Combining Gas Ch, Wood stove & Solar

[gspotts]

I have a Conventional Gas boiler (Valiant Thermo Compact) running at about 1bar and a flat plate solar system. The Gas boiler does the central heating and hot water and is controlled by a 3 way valve. The hot water is heated in a conventional copper cylinder with two coils. I wish to add a wood burning stove to the system. This would heat the hot water and also do the central heating. My problem is combining the gas CH circuit with the Wood burning stove circuit. The simplest way is to replace the copper cylinder with a 3 coil cylinder and run pipes from the stove to additional radiators as required. This would create an independent system but would be expensive. I was wondering if it was possible to fit a pressure vessel to the stove and add it to the Gas CH circuit with valves to either use the gas boiler or the wood stove but not both simultaneously. I have looked at the Dunsley Baker Neutralizer System but this is not suitable for a pressurised system. I cannot be alone with this problem. Your input would be much appreciated.

[lightfoot]

In a word....NO.  For safety reasons a wood stove back boiler must be connected to a open vented system.  If you wish/need to maintain a sealed/pressurised gas boiler circuit and require the wood stove to contribute to the CH load, then your only option is to install a appropriately sized, open vented, primary (DHW & CH) thermal store, which if sized and connected correctly can have many advantages over a conventional Y-plan or S-plan system and DHW storage cylinder, see these recent threads for more details and ideas....

http://www.navitron.org.uk/forum/index.php/topic,4069.0.html

http://www.navitron.org.uk/forum/index.php/topic,4177.0.html

For lots more info you could do a search on this forum for 'Thermal store' or 'Wood stove' etc

Good luck,

Lightfoot.

[Northbridge]

Even with make on rise stat and heat dump, what happens in a power outage or component failure......

However my constant argument with the solid fuel lot is how do you stop it over temp in winter when you are burning logs like they are going out of fashion.

Enter stage left, the dunsley.  It's basically a low loss header, you can have one welded up for abourt 150 quid.  But about three people in england know what a low loss header is.

[lightfoot]

Northbridge,

I may of misunderstood your post, but what possible use is a low loss header/Dunsley neutralizer etc, for someone that wishes to combine the input from both a sealed/pressurised boiler and a open vented wood stove/boiler to serve the central heating circuit etc.  The only practical way is to use a thermal store with a coil for the sealed/pressurised boiler circuit.  Also given the small volume of water contained in low loss header/Dunsley neutralizer etc , how could it protect the system from boiling....without an additional heat store/dump ?

The trick for burning logs etc, is to ensure there is always somewhere useful for the heat to go, so that the fuel can be burned both efficiently and cleanly.  This can be achieved by using an appropriately sized, well insulated, primary thermal store (preferably in conjunction with a dedicated, clean burning and efficient log batch boiler etc).

Lightfoot.


PS, Some biomass boilers also incorporate a thermostatically controlled, mains fed quench coil as another line of defense should the boiler ever reach a overheat condition.

[dhaslam]

There is no reason why the heat store for the wood burner shouldn't heat it's own separate cylinder.   You could just transfer the heat  to the radiators when it is needed.   

[guydewdney]

We are doing something similar - as as prompted by (i think)ajstone - it IS possible - but you need to be a bit sneaky:-

wbs connected to cylinder in attic, via 22 / 28mm pipes, in a gravity fed fashion. Heat leak rad installed in cool return leg. Thats a simple system, that actually does almost nothing. Now - the clever bit - add a heat exchanger Teed into the flow and return, with a pump. then, when the wbs is up to temp, the pump kicks in, and changes the flow from round the attic cylinder, to round the heat exchanger. At the same time, use the original coil from the conventional boiler, with some valves to separate off the boiler itself, and treat the HE as a 'heat source'. You can then use the wbs on the cylinder (obviously the pumps and valves need to be thermostatically controlled - which brings me to my question below:-)



Im looking at using a TDC3 to control the system above. Using hydraulic variant 11, pretending the panel is in fact the fire side of the heat exchanger, and the second tank is the buffer tank installed in the attic. Will it basically fire up which ever heat source is hotter than the main tank (sensor 2) - so if T1>T2 fire pump 1 if T3>T2 then fire pump 2 (ignoring all the complicated delta T and all that :-) )? Im sure it does - I would just prefer to ask.

[lightfoot]

Hi Guy,

In principle this may possibly be OK as a retro fit idea to indirectly heat your unvented DHW cylinder (UVC) using the wood stove, but for the wood stove to contribute to the space heating (assuming you want it to), then may I suggest the following....

If you are using a remote F&E tank, then connect the open vent and F&E pipe directly to the thermal store/buffer in the loft (or where ever you decide to put it).

The wood stove is directly connected to the thermal store using gravity flow and return pipework, with the option of using a charging unit/loading valve and pump.  Should you require a additional heat dump, then this should be fed from the primary flow, using gravity pipework.

The oil/gas boiler is also connected to the thermal store/buffer, preferably directly, but if you want/need to maintain a sealed/pressurised boiler circuit then you could connect it indirectly using a coil.  If the wood stove is not keeping up with demand, then the oil/gas boiler can fire up as necessarily, controlled by a stat on the thermal store/buffer.

The circuit for rads/ufh and UVC coil is fed directly from the thermal store/buffer, with two zone valves controlling the flow to each.

This way the thermal store/buffer will act as the neutral point in the system, avoiding any pumping conflicts and allow any heat source to contribute to both the DHW and space heating loads as and when required.  It may also improve the efficiency of the oil/gas boiler by reducing cycling etc at part load.  Should you wish to only heat the DHW in the UVC with the oil/gas boiler without charging the thermal store/buffer, then the thermal store/buffer could be bypassed and the flow diverted straight to to the UVC coil by using a additional motorised valve.


Hope that helps,

Lightfoot.

[guydewdney]

nope - that just confuses me. 

Ill scan my scribble and post it later / tomorrow, and you can pick it to bits

[lightfoot]

It really is very simple, you are basically putting the thermal store/buffer between the oil/gas boiler and the UVC/rads and also heating this thermal store with the wood stove.

What part do you not understand....don't say all of it, as I'm not sure I can explain it any more simply than that, without doing a drawing....and that's not easy to do on the computer for a old Luddite like myself 

Sleep on it and it may make more sense in the morning 


Lightfoot.

[guydewdney]

Ah - I have an UVC and no thermal store. I want to heat the brand spanking new shiney UVC with wood. so instead of directly doing it - im heating a slightly pointless cylinder in the attic, and shorting it out (in electrical terms) with a heat exchanger. as the HE is pumped - its a controled heat source.

[guydewdney]

Heres the start of my heating idea.

I havent shown details like f&e tanks etc, mains water in / out (of UVC) nor electrical controls.

I was thinking of using the coil in the upper vented cylinder to suck heat back out and put it into the lower UVC using the solar coil and pump etc. but I cant see a way I can do it whilst still allowing circulation of the solar. But then again - I suppose that if the solar want to circulate, I should be using that, not the stored HW in the store (I suppose thats what it is?). So IF the solar want to circulate, it over rides any control that wants to remove heat from the store.

I think....

[dhaslam]

If you connect the oil boiler to the new cylinder you would have a free coil in the other one.   You could then circulate heat from the upper tank.   

[lightfoot]

I was thinking along these lines....

(F&E tank not shown, but would be connected to T-store/buffer)

[Nomolos]

Hello all

A fascinating site and all very interesting, I too am interested in allowing a wood fired boiler to contribute to my current sealed combi central heating system, and was thinking possibly erroneously but my current system utilises a heat exchanger in the Central Heating loop to feed my wet under floor heating system.

Could a wood boiler not do the same? That’s to say utilise a heat exchanger to shift the heat from the wood boilers thermal store into the central heating’s direct loop? 

With a stat to trip a pump linked to the wood boilers temperature? 

Boiler goes out pumps off so the gas CH is not trying to heat the Wood Boilers thermal store, Wood boiler on pump kicks in starts to transfer heat into the central heating circuit with possibly a second stat to keep the gas CH off when there is sufficient heat from the Wood burner?

Basically having my cake and eating it.

I look forward to seeing what I have missed

[Pie Eyes]

 

Have set up a three coil system for heating water.  It provides the flexibility throughout the year and given the complex discussion above, i agree with the first reply.  Woodburner back boilers are not meant to have any restriction to their hot water transfer system.  The Victorians did it simply and i can confirm it works.

In the winter by firing a 4KW woodburner 4-5 times a week, we heat the house by the burner itself, it feeds three radiators and the heat also goes through the third coil on the cylinder.  My experience is that you just need to have some sense of fluid dymanics and faith in the power it generates

Result
Full tank of 172 ltrs in 45 mins (Winter) - Full tank each day (summer) curtosy of the sun and Gas powered HW and CH only when we want it.  Our gas bill for the worst 1/4 was only £115

I have to add that when we took out the old cylinder, it was already on it's last legs.
If in any doubt i would get a new one. We did through Navitron and it has been the best combined investment in our house since we moved in.

PS - Those that remember my bike generator - it works !!! Along with a 55w panel i can make my own power via 12v battery bank and have a switch over in the house for leach lighting circuit.  This gives approx 3-4 nights per week.

Cheers

Pie Eyes