My first ever post in the New Build category! What fun, I am helping a local Italian, with a distinctly un-Italien view on house building, design his new build - 11 months to breaking ground. Modest 3-bed, single storey, north facing with high wooded slope to south, big overhangs to prevent overheating - yup no solar gain to think of, solar panels for DHW, super-insulated wood frame (no doubt more on that sometime), airtight (but only to the level motivated first-timers utterly ignorant of all things airtight can achieve without supervision), not sure about the slab or windows doors yet but I have explained what a MVHR system is and he is convinced. So shaping up well but it will not be PH for sure.

Thing is, he has to heat with wood, he basically makes big things (like roofs) out of wood for a living and sells his bone dry offcuts pre-palletised for wood burners so it is pretty much inconceivable that he would use any other fuel source (not that there is gas available) and whilst he will pay extra to get a massive 6kW electricity supply it doesn't go far....

So decision made, underfloor heating, wood burner with hot water to a TS. Weeeeelll. He will have a low heating demand. First question - what does that look like? - I have no idea, my guess is under 5kW but can someone please speculate a more accurate range. All the kit required for his wet system really adds up (he has pencilled in 15k (euros). But I have been wondering: what about fans in the walls. The internal walls are not yet final but decided is one main living room (kitchen, diner, lounge with large picture window across amazing view) and shortest possible corridor. It has already been established on here that MVHR just doesn't move enough energy around to transfer any useful heat but what about fans in the internal walls!?

So given a starting point of a simple stand alone wood burner in the lounge, minimal use of electricity and no gas how do you make this house work? Fans at ceiling height between the lounge and the 3 bedrooms maybe - there will be a size that would work obviously. If my fluid dynamics knowledge is right these fans wouldn't affect the MVHR if the internal doors are open but correct me if I am wrong. Or maybe a really over-sized MVHR. Wife has decided tiled bathroom floors so perhaps infra-red and/or electric underfloor in these 2 rooms.

It is a weird one but if anyone can come up with a solution it will be you guys!

Posted By: ringiAs to moving the heat.... Without heat storage a wood burner needs a lot of love and care to keep it going at the rate the heat is needed.

Awwww bugger - I hadn't thought about that! low mass house too - Spoil sport!!

As for dust - masses! Bags and bags of it - not sure but I think he might sell that to someone for something too. Anyway - can't see the wood burner having beautiful dancing flames on sawdust!

We use our wood burning stove to heat our new build here in France - huge amount of harvested wood available. It's a RIKA passive stove, 2 - 6 kw, and heats our posh lounge very quickly! Unfortunately the heat goes out through the double doors and then up the open plan stairs - so make sure he avoids them!

Our MVHR needs to be balanced - can't find an expert at the moment - but I do feel it slowly balances the heat around the house. Perhaps consider vents above the lounge door to allow the heat to move out and spread around - open doors on the corridor will heat the other important rooms.

Also as he sells his of-cuts of wood, his heading wood is not free. Therefore what is the cost/benefit of insulating to a level that does not need a heating system?

I wonder if it's possible to combine “oversized MVHR” internally which circulates a lot of air within the building to move heat around with a normal-rate MVHR to exchange air with the outside. Probably more economical and easier to control than having two separate systems. Supply and extract might want to be in different places, though…

Maybe all the recirculation happens on the supply side: between the living rooms and bedrooms. Extract from the kitchen area of the main room and from the bathrooms goes straight out the MVHR.

Posted By: ringiTherefore what is the cost/benefit of insulating to a level that does not need a heating system?

Is that actually feasible without a reasonable amount of solar gain?

Thanks Cerisy - I'll point him at that make of stove (he already understands that he can't have an Italian WBS because they are crap). No chance of the heat going upstairs because, as I mentioned, this is a single storey building.

DJH - that is very helpful - the house is 170m2 so PH is 2.5kW heat demand ish. That's good to hear about airtightness, but he works full time, he will not have a main contractor or a project manager and whilst they will mostly be 'mates' working for him, and hence motivated, there will just not be supervision and it is hard to explain just how far back in time the knowledge base is hereabouts. Anyway the Rika 2-6 kW sounds on the money! Really good tip about the cellulose too - he was edging towards a natural product inboard and I gently suggested he wouldn't need the humidity buffering with MVHR and he knows it is more expensive....I am sure I can undo my poor advice at this stage.

All - selling the wood off cuts was about explaining he has very little (zero ish) work to get bone dry wood oak mostly - him making an income from this stuff is a red herring - hardly worth his effort he tells me, delivery, making the uniform size cages, customer hassle etc - and anyway he won't need much for his house will he!

Another point is that mostly people heat single storeys/with forced air around here BUT is is from an oversized wood burner in one room which always gets too hot - vents come out in the top of that room and for the modern ones a couple come out in the bedrooms - ceilings within 2m have to be repainted every 6 months (don't ask just accept) - keeping a fire ticking over is what everybody does as std around here.

With no solar gain and a youngster, a soon to be at least one dog and no 'air lock' and unsupervised trades one has to be realistic - what he is aiming for is like you guys trying for an interseasonal store. I'll work on 5kW. And the slab will yet become an issue.

There is no loft (all vaulted ceilings) so already MVHR will be tricky. Fans and noise is something I know a lot about. I am thinking: a slightly oversized MVHR but balanced, plus thermostat controlled (incl temp differential) through wall mounted Centra thingies (that's 10, 15 or for boost 25 dB @ 3m).

And cellulose insulation - any advantage in oversizing the slab a bit?

Ed - so sort of a bypass valve on the MVHR's 2 pipes to outside combined with perhaps boost (but using the Aircycle this can be set anywhere), thermostatically controlled from stats in the bedrooms (plus a temp differential test), theory being that one doesn't need 24/7 MVHR for high AQ - maybe with intermittent normal mode, maybe timed (ie after being on bypass for 2 hours switch to bypass 45 mins normal 15mins) or maybe/and/or a CO2/humidity over-ride. No issue whatsoever with the controls as REUK will make me exactly what I want for astonishingly little money.

Posted By: GotanewlifeEd - so sort of a bypass valve on the MVHR's 2 pipes to outside combined with perhaps boost

Yep, you could look at it that way. Or perhaps look at it as a recirculating heat transfer system between the living room and bedrooms into which the supply side of the MVHR feeds.

Posted By: GotanewlifeThere is no loft (all vaulted ceilings) so already MVHR will be tricky.

The roof will be quite thick though to contain the insulation so it may be possible to bury MVHR ducts in the insulation. Or place them directly under the apex and then add a bit of whatever boarding is used to form the vaulted ceiling horizontally underneath the duct. Or just leave the ducts exposed - Hockerton used clay drainpipes as ducts, suspended on bits of wire IIRC, if he fancies a different aesthetic to post-modern industrial.

On further thought, I think my suggestion is a bad idea. Still, what I was thinking of was to have ducts with fans between the living room and the bedrooms to redistribute heat from the wood burner. Return would be via open doors or gaps under doors, etc. Supply side from MVHR would feed into that duct. Problem would be finding a point within the circulation ducts with a sensible neutral pressure to match the extract side of the MVHR.

I'd agree with DJH that ducts under a vaulted ceiling needn't be a problem. Depending on the layout they could run above kitchen cupboards or otherwise be made to blend in most of the way. I'm planning vaulted ceilings for my living room and study with ducts and other services running across under the north-side ceiling boxed in with wood. Haven't decided how high up yet - originally though at the apex (as DJH suggests as a possibility) but now thinking lower down.

Posted By: GotanewlifeStill interested in over sizing slab to help with ups and downs of WBS output without TS - silly idea, worth it?

Directly (through UFH pipes) or indirectly, WBS heats the air which heats the slab?

I think downwards heat transfer from air is pretty slow, something like 6 or 7 W/m²·K. Given that you wouldn't want a big temperature difference between the air and the slab for comfort would that be enough to store useful amounts of heat?

Most WB will give out most of the heat into the room it is in, and needs wood cut to size. Some wood boilers can take wood as long a 1m, and put 90% of the heat into the water in the TS with only 10% going into the boiler room.
I therefore question using a WB to heat a TS.

Use the wood burner for a quick heat up when needed and to look nice….

A masonry stove is used in the USA to store heat and is often in the wall between two rooms, I don't know if a few masonry stoves would work.

Posted By: GotanewlifeI wouldn't know what slow or fast is - so that 6 or 7W per 1 deg difference per m2

Upwards the comparative figure would be about 10 W/m²·K. However, remember that that's only the temperature difference between the air and the surface of the floor. There's need for a further temperature gradient to drive heat into the floor so when heat is not flowing or flowing the other way the floor surface would be cooler still.

Posted By: Gotanewlifeno solar survey done yet

I think that is probably quite important. Mainly to see what solar power will be available from the roof but also to get some data about insolation through windows in various directions.