Barn Conversion Devon - Heating

[ChrisDL]

Afternoon everyone,

 

I have been dipping into the BuildHub Forums for quite a while and found it a great resource. Now though I think I am at the point in our build to ask for some direct opinions and ideas.

 

Briefly:

We are building a new home based around a class Q barn conversion. Currently we are up to DPC and having to make some decisions which I could do with some help with.

 

My main consideration at this point is heating. We will in all likelihood be Off-Grid for electricity purposes relying on a very large PV, batteries and back up generator, possibly a wind turbine if I can find a reasonably sized and cost effective option.

 

The result of this decision is that we can not rely solely on electricity for heating purposes. We do intend to fit a GSHP or ASHP (I have been around and around in forums on the merits of GSHP vs ASHP and believe I have settled for ASHP) to utilise the electricity generated from PV if the batteries are full. We will have to rely on more 'traditional' heating methods during the winter. Whether that be oil or gas. However we have also planned to install a wood burner and I thought it may merit being a boiler stove so we can dump the heat generated into a store to be used more purposefully in DWH and the UFH - this will mean installing a Thermalstore rather than a UVC.

 

My question is:

 

Is it worth bothering with a wood stove boiler and the thermal store?

 

• My plumber has advised that the wood stove will burn less cleanly until the return temperature is up to about 40 °C (however I don't see this being a massive problem since the store should never be starting from cold unless we have completely run out of alternative sources)
• Thermalstore vs UVC, seems to be a lot of opinions on this. Simplicity vs efficiency - common conception is that the thermalstore will leak more heat than a UVC - (recommendations on the highest insulated and performing ones welcome)
• Safety - Wood stove will need a pump to return the heated water - if this fails we don't want superheated water spraying over us.
•  For all the aforementioned considerations, even if overcome or positive, to save a few £100 a year on heating oil or gas are we overcomplicating the system when a UVC connected to a oil/gas boiler and ASHP will suffice?

 

I should mention that our objective is not going to be shovelling wood into the stove all day to offset the use of oil/gas. We will light it when it is cold/coldish/ and we want the atmosphere of a fire. I have been told by many overheating with a wood burner is likely to be our biggest worry, hence another reason to fit the stove boiler so the majority of the heat is absorbed into the water rather than towards the room.

 

Any thoughts and opinions very welcome,

 

Thank you

 

Chris

 

 

[joe90]

Hi and welcome Chris, firstly there is a big divide here on wood stoves, some here have one (like me) and others are vehemently against burning anything 🤷‍♂️. All I can say is my wood stove (non boiler) is for chilly winter evenings, looks good and as yet never overheated my near passive build. I have not installed PV yet but plan to. Being off grid is far more challenging and others here will chip in soon. Yes ASHP rather than GSHP IMO. Do post up photos and remember there is no such thing as a stupid question, stupid is not asking. The advise here is born from actually doing it, not sales talk. Looking forward to more info and progress 👍

[Nickfromwales]

26 minutes ago, joe90 said:

Hi and welcome Chris, firstly there is a big divide here on wood stoves, some here have one (like me) and others are vehemently against burning anything 🤷‍♂️. All I can say is my wood stove (non boiler) is for chilly winter evenings, looks good and as yet never overheated my near passive build. I have not installed PV yet but plan to. Being off grid is far more challenging and others here will chip in soon. Yes ASHP rather than GSHP IMO. Do post up photos and remember there is no such thing as a stupid question, stupid is not asking. The advise here is born from actually doing it, not sales talk. Looking forward to more info and progress 👍

+1

 

1 hour ago, ChrisDL said:

My plumber has advised that the wood stove will burn less cleanly until the return temperature is up to about 40 °C (however I don't see this being a massive problem since the store should never be starting from cold unless we have completely run out of alternative sources)

You‘d fit a thermostatic bypass aka Laddomat arrangement which lets the heat source recirculate back over until the device is up to a set temp. Only then does the heated water get sent to the store   

You may need a plumber who better understands this system, as that is quite basic knowledge in the world of wood-burning for heating.  
Eg;

https://www.ebay.co.uk/itm/Laddomat-11-100-for-wood-boilers-up-to-80kw-Biomass-Wood-heating-Burning-/163652025906?mkcid=16&mkevt=1&_trksid=p2349624.m46890.l6249&mkrid=710-127635-2958-0

 

Water will heat very quickly from the WBS, and that will likely not absorb all of the heat created by a burn, so beware of your thoughts there. 
 

If you’re going to fit an ASHP ( I’d defo not install a GSHP unless it’s a huge property ) then you’ll get an UVC anyways for DHW. Do away with the thoughts of getting DHW from the WBS then, as you sound like you’re not true “off-griddy” folk and want some life away from cutting, storing, drying and conveying wood to burn? If using wood for winter heating, you should budget about 1/2 a wheelbarrow a day, possibly 3/4, so would you be able to source and process that amount anyways? 

 

For DHW and heating you could just do away with LPG / oil auxiliary heating and just put all that money into PV and batteries to run the ASHP for heating snd hot water. 
 

Is the slab down with UFH pipes cast in? Is it a decent chunk where you can load shift heat energy into it? 

[SteamyTea]

You could use a CHP unit to generate electricity and supply thermal energy.

A diesel one would be reasonably cheap to run.

[JohnMo]

If you do intend to go thermal store, with an ASHP, you need to ensure you have either specify very large coils, which may not be practical, but better to have either internal or external plate exchangers for heating the store and for DHW take off, size to suit the flow temperature from an ASHP.  You need exchangers that give max of 2 degree approach temp (difference between heat source supply temperature and the secondary side outlet temperature).

 

If you are heating the thermal store at ASHP temperature it will need to be larger, but heat loss should be similar to an UVC as it will be at a similar temperature.

 

Boiler stove adds complication, and is only worth the expense if you intend to use a lot.  On only when really cold may be a waste of money and effort. Just install a non boiler.

 

To save £100/year, from the boiler stove you would need to heat a 400L cylinder from 35 to 70 around 100 times a year.  Assuming your wood comes free.

 

Keep it simple, cheaper to install, easier to manage.  Spend the money saved on insulation.  Aim to get the heat demand at or below 15W/m2, then your UFH flow temps can be sub 30 degs all the time.

 

 

[joe90]

1 hour ago, JohnMo said:

Spend the money saved on insulation.  Aim to get the heat demand at or below 15W/m2, then your UFH flow temps can be sub 30 degs all the time.

+1

[Iceverge]

What are your exact motivations for hoping to live off grid? 

 

It's rarely the most economical or environmentally sound method. Independence from grid has a psychological boom until you arrive at the situation 5-10 years down the road when you have to scale a wind turbine after it breaks and wait for parts to arrive from the far side of the globe. Going to a petrol station for Jerry cans of petrol and running it in a small generator at <25% efficiency is hardly off grid either. 

 

A better method on my opinion, rather than isolating oneself from the world is to actively contribute to it with excess PV power and a grid connection. Turn your house into a mini clean power station. 

 

 

@Marvins mantra is on the money. 

 

AIM and then APE That is Airtightness, Insulation, Mechanical Ventilation with Heat Recovery, and Air Source Heat Pump, Photovoltaics and Electric Vehicle.

 

 

 

 

 

 

 

 

 

 

[Nickfromwales]

5 hours ago, JohnMo said:

If you do intend to go thermal store, with an ASHP, you need to ensure you have either specify very large coils, which may not be practical, but better to have either internal or external plate exchangers for heating the store and for DHW take off, size to suit the flow temperature from an ASHP.  You need exchangers that give max of 2 degree approach temp (difference between heat source supply temperature and the secondary side outlet temperature).

You just wouldn’t go for a TS with an ASHP for both heating and DHW as the CoP would be out the window trying to get the stored temp up high enough to producer DHW. ASHP option needs to be at lowest possible temps for maximum CoP, so the PV in winter ( sub 25% of the same kWp in the summer ) remains effective. For every 1kWp of PV you should be aiming to produce at least 3kWh of heat energy, ergo a 4kWp array should be worth 12kWh of heat energy ( so installing a minimum of 12kWp of PV should get you, albeit conservatively, through a winter ) 

I would look to the slab as my thermal store, and incorporate a huge amount of thermal energy buffering at very low temperature.

DHW should be via an UVC without a shadow of a doubt. Heating and DHW need to be segregated in an off-grid design IMHO.

Generator for winter deficit should be a water-cooled unit, with that waste heat fed into a second set of UFH coils, or, if room for it, fed into a buffer cylinder that is shared between the ASHP and the generator. The buffer should really be a huge cylinder ( 1000L+ ) for bridging grey days where there is little ‘spare’ solar revenue.

 

I aim to be ‘off-grid’ by next simmer, but grid tied for reliability / fail-safe.

[SteamyTea]

3 minutes ago, Nickfromwales said:

I would look to the slab as my thermal store, and incorporate a huge amount of thermal energy buffering at very low temperature.

Needs a huge amount of insulation under it, and to the sides, as well.

[Nickfromwales]

Just now, SteamyTea said:

Needs a huge amount of insulation under it, and to the sides, as well.

Absolutely.

Apologies for the lack of info, has been a very long and arduous day……

 

EPS is cheaper than shoplifting so no excuses for not achieving as you suggest, eg 300mm all round aka passive raft / similar ethos.

[Jenki]

9 hours ago, Nickfromwales said:

EPS is cheaper than shoplifting so no excuses for not achieving as you suggest, eg 300mm all round aka passive raft / similar ethos.

On my build I'm planning a DIY insulated raft. How can you achieve 300mm all around the raft?(edit: I'm talking about the sides)

Is it stepped? otherwise I visualise a big chunk of EPS visible at ground level, not even thinking about door thresholds?

Edited July 7, 2022 by Jenki
Clarity

[joth]

10 hours ago, Nickfromwales said:

Generator for winter deficit should be a water-cooled unit, with that waste heat fed into a second set of UFH coils, or, if room for it, fed into a buffer cylinder that is shared between the ASHP and the generator. The buffer should really be a huge cylinder ( 1000L+ ) for bridging grey days where there is little ‘spare’ solar revenue.

This sounds fraught with daily prediction challenges.

To get a bit of heat into the UFH you'd first need to get 1000L of buffer up to temperature, which may take several hours of sunlight to achieve on a gray day. Then if the house battery runs flat your going to have to fire up the diesel genny anyway, at which point it's trying to cool itself from an already preheated buffer. 

So ideally you want ASHP to bypass the large buffer in normal use and just pull it inline when the genny is running.

But while the genny is running you need to make a call whether to run the ASHP too (higher load on the genny probably pulls it into a more efficient operating range plus you achieve the ASHP COP) or not 

To run it efficiently the control system will be unconventional and complex.

 

Given the OP only says it is "probably" going to end up being off-grid, I'd first put a lot of effort into seeing if you can actually go ongrid before committing to elaborate multimodal heating design 🙂

 

[Nickfromwales]

Off grid requires prediction / human intervention, that will never be out of the picture. Controls are only complex when you don’t understand them. 
Yes, ASHP direct to heating at first, and when the slab is up to temp ( satisfied ) the ASHP can run over into the TS / buffer for absorbing every drop of sunlight via that energy multiplier. 
Diesel genny would obviously not lose its cooling fan, just a 3-port motorised valve to toggle between that or the TS. 
Heat from the genny is incidental, not routine, and it’s waste heat if not captured. Can be further diverted into a solar coil in the UVC ( if it ever runs in the summer ), as ever, each instance is unique. 

[JohnMo]

A 2 port buffer doesn't require to be heated first, as the main flow goes direct to the heating, anything not required for heating will the heat the buffer.  Once buffer is up to heat the heating will be drawn from buffer as the ASHP return temp will be satisfied.

[SteamyTea]

I think that one problem of being 'off grid' is that people expect to get the same controllable levels of comfort that we have all come to expect.

 

It really does not matter of your house swings 4°C in temperature day to day, or you have to take a short shower, instead of 3 baths.

 

When I was a student in the early 1980s, we had flat that had a 2 kW bar heater in it, that was it, apart from the cooker.

We survived, through studying, fresh vegetables (one flat mate was a farmer) and alcohol (2 other flat mates died from it, eventually).

 

Just a change of lifestyle, and thermal underwear.

[Roger440]

On 06/07/2022 at 17:56, Iceverge said:

What are your exact motivations for hoping to live off grid? 

 

It's rarely the most economical or environmentally sound method. Independence from grid has a psychological boom until you arrive at the situation 5-10 years down the road when you have to scale a wind turbine after it breaks and wait for parts to arrive from the far side of the globe. Going to a petrol station for Jerry cans of petrol and running it in a small generator at <25% efficiency is hardly off grid either. 

 

A better method on my opinion, rather than isolating oneself from the world is to actively contribute to it with excess PV power and a grid connection. Turn your house into a mini clean power station. 

 

 

@Marvins mantra is on the money. 

 

AIM and then APE That is Airtightness, Insulation, Mechanical Ventilation with Heat Recovery, and Air Source Heat Pump, Photovoltaics and Electric Vehicle.

 

 

 

 

 

 

 

 

 

 

 

If you are going run a generator, you want batteries too. Then, when it runs, it runs at full load, its peak efficency. Without batteries, then yes, its hoplessly inefficent.

 

I did look at an off grid property. This looked helpful. Full turnkey solution. https://www.energy-solutions.co.uk/residential

 

For the OP, if you have to be off grid, and you are spending the money anyway, just make more electricity and bin the wood burning and just use the ASHP. The marginal cost cant be much more assuming it resonably well insulated.