Running Wet Underfloor Heating Using Multi-Fuel Stove with Back-Boiler

[titania]

Hello,

I have been told that it is not possible to run underfloor heating using a solid fuel stove with a back boiler.
What puzzles me is that wet underfloor really isn't any different from radiators in principle, yet you can apparently heat rads with this system? Surely underfloor is just a larger horizontal radiator, using plastic pipe instead of metal?
Ok, you cannot regulate the heat as efficiently, but it must be better than using a combi running from cylinders?
Underfloor heating needs to be on for a long time, which would use up the gas in no time. In winter, the log burner will be ticking over all day, all evening and some of the night, and be up and running again in the morning before the underfloor will have completely cooled.
It seems like a no-brainer to me ??
We have no mains gas here, and could only squeeze in a small water tank or whatever would be needed to run this system. Oil is out of the question as there is nowhere for an oil tank, and I wouldn't have it on principle anyway.
We want a combi so that we have instant hot water all year round,( have no room for a large water tank), and I like to cook with it. Using it to heat the cottage as well I don't think would be very cost efficient or practical. We have some electric underfloor also, plus two chimneys, so we have a few options, but oil and mains gas are definitely out!!!

 

[Solo Heating]

Hi,

Solid fuel stoves can only be connected to an open-vented system (one with a loft tank to fill the back-boiler). Underfloor heating systems don't usually work very well with open-vented systems because air becomes trapped in the narrow bore of the pipework and stops working. Instead, underfloor heating systems are connected to sealed/pressurised systems which keeps the air out.

There is a method which requires using a thermal store. A thermal store looks like a standard hot water cylinder (as you'd find in your airing cupboard) but works in reverse. The water stored inside it is the same water which runs through your back boiler on your stove and is connected to the loft tank which means it's all open vented. But the thermal store has in it a coil or multiple coils which can be connected to different systems such as underfloor heating. The coil and the underfloor heating can be separately pressurised thus overcoming the problem of air and a pump is used to push the water in the underfloor pipework through the coil where it heats and circulates through your floor. The downside to thermal stores is they can be expensive to buy and can be larger than a standard cylinder. You also need to use a pump and have space for the pipe connections for your underfloor heating.

I hope this helps for starters.

cheers,

Ben

 

[titania]

Thank you Ben. It is very complicated. You say that underfloor systems don't usually work well with an open-vented set up.
This implies that it is possible and has been done, just not as reliable? It does seem like a lot of hassle for just one room!
If the small pipework is an issue with the UF, why not just use a larger pipe? This would also give more heat.
If the rads were hooked up to a back boiler would I still need this same set up, as rads can be bled of air? I know that a pump would be required.
Sorry for my ignorance. I did have some of this run past me before, but at least in writing I have a chance of getting my head round it all!

 

[Solo Heating]

Thank you Ben. It is very complicated. You say that underfloor systems don't usually work well with an open-vented set up.
This implies that it is possible and has been done, just not as reliable? It does seem like a lot of hassle for just one room!
If the small pipework is an issue with the UF, why not just use a larger pipe? This would also give more heat.
If the rads were hooked up to a back boiler would I still need this same set up, as rads can be bled of air? I know that a pump would be required.
Sorry for my ignorance. I did have some of this run past me before, but at least in writing I have a chance of getting my head round it all!

Hi,

It is possible and you can buy single room kits for £400-500 (you need to ensure the kit contains a temperature regulator as underfloor heating works at a much lower temperature than your back boiler). The installer will need to ensure the pipe work is fully flushed of air and there are no areas where air can become trapped (even a small pocket of air can stop the circulation of water; in any system). There is then the cost of constructing the floor and adding insulation and ensuring you don't change your floor levels - this could become annoyingly complicated for you (but might be simple - I really don't know without seeing your layout). You also need space to put the pump/regulator and pipe connection manifold.

Considering a radiator can be purchased for a few tens of pounds (perhaps even for free if you scan the local papers/freecycle) and installing pipe work plus a pump for this will be similar to the cost of the underfloorheating kit, I'd go with this option. You can also run your radiators upstairs (if you have an upstairs) without using a pump if the installer can get in the necessary pipes for gravity circulation to work.

But the choice is yours.

Cheers,

 

[titania]

Hello Junior,
Having experienced UF heating in other peoples houses I would always prefer this against a rad, especially as the cottage is timber framed!
I have worked out insulation etc, that is not a problem! It was a kit I had in mind. Does the pump reg and manifold have to be in the same room as the UF heating?
There is space on the back of the chimney (which is now inside the extension) where the stove is.
Presumably I would still need a water tank somewhere, but as this is not for baths or showers could this be quite small? For example, about half the size of a normal copper tank? We could squeeze that in and build a cupboard round it.
Also, would the UF have to be on a different loop to the rads? If it were all on the same system perhaps any air that may be present could then be bled. The temp difference may not be an issue as the whole system would just be on slow burn, for 24/7 if I can keep the fire in. As we have wattle and daub walls once the clay has warmed up it retains heat for a long time, although the cottage would take a day or two to warm up properly.

 

[Solo Heating]

Hello Junior,
Having experienced UF heating in other peoples houses I would always prefer this against a rad, especially as the cottage is timber framed!
I have worked out insulation etc, that is not a problem! It was a kit I had in mind. Does the pump reg and manifold have to be in the same room as the UF heating?
There is space on the back of the chimney (which is now inside the extension) where the stove is.
Presumably I would still need a water tank somewhere, but as this is not for baths or showers could this be quite small? For example, about half the size of a normal copper tank? We could squeeze that in and build a cupboard round it.
Also, would the UF have to be on a different loop to the rads? If it were all on the same system perhaps any air that may be present could then be bled. The temp difference may not be an issue as the whole system would just be on slow burn, for 24/7 if I can keep the fire in. As we have wattle and daub walls once the clay has warmed up it retains heat for a long time, although the cottage would take a day or two to warm up properly.

The pump and manifold can be in another room or cupboard but that depends on the length of pipe in the kit.

The underfloor heating circuit will connect into the same pipe work feeding your rad system - treat it as a another rad on the circuit, but one which has it's own pump and regulator. Everything (back boiler, rad circuit, UF circuit) will be fed from the tank in the loft.

There's no need for a hot water cylinder but you will need a method of ensuring the back boiler doesn't overheat so at least one radiator is normally connected to the system which will work under gravity circulation and disperse the heat constantly without the need for a pump. The manufacturer of your stove will normally advise on this but if you don't know the manufacturer, check out other stoves of similar size and output and see what they say. Better to be safe than sorry.

 

[titania]

That is excellent news. Thank you. Quite a bit more expensive than just putting another rad in, but I am sure it will be worth it!

 

[AWheating]

many underfloor heating suppliers/manufactures will not only give you design advise but will also give you exact design details and material lists if you phone them up. I have even had the manufactures rep and 2 design engineers from Germany come to look over an ufh installation i was fitting to advise me (uponor)