Jimbouk Posted March 16, 2017 Share Posted March 16, 2017 Just wondered what people have been using as a duct for the air feed to a log burner? We will be putting one in on top of the concrete slab, between the celotex and below screed layer? Tricky element is is that I have not chosen a stove yet, Jotul in pole position. 100mm duct seems fairly standard though. Yes, I appreciate they are not environmentally sound and we run the risk of over heating, but we want one.... Link to comment Share on other sites More sharing options...
PeterW Posted March 16, 2017 Share Posted March 16, 2017 Talk to @Trw144 as he knows a lot about stoves ... And you also may find he has a stove or two at a price that is just too good to miss ... and yes I know as I bought one ..! Also bear in mind that if it's less than 5kw it doesn't need any external air Link to comment Share on other sites More sharing options...
Temp Posted March 16, 2017 Share Posted March 16, 2017 (edited) I think the stove will dictate the position of the duct so better check carefully. This site has a handy filter that allows you to list stoves that have a direct air supply together with diameter of pipe required. Click and scroll down... http://www.stovesonline.co.uk/direct-air-supply-stoves.html Edited March 16, 2017 by Temp Link to comment Share on other sites More sharing options...
joe90 Posted March 16, 2017 Share Posted March 16, 2017 I have put a 4inch underground drain pipe in under my floor insulation and will be bringing it up and into the air feed on the stove with 4inch semi flexible aluminium pipe. Our stove has a dedicated air supply even though it's sub( just) 5Kw, the suppliers were very good with their advice and flue planning, this is the stove we have -http://www.stovesonline.co.uk/wood_burning_stoves/ACR-Birchdale.html#entity_9162, p.s. I was advised to raise the air induction pipe up to 600mm above ground outside as this is best. Link to comment Share on other sites More sharing options...
Mikey_1980 Posted March 16, 2017 Share Posted March 16, 2017 We have a duct running under our slab for our Hwam stove, we had the hearth made to measure with a hole in it for the air feed, we made all the decisions on the stove/hearth etc before the slab went down so its worth planning it as early as you can. I will post a picture later. Link to comment Share on other sites More sharing options...
Trw144 Posted March 16, 2017 Share Posted March 16, 2017 1 hour ago, PeterW said: Talk to @Trw144 as he knows a lot about stoves ... And you also may find he has a stove or two at a price that is just too good to miss ... and yes I know as I bought one ..! Also bear in mind that if it's less than 5kw it doesn't need any external air Very little left now I m afraid - we ve sold 1400 stoves in around 6 weeks, and just 15 Franco Belge Belforts left. You do still need either an air brick or direct air supply on sub 5kw stoves if the house is above a certain level of air tightness (pretty much any new build and I m sure everyone on this forum). For direct air supply under the slab - like others have said, any circa 100mm soil pipe will suffice and you can then find a means of connecting a 100mm flexible ducting from this onto a stove. Link to comment Share on other sites More sharing options...
readiescards Posted March 16, 2017 Share Posted March 16, 2017 Got an ACR Oakdale as smallest direct air room sealed solution I could find (in cream a SWMBO 'must have'). It was not till we unpacked it after 6 months of ordering we learned the direct air feed vent was at the rear instead of underneath so we fabricated a sliding adapter that feeds the stove from the void below the beam and block floor. This was installed before the functional concrete hearth and screed, getting on for a 700mm long duct! Link to comment Share on other sites More sharing options...
Crofter Posted March 16, 2017 Share Posted March 16, 2017 1 hour ago, readiescards said: Got an ACR Oakdale as smallest direct air room sealed solution I could find (in cream a SWMBO 'must have'). It was not till we unpacked it after 6 months of ordering we learned the direct air feed vent was at the rear instead of underneath so we fabricated a sliding adapter that feeds the stove from the void below the beam and block floor. This was installed before the functional concrete hearth and screed, getting on for a 700mm long duct! Burley Springdale is smaller, at 3kw. Great little stove but you need to chop very very small logs for it! Link to comment Share on other sites More sharing options...
Mikey_1980 Posted March 17, 2017 Share Posted March 17, 2017 Here a pic of our stove Link to comment Share on other sites More sharing options...
TerryE Posted March 18, 2017 Share Posted March 18, 2017 On 16/03/2017 at 19:20, Jimbouk said: Yes, I appreciate they are not environmentally sound and we run the risk of over heating, but we want one.... That might be true for the range of stoves up to and including the one in @Mikey_1980's pic above (and fair disclosure we have a similar one albeit 15 year old technology in our current farmhouse). The problem is that physics is against you if you want to see a nice looking ember heart. For wood to break down all its nasty combustion products, then you need to get the exhaust gas stream up to around 1,000°C. At this point pretty much everything ends up as CO2 and H2O in gaseous form. You can do this by using closed technologies such as a rocket stove. In contrast look at the exposed radiant area in this picture and do the maths -- for a heart of say 0.1× 0.1 m2 radiating at 1,000°C. The radiant heat output is tremendous -- far hotter than an average BReg 2103 house can cope with, let alone a passive house. So you have to use a lot of wood and then let it burn at a far lower temperature -- so as a consequence to you get all of those nasty combustion products up the chimney. I've been thinking about this, and if I really had to heat the house with wood, then I'd use a rocket stove technology, with a short burn (say 1-2 hrs) with the exhaust gasses going through a cob or concrete thermal store. If you can get the exhaust gases cooling down well below 100°C, then you've got a condensing boiler and the efficiency of burning even damp wood goes up to nearer 100%. OK, you will need to fan assist, but that extra 60W of electricity will get the burn efficiency up from ~30% to ~90% so why not? And then thread one of the UFH loops through the same thermal store so that you can control the transfer of the stored heat into the the main UFH. Mind you, this wouldn't help to cool the house during the high summer ;-? Link to comment Share on other sites More sharing options...
Trw144 Posted March 18, 2017 Share Posted March 18, 2017 (edited) 5 hours ago, TerryE said: That might be true for the range of stoves up to and including the one in @Mikey_1980's pic above (and fair disclosure we have a similar one albeit 15 year old technology in our current farmhouse). That one in @mikey_1980 s picture actually has a lambda sensor and flue temperature sensor built in. One of the biggest issues with woodburning stove efficiencies/emissions is the user - not knowing when it should be refilled and what air should be added and when. Mikeys stove takes control of all of this and will bleep at him when it is the correct time for reloading. This was all introduced as a result of environmental concerns and a lot of pressure/bad press on woodburning stoves in Denmark. Edited March 18, 2017 by Trw144 Link to comment Share on other sites More sharing options...
TerryE Posted March 18, 2017 Share Posted March 18, 2017 @Trw144, maybe I shouldn't post after drinking with my sons! However it uses sensors, the stove still has roughly a third of its firebox as a radiant panel. As I said, do the radiant heat calculations. It will be very hard to modulate the head output of a stove like this down without significantly dropping the heart temperature, and getting a lot of crud and particulates into the exhaust stream. You still lose at the latent heat of evaporation, so the efficiency will be terribly unless the wood has been air dried for 2 - 3 years. I agreed that this stove mightbe best in its class, but it is the use of this class that I am questioning: too dirty, too inefficient and too much heat output. Link to comment Share on other sites More sharing options...
Jimbouk Posted March 20, 2017 Author Share Posted March 20, 2017 Thanks for all the advice. Much appreciated! Link to comment Share on other sites More sharing options...
Frogeye Posted March 21, 2017 Share Posted March 21, 2017 I am amidst a bit of a problem trying to get my stove signed off because of its direct air feed. I've got the Jotul F373 and this has an air feed through the pedestal from the void under my beam and block floor. I'm pretty sure that I've seen a picture in the building reg book depicting an air intake from a subfloor void - however my installers are telling me that this doesn't apply to direct air feed stoves and that I will have to have a continuous pipe under the sub floor to the outside wall. I'm contesting this but being told that this is coming from the HETAS regs. I don't know if anyone else has run into this issue? I'm hoping to find a more understanding installer who would be happy to sign it off. 1 Link to comment Share on other sites More sharing options...
Roundtuit Posted March 21, 2017 Share Posted March 21, 2017 I think I'm about to have exactly the same issue. The installer says the air feed has to be from an external vent, but I checked with Building Control today and they're happy with taking air from the ventilated void under the block and beam floor as per my original plan. I'll get it writing from BC, and then have another chat with the installer and see where we get to... Link to comment Share on other sites More sharing options...
TerryE Posted March 21, 2017 Share Posted March 21, 2017 1 hour ago, Roundtuit said: they're happy with taking air from the ventilated void under the block and beam floor Can you guarantee that the ventilation into the void will match the airflow through the stove? Link to comment Share on other sites More sharing options...
PeterW Posted March 21, 2017 Share Posted March 21, 2017 2 hours ago, Frogeye said: I'm contesting this but being told that this is coming from the HETAS regs. What size is the stove...? Link to comment Share on other sites More sharing options...
PeterW Posted March 21, 2017 Share Posted March 21, 2017 And BRegs say that under floor void is fine, if it's not a truly room sealed stove just point your installer at your MVHR intake and ask them to measure that - usually far more than the mm2 required ..! Link to comment Share on other sites More sharing options...
Frogeye Posted March 22, 2017 Share Posted March 22, 2017 (edited) 14 hours ago, PeterW said: What size is the stove...? Its max output is 7kW. I think I'd need about half an air-brick to meet this requirement. As it stands my stove is being supplied by a 110mm soil pipe which pokes in to an underfloor void fed by about 38 air bricks. Edited March 22, 2017 by Frogeye Keeping on topic 1 Link to comment Share on other sites More sharing options...
ProDave Posted March 22, 2017 Share Posted March 22, 2017 This is an interesting one. So BR day they are happy for combustion air to be drawn from under a ventilated floor but HETAS say not. How will that change if you are installing a little 3KW stove. BR for that say you don't need to make any provision for combustion air. Would HETAS agree with that? If so would HETAS also agree that even if you don't need to make provision for combustion air, they would not object to you drawing it from under the floor? Link to comment Share on other sites More sharing options...
Trw144 Posted March 22, 2017 Share Posted March 22, 2017 Building control are able to certify the install so you don't have to rely on the self - certification of the installer. Link to comment Share on other sites More sharing options...
Frogeye Posted March 22, 2017 Share Posted March 22, 2017 ...... if only my building control inspector (independent) was so obliging! Link to comment Share on other sites More sharing options...
readiescards Posted March 23, 2017 Share Posted March 23, 2017 (edited) My independent BI is ok with venting my log-burner from the underfloor void - likewise fed with 30+ air bricks. And as the 'direct air feed kit' supplied with the stove is pretty poor me and the HETAS installer have built this metal channel that sits in another metal channel installed before the insulation, constructional hearth and screed went in. The channel then being 'roped sealed'. (The csa of the rectangular part being greater than the circular connection to ensure no reduction in air flow). Untested admittedly at the moment though! Edited March 23, 2017 by readiescards added rope seal Link to comment Share on other sites More sharing options...
Roundtuit Posted March 23, 2017 Share Posted March 23, 2017 Hmm. Following my request for BC to confirm in writing that taking an air feed from the void is OK, I've just had a call from someone different now recommending that the air feed is ducted in from an external wall after all. I can run a duct in the insulation layer on top of the block and beam OK, but this will bring the vent out in a lower position than the air bricks (telescopic vents) for the floor void. Any suggestions please? Link to comment Share on other sites More sharing options...
joe90 Posted March 23, 2017 Share Posted March 23, 2017 I was recommended by my woodstove supplier to bring the vent pipe up 600 mm outside. Not sure why? ( forgot to ask!) Link to comment Share on other sites More sharing options...
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