall immediately commence restoring the area in which the work was carried out to the condition it was in prior to the commencement of the work, including undertaking any remedial work that WMM decides is required, and shall complete this within a reasonable timescale.

[wakey_wake]

Some numbers which will tend to be more precise than accurate...  these expressions will paste into GNU Units 2.19 so anyone can come back and re-work them.

• Assuming a 66ft narrowboat as a cuboid 6'10" wide and 9' tall, with 1.5" insulation with R(SI) of 0.85 picked from a list, ignoring the fact that bilges & non-cabin areas will be left cold, thermostat is at 20°C (and that's effective across the whole place with no cold spots inside) and its exactly freezing 0°C outside,
• 2(6ft + 10in -3in)(66ft -3in) + 2(6ft + 10in -3in)(9ft -3in) + 2(66ft -3in)(9ft -3in) = 198 m² of interior surface
• total R(SI) is 0.85*1.5 = 1.275 degC m^2 / W
• 20 degC 198 m^2 / 1.275 RSI = 3.1 kW for heating the box with zero ventilation

I don't have any air changes per hour (ACH) figures, but I can guess and it's a starting point

• Taking the same internal volume (ignoring unswept volume for cupboards, tanks, mattresses etc.), 5 ACH at 60% of the swept volume based on numbers that pop into my head after doing minimal research, and taking figures for heat capacity for air at given humidity,
• ACH figure will greatly depend on the wind. I suspect that, due to indoors combustion, boats are made with generous ACH.
• I've conveniently put the humidity close to zero by declaring that outside is frozen
• I don't understand GNU Units' "airmass(alt)" formula so I'm going with this page and taking air at 1.225kg/m³

• (6ft + 10in -3in)(66ft -3in)(9ft -3in) = 107.25 m³ swept volume

• 5/hr *60%*107.25m^3 * 1.225kg/m^3 * 1005J/K/kg * 20degC = 2.2kW for heating air as it passes through

 

I need another section for "air heated by the stove and sent up the chimney", which I will tend to assume is additional to the ACH figure, but we are at least in the ballpark of the 5kW output stove.

[DHutch]

An interesting topic.

 

I have grown up in a house with a MHRV system, fitted another to a family members bungalow, and looked into the options of retrofitting it to houses I have owned, including reard and posting about it for years.

 

Basically it is good; and with a suitable power supply (renewable elec, on grid) on a reasonably airtight space, it make sensible economic sense in savings.

Even if the space is not that airtight, a reasonably built house with airtightness inline with current regs and or a well built house since the 80s it makes it feel nicer, dry clothes, etc.

If designing a seriously airtight 'passiv house' type build you have to be quite careful as even if you comply with regs you can have an air turnover which is not a healthy environment.

 

The same broadly speaking does for heat pumps (GSHP or ASHP) although the current economics really only work for on-grid properties without mains gas.

 

However with all that said, while it would be a lovely option for reducing condensation especially on single glazed windows, I agree that it is unlikely you could make it work off-grid on a narrowboat. At which point I think its application would be limited to a standalone 'fine wire' FiWi unit or two, in a boat predominantly moored with a shoreline. Switched off when unplugged.

 

 

Daniel

[IanD]

On 01/02/2021 at 15:46, IanD said:

Not an available off-the-shelf shoebox, but saying what *should* be possible... ?

 

Depending on what conditions are and which model, the Kensa "shoebox" looks to have a COP of between ~3 and ~4. Unfortunately they don't give enough details of exactly what the conditions are...

 

If COP is 3 then there's no fuel saving compared to burning diesel. But the energy source can be solar (or renewable if plugged in), which is the way the world is going.

 

Same for houses -- the driver is the need to switch from fossil fuels to renewable energy, which basically means electricity, and a heat-pump is at least 3x more efficient than electric heating (but a *lot* more expensive to install...). Running costs are not really what matters, nothing is as cheap as burning fossil fuels...

I found some figures for Vaillant air and water source heat pumps, these are not really aimed at boats but show what is possible:

 

Air source 5kW COP 4.06/3.66/3.25/3.14@35C/40C/45C/50C output temperature

Air source 8kW COP 4.58/4.22/3.86/3.4@35C/40C/45C/50C output temperature

Water source 5kW COP 4.82/4.39/3.96/3.89/3.81@35C/40C/45C/50C/55C output temperature

Water source 8kW COP 5.1/4.59/4.08/3.96/3.83@35C/40C/45C/50C/55C output temperature

 

So the bigger water source one can reach COP=5 so long as the output water is relatively cool, which in practice means either underfloor heating or huge radiators. With more normal heating water temperature (normal radiators) the COP is around 4 or lower.

 

8kW is really too big for a narrowboat, unless it's pulsed on and off which you could do with underfloor heating but not radiators. But the 5kW one gets pretty close on CoP.

 

Also they're pretty big, about 500l volume for the air source and 420l for the water source. And expensive, especially the water source ones... ?

 

I remember somebody posting about an underfloor heating setup that worked well for them, this is really what's needed to take advantage of a heat pump. With this a COP of about 4 should definitely be achievable in practice with 45C output.

[TheBiscuits]

11 minutes ago, IanD said:

So the bigger water source one can reach COP=5 so long as the output water is relatively cool, which in practice means either underfloor heating or huge radiators. With more normal heating water temperature (normal radiators) the COP is around 4 or lower.

 

Just be wary of sales literature - that's quoting SCoP (Seasonal CoP) so gets the benefits of warm source water in summer.  When the water source temperature is near zero it'll be more like 2.5 CoP.

 

SCoP is a fair comparison between different systems, but it's an average over a year. 

 

Add: And the quoted CoP/SCoP for heat pumps is usually just for the heat pump compressor itself - if you need one or more separate system pumps to shove the warm water around your heating that's not included in the energy use measurement!  If the system pumps are built into the heat pump unit the power will be included in the quoted figures.

[IanD]

2 hours ago, TheBiscuits said:

 

Just be wary of sales literature - that's quoting SCoP (Seasonal CoP) so gets the benefits of warm source water in summer.  When the water source temperature is near zero it'll be more like 2.5 CoP.

 

SCoP is a fair comparison between different systems, but it's an average over a year. 

 

Add: And the quoted CoP/SCoP for heat pumps is usually just for the heat pump compressor itself - if you need one or more separate system pumps to shove the warm water around your heating that's not included in the energy use measurement!  If the system pumps are built into the heat pump unit the power will be included in the quoted figures.

You're correct that the figures are SCoP, probably at 10C input (not clear from spec), in this case they do include both the pump to circulate water from heat-pump to keel cooler and around the central heating.

 

Heat pump CoP depends almost entirely on input-output temperature difference, so the 5kW water source CoP should be as follows:

 

10C/35C : 4.82

10C/45C or 0C/35C : 3.96

10C/55C or 0C/45C : 3.81

 

For sure, an advertising "headline CoP" is often deceptive, but I think these figures are right...

[wakey_wake]

 

On 02/02/2021 at 13:27, wakey_wake said:

I need another section for "air heated by the stove and sent up the chimney", which I will tend to assume is additional to the ACH figure, but we are at least in the ballpark of the 5kW output stove.

Using mostly this first data source of three top search hits

• https://www.brighthubengineering.com/power-plants/20189-burning-coal-how-much-air-is-required/ says "around 0.332 kg of air for one MJ of heat", and (next page) coal fired power plants burning pulverised coal use 20% extra air to ensure complete combustion without heating air for no gain
• https://coalpail.com/coal-forum/viewtopic.php?p=81586&sid=cdb05b7b279e06a200febae320d5a639#p81586 on the burning of coal in stoves in houses, discussion of air input
• https://www.sciencedirect.com/science/article/pii/S2352484719308704 very detailed but TL;DR

and air figures from my previous post, assuming the 5kW stove output fueled by coal

• assuming initially that the stove is damped down to minimum output and hence only combustion air goes up the chimney,
• but somehow we retain stoichiometry of full combustion
• assuming air enters the stove at 20°C and exits the chimney at 200°C
• 5kW * (0.332 kg / MJ) / (1.225kg/m^3) = 1.35 litre/sec
• 5kW * (0.332 kg / MJ) / (1.225kg/m^3) / 107.25m^3 = 1/22 hr (i.e. 0.04 ACH)
• 5kW * (0.332 kg / MJ) * 1005J/K/kg * 200degC = 0.33 kW

This seems remarkably low. Maybe the output is hotter? Maybe normal stove air throughput is rather more than the 120% air supply used in carefully managed power stations?

It's also possible the combustion air heatup is already accounted for in the (0.332kg//MJ) but my interpretation is that the whole megajoule would have to be recovered by bringing the exhaust back to 20°C.

 

Then looking at the other unrealistic assumptions,

On 02/02/2021 at 13:27, wakey_wake said:

Some numbers which will tend to be more precise than accurate...  [...]

• Assuming a 66ft narrowboat as a cuboid [...] ignoring the fact that bilges & non-cabin areas will be left cold, thermostat is at 20°C (and that's effective across the whole place with no cold spots inside) [...] 3.1 kW for heating the box with zero ventilation

Suppose only 66% of that boat is actually heated, which I could try to justify as subtracting the bow & stern (5ft each) and the bilges & floor (lower 1ft); and then claiming rather arbitrarily that a further 10ft of the interior is effectively unheated because the radiators don't reach / cupboards aren't heated / the ACH draught carries the heat the other way.

• 2(6ft + 10in -3in)(66ft -3in -10ft) + 2(6ft + 10in -3in)(9ft -3in -1ft) + 2(66ft -3in -10ft)(9ft -3in -1ft) = 158m² = 80% of 198m²
• 2(6ft + 10in -3in)(66ft -3in -10ft -10ft) + 2(6ft + 10in -3in)(9ft -3in -1ft) + 2(66ft -3in -10ft -10ft)(9ft -3in -1ft) = 131m² = 66%
• taking a reduced 2kW to heat the box.

Then when the stove output is 5kW, that means 3kW must blowing through, indicating

• ((3 - 0.33) kW / 2.2kW) * (5/hr) = 6.1 ACH, not a huge change.

which seems to support the ACH assumptions being around 50% of heat loss. So how much power can be saved and what's that going to mean?

• Assume a marvellously optimistic heat recovery of 90%, (3 - 0.33)kW * 90% = 2.4kW max saving
• With a "first pass home brew" heat recovery of 25%, (3 - 0.33)kW * 25% = 700 W "could do better"
• I should run the numbers again with 10°C outside. Then maybe I have to consider also humidity.

but when effective stove output is regulated by opening windows ? or if you can't run the heater at 2.5kW (and less in warmer weather) then we're eating electricity but not achieving anything.

 

The way I look at it this is: having an MVHR requires a heater with a modulating or averaging output - fluid fuel with a thermostat, or something else.

[alistair1537]

On 31/01/2021 at 16:59, wakey_wake said:

but IMO all other combustion must have a verifiably independent air supply.

I have installed a Stanley Waterford Oisin Multifuel stove in the centre of my Wide beam with a external air kit fitted - factory option. I run a 40mm pipe from an engine bay vent 6m and this has proved to be ample in providing enough "fresh air" for the stove. The only time I get any smoke in the cabin is when I open the door too fast to load more fuel...If I remember to do it gently there's no back draught at all.

 

I am in the process of fitting MHRV in a few apartments - around 160mm dia through wall individual units. I think there is scope for installation on a barge - space is an issue, obviously, but these units are not much bigger than a couple of stacked cake tins...

 

As to power requirements, I'm not sure, I haven't checked the consumption yet. I run ample solar so summer running would not be a problem - winter time I run a generator to charge the LiPO4 batteries, usually an hour a day - so I'm sure that adding a MHRV system would not add greatly to that regime.

[Sea Dog]

I love it when these energy saving ideas come along to make boating life cheaper and simpler...!