Heat recovery systems & condensation

[Verismo]

A year into boat ownership (loving it) and researching heating and condensation. Has anyone fitted a MVHR heat recovery system? Like this - https://www.ebay.co.uk/itm/251159375045. A 20W trickle fan sucks warm damp air out and transfers up to 80% of the heat to incoming fresh air. A few holes and some flat trunking could suck damp & hot stove, cooking and showering air from living and bathroom and pump warm fresh air into bedrooms on my 57' x 10' liveaboard. Circulating warm fresh air and removing damp air. Feeling I'm missing something as it seems all too easy!!

[blackrose]

I think what we're all missing are any details or specifications for this product - at least the eBay mobile link didn't give me any details.

 

How does this system work, how does it dry or warm the air coming into the boat? Where does that power come from?

 

Personally I don't suffer from condensation issues on my widebeam and the foggy air outside is far damper at the moment than the warm air in my boat.

Edited December 31, 2016 by blackrose

[Alway Swilby]

20 watts at 12 volts is getting on for 2 amps current draw. That's 48 ah per day, 2/3 of a battery. But Perhaps you're plugged into a shore line.

[Verismo]

Thanks for prompt replies! Good point on the battery drain. I'm on shore power when not on the move so maybe less of a problem. There's lots of info on these systems - mainly designed for house ventilation - one of many sites online with info http://www.greenwood.co.uk/range/5/heat-recovery-mvhr.html

[nicknorman]

Heat recovery systems seem a really good idea to me - for houses. However I'm less sure about narrowboats. Things to consider:

 

Space. The heat exchanger needs to be physically quite big to be effective. If it is, it is going to be a problem finding space for it. If it isn't, it isn't going to work very well.

 

Condensate removal. Presumably where the cold incoming air and warm moist outgoing air meet, condensation can form and would need somewhere to drain away to. This would be a problem on a boat unless the exchanger is located well above the water line.

 

Power: 20w for the fan doesn't sound that much, but 24/7 it is around 40AH per day which is significant.

 

Noise. The small confines of a narrowboat means that any fan noise that might be inconsequential in a house, could be a nuisance on a boat.

 

Fixed ventilation. These systems are designed to be used in houses which are reasonably well sealed. On a boat there has to be a lot of fixed ventilation to be safe and meet the BSS. I think this might have an effect on the effectiveness of the system.

 

Finally there is no data about this system in terms of the exchanger efficiency and noise. It is obviously a budget system and I'd be sceptical about what was inside the box.

and the foggy air outside is far damper at the moment than the warm air in my boat.

Don't confuse relative humidity with absolute humidity. the foggy cold air will probably have less humidity than the warm air inside your boat, but of course the foggy air's relative humidity is 100%. Edited December 31, 2016 by nicknorman

[nicknorman]

As an example on the humidity thing:

 

Foggy air at 5C 100% RH, 1020 mB has 0.007 kg/m^3 of water.

 

Take the same air and warm it to 22C and the relative humidity becomes 36%. RH below 50% is getting uncomfortably dry. More likely the RH inside a warm boat is 50 or 60% and so has much more water content than the foggy air outside.

[Chewbacka]

Which is why the fog will often clear very quickly once the sun has a put bit of warmth into the air.

[Alan de Enfield]

As an example on the humidity thing:

 

Foggy air at 5C 100% RH, 1020 mB has 0.007 kg/m^3 of water.

 

Take the same air and warm it to 22C and the relative humidity becomes 36%. RH below 50% is getting uncomfortably dry. More likely the RH inside a warm boat is 50 or 60% and so has much more water content than the foggy air outside.

 

Hence the formation of cumulus clouds and thermals (moist adiabatic lapse rate)

[mross]

A condensing boiler would make more sense. You could also look at air-to-air heat pumps which can be 400% efficient (COP = 4). But a solid fuel fire is tops and ensures good ventilation and minimum condensation. You could use a water to air heat pump but this would be problematic if the canal froze! The eBay link seems to be just a simple heat exchanger - it might reduce heating bills a little but it won't really warm the boat. You could also try and recover heat from the engine's exhaust and solid fire's flue but remember that lower gas temperatures will cause corrosion unless you use exotic materials.

 

As an example on the humidity thing:

Foggy air at 5C 100% RH, 1020 mB has 0.007 kg/m^3 of water.

Take the same air and warm it to 22C and the relative humidity becomes 36%. RH below 50% is getting uncomfortably dry. More likely the RH inside a warm boat is 50 or 60% and so has much more water content than the foggy air outside.

Excellent advice! Also, as the relative humidity gets lower you need higher air temperatures to feel warm. If humidity is too low all the wood will shrink and crack.

[Verismo]

Boy you have to love these forums (or is it forii) Lots of really good advice here. I wasn't thinking of using for heating - more for ventilation and heat distribution. My lovely stove gets the lounge area really warm (sometimes too hot) so a system that trickle pumped warm air to the colder areas and helped ventilate looked interesting. Wondered if lack of usage of MHVR systems on canal boats was down to something I'd missed or just not a big enough benefit. Looking like maybe the latter but jury is still out!

Edited December 31, 2016 by Verismo

[nicknorman]

A condensing boiler would make more sense. You could also look at air-to-air heat pumps which can be 400% efficient (COP = 4). But a solid fuel fire is tops and ensures good ventilation and minimum condensation. You could use a water to air heat pump but this would be problematic if the canal froze!

The trouble with heat pumps is that they require lots of electricity. So to replace a 4 kw stove with a 400% efficient heat pump, you still need a steady 1kw of electricity. Great in a house, hopeless on a boat unless permanently on shore power.

[WotEver]

Boy you have to love these forums (or is it forii) Lots of really good advice here. I wasn't thinking of using for heating - more for ventilation and heat distribution. My lovely stove gets the lounge area really warm (sometimes too hot) so a system that trickle pumped warm air to the colder areas and helped ventilate looked interesting. Wondered if lack of usage of MHVR systems on canal boats was down to something I'd missed or just not a big enough benefit. Looking like maybe the latter but jury is still out!

Two small PC fans in series in a box/trunking/cowl above the stove will be very quiet and move the air down the boat for minimal power.

 

Or you could buy a peltier stove fan which would look pretty...

[cuthound]

Or you could buy a peltier stove fan which would look pretty...

And tell you when the stove needs more coal putting in it

[WotEver]

And tell you when the stove needs more coal putting in it

Yeah, that's what they're for, innit?

[DHutch]

Boy you have to love these forums (or is it forii) Lots of really good advice here. I wasn't thinking of using for heating - more for ventilation and heat distribution. My lovely stove gets the lounge area really warm (sometimes too hot) so a system that trickle pumped warm air to the colder areas and helped ventilate looked interesting. Wondered if lack of usage of MHVR systems on canal boats was down to something I'd missed or just not a big enough benefit. Looking like maybe the latter but jury is still out!

My parents have a MHRV system in their house.

 

A lot of people say the majority of houses do not have the airtightness levels required to make fun use of the system, and this may be true and hence is likely to be true for a boat with BSS spec ventilation. However I cab vouch for the fact it makes the house a lot nicer, no condensation but yet not too dry either, laundry dries overnight all year round with damp issues, etc. The system was installed from new, when the house was built in around 25 years ago and is still working well, the two fans where replaced 6months ago as the bearings where beginning to fail, I guess 250,000hrs run time is reasonable.

I have also retro-fitted a similar unit from the same supplier (Villavent) in my grans bungalow, which was reasonably easy to do within the loft void due to the single story nature of a bungalow.

 

On a boat the issue is always going to be powering the unit, as well as to an extent, where you physically house it if it cant go in the loft! If you are on mains, a small unit may fit somewhere and do what you need to do and make it nicer over winter, if you are off-grid or only using the boat infrequently or over summer months I would forget about it.

 

There are some small standalone units marketed for use as bathroom extractors and the like which use a small thin-wire heat exchanger, which might be more suited.

 

Daniel

[Arthur Brown]

There is much to be said about heat recovery, it's good economic sense! BUT on a boat there is too little spare volume to fit a big enough heat exchanger system. It's common in building HVAC to recover heat from the out going air but it can take an ISO container sized lump on the roof, there isn't a spare cubic foot on most narrow boats, -if there was it would have three uses already!

[DHutch]

Depends on the boat I guess, but we have a space on our boat you could fit a small unit under the deck, getting the ducting to the living space would be interesting, but not beyond the witt of man during a major referb job.

 

 

Daniel