Wood burning stove

I have a gas heating system which is split into 3 zones, hot water 1, upstairs 2, and downstairs heating 3, all controlled through a timeclock which then operates the 3 motorised valves. Iam wanting to add a wood burning stoves to my system as a secondary form of heat. How will i wire a pump for it into my heating system and also open the 2 heating valves??? Help.

I think this would be better off in plumbing because there are special measures when adding a wood burner due to the lack of control.

I will be interested in the reply as I am also involved in setting up a wood burning stove with a oil fired back-up boiler.

As far as I can work out there seems to be two types of cistern mega-flow and glenhill plus a system used in Ulster which has a secondary tank.

The mega-flow needs annual inspection because the whole tank is pressurised but the glenhill is open vented so not required only the pipes inside the cylinder are pressurised.

The whole idea is to have a heat store and only when the water cools too much is the secondary system used.

However in the system we want to fit there is no room for such a huge tank so the idea at moment is to keep the two systems independent for central heating and we hope to be able to run around about 4 radiators by thermosyphon. We were going to use a car header tank and use anti-freeze in the water. We would like to use some form of heat exchanger to heat the domestic water but to date we have not found a small enough unit.

Consideration is being given to running two pipes one inside the other the inner pipe being for domestic hot water which will be pumped from the holding tank.

Also consideration is being given to a way to sink any unwanted heat loads of dirty cold water is available just the other side of a steel plate so should not be a problem.

However for safety reasons one can not rely on any pump or motorised valve as you can't turn the stove off in a hurry and that is a very limiting factor.

I would like to work out how much heat the stove can produce when full before it runs out of fuel does not really matter if KWh or BTU and then select a header tank which will have enough water to boil off not to run out of water before the fire goes out.

Of course you can't use any header tank which can't hold with boiling water and any immersion heaters will need to not have the safety device as with the non resettable over temperature bit in a thermostat every time the back boiler over heats the thermostat will need replacing.

In my system there will not be a thermostat but there will be three ways to heat the water main engine, diesel water heater and the stove.

Mine is slightly different as very little room on a narrow boat but principle will be same. I think in a house I would fit GlenHill system see http://www.willis-renewables.com/immersion-how-it-works.htm and here http://www.gledhill.net/building-products/bp-index.htm and I will monitor to see any other ideas.

If my calculations are correct 1kg of wood will boil away 14 pints of water assuming no loses else where so I reckon to be safe need about 1 gallon in header tank so if something goes wrong while away the fire will burn out before the water all boils away.

Can anyone confirm my figures please I have taken wood at 15MJ/Kg and Latent Heat of Vaporization for water as 2260 KJ/Kg of course approx but should be close enough?

ericmark said:

If my calculations are correct 1kg of wood will boil away 14 pints of water assuming no loses else where so I reckon to be safe need about 1 gallon in header tank so if something goes wrong while away the fire will burn out before the water all boils away.

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I think you would need more than a gallon, when a gravity system was installed for a friend there was a 20 gallon contingency tank of cold water in the loft to be used when the stove heated water had to be dumped and the fire drawn ( put out ). IIRC there was an automatic thermostatic dump valve that opened when circulating water became dangerously hot. I would think depending on boiling off water to dissapate excess heat would be a high risk process

Sorry was not going to dump water but allow it to boil away and use latent heat of evaporation. So assuming incoming water at 20deg and dumping at 100deg that's 80Kcal per kg at 4.2 joules per calorie 337KJ/kg.

20 gallon / 2260 KJ/Kg x 337KJ/kg = 3 gallons

What I have to remember is power failure means no water so no option to dump water as no way to replace it.

Of course in reality steam will not be only thing to escape through radiator cap and some water will be ejected.

I am not altogether happy that the overflow pipe can take all the steam generated. But how can I test it before building?

I had enough job convincing my son it needed a header tank never mind a 20 gallon one where would the people go 20 gallons is rather big and heavy.

It is not as simple as how much water must boil away to remove the energy from the burning wood. The effects of steam generation must be fully considered.

Once the water starts boiling the steam will be produced on the walls of the boiler and this steam will need to escape from the boiler. The only exit route is via the pipes that bring water to and from the boiler. The steam will displace the water from the pipes possible blowing a quantity of hot water out of the system. More important and potential dangerous ( depending on layout of pipes ) the steam exiting from the boiler could prevent water getting to the boiler to replace that being boiled away. The boiler will then become very hot and when no more steam can be produced the pipes become free of steam and water will be able to come down the pipes into the now very hot boiler. Putting water into a over heated dry boiler can create a steam explosion that may rupture the boiler. Cast iron suddenly cooled can fracture or be permantly weakened. Not a good idea if the boiler that is also containing a quantity of burning material ruptures and spills burning materials where they can start a fire in the building.

Sorry, a bit slow to see this thread.

I have successfully plumbed a wood-burner with a back (and top, and sides) boiler into our existing oil-fired central heating system.

The existing CH system was open and vented, stop here if yours is not!

It was done as follows:

Install a heat store. I used a bog-standard 210 litre direct hot water cylinder with five tappings: two on each side (one at top and one at the bottom) and obviously a single tapping at the very top.

Install this above the stove so that flow circulates by gravity convection using the tappings on one side.

Connect the tappings on the other side so that, in effect, they are in parallel with your existing oil-fired boiler. In other words tapping at top needs to run to pump suction side, tapping at bottom return from flow around system.

The tapping at the very top is your vent pipe and should run up to the existing CH header tank.


Install two motorised valves, one in the pipe from boiler to pump and one in the pipe heat store to pump.

Put a thermostat at the top of the heat store wired up so that when it is cold it connects to the motorised valve on the boiler, and when hot to the motorised valve to the heat store.

Rewire the existing "demand" from the CH timer so that it goes via the thermostat at the top of the heat store, meaning that when heat is requested it opens the boiler valve when the store is cold, and the heat store valve when the store is hot.

The pump should be switched on when either of the two motorised valves opens, so you need valves with built-in microswitches that make a contact when the valve is fully open. The switch in the heat store valve can connect direct to the pump, but that from the boiler valve needs to run a relay which, in turn, switches on both pump and oil-fired boiler.


Hopefully you've followed me so far, so it should now be clear how the system works:

If you light a fire you warm up the heat store, and when it gets hot enough the thermostat at the top switches so that when the CH system demands heat it can draw it from the store. If you haven't lit a fire, or the store isn't warm enough yet, then the system reverts to firing up the boiler.

The heat store also acts as a buffer in that it absorbs heat when the wood-burner is running but the CH system doesn't need it, and also stores it for when the wood-burner has died down yet the CH system needs to run.

To stop the water in the heat store getting too hot you also need an upper temperature limit thermostat at the bottom of the heat store, set at about 85 deg C. This needs to be wired up so that, regardless of what timers or thermostats elsewhere may be requesting, water is circulated from the heat store around the CH system. This is to dump the heat before the water in the store boils, and if you have an existing frost-stat in the system (which you should have) wiring it in parallel with this should do the job.



I'll get shouted at for saying this, but unless your wood-burner is absolutely massive and your house microscopic you really, honestly don't need to worry about boiling the water in the heat store unless you are catastrophically stupid.

This is because boiling 210 litres of water requires a *lot* of heat. The figures work out like this:

The specific heat capacity of water is 4200 Joules / kg / deg C, or in plain english it takes 4200 Joules to raise the temperature of 1 kg (= 1 litre) of water by 1 deg C.

Lets say that your store is already at 50 deg C, then to raise 210 litres by another 50 deg C to boiling point requires

210 x 4200 x 50 = 44.1e6 Joules, = 44.1e3 kiloJoules (kJ)

Stove outputs will be rated in kiloWatts (kW). 1 watt = 1 Joule for 1 second, so 1 kiloWatt = 1 kiloJoule for 1 second.

So let's say that your stove generates heat in the water jacket at the rate of 10kW, then it will take 44.1e3 kJ divided by 10 kW seconds to deliver all that heat, which is 4410 seconds, or 73.5 minutes, or about 1 and a quarter hours.

So you would have to blaze your stove at its rated full capacity for an least an hour and a quarter to boil that tank if it was already at 50 deg C, and for at least 2 hours if the water in the tank was at room temperature.

In practice the system will lose heat in pipework and elsewhere, meaning that it will take longer to heat the water, and it is pretty much impossible to run a wood-burner at full capacity for that length of time anyway since it consumes the wood too fast. So you would probably have to run the thing flat out for between 2 and 3 hours, refuelling it at least two times, while simultaneously not drawing any heat at all out into the CH system, for the tank to get anywhere near boiling.

If you *did* chose to do this you would get lots of audible warning from the stove itself as the water in its boilers started to hiss and bubble.

You would have to be completely nuts (and deaf) to get into this situation, and since a wood-burner has to be manually refuelled it simply is not a problem in practice.


As I said at the top of this post all of the above is based on actual experience: we have a wood-burner rated at 20kW, of which (nominally) 3kW goes to the room and 17kW to the water jacket, and an oil-fired boiler rated at 15kW. Last winter the wood-burner heated the whole house so long as I fed it enough wood.

That last point is worth considering: we burned about 1.5 tons of wood last January when it was really cold, and averaged about 1 ton/month through the winter as a whole. That's a lot of wood, and if you haven't got anywhere to store it you need to think long and hard about this whole exercise.


I hope this helps.

Great post cbell.

What do you do for hot water? I presume you have an indirect cylinder as well? How do the controls for that work?

We have an oil-fired Aga that heats the domestic HW cylinder via an indirect coil.

The CH system also has a (second) indirect coil in the same cylinder, so the wood-burner can be used to heat domestic HW via the CH piping and pump.


Incidentally the domestic HW cylinder is not the same as the cylinder used for the heat store in the roof.


Not how I'd have arranged it if I were starting from scratch, but good enough.


The best system of this type I've seen so far has a purpose-built heat store which is directly heated via a convection circuit from the wood-burner, and also (if needed) via an oil-fired boiler. The CH system runs directly from the water in this cylinder; so water in wood-burner, oil-fired boiler, radiators and heat store is all common.

Then there are further indirect tappings in the heat store:

#1, near the top, supplies domestic hot water with a mixing valve to limit temperature.

#2, roughly in the middle, runs from solar panels in the roof. This circuit is filled with a mixture of glycol and water to prevent freezing in winter, and is also pressurised to stop problems of the wood-burner boiling it off and/or melting its header tank.

#3, lower middle, is currently unused - don't know what they had in mind for that!


Mind you, it's an expensive solution.

I have fitted two of these during the last four weeks and have another to do in December. Gledhill Torrent RE solar in both cases coupled with wood burner and oil boiler. They really do work well. In both cases I will be going back to add solar. The one I have not done yet is wood burner/solar/lpg boiler and is a week long job in December.

I think the concept is definitely the right way to mix multiple heat sources, I just object to the cost of the turn-key systems.

Over £1000 for a "heat store" is silly money.

Also the lack of sensible and/or sensibly priced control systems: I built my own for a grand total of £50, of which the most expensive part was the plastic box.

What do you use?


Where do people get and/or keep their wood though? Here in Devon I can get green corded hard-wood at £40/ton delivered, which I log and split myself and then season in our dutch barn. But how many people have access to reasonably priced timber and space to store it?

I was just ready to ask how much a ton of wood costs you.

A client had a flat back of a pickup full of ready cut and dried hardwood delivered for £65 nominally about 1 m³ but I dont know what the weight would have been about 200 kg ?

That ton of wood for £40, how long does it take you to prepare it for use and how long do you have to dry it for before use?

My friend in France has the wood stored and dried in logs about 100 mm diameter but cutting an evenings supply to length takes about 30 minutes. So running the stove all day would mean about an hour a day just cutting to length.

I have a large felled oak tree in London but so far found no one to buy the wood!

Tony

Thanks cbell, I think you have cured my insomnia!!!

Your system is dangerous & will not comply with British building regs or water bye-laws!!

Seasoned and split wood ready for burning around here was at least £100/ton delivered last winter although, as you say, it is difficult to relate volume to weight since what you get is a "load". I should think it is more this winter, although I haven't tried to buy any.

I've read elsewhere the 1m3 of randomly stacked split logs is about 1/2 ton, and measurement of the wood pile in my barn suggests that if you stack it neatly then 1m3 is about 2/3 ton.

I think your French friend must be cutting it by hand. I have a chainsaw and an electric log-splitter, and I reckon I can process (as in cut, split and stack) about 2.5 tons a day. So three days of (hard) work is my fuel for the winter sorted. Cutting is actually the quick bit, it is the splitting that takes the time as my log-splitter (electrically powered hydraulic ram) will only do about 100 logs/hour.

There is an old saying that wood warms you three times: one when you cut it, once when you stack and once when you burn it. I think I'll stick to my chainsaw!

As for seasoning time, the normal recommendation is at least 18 months; and I have also read that wood dries at the rate of 1 inch/year, meaning that a 4 inch/100mm diameter log takes 2 years to dry through.

Since I only installed the wood-burner last November and embarked upon large-scale wood buying last summer I can't speak from experience yet, however the logs I split last June are developing cracks and visibly drying, whereas those I did in September have not done so well, so I think that if "18 months" were construed as "two summers in a well ventilated place" you'd be about right.

A lot depends on the type of wood, for example you can burn Ash when green, but you'd be nuts to burn a very resinous wood like pine when green as it will tar everything up, and if you have any cold spots in your chimney that could be dangerous. Green elm simply won't burn at all when wet, but it's a good firewood when dry.

If I were a bit nearer to London I'd happily take that oak tree off your hands, and I'm amazed that you haven't sold it. I'm sure that if you rang a firewood supplier he would make you an offer.