What size multifuel stove,?

[Ltec]

What size multifuel stove should be in a room that measures 5.5 x 4 x 2.5m
It is a new build, so 6 inch cavity and about a ft insulation on ceiling. From calculators online I've got anywhere from 2.2kw to 4.
I currently have a boru carraig mor 20kw which is supposed to be putting out 4 to the room but the heat is unbearable even with the doors and windows open. It is heating the rest of the house fine about 14 radiators I think.
We now don't light the stove as its too warm in its room so its an expensive ornament.
What should I do.
Most boiler stoves put out at least 4kw to room but I've seen the stanley tara puts out 2kw to room and 8 to radiators.
Would this cure my problem.
The house is zoned so I could cut the radiator heat in half needed.
It would be an expensive change so I'd need to be sure

 

[Brambles]

These are expensive installations, you are far better getting a decent Hetas installer to design the system for you and warrant it’s performance. That does not stop you buying the stove on Ebay et al, but it does mean that
you will get an installation that performs to your requirements.

Even the type of internal flue installed can make a significant difference to the convected heat from the stove.

If it is installed incorrectly or not performing to suit the room, post installation modification to correct issues are not cheap.

 

[Ltec]

These are expensive installations, you are far better getting a decent Hetas installer to design the system for you and warrant it’s performance. That does not stop you buying the stove on Ebay et al, but it does mean that
you will get an installation that performs to your requirements.

Even the type of internal flue installed can make a significant difference to the convected heat from the stove.

If it is installed incorrectly or not performing to suit the room, post installation modification to correct issues are not cheap.

I think hetas in only available on uk mainland not northern Ireland.
Most plumbers here don't want to touch stoves.

 

[Chuck]

In my opinion, that type of stove is a poor choice for a highly-insulated modern house. If it were me, I'd want something that can be controlled by a room thermostat. If you need 2kW when it's -5°C outside, which is what I assume you've caclulated, you're not going to want much more than a few hundred watts most of the time.

 

[Ric2013]

Some considerations:

There are other certified solid-fuel installer schemes such as OFTEC, for example, not just HETAS.

If the room is really airtight as well as insulated, it can be difficult to avoid overheating. Particularly if the room has little thermal mass to soak up the excess heat and later release it. Room heat losses tend to be calculated for mid-winter conditions, and any milder weather will result in less heat loss, something automated central heating copes with better than a stove that is not so easy to switch on and off.

If you can find a stove that puts very little out to the room and couple the hot water output to a thermal store, then you might be onto something. That said, the eco credentials of wood stoves is often questionable, and since you have a modern insulated building, solid fuel may just not be the best solution.

Do let us know what you try out and how it goes. My personal experience is that even in relatively old and inefficient houses, solid fuel stoves tend to cause overheating. If there is thermal mass, you can get away with a quick hard burn and the room stays warm for a considerable time, but if not, it doesn't really work. I would be interested in how you tackle this problem.

What I would comment as a possibile consideration, however, is that my father's house in Italy has a large woodburning stove with 9kW output. However, the metallic parts of the stove are surrounded by heavy ceramic slabs that result in a much slower release of heat to the room, and lower stove surface temperatures. It isn't actually a very good stove and I wouldn't recommend the brand (which, in any case has now closed down), but the point remains that the ceramic parts tend to delay the heat output and spread it over a longer period. The disadvantage is the room takes longer to warm up in the first place.

 

[Brambles]

Hetas has a had a registration scheme acrosd Ireland for a number of years. Oriel Flues can put you in touch with some

 

[Ltec]

In my opinion, that type of stove is a poor choice for a highly-insulated modern house. If it were me, I'd want something that can be controlled by a room thermostat. If you need 2kW when it's -5°C outside, which is what I assume you've caclulated, you're not going to want much more than a few hundred watts most of the time.

Sorry for being so stupid, are you saying you wouldn't put in a multifuel stove. Is it an electrical stove or something that is controlled by a room stat. Could you give me an example please.
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Some considerations:

There are other certified solid-fuel installer schemes such as OFTEC, for example, not just HETAS.

If the room is really airtight as well as insulated, it can be difficult to avoid overheating. Particularly if the room has little thermal mass to soak up the excess heat and later release it. Room heat losses tend to be calculated for mid-winter conditions, and any milder weather will result in less heat loss, something automated central heating copes with better than a stove that is not so easy to switch on and off.

If you can find a stove that puts very little out to the room and couple the hot water output to a thermal store, then you might be onto something. That said, the eco credentials of wood stoves is often questionable, and since you have a modern insulated building, solid fuel may just not be the best solution.

Do let us know what you try out and how it goes. My personal experience is that even in relatively old and inefficient houses, solid fuel stoves tend to cause overheating. If there is thermal mass, you can get away with a quick hard burn and the room stays warm for a considerable time, but if not, it doesn't really work. I would be interested in how you tackle this problem.

What I would comment as a possibile consideration, however, is that my father's house in Italy has a large woodburning stove with 9kW output. However, the metallic parts of the stove are surrounded by heavy ceramic slabs that result in a much slower release of heat to the room, and lower stove surface temperatures. It isn't actually a very good stove and I wouldn't recommend the brand (which, in any case has now closed down), but the point remains that the ceramic parts tend to delay the heat output and spread it over a longer period. The disadvantage is the room takes longer to warm up in the first place.

I left the flue and around the stove fairly exposed as I thought it would waste heat into the wall. I didn't realise it would be too warm.
I have enough firewood for the next ten years so its not costing me very much to fuel.
I'm completely fed up with the whole stove thing as its a very expensive ornament.
I feel like ripping the whole thing out and putting in an open fire which wouldn't give out as much heat.
Its not lit all the time. Its more of a novelty for snow and Christmas etc.

 

[Ric2013]

I have enough firewood for the next ten years so its not costing me very much to fuel.
I'm completely fed up with the whole stove thing as its a very expensive ornament.
I feel like ripping the whole thing out and putting in an open fire which wouldn't give out as much heat.
Its not lit all the time. Its more of a novelty for snow and Christmas etc.

If this is the case, that it's just for snow days and Christmas, perhaps just consider it as an expensive ornament, leave the window open a crack and enjoy it for what it is, especially as you've already made the investment both in the installation and in the storage area for the firewood. It isn't your main source of heat, so it could have been worse, and if there is ever an interruption to the gas supply you can be smug and snug.

It does sound like a very frustrating situation, and you have my sympathy, but it seems to me that there's no getting round the fact that low-energy houses do not need much heat. It would be lovely if there were a manufacturer making 1-2kW output stoves that met with building regulations and could be installed in houses, but I'm not aware of any. Please do let me know if you find one though.

 

[gmartine]

20kw 🤣. I'm coming round for the spit roast, whose cooking? Sorry, couldn't help it.

Your calculations are about right, for a well insulated room it's volume divided by about 15 so anything beween 2-5kw should do. for the stove room. I have a 4kw MF stove in a similar sized less well insulated room and it's about right. You'll have to work out what you need for the rest of the house but I'd persevere. Replace it with something much smaller and sell the 20kw stove on, there's a strong market for used stoves from what I can tell. Do a heat loss on the whole house and I bet it's no more than 12kw.

 

[Ric2013]

Your calculations are about right, for a well insulated room it's volume divided by about 15 so anything beween 2-5kw should do. for the stove room.

If you are suggesting cu m / 15, that seems excessive to me if that heat input is sustained for any extended period of time. Unless you are basing that on a very windy cold site, unlike my experiences in the sub-urban South-East?


Case study 1: House I grew up in. The standard Morso 1410 (Squirrel) has a 5kW output. The version with the 7000btu (2kW) backboiler reduces the heat output to the room to some degree (a friend of mine had a similar stove without backboiler and there was a perceptable difference). I installed the backboiler model.

The house was built in 1962 and had brick/brick (not brick/block) cavity walls, uninsulated, and the stove room had three external walls with the remaining internal wall being to unheated rooms; the ceiling was again below unheated rooms (and not insulated). The front window was double-glazed, while the back wall had the original timber/single-glazed door and steel-framed (i.e. draughty) single-glazed window.

The room was around 20' x 10' x 8'=1600 cu ft, so 46 cubic metres. Divide that by 15, and we get around 3kW, so by your rule of thumb, the stove should have been approximately correct had the house been a modern one. Contrarily, the room in question actually tended to overheat after around an hour all year round despite the poor thermal efficiency. The solution was to open the door to the rest of the (unheated) house and allow heat to go upstairs, meaning once we had warmed the stove room, the rest of the house could begin to warm up too. Thus we used the stove or the central heating, but never both.


Case study 2: My current house, also in Colchester (much older, but insulated to around 1990's standards and centre-terraced). The 85 cu m of ground floor (i.e. living room plus two other rooms) is heated by around 1.4 kW (IIRC) of UFH, with a backup of a 1.3kW radiator used only if a a neighbour goes on holiday or in mid-winter (heat losses were calculated by me based on thermal efficiency of building elements and assumed air changes per hour, and that was before the ground floor was insulated and draughtproofed). Your rule of thumb would suggest nearly 6kW would be required.


The OP has 4kW (plus the fluepipe) in a room that is the same size as my living room. 2kW of radiators would be excessive, but radiators can be thermostatically controlled and oversizing can be useful to reduce heat-up times: a stove is much harder to regulate.

 

[Brambles]

Ric2013’s calculations are very much in line with mine.

We have had a lot of success with oversized stoves by installing insulated internal flues from the stove spigot. It significantly delays the period before the room becomes overheated.

It seems to be (in Oxon) that people light the stove and sit in the room for 6 or so hours.

Useless on the fuel efficiency front, but a lot of our Customers just have them for the wow factor - so a slow(er) heat build up over that 6 hour evening is all that they are looking for.

 

[Ltec]

20kw 🤣. I'm coming round for the spit roast, whose cooking? Sorry, couldn't help it.

Your calculations are about right, for a well insulated room it's volume divided by about 15 so anything beween 2-5kw should do. for the single room. I have a 4kw MF stove in a similar sized less well insulated room and it's about right. You'll have to work out what you need for the rest of the house but I'd persevere. Replace it with something much smaller and sell the 20kw stove on, there's a strong market for used stoves from what I can tell.

Ric2013’s calculations are very much in line with mine.

We have had a lot of success with oversized stoves by installing insulated internal flues from the stove spigot. It significantly delays the period before the room becomes overheated.

It seems to be (in Oxon) that people light the stove and sit in the room for 6 or so hours.

Useless on the fuel efficiency front, but a lot of our Customers just have them for the wow factor - so a slow(er) heat build up over that 6 hour evening is all that they are looking for.

So I should find out if my flue is insulated and if its not and I insulate it it should lower my temperature build up a bit.

 

[gmartine]

@Ric

We all know how vague and shot in the dark these calculations can be as well as highly dependent on how the stove is used and "about right" meaning a stove rating of between 2-5kw for which I'm happy to stand by. The OP's heat loss calcs varied from between 2.2 - 4 kw. My use of a constant gave a reading 3.6kw at the upper end and the OP's installation has an exposed flue increasing the stoves output into that room probably by another KW or more especially as flue gases are hotter than the actual masses they are being emitted from. I suspect without any ventilation too which is obligatory with stoves rated over 5kw, no idea of the size or type of windows or whether the cavity is filled etc etc so you see we are all piddling in the dark somewhat.

As it's now been mentioned I have an insulated flue which obviously increases the flue's efficiency, would it help should the OP go down that route, certainly but I'd be surprised considering how extreme the OP's overheat seems to be that it would be enough and as Brambles said it only delay's the onset of overheat.

MF stoves aren't exactly precise and yet can glibly claim 4Kw out the front and 16kw out the back especially without it seems much if any control gear. Chances are when the OP's stove get's loaded and the flue is up to temp the OP's stove is pumping out at least double the claimed output into that room initially and causing the considerable overheat. The OP burns wood meaning the stove will initially burn very hot indeed. I don't, only using wood initially to create a layer embers so I can load it with smokeless coal and that suits the room just fine as coal is far easier to regulate.

Does the room overheat sometimes? Certainly, it is not heated in isolation but nothing opening the door won't sort out and as you say they don't make 2kw MF stoves as far as I know and practically speaking the OP would have to cut his wood down to matchsticks to be able to fuel one. Having said that depending where the OP lives it's barely been cold enough to use these past few winters and the OP's house is far better insulated than mine.