Wood for whole-house heating

[jamieled]

For our new build we will making use of the large quantity of timber that we have access to for heating.  While we are not planning on building a passive house as it is commonly known, we are planning on something well insulated and airtight.  For several reasons I would prefer not have any wet central heating system (cost, faff) other than a dump radiator if we need it.  Has anyone else done this or are we a bit too optimistic that this will suffice in a potentially fairly cold area of Scotland?   I'll be looking at heating requirements in more detail once we have a  finalised design, I'm keen on understanding whether anyone else has done this and what they think about it.  Did you use any other form of heating such as electric panel heaters in the bedrooms?  In winter it will also be doing the DHW.  The house will be ~130m²

 

We've had several varying opinions up until now ranging from no problem to bad idea.

[PeterW]

If you get a decent log boiler that takes big stuff such as 600mm logs, and couple it to a store of 3,000 litres then add in UFH you have the basis of a workable system. 

 

Asking a single stove to heat a house by convection alone is a tall order ...

[Alexphd1]

I have asked myself a similar question hundreds of times!!!

 

By the time you get enough insulation/air tight in your property for very little heating in bedrooms etc even a smallest log boiler overheats the room.  A log boiler does create a nice center peace.

 

 

Edited April 9, 2018 by Alexphd1

[Stones]

I know of one chap who built a house in Perthshire that initially went with a small WBS for his DHW and minimal heating (it was a passive standard house). IIRC he had electric towel rails in the bathrooms and had wired for electric heaters in the bedrooms. He  reduced the WBS output to the lounge by placing a couple of firebricks inside to direct more of the heat to the back boiler which fed a TS for his DHW. He ended up fitting a couple more radiators to the initial  heat dump to get a better distribution of heat in the house.

 

An easy option for you if you don't want full CH may be an electric post heater in your MVHR supply ducting.

[ProDave]

One that worked well in a house I wired a few years back was a stove that put 10KW to water while only putting 2KW into the room. This heated a massive thermal store that provided DHW and UFH. They only needed to light the stove every few days to heat the store.

 

What is it that puts you off wet UFH?

[jamieled]

@ProDave it's mainly that I don't like the idea that I might need a pump, and hence use electricity to heat the house, even if it's a fairly low running cost.  However,  we'd have a wood burner and you'd hope any power cuts would be limited.  If it looks like the best way to go then we'll do it, but I'd rather see if there's something simpler out there before we do.

[SteamyTea]

Yesterday I used 8.1 kWh of electricity to heat my house and all the hot water I need.

Timber has around 4.5 kWh of energy per kg, and if you are luck, you will get around 65% efficiency from a WBS.

So what will be around 3 kg of dried timber.

My worst day of heating was around 40 kWh, so about 14 kg of dried timber.

If the timber has a density of 600 kg.m^-3, and my usual winter load is 20 kWh, I would need 7 kg of timber a day, and say I heat for 150 days, that is about 2 m³ of storage needed.

 

And that is before I get onto the environmental and personal health reasons.

Find a better method.

[Jimbouk]

I was tempted by a similar approach, house has been steady above 10 degrees all winter despite , sitting there empty with no heating all all. The cost of a gas connection and boiler really irked me, however unless I choose to retrofit  a pv system there was no way I could get a high enough TER (?) to pass building regs....

[jack]

One thing to consider: it you ever needed to sell it (never say never), the lack of a central heating system is likely to be seen as a significant negative for many/most buyers.

[ProDave]

5 hours ago, jack said:

One thing to consider: it you ever needed to sell it (never say never), the lack of a central heating system is likely to be seen as a significant negative for many/most buyers.

That is so true.

 

I did some upgrades in a house recently that had just changed hands, and the new owner was about to rip out the not very old wood pellet boiler and put a gas boiler back in.

[jamieled]

Thanks to all, some useful thoughts to ponder.  The selling aspect is a good one.  We are in a slightly different situation to many in that the house will probably be tied to some woodland and hence any future owners would also have some wood supply although they may not fancy chopping it.  Most of the wood fire timber is excess from the main felling, rather than chopping down stuff particularly for burning.  Space is not an issue.  @SteamyTea I reckon you'd need at least 6m³ storage as you'd need two years worth of drying storage (assuming you weren't buying it dry).

 

The main plus point I see is the ability to be in control of our heating to some extent, rather than reliance on an electricity supply (even if it is small usage).  Power cuts are not unheard of, we will be on a line that few others connect to and I want to be able to have some heating which is not reliant on having an electricity supply, even if most of the time we might use renewables for a central heating type system.

[Alexphd1]

UFH pipe is pretty cheap in the grand scheme of things, why not install pipe and cover your bases for future. You could always run a UPS back up system which would keep the heating/ufh running in a power cut.

[SteamyTea]

An emergency generator and fan heater is pretty cheap in the scheme of things too

[ProDave]

The longest we were without power was 4 days following a big storm a few years ago. It was the WBS that kept the house habitable for that time. 

 

If you are heating water with a stove you need to design the system very carefully so everything flows by gravity including the dump load. So many I see rely on a circulating pump and would be unusable in a prolonged power cut.

 

So WBS heating a thermal store by gravity and over heat dump rad fed by gravity. Work out a valve arangement that does not need electricity to open the flow to the dump rad and you have got it sorted.  Such a system can then drive wet UFH when power is available but can safely run with just the stove heating the house in a power cut.  Best of both worlds.

 

It's a valve to open the over heat dump without electricity that I have no idea about.

[richi]

11 hours ago, ProDave said:

It's a valve to open the over heat dump without electricity that I have no idea about.

 

I'd naïvely assume you can use a normal 2-way motorised valve, capping off the outlet that gets opened when it's energised. Do I win the banana or the wooden spoon?

Edited April 11, 2018 by richi
missing word

[ProDave]

3 minutes ago, richi said:

 

I'd naïvely assume you can use a normal 2-way motorised valve, capping off the outlet that gets opened when it's energised. Do I win the banana or the wooden spoon?

The dump valve needs to open when the thermal store is getting too hot. In this scenario it needs to do it when there is no mains power so a motorised valve is out.

 

It needs something like a capillary operated valve that opens at a set temperature with no electricity. Does such a thing even exist?

[richi]

Yeah, my point was to plumb it so it opened when not energised. But I missed the point about being temperature sensitive, so dumping even if there's power.

 

But could you do what I suggested but add a thermost that cuts the power if overtemp? It would mean your dump rad would always warm up if there was a power cut, but that could be no bad thing, I guess.

[PeterW]

44 minutes ago, richi said:

Yeah, my point was to plumb it so it opened when not energised. But I missed the point about being temperature sensitive, so dumping even if there's power.

 

But could you do what I suggested but add a thermost that cuts the power if overtemp? It would mean your dump rad would always warm up if there was a power cut, but that could be no bad thing, I guess.

 

 

55 minutes ago, ProDave said:

The dump valve needs to open when the thermal store is getting too hot. In this scenario it needs to do it when there is no mains power so a motorised valve is out.

 

It needs something like a capillary operated valve that opens at a set temperature with no electricity. Does such a thing even exist?

 

You could use a 22mm Normally Open 2 port zone valve which is energized to close with the valve connected to the NC terminals on the thermostat. When the stat opens at high temp then the valve would open. If the power was lost it would do the same making it fail safe. 

 

I’m not keen on any valves in a dump circuit  but it’s a possibility. 

[Nickfromwales]

23 hours ago, ProDave said:

It needs something like a capillary operated valve that opens at a set temperature with no electricity. Does such a thing even exist?

Check out this post to see the pressure reducing valve and capillary probe operated quench valve that ran through a 12kw coil in that TS. The company Dedicated Pressure Systems have the patent ( or did have maybe ) for that concept / design. We binned it as there was no need for it, the PRedV had failed, and the dry pocket for the capillary probe was rotted through and about to dump a few 1000 litres of water into the house whilst M'lady was out at work none the wiser

 

Could easily be implemented in a solid fuel arrangement and tbh its a far better arrangement imo, to quench rather than dump, or to have a combination of the two. IIRC the quench valve operated at 95oC, so you could easily have a dump to a second thermal store ( what id do ) or to a heat loss rad in the attic. Theres no way id ever have a heat dump radiator inside my house as it would be lethal at those temps. Heat dump rads go in attics or garages, and should be caged even if in the garage.

22 hours ago, PeterW said:

I’m not keen on any valves in a dump circuit  but it’s a possibility. 

You'd have no choice or it would be continuously dumping heat by convection when you didn't want it to. +1 on the energise to close type, aka stored energy, and its the same as fire dampers which close using the stored energy that was used to open then eg by charging a powerful spring ( hence they're always low geared motors which take an age to open ).

  

As Peter says, if I was ever looking at this as a solution id have the 3000L TS and use that as a buffer / dump. From there you'd be able to do 24 / 48 hrs of timed space heating and have DHW, but id also fit ufh pipes into a 200-250mm thick concrete slab too as a secondary storage / buffer and to regulate heat output into the rest of the house in a far more comfortable and manageable way. UFH pipe is just too cheap to not do that, its a no brainer. With the thicker slab and the huge TS you'd be able to become far less reliant on the stove actually being lit and burning so often, and instead just relax a bit and live  Other than that you may find yourselves slaves to the WBS and thats not cricket. It would be nice to go out, and still have a nice warm house to come back to with oodles of DHW available whenever you wanted it, and then just lighting the stove for a few hours of an evening when it suited you best. 

A Solar thermal or solar Pv system would compliment this arrangement beautifully. Sun shines on the roof and charges your heat battery ( TS and slab ) and away to go. Even a small array of either to offset losses would be a massive benefit. 

Edited April 12, 2018 by Nickfromwales
concept not concert lol

[Nickfromwales]

To garnish the above, if you were concerned about regular interruptions of the cold water mains supply as well as regular power failure, this overheat solution could be fortified with a cold water accumulator for any serious paranoia. These ramblings will no doubt have to be in accordance with the guidelines set out by HETAS so please dont take these all as gospel without prior consultation with a suitably qualified individual.  

The side effect of fitting the accumulator would result in you having a top-notch water supply too.