What size of woodburner?

[MdB]

You are generally correct Penners but you have missed the heat store effect of the underfloor. This means that when you light the stove and cause the room temperature to rise enough for the thermostat to switch off the underfloor, it will still be chucking out roughly the same amount of heat - at least in the short term. This means that the stove will be additional to the underfloor output rather than replacing it and the room temperature will rise accordingly. The next night / day, the underfloor will of course be on for less time to compensate for the heat produced by the stove.

In our house, we have the thermostat set to 20 degrees (during the cheap electricity period) and currently it is easily achieving that. When we put the stove on, the temperature will rise to 21 or 22 degrees which for us is about right. Note: downstairs for us is completely open plan and a lot of heat goes upstairs also so our 4.5kW stove is never going to make a large difference in temperature as it is effectively heating the whole house. If the room was enclosed - like a standard living room, I would expect the temperature to rise far more.

 

[AMc]

Thank you so much, Penners. That's the most helpful piece of information I have read during my tedious hours of research on woodburners. As you are obviously the fount of knowledge on this subject, would you be kind enough to offer me some advice? Our house is stone and will have, any day now..., underfloor heating under flagstones. We want a woodburner both to top up when it gets very cold, but also to be a point of focus in our very large stone fireplace. The chimney place is very large so I would like a large sized burner to take up the space, but am concerned that it will be too hot with the UFH. I suspect I want the impossible. I've seen the Dovre and Jotul stoves that you can use open as well as closed though they are a bit expensive - given that the renovation has guzzled much more money than we expected.

I'd be very grateful (ha ha) for any advice.

I have underfloor heating and 2 stoves (different rooms). Echoing Penners comments thermostats in the room will be crucial.

If your UFH is up to the job of heating the room alone then the woodburner will remove the load partially or entirely as it warms up.
If the UFH doesn't react to the stove quite fast then you might get uncomfortably hot while both are putting out their full heat.
In a conventional radiator system with a thermostatic valve that system will turn off almost immediately and cease to add heat in minutes after lighting the stove.

UFH tends to be slow acting, taking a couple of hours to reach temperature and the same to cool to ambient temp (depending on the flooring material and flow temp). Your flagstones will act as a giant storage heater holding a lot of energy for a long time.
By contrast a stove should reach full temp in minutes and will drop off slowly as the fire dies down if you don't refuel.

In one large open plan room (8m x 4m) we have an 8kw stove but this area has a large glass roof and glass wall so loses heat fast. We tend to light this one at or before dusk in the winter as the UFH begins to kick in if we're going to use that room for the evening. As the stove works faster the underfloor pumps a little extra heat into the room but the stove does most of the work and the room stays warm but not too hot providing the fire is refueled. If we don't light the burner and its cold outside the UFH keeps the room comfortable but we retreat to a smaller one to watch TV.

The other room is about 3m x 4m with a stove that is at least as large (but is too old to know). Even with no heating at all in that room the stove can get the room to 26-27C in under an hour (when it's 0C outside). If the UFH has been warming the room already then this happens faster. The stat turns off the UFH almost as soon as the fire is lit and the floor cools quite fast. The stove can keep the room at 26C with all the vents closed and the baffle shut so I can only imagine that the flu temp is very low. It's also quite irritating to have to open a door and introduce a draught at the same time as feeling a bit hot and bothered. The flu is solid metal in a brick chimney (no insulation) so I don't like to think what is happening to it - at some point I daresay we'll change the stove and replace the liner if it's badly corroded.

Most of the stove calculators using room sizes assume the stove is the sole source of heat in a modern, insulated room with a margin of over capacity. If your renovation has brought your room up to 2006 or newer Part L1B building regulations levels of insulation then you may need a smaller stove than the ready reckoners suggest. If you have heat loss calculations for your underfloor that should give you an idea of the size of stove you'd need but unless its a huge room I would look at a smaller stove working harder than a large one.

If you're worried about filling the space then a good stack of firewood on either side looks good, makes you more likely to light and refuel (I don't like going out in the dark, cold and rain to get wood). Having the firewood warmed and dried by the stove makes it easier to light too.

I really would recommend you go and speak to a couple of HEATAS registered installation companies. You can get free quotes and good advice. I know stove installation is not rocket science but an new installation should comply with building regulations and you may have issues with signing off any other work if you can't show compliance not to mention potential issues with insurance and resale.

 

[Zebra]

Thanks for the referral Penners, I don't know that firm. I do know another nearby but as I am hoping to save money by buying a reconditioned stove from a dealer elsewhere in the UK, the local firm seemed to think that a registered installer would refuse to install it! So the retailers wouldn't be interested in advising me as I'm not buying from them, so that leaves me rather on my own.........

 

[skier-hughes]

you pretend you are going to buy one from them and then don't..........

 

[Penners]

You are generally correct Penners but you have missed the heat store effect of the underfloor. This means that when you light the stove and cause the room temperature to rise enough for the thermostat to switch off the underfloor, it will still be chucking out roughly the same amount of heat - at least in the short term.

For deeply buried UFH I'm sure this is the case - a situation for which you have to adjust the on/off timing of your heating, presumably.

But I was speaking to someone who had UFH installed recently, and he told me that the pipes are tending to be buried at a much shallower depth, these days, thus reducing the heat sink effect.

I can't comment on this, just report it.

 

[tobydog]

I concur with that. Our UFH sits in a 3-4" screed about 4" of Celotax. On top of the screed is a fairly thick engineered oak floor. I know wood is a good natural insulator but it still has the desired effect. We do notice though that if the heating goes of in the evening, the floor has gone cold by the morning.

 

[MdB]

I concur with that. Our UFH sits in a 3-4" screed about 4" of Celotax. On top of the screed is a fairly thick engineered oak floor. I know wood is a good natural insulator but it still has the desired effect. We do notice though that if the heating goes of in the evening, the floor has gone cold by the morning.

Ours it about the same. 75mm screed on 100mm Celotex. Our floor would be cold in the morning too if it switched off, but it would stay warm for 2 to 4 hours which is the point I was making.

 

[AMc]

Our house is stone and will have, any day now..., underfloor heating under flagstones.

The heat storage capacity of the flagstones will be significant. You can obviously mitigate this by lowering the thermostat well ahead of lighting the fire or simply allow the heat into other rooms until the floor cools/stove warms.

It will still be easier to overheat the room just lighting a large stove compared with a smaller one - my large stove/small room combination isn't possible to control to keep the room temperature comfortable.
I would prefer a smaller stove, opening the vents and more frequent refueling to deal with being too cold than the situation of having the stove smouldering and still too hot. I suspect that the stove in this room is MUCH too large though.

Some of our suspended floors with boards on top have as little as 25mm screed and are quite "fast" and those rooms heat up and cool down almost as quickly as the radiators. The floors with solid screeds 75mm thick with floor bricks and pamments are much slower.

As a side note I had an electricity energy monitor logging to my computer in a radiator room when I was debugging my heating it also logged room temperature. That room could take 2 hours to get from 16C (unheated) to 20C I suspect as a result of the uninsulated solid walls and significant ventilation losses

 

[Zebra]

I bought the 8kw fire in the end. Am going to install it myself, with a Building Control Notice to cover the legality. However, means I have to sweep chimney myself. Anyone near me got 16 inch brush and rods set I could borrow?!! Perhaps I'll buy one on ebay. Or hire one. Or lure a squirrel to run up and down the chimney (offering nuts first at one end, then the other) with hairspray in its tail.

 

[Feltwell]

I'm sure I remember reading somewhere that one traditional way to clean a chimney was to drop a cockerel down it, letting it flap like mad to clean the chimney. Presumably the filthy and probably injured bird once it emerged became Sunday lunch. Might be an old wives tale, but if true then not a method I'd recommend to use now!

 

[Penners]

You both seem to have omitted the obvious option of a ladder and a ten-year-old boy wearing a wide-brimmed hat.

 

[Zebra]

:lol: Imagine asking my neighbour if I could borrow one of her chickens for a short time, and returning it filthy with broken wings! I suppose I could see if anyone would lend me a small child...
But actually, the wide brimmed hat idea won't work. I myself can fit up the chimney, the problem is that you can't get a ladder into the fireplace.....

 

[Feltwell]

the problem is that you can't get a ladder into the fireplace.....

http://www.amazon.co.uk/Telescopic-Ladder-Unique-Integral-Stabiliser/dp/B001O3CJD8

You both seem to have omitted the obvious option of a ladder and a ten-year-old boy wearing a wide-brimmed hat.

Ten years old? Feeling charitable today are we Penners? Surely younger ones are more nimble and easier to get round the bends in the flue?

 

[Penners]

Ten years old? Feeling charitable today are we Penners? Surely younger ones are more nimble and easier to get round the bends in the flue?

I suppose it depends on the dimensions of the chimney and the girth of the boy. I'm amazed that chimney builders have never thought to engrave into the brickwork the optimum age of child required for an effective sweep.

 

[Feltwell]

Choose your 10 year old carefully. Some come with a natural ability to scrape the sides, but cause problems with your fire smoking terribly into the room when they inevitably get stuck.

 

[Penners]

But surely they only stay stuck for a few weeks. Nature will eventually solve the problem.

 

[Feltwell]

True, with the added bonus that their now-outsize clothes act like a kind of parachute so they come down the chimney gently and don't risk damaging your fireplace.

 

[Zebra]

That telescopic ladder would work yes. But for £89.99 I could surely get quite a few chickens.

 

[Feltwell]

Fair point. And they taste much better when roasted as well, truly dual purpose.

 

[Zebra]

They probably do. I've never tried roasted ladder.