CH/DHW system using Thermal Store and Stove backboiler

[Schwaa]

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

[Boater Sam]

Does the boat ever move?

 

Not a facetious question. If you intend to move how are you going to power  2 circulation pumps?

 

If its a boat, you cannot expect to run domestic land based systems if not  connected permanently to a land line.

[Jen-in-Wellies]

Just a couple of thoughts. The 190l thermal store is going to be very heavy. Ballasting of the boat and the location of this and other heavy components would need to be carefully planned to stop the boat having a permanent list.

From the description, this boat is planned to not be on a shore line. It would need a very substantial alternator to run even a 1kW immersion heater. Such a small immersion would take a very long time to heat 190l. The engine cooling circuit would be doing most of the heating. Better to let the alternator just charge batts.

How much solar photovoltaic capacity is being planned? Would need a lot for the immersion to be useful as an excess power dump.

 

Jen

Edited November 19, 2018 by Jen-in-Wellies

[Schwaa]

They are 12v pumps. Yep, I move a bit and I dont have shore power on my permanent mooring, it's all off grid 

[Jen-in-Wellies]

Forgot to say  welcome to the forum!

[Schwaa]

1 minute ago, Jen-in-Wellies said:

Just a couple of thoughts. The 190l thermal store is going to be very heavy. Ballasting of the boat and the location of this and other heavy components would need to be carefully planned to stop the boat having a permanent list.

From the description, this boat is planned to not be on a shore line. It would need a very substantial alternator to run even a 1kW immersion heater.

How much solar photovoltaic capacity is being planned? Would need a lot for the immersion to be useful as an excess power dump.

 

Jen

Hi and thanks.

 

I'm taking the weight of the store into account in the layout.

Domestic alternator is 175a. I'm thinking of using a 240v 1kw element through a pure sine inverter, I figure I can spare the overhead as engine will be running or solar 'over-producing' when it is operating.

I'm planning 1.2kW of solar (with an 880ah 12v battery bank). This is something I'm wondering about... willl it ever actually spill over? (I know it depends how much power i use)

[Boater Sam]

Pumps and immersion element are available in 12v but they still use the same watts as mains ones, the current will be 20 times what it would be  when  they were mains.

Just how are you going to charge batteries fast enough?  Inverters are less than 100% efficient.

 

Solar in winter? Forget it.

Edited November 19, 2018 by Boater Sam
added more

[Schwaa]

2 minutes ago, Boater Sam said:

Pumps and immersion element are available in 12v but they still use the same watts as mains ones, the current will be 20 times what it would be  when  they were mains.

Just how are you going to charge batteries fast enough?

The pump on the engine calorifier circuit will only be running when the engine is, i'm thinking of putting it on the starter alternator/battery circuit (twin alternators).

The ch pump will need adequate cable but the draw is not crazy, can't remeber exactly, around 2 amps I think, and the run is approx10m one way. It wont be on anything like 24/7. I dont think this is a real concern but thank you, I want to be thoroughly grilled as I want to make sure I havent missed anything!

[Schwaa]

The boat's got twin alternators, 75a and 175a. It has occurred to me that the starter alternator has a lot of spare power once it's topped off the battery, which could perhaps be diverted to a, say, 600w 12v element all of its own. Has anyone done or heard of such a setup? The cables would of course need to be huge in my case.

[Boater Sam]

Do a power audit. You will be surprised how much power you need and how long your engine will need to run everyday to maintain your batteries.

[Jen-in-Wellies]

Have you considered solar thermal for your summer hot water needs? Dedicate your 1kW plus of photovoltaics to battery charging and let a thermal panel heat the water. Much more efficient than photovotaic pqnels for getting heat in to water. That way, while the sun shines in summer there may be no need to run the engine for days on end.

A recent topic:

 

[WotEver]

17 minutes ago, Schwaa said:

I'm planning 1.2kW of solar...

So in the summer you can expect an average of around 500Ah at 12V per day from that setup. In winter, 25Ah per day. Just some realistic figures for you to work with. 

6 minutes ago, Schwaa said:

The boat's got twin alternators, 75a and 175a. It has occurred to me that the starter alternator has a lot of spare power once it's topped off the battery, which could perhaps be diverted to a, say, 600w 12v element all of its own. Has anyone done or heard of such a setup? The cables would of course need to be huge in my case.

Just parallel up the alternators. If the batteries are in bulk then the two alts will give you up to 250A. If the batteries aren’t in bulk then you don’t need both alts anyway. 

[Alan de Enfield]

24 minutes ago, Schwaa said:

Domestic alternator is 175a.

You may well be aware , if so apologies.

Many people think that a 175A alternator will 'put in' 175aH in an hour, 350Ah in 2 hours etc etc

 

It doesn't - just as an example, 

You may get 150a for the first 30 mins, then 100a for the next 30 mins, then 50 a for the next 30 mins then ever decreasing amounts such that it may well take 6-8 hours to put back (say) 350Ah.

the last 10-15% takes 'forever'.

 

Remember the alternator rating is based on about 6000 RPM

Edited November 19, 2018 by Alan de Enfield

[Bee]

I have successfully put heating systems in 4 boats. They were all thermosyphon systems run from small stoves with backboilers. Simple, boringly reliable, quick to heat, cheap, easy (ish) to install and would work when batteries went flat in January and when the gas froze. As for hot water, Palermo gas heaters were good, not sure what is available these days. There is a lot to be said for keeping things as simple as possible.

[Tony Brooks]

It all seems needlessly complicated to me. Why not get a huge twin coil calorifier made with extra insulation if required. Then pipe the radiator circuit for gravity circulation so no pump required. The engine water pump will push coolant through the second calorifier coil so again no pump required. Then you will not be cooling your hot water by drawing it off to circulate through the rads. If you specify a three coil calorifier you could then use a low current 12V solar pump and solar water panel to heat the water as well.

 

A recent topic showed (I think) electricity from on board means cost about £14 per kWh. That makes an immersion very expensive to run unless on a shoreline.

 

There are good reasons boats use the systems they do but it does not stop people regularly trying to reinvent the wheel. Remember that when away from a shoreline it is best to consider ANY form of electrically derived heat as a fast way to destroy batteries unless you are very, very rich or exceptionally careful. Battery charge tends to be in short supply so every Amp hour you use for something else is an Amp hour less charging.  The only way an immersion heater will work is of you ensure it is never on when the batteries are charging at high current and you only ever power it with the difference between the charging current and the alternator/generator's/solar maximum output. Remember to fully charge a lead acid battery will take at least 8 hours and likely longer.

[Schwaa]

Thanks for the input so far, its much appreciated. I'm pretty good with electrics, it's the plumbing I'm more concerned about, and the overall safety of the system.

 

2 minutes ago, Jen-in-Wellies said:

Have you considered solar thermal for your summer hot water needs?

 

 

I havent given it too much thought to be honest, mainly because I've known others to try and be disappointed. I'll give that a read though, looks interesting.

I dont really intend to rely too heavily on solar for DHW anyway and I'm happy to run the engine for an hour or so most days for hot water and a battery boost all year round.

 

[Boater Sam]

1 minute ago, Schwaa said:

Thanks for the input so far, its much appreciated. I'm pretty good with electrics, it's the plumbing I'm more concerned about, and the overall safety of the system.

 

 

I havent given it too much thought to be honest, mainly because I've known others to try and be disappointed. I'll give that a read though, looks interesting.

I dont really intend to rely too heavily on solar for DHW anyway and I'm happy to run the engine for an hour or so most days for hot water and a battery boost all year round.

 

With thar charging routine you are going to destroy a lot of batteries very quickly!

[Schwaa]

22 minutes ago, WotEver said:

So in the summer you can expect an average of around 500Ah at 12V per day from that setup. In winter, 25Ah per day. Just some realistic figures for you to work with. 

Thanks, in the winter the solar wont be contributing to the heating system at all. the stove will be doing most/ all of the work.

[WotEver]

Just now, Schwaa said:

Thanks, in the winter the solar wont be contributing to the heating system at all. the stove will be doing most/ all of the work.

It won’t be contributing to charging the batteries by any significant amount either. Just so’s you’re aware. 

[Schwaa]

1 minute ago, Boater Sam said:

With thar charging routine you are going to destroy a lot of batteries very quickly!

Not in the summer, when the panels are outputting. In the winter i rely much more (entirely) on the engine for power anyway, it gets run for a full day every few days

[Sea Dog]

2 minutes ago, Schwaa said:

Not in the summer, when the panels are outputting. In the winter i rely much more (entirely) on the engine for power anyway, it gets run for a full day every few days

Ooh, that's unlikely to win you many friends! 

 

This looks like a very complex and expensive solution to a problem most boaters get over far more simply and cheaply.  That said, I rather like these experimental boat ideas - one day someone might actually pull one off! 

[Schwaa]

12 minutes ago, Tony Brooks said:

It all seems needlessly complicated to me. Why not get a huge twin coil calorifier made with extra insulation if required. Then pipe the radiator circuit for gravity circulation so no pump required. The engine water pump will push coolant through the second calorifier coil so again no pump required. Then you will not be cooling your hot water by drawing it off to circulate through the rads. If you specify a three coil calorifier you could then use a low current 12V solar pump and solar water panel to heat the water as well.

 

A recent topic showed (I think) electricity from on board means cost about £14 per kWh. That makes an immersion very expensive to run unless on a shoreline.

 

There are good reasons boats use the systems they do but it does not stop people regularly trying to reinvent the wheel. Remember that when away from a shoreline it is best to consider ANY form of electrically derived heat as a fast way to destroy batteries unless you are very, very rich or exceptionally careful. Battery charge tends to be in short supply so every Amp hour you use for something else is an Amp hour less charging.  The only way an immersion heater will work is of you ensure it is never on when the batteries are charging at high current and you only ever power it with the difference between the charging current and the alternator/generator's/solar maximum output. Remember to fully charge a lead acid battery will take at least 8 hours and likely longer.

Hi Tony, thanks,

 

This system looks complicated but it's just combining the engine's heat output with the stove's in one place to serve both radiators and DHW. It is bringing together already very common boat systems and replacing the typical calorifier tank with a thermal store. Apologies if my diagram makes it look more complicated than it is!

 

In the winter there will be no more electrical demand than in most boat heating systems, just the ch pump for a few hours each day, less than many boats I've known. the summer is trickier although it will only be providing DHW so input will nned to be considerably less.

1 minute ago, Sea Dog said:

Ooh, that's unlikely to win you many friends! 

Haha, I'm not trying to offend. Why?? What do most people do for power in the winter on a boat?

[Alan de Enfield]

21 minutes ago, Schwaa said:

Not in the summer, when the panels are outputting. In the winter i rely much more (entirely) on the engine for power anyway, it gets run for a full day every few days

If looking to use electric power I think the concensus will be that you will need about twice that.

It all depends on your audit, but I would have thought 6 hours + EVERY day is more than likely needed to meet your needs,

 

Just remember that to Heat 1 litre of water by 1 degree C = 0.00115Kwh

You are looking at 190 litre cauliflower.

 

It will take approx. 18Kw to heat 190 litres from 10 degrees C to 90 degrees C.

 

18 Kw at 12 volts is about 1500 AH (one thousand five hundred Ah)

To support that water heating alone you will need some 6,000Ah of battery bank, and some method of producing 1650Ah ( usage +10%) to put back into the batteries.

 

I realise that some heat will be going into the cauliflower / heat store from the engine water,

But if using simply electric power …………………………………………….

 

The numbers are so huge I may have slipped a decimal point somewhere - could a responsible adult check my working ?

Edited November 19, 2018 by Alan de Enfield

[WotEver]

9 minutes ago, Alan de Enfield said:

The numbers are so huge I may have slipped a decimal point somewhere - could a responsible adult check my working ?

While we wait for one I can confirm your maffs. 

[Schwaa]

2 minutes ago, Alan de Enfield said:

I think the concensus will be that you will need about twice that.

It all depends on your audit, but I would have thought 6 hours + EVERY day is more than likely needed to meet your needs,

 

Just remember that to Heat 1 litre of water by 1 degree C = 0.00115Kwh

You are looking at 190 litre cauliflower.

 

It will take approx. 18Kw to heat 190 litres from 10 degrees C to 90 degrees C.

 

18 Kw at 12 volts is about 1500 AH (one thousand five hundred Ah)

To support that water heating alone you will need some 6,000Ah of battery bank, and some method of producing 1650Ah ( usage +10%) to put back into the batteries.

 

The numbers are so huge I may have slipped a decimal point somewhere - could a responsible adult check my working ?

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.