Back boiler/Wood burning stove and gas boiler with thermal store?

[Tony99]

I recently posted about heating requirements and got some very helpful answers. They also mentioned a thermal store which I was unaware of before. We are building a well insulated fairly air tight house-mainly 2 of us in it but 3 or 4 in the uni holidays. We will be on mains gas and will be fitting at least 4 kW PV-possibly more and having UFH. Looking at previous posts, an UVC set up has been recommended. 

We are looking at having a log burner and I appreciate that we probably won't use it often (esp after reading posts on this site!) but we love using our current log burner at weekends etc. We are going to have loads of wood available due to trimming and cutting trees down for the build so I was wondering:

 

is having a wood burner/back boiler with a thermal store a sensible idea or is it just feeding my 'fantasy' of using 'free' wood to heat water/the house?!

 

There do seem to be various companies that specialise in thermal stores particularly for wood burners and PV but I get the feeling that they are mainly aimed at people who are using the wood burner as a primary heat source i.e. not on mains gas. We would be looking to use the wood burner to heat a thermal store in winter when there is little PV available but probably only on some days, knowing that we have the gas boiler to top up or use as the main heat source if we aren't in or forget to load the burner! 

And how much do I actually need to burn to keep a ?300 litre thermal store hot? I can't see myself using the wood burner every day in the winter-perhaps just weekends.

 

The problem is that speaking to the guys selling the thermal store/wood burners (Chelmer Heating, McDonald, TMS etc) only results in them telling me how good the system is! I'm actually wondering whether it is an expensive, time consuming and complex way to use 'free' wood! As you probably realise, I am not a plumber/competent DIYer so any system would have to be fairly straight forward so that if it did go wrong, a competent plumber could sort out!

Many thanks for any advice!

 

[Carrerahill]

A friend uses a Bosky, installed it all himself, couple of tanks in the loft, has heated his house for free for years. Wasn't overly costly to install, UFH in his kitchen etc. he has immersion heaters for electric water heating in the summer. His situation works well for him though, unlimited access to wood, lives on a smallholding with a tractor and log-splitter so he can prepare the firewood himself. He also uses some smokeless ovoids in the winter so he can leave it unattended for longer but only £100 a year or so in smokeless.

 

These look good too, interesting video of an old guy on YouTube who has one of these installed and has had it for 40 years or something:

 

https://www.garn.com/

 

 

Edited April 2, 2019 by Carrerahill

[billt]

1 hour ago, Tony99 said:

IWe will be on mains gas and will be fitting at least 4 kW PV-possibly more and having UFH. Looking at previous posts, an UVC set up has been recommended. 

We are looking at having a log burner and I appreciate that we probably won't use it often (esp after reading posts on this site!) but we love using our current log burner at weekends etc. We are going to have loads of wood available due to trimming and cutting trees down for the build so I was wondering:

 

is having a wood burner/back boiler with a thermal store a sensible idea or is it just feeding my 'fantasy' of using 'free' wood to heat water/the house?!

 

Yes, it's just feeding a fantasy.

 

You will be spending a lot of money on a system which will give you no benefit, given that you are on mains gas and not burning wood frequently. You will also end up with a needlessly complex and bulky system. (Oh, and wood is a polluting fuel.)

 

If you want to burn wood, just use a stand alone heater, it's a lot simpler and a lot cheaper.

 

We've been burning wood to heat our house for nearly 10 years, using a log boiler and a large thermal store supplemented by feeds from solar thermal panels, a PV fed immersion heater and a Rayburn (with provision for adding an oil boiler).

 

Despite having free fuel I wouldn't do it again. The system is complex, bulky and a lot of work.

[Vijay]

@billt, have you been using the log boiler to supply UFH and DHW?

[PeterW]

@Tony99 you could use a big - 800 litre or so - thermal store with a number of coils to segregate the heating requirements and sources. 

 

I would have this in the garage (!!) but make sure it’s well insulated and it won’t take up floor space or potentially overheat the property. 

 

I would run the TS directly connected to the back boiler, and this would be open vented. 

 

Coil 1 - at the top - is your DHW mains pressure coil, and no moving parts to get blended hot water.

 

Immersion 1 - below this coil to provide a hot water boost if required, and the primary immersion for PV

 

Coil 2 - gas powered coil from the boiler, means you can heat just the top 2/3 for DHW

 

Immersion 2 - below the second coil, this is the second dump load for PV. 

 

Coil 3 - this is the UFH coil, and I would link it to coil 2 using a motorised valve for the ability to push the boiler feed directly into the UFH if required if the TS is below temperature to heat the floor. 

 

I’d say this isn’t overly complex, but big thermal stores have big losses so you need to manage this carefully. 

[Tony99]

@PeterW That's a big TS....the TS websites I looked at were stating sizes of around 300 litres.....I suppose something smaller would loose heat too quickly compared to 800 litres?? Our garage will be several yards away from the house so would be difficult, I imagine, to house the TS there but we could find a space for it in the utility room with a bit of imagination...

[Tony99]

3 hours ago, billt said:

 

Yes, it's just feeding a fantasy.

I'll tell my wife-she'll say 'I told you so'!! But seriously, I very much do appreciate your honesty: it is very helpful to have your views as you are doing this first hand. I think with lots of aspects of a self build, pretty much anything is possible-it boils down to cost/complexity/and how much one really wants something! 

[Carrerahill]

23 minutes ago, Tony99 said:

I'll tell my wife-she'll say 'I told you so'!! But seriously, I very much do appreciate your honesty: it is very helpful to have your views as you are doing this first hand. I think with lots of aspects of a self build, pretty much anything is possible-it boils down to cost/complexity/and how much one really wants something! 

 

There is the flip side in my reply.

 

Nothing wrong with heating with wood. When the power plants go down and gas is banned people with wood heating will be the ones that survive the cold winters!

[Alexphd1]

We have 2x 500l thermal stores in the house heated by 20kw log gasification boiler in garage and solar thermal . PV/ashp to be added. Log and thermal seem to work pretty well. All done DIY. I am slightly concerned about hooking up the ashp to 1x 500l thermal store for ashp efficiency.

 

We are installing a bioethanol fire in the house for the caveman instinct of light a fire to be warm.

[billt]

5 hours ago, Vijay said:

@billt, have you been using the log boiler to supply UFH and DHW?

 

In a manner of speaking.

The thermal store is used to decouple the heat source from the heat sinks, among other things. All the heating requirements are met from the thermal store; there are 2 coils to supply DHW, there's a weather compensated supply to feed the radiators and another supply to feed the 2 UFH areas.

 

If it's not too cold and there's a fair amount of sun the solar thermal panels and diverted PV supply more than enough energy. The boiler hasn't been lit since the 22nd of March. Unfortunately it had to be lit this afternoon. Even in the depths of winter it doesn't usually have to burn for more than 8 hours a day (3 fills). Last year the last burn was on the 28th of April and the next one was on the 9th of October.

 

Bear in mind this is in an old house with a much higher heat load than a modern house should have.

 

It's possible to justify a system like this if you need to spend a lot of money on fuel. In a modern house heated by gas I'd expect the annual heating cost to be pretty low. Spending multiple thousands on a complex wood system isn't going to make financial sense.

[Carrerahill]

On 02/04/2019 at 17:08, Alexphd1 said:

We have 2x 500l thermal stores in the house heated by 20kw log gasification boiler in garage and solar thermal . PV/ashp to be added. Log and thermal seem to work pretty well. All done DIY. I am slightly concerned about hooking up the ashp to 1x 500l thermal store for ashp efficiency.

 

We are installing a bioethanol fire in the house for the caveman instinct of light a fire to be warm.

Photos! Sounds good. I intend to go down a similar route but I will keep a real log burner in the living room.

Edited April 3, 2019 by Carrerahill

[Alexphd1]

Unfortunately that's not our photo but that's the idea.

[Nickfromwales]

One thing that's not really hammered home is a log gasification boiler commands an RHI revenue. Much as I think the system is perverse it's there for the taking.

If you have wood to burn.....

On ‎02‎/‎04‎/‎2019 at 11:03, Tony99 said:

We are going to have loads of wood available due to trimming and cutting trees down for the build

…..that is not sustainable so is not to be considered a source of fuel ……….

…...then I would say go for log gasification every day of the week.

On ‎02‎/‎04‎/‎2019 at 17:08, Alexphd1 said:

We have 2x 500l thermal stores in the house heated by 20kw log gasification boiler in garage and solar thermal

I have just done an install with a 40kW LGB and a 2600 L buffer tank. The results are staggering, with 2 barrows of wood per day heating a 200 year old stone house ( 3 floors ) which is as energy efficient as a block of swiss cheese. Got the chap off oil, so a projected saving of >£30k over 30 years, a PV install ( 9kWp ) with a revenue of >£20k over 25 years and £12k of RHI off the LGB install. 

I really like these things now, and would recommend anyone considering setting fire to wood to do so with an LGB. RHI, it's outside the house, and the things are crazy efficient as they incinerate what they burn.

 

You'll need wood....lots of it.....and not just what you cleared or have laying around, so choose wisely. Buying wood is not an option.

 

 

 

[Tony99]

@Nickfromwales Yes-the wood available would probably last a few years/seasons just using it now and again for a log burner: certainly not enough wood to use regularley in a log boiler/gasification i.e. using wood as the main source of fuel if we were not on gas. I suppose what I was thinking of was having a mains gas boiler but also having a WBS that could be used sometimes in the winter to 'top up' a thermal store or similar so I wasn't using the gas boiler all the time. I appreciate it would actually be more expensive to have all the hardware/plumbing in place and that savings on gas would be fairly minimal. But it is a lovely idea-to be using wood as a source of heating!

[Nickfromwales]

3 hours ago, Tony99 said:

But it is a lovely idea-to be using wood as a source of heating!

In a nutshell, yes Ask your neighbours and you may get a different reaction ?

3 hours ago, Tony99 said:

I appreciate it would actually be more expensive to have all the hardware/plumbing in place and that savings on gas would be fairly minimal.

I doubt you realise quite HOW expensive / obtrusive that kind of installation would actually be..... A WBS is not eligible for RHI so its just a "thing of beauty" which occasionally heats a bit of water. Unless you have a serious sized, part year round redundant TS then forget gleaning heat and just have a WBS that heats the room its in / immediate surroundings and save a shit-load of cash.

You'll need a BIG galvanised header tank and then to get that plumbed in through the house to the stove, then install the TS with multiple coils etc and on and on. If the heart really wants it, it'll get it, but the head will likely disagree, along with the wallet  

[Tony99]

22 hours ago, Nickfromwales said:

In a nutshell, yes Ask your neighbours and you may get a different reaction ?

I doubt you realise quite HOW expensive / obtrusive that kind of installation would actually be..... A WBS is not eligible for RHI so its just a "thing of beauty" which occasionally heats a bit of water. Unless you have a serious sized, part year round redundant TS then forget gleaning heat and just have a WBS that heats the room its in / immediate surroundings and save a shit-load of cash.

You'll need a BIG galvanised header tank and then to get that plumbed in through the house to the stove, then install the TS with multiple coils etc and on and on. If the heart really wants it, it'll get it, but the head will likely disagree, along with the wallet  

I do agree @Nickfromwales....!  Of course one of the good things I've found about this forum is getting a pretty unbiased opinion about many aspects of self build for a complete novice self builder like myself. Posting on here and reading some posts is a bit like thinking aloud and having helpful comments/reality check coming back. So thank you all for comments ?.

 Isolated WBS it will be [and probably won't use it that much anyway!!]-and save a lot of cash!!