CH/DHW system using Thermal Store and Stove backboiler

[Jon12345]

 

Hi, I would not use the plate heat exchanger on the engine and buy a themal store with an indirect coil so you could attach the engine direct. Hopefully if the engine is close enough to the store the engine water pump should take care of it.

 

In my opinion you don't need the vent pipe on the heat sink rad or on the gravity hot water flow. But to be safe and because it's easy would put a vent pipe there ( See Pic ) 

 

I think it's workable but the one problem I can see is keeping the store at an adequate temperature for hot water as required. I have fitted only a few Thermal stores in houses and they have all had a controllable source of heat like an oil/gas boiler incorporated. Which can be timed say before you get up in the morning to heat the store for hot water and the solid fuel/wood burner is a bit of a evening focal point and not a main source of heat.

 

 

 

 

 

 

 

 

 

 

 

 

Edited November 19, 2018 by Jon12345
Inserted pic twice

[Tacet]

The idea is to store surplus kwh for DHW or space heating for later use.  In principle, this is just pushing the tried and tested calorifier store of DHW a bit further.  It is improbable that there will be spare kwh in electrical form when space heating is required (and you are off grid) – but heat generated by engine or stove/boiler is more practical.

 

Once the storage tank drops below, say 40 degrees, it is not going to be much use for heating or hot (warm) water. Taking 90 degrees as the maximum storage temperature, gives a usable range of 50 degrees.

 

From it taking 1 calorie to raise 1 gramme by 1 degree – (a calorie is 4.184 Joules, and Joule is a watt second) means that the store will hold a theoretical 9.3 kwh.   You could payback at, say, 0.5kw over the 18 hours that the engine/stove was not operating.

 

But allowing for say, 50 litres for the DHW – and you are down to 6.4 kwh – or about 1/3 kw for the 18 hours.  I doubt this is enough to keep a boat (well, a sufficiently sized boat to hold the kit) on even a typical winter day.  It’s around the same output as a single panel radiator of 400 x 500.  Maybe 1 or 2kw would be a reasonable target?

 

Allowing for various inefficiencies and you could be needing maybe 5 or 10 x 190 litres to make this work.  And, of course, you need to find the energy from somewhere to shove it back in.

 

I am all for other people experimenting with new ideas, but there is much to be said for the solid fuel stove (with or without a back boiler) gently producing 24 hours a day.

 

All assuming the numbers are anywhere near correct.

Edited November 19, 2018 by Tacet

[nicknorman]

I've only skimmed this thread but a couple of things spring to mind:

 

Firstly, although 175A alternators are great (we have one), they do run very hot at full output. I am not sure the life will be very long if you routinely make it work flat out for hours. They were not really designed to do that.

 

Secondly, have you calculated how much usable heat is actually stored in 160l of hot water and thus how much heat it could put into the boat?  I suggest that it is "not a lot" and you are adding a huge amount of complexity, weight and lost space for very little energy storage. Without doing any calcs myself, my gut feeling is that the amount of kWH stored might keep the rads hot for about an hour, warm for a bit longer. And of course as the temperature in the thermal store decreases, so the ability to dump heat into the cabin decreases (because heat transfer is proportional to the temperature difference). I think you could work on 4 or 5 kw needed to keep a widebeam reasonably warm. If you have 160l or water at say 80C and extract 4kw of heat from it, how rapidly does it cool? Sums say that 160l of water will cool 25C in an hour if you extract ~4.5 kw from it. So  by the end of the first hour, if you can manage to continue to extract 4.5kw, the temperature will be down to 55C.. Another hour (if you can still manage to extract 4.5kw) will see it at 30 which is barely tepid.

 

Unfortunately storing heat in water is very inefficient (in terms of mass and volume), compared to storing chemical heat energy in diesel/gas/coal.

 

I'm all for new ideas and ways of doing things, but they have to pass the tests of basic physics first!

Edited November 19, 2018 by nicknorman

[Stilllearning]

2 hours ago, Schwaa said:

I think perhaps I've not explained myself very well. Sorry if I'm failing to make sufficient sense.

 

At no point will I be trying to heat 160l of water electrically or in one go.

 

In the colder months, the body of water in the thermal store (let's call it thermal fluid to differentiate it from the DHW) will be heated by the wood burner. This thermal fluid will be used to heat DHW in a high efficiency coil through the store, like a reverse cauliflower, driven by the freshwater pressure pump and with a thermostatic mixing valve on the output to regulate the DHW temp.. The thermal fluid will also be circulated through the radiator system. When the engine is run, which it is regularly in the winter, there will be the option to feed thermal energy in via the engine's calorifier takeoff, to top up the store (ther reason for the heat exchanger is because of the distance from the engine to the thermal store). THe aim with a thermal store is to kkep it's contents at around 70 to 90degC as much as possible, in the winter anyway.

 

In the summer, when demand for hot water is slightly less and heating is virtually nil, the water in the store (let's call it thermal fluid to differentiate it from the DHW) will be heated when the engine is run. It will not need to heat the whole store. THe idea is that the contents of the store is stratified as much as possible to keep the heat where it's needed, around the DHW coil.

 

The immersion element is very much a secondary source for a short boost and an opportunity to collect any spill-over from the PV output on really good days.

 

I was hoping for some help with the technicalities of the plumbing and control system, i.e the placing of stats and valves, more than anything else really.

 

Thanks all for taking the time though.

As you say, you are hoping for assistance and suggestions on the technical side of your proposed system. I leave that to others who are better informed on the subject, and will add my feelings on the keep it simple principle. From past experience, having a complicated system means that at some point it will go wrong, and béa so-and-so to fix. A decent sized stove with back boiler, plus a gas powered instant hot water boiler works really well, and crucially needs next to no electric capacity. 

By all means try out your planned system, and if it works, tell us all about it.

[MtB]

What's the point of a thermal store in a boat?

 

In a house, it is to avoid hving to install all that complicated and awkward G3 discharge pipework on an unvented cylinder in order to comply with Building Regulations. On a boat no-one gives a toss about this, or even understands the point of it.

[peterboat]

15 hours ago, Schwaa said:

Hello everyone,

 

I'm new to the forum but been on boats for a while. I'm currently fitting out a sailaway widebeam and I'm on a search for some thoughts on my plan to build the central heating and domestic hot water system around a thermal store and boiler stove. Someone on a facebook boaters' group suggested this might be a good place to ask. On reading around, you seem to be a pretty friendly, helpful and knowledgable bunch so any advice or opinions  on the attached diagram would be very gratefully received.

 

For context, the stove, store and heatsink are all next to each other, in the middle of the cabin, with the bathroom bulkhead between them

Hi send me a PM with your phone number I have a similar system on my widebeam it's easier to chat about it than endlessly exchanging mail Peter

[Jen-in-Wellies]

One potential problem with the system as designed is starting up from cold on a cold day. For example, you go away for a week in winter and come back to a stone cold boat and heat store. It is going to take many hours heating up the 190l before you get any heat going in to the boat and you'll be shivering in the cold. I get this on my boat. The 5KW stove has a back boiler that gravity feeds a calorifier, followed by finrads. When starting up again from cold, most of the stoves heat is dumped in to the calorifer, warming around 60l of water. It takes several hours before the stove produces significant heating in the cabin around it, or from the finrads. You'll have over three times the volume of water to heat, with only 2kW of extra stove power. Could mean shivering in the cold for quiet a while.

 

As I understand it, the idea of having the heat store in winter is that the stove can be run hard to heat the store, then allowed to go out and the heat store will supply the radiators and heat hot water until it cools down. Those of us with more conventional systems will bank the fire up and run it with the air intakes low so that it provides moderate heat all the time. Your proposed approach has good and bad points. Good point is that running the stove hard will reduce the amount of sooting and clinker formation in the flue and on the glass, needing less frequent sweeping. A potential bad point is that there will be a lot of thermal cycling on the stove, potentially shortening its life and leading to cracking. Also, only a certain proportion of the stoves 7kW output will be going to the back boiler and heat store. The amount will be depend on the stove model. The rest will be going direct in to the boat cabin. You may find that running it flat out in normal winter weather makes the boat insufferably hot, long before the heat store is up to temperature. If that is the case you'll end up running the stove at less than 100% for longer, which makes the case for having a heat store hard to justify, when you could have installed a cheaper more conventional system.

 

Jen

[Schwaa]

1 hour ago, peterboat said:

Hi send me a PM with your phone number I have a similar system on my widebeam it's easier to chat about it than endlessly exchanging mail Peter 

Thanks very much Peter, I've sent you a PM. A chat would be excellent.

 

15 minutes ago, Jen-in-Wellies said:

One potential problem with the system as designed is starting up from cold on a cold day. For example, you go away for a week in winter and come back to a stone cold boat and heat store. It is going to take many hours heating up the 190l before you get any heat going in to the boat and you'll be shivering in the cold. I get this on my boat. The 5KW stove has a back boiler that gravity feeds a calorifier, followed by finrads. When starting up again from cold, most of the stoves heat is dumped in to the calorifer, warming around 60l of water. It takes several hours before the stove produces significant heating in the cabin around it, or from the finrads. You'll have over three times the volume of water to heat, with only 2kW of extra stove power. Could mean shivering in the cold for quiet a while.

 

As I understand it, the idea of having the heat store in winter is that the stove can be run hard to heat the store, then allowed to go out and the heat store will supply the radiators and heat hot water until it cools down. Those of us with more conventional systems will bank the fire up and run it with the air intakes low so that it provides moderate heat all the time. Your proposed approach has good and bad points. Good point is that running the stove hard will reduce the amount of sooting and clinker formation in the flue and on the glass, needing less frequent sweeping. A potential bad point is that there will be a lot of thermal cycling on the stove, potentially shortening its life and leading to cracking. Also, only a certain proportion of the stoves 7kW output will be going to the back boiler and heat store. The amount will be depend on the stove model. The rest will be going direct in to the boat cabin. You may find that running it flat out in normal winter weather makes the boat insufferably hot, long before the heat store is up to temperature. If that is the case you'll end up running the stove at less than 100% for longer, which makes the case for having a heat store hard to justify, when you could have installed a cheaper more conventional system.

 

Jen

Starting from cold will always take longer with any system of course but these are really good points, thanks. Just to clarify, the stove is in fact rated at 12kw total, with roughly 7 going to boiler. It is in the main living space and the main source of heat. The living room radiator is an afterthought in many ways, and a relatively small additional expense for greater flexibility. In reality, the stove would be run hard in the evenings, but still kept ticking over full time in the winter, as is conventional.

 

12 hours ago, Tacet said:

Once the storage tank drops below, say 40 degrees, it is not going to be much use for heating or hot (warm) water. Taking 90 degrees as the maximum storage temperature, gives a usable range of 50 degrees.

 

From it taking 1 calorie to raise 1 gramme by 1 degree – (a calorie is 4.184 Joules, and Joule is a watt second) means that the store will hold a theoretical 9.3 kwh.   You could payback at, say, 0.5kw over the 18 hours that the engine/stove was not operating.

 

But allowing for say, 50 litres for the DHW – and you are down to 6.4 kwh – or about 1/3 kw for the 18 hours.  I doubt this is enough to keep a boat (well, a sufficiently sized boat to hold the kit) on even a typical winter day.  It’s around the same output as a single panel radiator of 400 x 500.  Maybe 1 or 2kw would be a reasonable target?

 

Allowing for various inefficiencies and you could be needing maybe 5 or 10 x 190 litres to make this work.  And, of course, you need to find the energy from somewhere to shove it back in.

 

I am all for other people experimenting with new ideas, but there is much to be said for the solid fuel stove (with or without a back boiler) gently producing 24 hours a day.

 

All assuming the numbers are anywhere near correct.

and

12 hours ago, nicknorman said:

Secondly, have you calculated how much usable heat is actually stored in 160l of hot water and thus how much heat it could put into the boat?  I suggest that it is "not a lot" and you are adding a huge amount of complexity, weight and lost space for very little energy storage. Without doing any calcs myself, my gut feeling is that the amount of kWH stored might keep the rads hot for about an hour, warm for a bit longer. And of course as the temperature in the thermal store decreases, so the ability to dump heat into the cabin decreases (because heat transfer is proportional to the temperature difference). I think you could work on 4 or 5 kw needed to keep a widebeam reasonably warm. If you have 160l or water at say 80C and extract 4kw of heat from it, how rapidly does it cool? Sums say that 160l of water will cool 25C in an hour if you extract ~4.5 kw from it. So  by the end of the first hour, if you can manage to continue to extract 4.5kw, the temperature will be down to 55C.. Another hour (if you can still manage to extract 4.5kw) will see it at 30 which is barely tepid.

 

Unfortunately storing heat in water is very inefficient (in terms of mass and volume), compared to storing chemical heat energy in diesel/gas/coal.

 

I'm all for new ideas and ways of doing things, but they have to pass the tests of basic physics first!

again, really good points and making me triple check all of my thinking, thanks.

 

In reality, the rads will not be expected to do much during the day. They are primarily for putting some heat into the bedrooms in the evening and first thing. I've done loads of calculations for different scenarios and I'm more concerned about being able to dump enough heat in the evening when the store is at 90 and the boiler is still going strong!

 

13 hours ago, Jon12345 said:

In my opinion you don't need the vent pipe on the heat sink rad or on the gravity hot water flow. But to be safe and because it's easy would put a vent pipe there ( See Pic )  

I can see why that would make sense (on the TS thermosyphon), great input, thanks. Do you not think a vent on the heat sink side would help to reduce the risk of knocking in an overheat situation? Hmm, now I'm tempted to do both.

[Detling]

Something else to consider. Your engine running lightly loaded charging at whatever revs will not generate loads of heat to go through your heat exchanger. I have a heat exchanger, in the central heating circuit, and it is wonderful when we are cruising but at tickover passing moored boats or in locks/waiting, it exchanges more heat than the engine produces so the engine temp gradually declines. This is not good for the engine, it runs 85 to 90 degrees but on tickover will struggle to get above 65,  I have a switch to turn the pump off which solves my problem.  The Central heating is 4kWatts and the calorifier(30 litre) which heats up in under 30 mins. and my 42 HP engine obviously does not give out 4kW at tickover or just charging at 1500 rpm.

[cereal tiller]

17 minutes ago, Detling said:

Something else to consider. Your engine running lightly loaded charging at whatever revs will not generate loads of heat to go through your heat exchanger. I have a heat exchanger, in the central heating circuit, and it is wonderful when we are cruising but at tickover passing moored boats or in locks/waiting, it exchanges more heat than the engine produces so the engine temp gradually declines. This is not good for the engine, it runs 85 to 90 degrees but on tickover will struggle to get above 65,  I have a switch to turn the pump off which solves my problem.  The Central heating is 4kWatts and the calorifier(30 litre) which heats up in under 30 mins. and my 42 HP engine obviously does not give out 4kW at tickover or just charging at 1500 rpm.

A good example of this was when Moonraker Yachts used Matrix Heaters connected to Each one of a pair of Perkins 145 Bhp Turbo Diesels in their 36 Foot motor Boats.

At Sea when the Engines where at 75% or more power output the Heaters would blow nice warm air but on rivers and canals the Heaters would emit tepid Air only

[WotEver]

14 minutes ago, cereal tiller said:

This is not good for the engine, it runs 85 to 90 degrees but on tickover will struggle to get above 65...

Is the circuit not after the engine stat then?

[cereal tiller]

Just now, WotEver said:

Is the circuit not after the engine stat then?

 

1 minute ago, WotEver said:

Is the circuit not after the engine stat then?

Those old Perkins were seriously Overcooled and the were 1950's Technology ,the take off was before the Thermostat so the Engines never got to a decent operating Temp.whether the Matrix Heater was in circuit or not.

[Detling]

On 20/11/2018 at 13:13, WotEver said:

Is the circuit not after the engine stat then?

Yes but the stat once open seems not to shut (it must do sometime but only when the engine is stopped and has cooled down). So engine heats up then stat opens and engine cools down but not enough to shut the stat, so it runs cool.

[Woocash]

Hi, was there any update on this thread at all? Was the system a goer? Thanks.

[WotEver]

1 hour ago, Woocash said:

Hi, was there any update on this thread at all? Was the system a goer? Thanks.

Satisfactorily answered in post 37 I believe.

[Woocash]

3 hours ago, WotEver said:

Satisfactorily answered in post 37 I believe.

Thanks, but I’m not sure what you mean. I was referring to the OP’s thermal store system. I wish to utilise something similar, but I may start another thread.

[WotEver]

29 minutes ago, Woocash said:

Thanks, but I’m not sure what you mean. I was referring to the OP’s thermal store system. I wish to utilise something similar, but I may start another thread.

That would be your best bet rather than resurrecting a 2 year old one  

[Bee]

My thoughts are that it might work but bitter experience tells me that almost every bright idea that I've ever had or clever scheme I have installed in a boat ends up as a heap of scrap on the towpath and the money it cost would have been better spent on a few bags of coal rather than chasing a few percentage points of efficiency.