o/t backboilers on woodburners

[FamilyWiggs]

I have a 14kw Villager woodburner, burning wood nicely and heating the lounge (32 sqm). It has a 45,000 BTU backboiler fitted, that is not currently conencted to the oil-fired biler/central heating system.

I'd rather burn free wood than oil, obviously.

Has anyone got their DHW/CH system connected to a woodburning stove with a backboiler? Does it work well? What does ths system look like? Can I link it in to the existing system, so that the boiler only fires when the stove is not lit?

I do have an manifold in the boiler room that originally connected in the backboiler - but I disconnected it all when I took the stove out during the lounge/dining room works a couple of years ago. When used, I could hear the water boiling in the pipes, which worried me and so I haven't reconnected it now the stove is back in...

 

[Jon Maine]

Usual setup is a gravity system, yes can boil well be careful about using non plastic header/expansion tank.

This may help

http://images.google.co.uk/imgres?i...n&rlz=1B3GZFB_enGB280GB280&sa=N&start=40&um=1

 

[JoceAndChris]

We have a back boiler which came as rather a nice surprise. When we rebuilt the 30s fireplace the flames just whooshed up the back and a few hours later there was a nice tank of hot water and warm rads. Don't ask me how it works or even where it is!

Sorry that's not a helpful post, but hello! And hope you can get this coz it feels good.

Why is this off topic? I wonder what that makes half of my questions if this is off topic?!

 

[Flyfisher]

I've been looking into such hybrid systems and have got as far as confirming they are certainly feasible. My plan is to install a stove with a back boiler together with a suitable (i.e. multi-coil) hot water tank and see how it performs. If necessary I'll add a gas boiler at a later stage, although I'm hoping it won't be necessary and we can use an immersion heater for summer hot water. This might cost a bit more to run but would save a lot on the installation. I haven't really decided on that particular aspect yet.

A key requirement is that the stove boiler cannot be part of a pressurised system and it should also have a gravity 'heat-leak' radiator in the primary circuit. I understand a bathroom radiator is often used for this. I'm less sure about the gas boiler side of things as I've been told that this circuit may or may not be pressurised.

Have a look at http://www.dunsleyheat.co.uk/linkupsys.htm for more information on the principles involved.

 

[FamilyWiggs]

Jon, thanks. I think a heatbank will involve a reasonable amount of additional plumbing (which I'm happy and reasonably competent to do), but I was hoping to reconnect the existing manifold into the flow/return of the backboiler. But I'm worried about the boiling - although the I presume the manifold connects the backboiler to the primary circuit and thus into the vented DHW system via the tank above.

Since I have replumbed the CH system with plastic pipe, I'm a little concerned that the backboiler may overheat the water. How is that controlled in a vented system? At present the oil boiler stops firing when the water temp in the system exceeds the thermostatically-set value. How is this achieved in an uncontrolled woodburinging stove?

Joce - I have been looking at backboiler posts on a Landrover forum; in comparison your posts are veritably on-topic!

 

[jocelyn plummer]

My 12 year old Landrover is beginning to sound like an old boiler, so we are not so far off topic.

 

[FamilyWiggs]

FF - thanks. I'm wondering if the 8-way manifold in my boiler room is some form of neutraliser. Need to investiagte that a little further.

I've been thinking about the multi-fuel aspect. I have a standard immersion tank with primary coil from the oil boiler, and an electric element too. I was thinking of a system with a primary coil fed by either the oil boiler or wood-burner(or at least supplemented by the woodburner). If I could connect a solar source into the immersion tank as well, I wouldn't need to use the oil boiler or electric element in summer.

I could have almost free hot water, as wood is plentiful here (although sun is not necessarily so in summer!).

One thing that crops up on the forums is the drop in room-heat output when the boiler is on. I would imagine a 14kw stove with 45,000 BTU backboiler should be man-enough? There are currently 10 rads on the circuit although I may add in a couple more in time.

 

[FamilyWiggs]

My 12 year old Landrover is beginning to sound like an old boiler, so we are not so far off topic.

I once had an old boiler in my Landrover... :lol:

 

[Flyfisher]

FW: yes, sizing the stove/boiler seems to be a bit of an art for the reason you mentioned. I originally hoped that starting from a clean sheet would make things easier for me but it doesn't seem to necessarily work that way. I suspect that hybrid systems are not that common and thus it's not so easy to find experienced people out there to help design a system. I also suspect I'm beginning to suffer from 'analysis paralysis' when I really should just make a decision and see how it works out, tweaking it later if necessary.

 

[Jon Maine]

Since I have replumbed the CH system with plastic pipe, I'm a little concerned that the backboiler may overheat the water. How is that controlled in a vented system? At present the oil boiler stops firing when the water temp in the system exceeds the thermostatically-set value. How is this achieved in an uncontrolled woodburinging stove?

Thermostats in the rooms connected to a buzzer so you know when to douse the fire :lol:

 

[Flyfisher]

In an unvented system the water should not be able to exceed 100C. I don't know the temp ratings for plastic plumbing but it should be easy to find out (and I'd be surprised if it was below boiling point). Even if it doesn't totally fail, it might become 'soft', hence the previous comments about taking care with the expansion tank.

But you're right about the overheating issue due to the relative lack of control over the stove. My understanding is that the gravity operated 'heat leak' radiator (basically an upstairs radiator) should be sized such that boiling can't occur.

 

[skier-hughes]

I'm having a nightmare trying to get someone to install a similar system.

I want.
New stove with back boiler.
Solar
Rayburn

To remove the need for firing up the rayburn, which is far too small for our 14 rad system, I was advised to have a heat store/accumulator tank of around 1000/1500 litres, which would store enough heat from the back boiler to give us plenty of hot water and central heating in the morning before the fire is lit and going.

Can I find ayone to do it.


NO.

I started in March and thought I'd be warm this winter, I just wish I had analysis paralysis, but I have plumber paralysis.

Nobody seems to be able to spec a system.

 

[JohnB]

Can I find ayone to do it.
NO.
I started in March and thought I'd be warm this winter, I just wish I had analysis paralysis, but I have plumber paralysis.
Nobody seems to be able to spec a system.

You might find someone through the Green Building Forum (http://www.greenbuildingforum.co.uk/newforum/) or the AECB (Association for Environmentally Concious Building).

 

[Penners]

I've been mulling over and reading up on a similar problem, recently, in relation to my wish to install some solar water-heating panels. The heat source doesn't really make any difference - woodburner, solar panels, etc. It's all producing hot water.

While I profess no expertise in this subject whatsoever (so nothing new there!), I agree with Flyfisher. The most logical way to incorporate two heat sources into your system appears to be to have a dual coil indirect cylinder. One coil is heated by the boiler, in the normal way, and the other is supplied from the woodburner.

The boiler circuit is controlled by a contact thermostat on the cylinder. So when the stat is satisfied, the boiler stops firing.

The woodburner circuit has a contact stat on the pipework near the woodburner, and if the temperature of the water in that circuit exceeds your required level*, a motorised valve redirects the water flow to a heat leak radiator.

Ideally, it would be good if the woodburner also had a motorised damper, so that when the thermostat is satisfied, the air supply is automatically closed. I wonder whether such a thing exists? (That's idle speculation, by the way - I realise that your woodburner is already in situ.)

I would imagine a 14kw stove with 45,000 BTU backboiler should be man-enough?

I should have thought so, too. 32m^2 is a sizeable room, but then 14kW is a sizeable woodburner.

* The recommended water temperature in the primary circuit of an indirect system is, I believe, 60 deg. C. This is sufficient to provide domestic heating and hot water at a satisfactory temperature, but cool enough to minimise calcium build-up in the pipework. So you would need to set both thermostats at this temperature.

 

[Aidan]

A key requirement is that the stove boiler cannot be part of a pressurised system and it should also have a gravity 'heat-leak' radiator in the primary circuit. I understand a bathroom radiator is often used for this. I'm less sure about the gas boiler side of things as I've been told that this circuit may or may not be pressurised.

True; the solid fuel stove must be open vented so as to allow it to dissipate heat by boiling off water. Many solid fuel stoves are cast iron and have no pressure rating. I would suggest checking that a stove has a working safety valve fitted; there has been a fatal explosion of a disused back-boiler in recent years. Dripping safety valves will concrete themselves shut with limescale and the open vent pipes can freeze or get blocked. Safety valves are both cheap and invaluable.

The gas/oil boiler could be open vented, sharing water with the solid fuel burner.
If it is a pressurized sealed system, it could only be linked to the solid fuel system via a heat exchanger (plate, tube-in-shell or multi coil thermal store). A plate heat exchanger is usual, tube-in-shell types being much more expensive. The simplest way to shift heat from the solid fuel system to the gas/oil system would be with an variable-speed injection pump connected to closely spaced tees on a manifold/low-loss-header/neutralizer/heat store. You also need to ensure that excess heat was not removed or the return temperature would be reduced to the extent that it caused condensation and back-end corrosion.

The famous neutraliser is an expensive , small thermal store, or an expensive large low loss header. I would control such a set up with a BMS outstation, but then I know how to set them up. The open vented system should still work safely in the event of mains &/or control system failure. I am infuriated by the lack of electronic domestic heating controls.

You will find a lot of information about interlinking wood burners and solar on US sites; John Siegenthaller's articles (not sure if I've spelt that right) are very informative.

Sorry if that's techno-waffle, it makes sense to me (I think).

 

[Flyfisher]

I must admit that I'm not entirely sure about the neutraliser device; I'm sure it must work but I can't entirely see why it is needed if a multi-coil tank is used.

If such a system is based around a large, well-insulated hot tank (or 'heat-store' as they are seemingly called these days) with separate coils for a stove/boiler, a conventional boiler and solar panels (I've seen such heat stores with multiple coils), is it not possible to just treat each heat source as a separate system?

Thus, the stove/boiler heats the water tank and excess heat is channelled, by gravity, to a 'heat-leak' radiator in an open-vented system. The conventional boiler is controlled as normal, switching on and off depending on the heat store temperature, and could (perhaps??) be part of an open or pressurised system. The solar panels would also operate as normal, transferring their collected heat through the third heat-store coil. DHW would be drawn off from the heat-store itself and all other radiators would be connected and pumped via a 4th coil to distribute the required heat around the house.

Or is this too simplistic?

 

[Aidan]

I must admit that I'm not entirely sure about the neutraliser device; I'm sure it must work but I can't entirely see why it is needed if a multi-coil tank is used.

It is only needed if the various heat input devices share the water. If they're separated by a heat exchanger it's not needed.
A multiple coil cylinder is a heat exchanger but I didn't think of it when I wrote my previous post because I've never used one. I've edited the previous post to make more sense; apologies if it was confusing.

The woodburner circuit has a contact stat on the pipework near the woodburner, and if the temperature of the water in that circuit exceeds your required level*, a motorised valve redirects the water flow to a heat leak radiator.

Ideally, it would be good if the woodburner also had a motorised damper, so that when the thermostat is satisfied, the air supply is automatically closed. I wonder whether such a thing exists?
* The recommended water temperature in the primary circuit of an indirect system is, I believe, 60 deg. C.

The heat dump radiator has to be permanently in the gravity circuit. I would not fit motorized valves in the gravity circulation circuit. The injection pump system suggested is on a branch off the gravity circuit and so doesn't affect its operation.
There are wood burners with air dampers, but they're more of the wood-fired boiler type rather than a stove, with prices to match. The US makers offer them and there's also a Czech (I think, forget the name) maker. It wouldn't be advisable with an in-house boiler because it would produce a lot of carbon monoxide. 60 degC is the recommended storage temperature for domestic hot water (secondary), the primary would have to be higher than that. If the DHWS cylinder is used as a heat dump for the wood boiler (as it usually is) the stored DHW temperature must be able to rise above 60 degC if necessary. The DHW temperature can get dangerously hot (so a TMV would be a good idea) and the cylinder can accumulate limescale.

 

[Flyfisher]

Thanks for that clarification Aiden. Sounds like a 'heat store' with multiple coils is the simplest system to go for - in a logical sense at least.

Can I ask why limescale accumulation is more of an issue if the water exceeds 60C (which I guess is quite likely with a solid fuel back boiler)? I would have thought the solubility of water was greater at higher temperatures, making it less likely for dissolved solids to come out of solution.

 

[Aidan]

Thanks for that clarification Aiden. Sounds like a 'heat store' with multiple coils is the simplest system to go for - in a logical sense at least.

A heat store is sensible with a solid fuel boiler, if you have room. You could then distribute water from this to the central heating via a mixing device. You still have to be wary of nuisance gravity circulation, typically hot water flowing through the solid fuel stove and heating the atmosphere when the oil/gas boiler is on.

I knew you'd get more limescale deposits with higher temperatures, and 60 degC is the recommended setting for legionella control.
Wikipedia says the dissolved calcium bicarbonate turns into less soluble calcium carbonate at higher temperatures so that's probably right.

 

[Flyfisher]

Couldn't the CH water just be pumped through a coil in the heat store, thus extracting the heat without requiring a mixing device?

Similarly, the gas boiler would only 'drive' a separate coil in the heat-store, so there wouldn't be any heated water being circulated through the stove/boiler. Indeed, all the water circuits would be completely separate and isolated from each other, each dumping their heat into the heat store through separate coils (or extracting it through a separate coil in the case of the radiator circuit).

I knew about the temperature recommendation regarding legionella control (a potential issue with solar panels, I've read) but wasn't aware of the chemical changes resulting in more limescale at higher temps, although it makes sense now you mention it. Thanks.