Low room output radiant (not convective) wood burning stove

[davidc]

Can anyone recommend a low room output radiant rather than convective wood burning stove for a low energy airtight build ?

 

Preferably with the capability of heating a thermal store and also DIBt certification ?

 

I've already got some Woodfire models on my list and would also have Tonwerk on it were it not for their astronomical price tag.

 

Thanks.

[JohnMo]

I have a Heta Scanline 8, with soapstone top and sides, to slow heat output.  Couple of small logs and min air, puts out a gentle (ish) heat.  Have a managed a hour with it on, with all the internal doors open, before melting and having to open the windows.

 

Not sure a boiler stove would be on enough or for long enough, to be worth the added expense.

[PeterW]

3 hours ago, JohnMo said:

Not sure a boiler stove would be on enough or for long enough, to be worth the added expense.

+1 to this - you’ll get no benefit and a TS in a thermally efficient house is also an odd choice, what is heating it ..?

[dpmiller]

I'll go against the accepted wisdom here as I went ASHP/TS/ boiler stove. At least that's the plan, the stove is a wee bit down the things-to-spend-money-on list yet.

 

Likely Woodfire Passiv, but I'm holding out for the option of a FHT Stove actually coming to market (or having a play at building one myself..)

[JohnMo]

There is no reason a TS cannot be used, but you will need to watch a couple of things.

 

Overall heat capacity is reduced due to lower temperature within the store.

 

With ASHP, you will need to either direct charge from ASHP or go via a plate exchanger, as the normal TS coil is designed for 70/75 degC + input temperature. At ASHP temperature, you will not get the heat transfer rate required to manage the supply and return deltaT to the ASHP.

 

Also same is true for DHW supply, you will need to oversized the heat exchanger to get a decent flow temperature. So PHE is the easiest option or a a huge coil.

[davidc]

@JohnMo thanks, I didn't have that on my list but it may have a tad greater output that I'd like - hopefully less than 2kW to room

 

20 hours ago, PeterW said:

+1 to this - you’ll get no benefit and a TS in a thermally efficient house is also an odd choice, what is heating it ..?

 

The benefit, as I see it, lies in the fact I have unlimited Scots Pine within 200m of the plot and if possible I'd like to use this to provide some of the DHW and heat, the latter (with UFH) in smoothed fashion that also reduces the peak stove output to the room. I don't mind felling, chopping, splitting and storing the lumber. As regards what heats it when a potential stove doesn't, I've oscillated between ASHP and just a simple electric heating element.

 

Thanks, @dpmiller, I was unaware of the FHT stove. 

 

11 hours ago, JohnMo said:

 will need to either direct charge from ASHP

 

Sorry, I'm too daft to know what is meant by direct charge, if asked I'd of said that *was* a coil - but obviously not.

 

Digressing a but, I did look at an Valiant AroTHERM plus .

Edited May 9, 2022 by davidc

[JohnMo]

By direct charge I was referring to heating the contents of the store directly, instead of by heat exchanger (coil or plate heat exchanger).

[ProDave]

I wired a straw bale house some years ago.  That was heated with a WBS that claimed to put 10kW to water but only 2kW to the room, and that heated a massive thermal store (1000litres iirc)  and the thermal store ran under floor heating for when the stove was not burning.

 

Unfortunately I don't know the make or model of stove.

[davidc]

28 minutes ago, ProDave said:

 WBS that claimed to put 10kW to water but only 2kW to the room

Several Woodfire models have that split so it may have been one. They aren't DIBt I believe.

 

I'm also trying to accommodate countervailing requirements here as as I'd also like to keep the house as cool as possible over the warmest months - which I think a TS may hamper.

[jamieled]

We spent quite a while considering this for our place - we have a lot of timber available. In the end I was persuaded to keep space heating and hot water seperate. We have a ~5kw woodburner which is our only source of heat. Hot water via direct electric immersion, a lot from solar PV.

 

I'd be interested to know whether it can be done well though - my main concern was around overheating if we were reliant on it for hot water, but if there's another source like an ashp then that's dealt with. We only ever considered the boiler bit of the stove for DHW and I think it's maybe trickier that way.

 

Our hot water tank gives out a bit of heat - it's set to a high temp but we make good use of the heat in an airing cupboard and it doesn't noticeably effect the rest of the house.

 

[PeterW]

@davidc have you considered a log gasification boiler and a larger (~3000 litre) TS in an outbuilding and then you can utilise the biomass much better. I would still put a smaller UVC in the main house envelope as the primary hot water source along with the primary solar dump but it would even out your usage. You may find yoh can get RHI for the right set up and also it allows for segregation of heat sources. Your log burner then becomes a smaller focal point / secondary heat source 

 

[davidc]

@jamieledI'm starting to think the only way I can do it well with a WBS and TS is the Tonwerk route which put about 1.3 kW to the room and approximately the same to water but at cost of ~£10k for the stove if my currency conversion is right, seems to me, like the tail would be wagging the dog in the sense I'd be spending a fair bit of extra cash just to save some cash.

 

@PeterW that is all new to me, thanks, the house is only 125 sq. m and the budget is fairly modest so perhaps going down that route I'd end up in the same position as above in terms exploiting the free fuel but adding to the build costs a fair bit ?

 

I did think about trying to find out if there was any mileage in getting a pellet maker and seeing if a pellet burning stove would change anything.

 

I may just end up having to have a WBS as secondary heat source - if I can find one with low enough output that is and a better understanding of the supposed problem of indoor particulates and which models mitigate that best.

Edited May 10, 2022 by davidc

[Thedreamer]

@davidc

 

Similar to your circumstances I have a lot of free wood.

 

My space heating comes from a centrally place wood stove (mine is surrounded by dense concrete blocks). This goes on from October to March. The rest of the year I rely on solar gains and waste heat from appliances and us. 

 

Having access to free wood has resulted in the space heating bills being no more than £5-£10 since we moved into the house in July 20.

 

My hot water comes from an exhaust air source heat pump which sits on a good quality hot water cylinder. 

 

My approach is not for everybody but I feel in the right location it could work for others. 

[Iceverge]

I spent ages looking at this, in a similar situation re free wood. 

 

Didn't bother in the end. Too complex, expensive and oversized.  Just used a direct cylinder with an immersion and plug in rad. 

 

The Olsberg tolima Aqua II or Spartherm A4 H2O are both Dibt rated as far as I can remember. You may need to be on mains water to avail of the ability to connect to an UVC for the quenching function in the event of overfiring however. An alternative is a gravity fed open vented thermal store but these are expensive and need to be very large or kept at very high temps to get lots of hot water. 

 

@Thedreamer has a nice solution. Would it suit you?

 

My guess is 4kw ish of PV diverting to a 300l direct cylinder would be good and cheap with a <5kW dry stove in a large room downstairs for space heating and an electric rad for backup. 

 

 

 

 

 

 

 

[davidc]

Thanks, @Thedreamer and @Iceverge .

 

I did consider an EAHP but concluded that the air flow rates required (as air has a much lower heat capacity than water) were likely to present some challenges in an air tight home with MVHR.

 

I'm starting to regress back to the mean position of, if a having an WBS, just having it for space heating but the issues surrounding <2kw output, particulate leakage and knowing that the installation and stove itself won't make a sizeable dint in the airtightness remain !

[Iceverge]

Here is a link to a German retailer that with a bit of Google translate help you can filter by Dibt compliant stoves.

 

https://www.kamdi24.de/kaminoefen-bis-5-kw/FILTER_VERBRENNUNGSLUFT:FILTER_VERBRENNUNGSLUFT_RUU_DIBT

 

However I came to the conclusion an external air supply wasn't as important or useful as it first appears, although it is required by Bregs. 

 

 

Bregs require 550mm2/kW of permanent ventilation for  stoves. At 0.31ACH our airtightness result still left 5000mm2 of equivalent leakage in the envelope. 

 

In theory enough for a 9kW stove.  I'm pretty sure you'd have a tough time arguing that with building control but practically with a balanced MVHR system I can't see a small stove ever struggling for air. If the MVHR is highly biased towards extract you risk pulling fumes out of the stove. Not ideal. 

 

I have seen 2 examples of passivhaus with stoves that have good airtightness results. 

 

0.59ACH with a boiler stove. 

 

https://mollyglass2012.tumblr.com/page/9

 

An extract from "Old Holloway" passivhaus. (one of my faves) 0.39ACH. 

 

 

 

Interesting both use poujoulet flues of different flavours. Maybe no more than coincidence. 

 

 

 

Edited May 12, 2022 by Iceverge

[SteamyTea]

From this week's comic.

 

Trees beat energy crops as CO sink

12 May 2022

Nearly twice as much carbon could be removed from the atmosphere by using available land in the US to plant forests rather than grow fuel crops and capture the carbon dioxide emitted when those crops are burned, says a study. Trees can also help to avert water shortages (Science Advances, doi.org/hsxs).