Woodburner with hotwater/radiator(s)

[Andrew]

Does anyone have any recent experience of specifying a small woodburning "wet" system - say 5kW woodburner with 2kW to room and the rest to hot water/radiator(s).

I could fit one using a redundant kitchen chimney, but I estimate my kitchen only needs 2kW - so it would be useful/necessary to divert the rest of the heat elsewhere. However the cost/complexities of a "wet" system seem to make the whole project "too expensive / too difficult". For example a dual coil hot water cylinder will cost several hundred more than the standard item.

I currently have a log burner in my lounge which (with internal doors open) heats about 2/3 of my bungalow. The aim of an additional stove would be to further minimise the use of gas central heating. My economic case makes assumptions about burning my own wood - in my experience logs bought commercially cost more per kWhr than gas at its present price - in fact they track the price of other fuels!

Any comments/ experiences?

Andrew

[harryagain]

The ratings given for wood burners are max, ie with a roaring inferno inside, not practical or neccessary. Try to get a room sealed one ie with an air pipe coming to the stove from outside. Avoids drawing cold air into the room hence much more effective

[Dave Liquorice]

I think you will struggle to find that combination and possibly a boiler model that small. We have a Stovax Stockton 11. Nominal 11 kW total with 3 kW to the room.

Why do you need a dual coil cylinder? The stoves boiler shares water with the gas boiler CH circuit. All you need is a "Dunsley Neutraliser" or similar where all the flow/returns meet and mix at the same level and thus pressure. This stops the pumps circulating water via the stove when it's not hot. The DN needs to be above the stove so gravity can circulate the water and with only a low thermal capcity DN it might be wise to have a dump radiator across the gravity loop with a passive thermostatic valve that opens at around

85 to 90 C.

Our stove is linked into a 300 l thermal store also heated by solar and oil. I reckon the stove saves about 1000 l of oil over the 6 month winter period mid October to mid April. Lighting the stove about 1500 and keeping it going with a good burn through to about

2200. Note that is roughly 7 kW to water rather than 3.

I'd agree with that. Buying seasoned hardwood logs at £75/dumpy bag (approx 500 kg) the cost is comparable to oil. Oil is around 6p/kWHr. Last time I paid a mains gas bill it was about 3p/kWHr but thats nearly 15 years ago...

[newshound]

Fascinating, never come across them before.

[Fredxx]

How would that work? If all ins and outs are at the same pressure, then flow would be only dependent on resistance?

The way I've done it for 2 gas boilers is to have a pump and a non-return valve for each boiler.

[Dave Liquorice]

No pressure difference between an in and out on a given loop means there will be no flow in that loop. CH/DHW loop has pump to circulate the water and stats to control a motorised valve to direct the flow. The gas boiler loop also has a pump. The stove loop is just gravity.

Gas boilers are very controlable, turn it of it goes off. A stove is not so easy to turn off. The heat loop needs to be open and a heat dump rad connected by a passive thermostatic valve. ie one like a wax TRV, not an electrical thermostat and motorised valve.

As the OP is only looking at 3 kW to water it might be simpler just to have a good sized radiator on the landing up stairs that is only connected(*) to the stove boiler and leave bedroom doors open to distribute the heat. The normal room stat on the CH system will hold the gas boiler off if the place is warm enough.

(*) *NOT* sealed this needs to be an open vented system with feed/expansion tank and feed/expansion pipes somewhere.

[Steve Firth]

Yeah, they're the expensive alternative to having a thermal store. Hideously overpriced.

[Steve Firth]

I've done this more than once. A thermal store with a coil for gas/oil boiler on a sealed system is the best way to do it. Not cheap.

I'm about to do it all over again with a 16kw stove, I have an inexhaustible supply of wood and gas is expensive.

Manufacturers estimates for stoves and the split between hot water and space heating are always optimistic. I'm unconvinced that any installation puts more heat into the water than it does into the room.

[Andrew]

Thank you for your insights. I think this sums up my dilema. My existing woodburner in my lounge provides about 2/3 of my heating requirements. Therefore installing another one to cover the remaining requirements gives rise to dimishing returns - unless it is straightforward.

My other limitation is how much heat can be output in kitchen - and as you say the manufactures split between hot water and space heating is probably optimistic.

Andrew

[Andrew]

Not easy to explain complicated situation in a few words - but this is the root of my problem. It would be simple to just add a heat leak radiator in a hallway to dump the extra heat, which would then find its way into the bedrooms. But I live in bungalow, and without major building work the only suitable location in the hallway for gravity feed is just 450 mmm wide, necessitating a tall thin radiator. Even this is awkward as it would obstruct the full opening of an internal door.

The bathroom is another possibility - but it is very small and would result in some awkward arrangement of a radiator directly above the bath.

Plumbing into hot water cylinder would be easy (if costly), but I recon my annual hot water costs with gas are not much more than £100 - and the use of a stove will not always co-incide with hot water needs. Also these days it is increasingly difficult to avoid heating water by electricity - practically no washing machines or dishwashers have a hot fill.

The cost of gas is of course very non linear if one only uses a small amount due to standing charges/ increased unit rate on the first units in lieu of standing charges.

Andrew

[Andrew]

Your comments are much appreciated.

I was thinking perhaps in terms of a Clearview Pioneer 400 - 5kW with approximately half of that to hot water

Andrew

[Andrew]

I do understand however that woodburners are most efficient when working towards the top of their output ranage - and that continual use on a low setting increases the production of pollutants/creosote which is damaging to the flue.

Andrew

[Dave Liquorice]

Ah, that does make it more complicated and could well make a boiler option on the kitchen stove a non-starter.

The neutraliser needs to be above the stove and not far from it horizontally, the CH/DHW flow/return and the boiler flow return also need to be connected to it (and the volume of the neutraliser forming part of each circuit) but as they are pumped getting circulation through them isn't a problem. The heat dump rad is.

TBH seems like a lot of work for very little benefit. 3 kW isn't a great deal in space heating terms.

I wonder if just installing a small stove, sans boiler, and leaving the kitchen door open would be just as effective at reducing the gas bill.

You don't need a double coil cylinder or even a replacement cylinder. All you need is a Dunsley Neutraliser or functional equivalent. DNs are fing expensive for what they are, a water tight box with lots of pipe connections (6 in your case).

This is very true it can be offset by using a thermal store but for just 3 kW of occasional/intermittent wood burner it's probably not worth going down that route. Starts getting more interesting if you add solar thermal. The solar thermal and the 300 l store here save about 10 l oil/week in the summer period or about £150 at 60p/l.

If you are a low user take a look at Equipower/Equigas. No standing charge at all (not even the "hidden" one with high priced first x units). You pay only pay for what you use, slighly higher rate than the cheapest standing charge tarrifs out there but not stupidly expensive.

[Dave Liquorice]

I agree the figures are a bit "vague", having said that ours seems to match the nominal 3 kW to room and 7 kW to water. IIRC the heat calcs for the room indicated about 2.5 kW. With the stove buring well all evening the room is "toasty" but not over heated. And you can see the store temp climbing fairly quickly when the CH is not calling for heat.

The 38 kW oil boiler will raise the store from 60 to 70 in about 20 mins, the solar (suposidly about 3 kW) will take most of a bright sunny day say 6-8 hours, the stove is between the two maybe 3 hrs,

[Steve]

practically no washing machines or dishwashers have a hot

What you can do is run a thermostatic mixer valve with a return loop and pump provided you have a hot water cylinder.

Basically run three pipes to the TMV and the TMV must be as close as possible to the washing machine/dishwasher.

One pipe will obviously be cold, the 2nd will be hot, and the third pipe is the hot water return via a small pump immediately before it enters the hot side of the TMV.

This small pump can be a 12 volt job powered by a solar panel and rechargeable battery.

Set the TMV to the desired temperature infact, you probably can get away with setting the temp 10 degrees higher to account for teh thermal mass of the washing machine/dishwasher.

When you want to use washing machine or dishwasher, start the hot water return loop pump on. As soon as the hot water and hot water return pipe is piping hot, the start the dish washer/washing machine.

As soon as the cycle has finished, switch off the hot water return loop pump.

Less electricity will be used to heat the water and the washing cycle will be shorter as a result. Gas is 1/4 of the cost of electricity for heating water (assuming condensing boiler, or would be even cheaper if you have free wood.

[harryagain]

A lot depend son the design of the stove. Creosotes etc are caused by unburned distillates condensing in the chimney. Wet wood makes things worse. Modrn stoves have this problem much reduced, there are two zones in the stove, the pyrolising zone (where the wood is broken down) and the combustion zone where the resulting vapours are burned off as a separate process. A bit here on the topic.

Most of these pellet stoves have the two separate zones.

[Andrew]

Yes it probably would if the kitchen led directly into the hall - and then the bedrooms. However it leads directly into the lounge/dinner which already has a stove - and only indirectly into the hall, slthough the two doors are very close together and perpendicular to each other.

Very good point.

Andrew

[Andrew]

Thanks to everyone who has contributed to this thread. My conclusion is that installing another woodburner is probably too much effort for an uncertain gain.

What is interesting is that when the bungalow was originally built in the early 1960's there would have been a solid fuel stove for hot water/radiators in the kitchen, which presumable did not give out too much heat. But in those days there would have been no double glazing, cavity wall insulation, or sensible amount of loft insulation. I am also suspect the single pipe central heating (now pumped - and about the be replaced) was originally designed for gravity circulation, but no doubt in those days there were different expectations about both warmth and system safety.

Andrew