Okay, just been tinkering with this idea in my head for a while.

- Bungalow renovation with insulation where I can (floors, EWI, ceiling) and 3G windows.
- I'm not going to end up with an airtight house though and may opt for log burner.

Plan to live in the house for a year and experience a winter (2017 hopefully) to see how the MHVR and UFH perform, but I may also add an air tight log burner (with own air supply) into the mix (wood is onsite and I have a regular supply). But rather than losing a lot of that heat in the flue, surely there must be a way to use up some of that heat b4 it escapes into the atmosphere by way of connecting it to the heat exchanger of the MHVR? Not talking about mixing /exposing the hot air with the incoming air duct, but by running the flue parallel to incoming cold duct of the MHVR. I know the flue will go up vertically obviously but the incoming air duct could possibly also have a portion of it vertically before it goes into the MHVR unit.

MHVR duct would have to be metal obviously and probably no point having an insulated flue pipe at the 'heat exchange' point. Would have have temp guage fitted to ensure the temps stay within a 'safe range' to ensure maximum heat intake is not exceeded as the air hits the MHVR unit as well as to ensure things don't get too hot in the loft (smoke/heat detector units, fire extinguisher!!).

Most MHVR units appear a bit plasticy where the heat exchange takes place, so would need to ensure that the incoming heat is not too high, or get hold of an MHVR unit that has a metal matrix.

Perhaps there's something on the market already that does this? Or maybe I just need to add this to my retirement list of things to try out (okay, in the shed, not the house!). I recall someone on here doing their own DIY MHVR unit?

And no doubt HETAS would not approve of any of this!

No, I haven't had a drop yet! But just wanted someone to blast this idea out of my head as I should be having my mind trained on other pressing matters like windows and floor insulation!

If, on the other hand, you want to warm the supply air after it's gone through the MVHR heat exchanger then that might be a very good idea.

I'd be very very nervous about pumping carbon monoxide into the bedrooms if anything leaked, though.

Thannks for your thoughts Ed. Not sure if I understand all that right now, not touched a drop yet either!) but will mull it over.

Posted By: Ed DaviesIf, on the other hand, you want to warm the supply air after it's gone through the MVHR heat exchanger then that might be a very good idea.

I'd be very very nervous about pumping carbon monoxide into the bedrooms if anything leaked, though.

That would only happen if the flue leaks and if the ducting has a leak (unlikely?). CM detector in loft and also one at each outlet in the rooms?

Posted By: joe90Or, warm the exhaust air before it goes through the heat exchanger, ( which would not risk fumes getting into the house)

That would be safer, but probably still worth having CM detectors at each extract vent in case of leaks and air flow ceases to be extracted from rooms.

So may be worth adding onto the retirement list after all.

Posted By: kentishgreenThannks for your thoughts Ed. Not sure if I understand all that right now, not touched a drop yet either!) but will mull it over.

Just remember that, if certain simplifications are assumed, the reduction in temperature from the extract air down to the exhaust air is the same as the increase in temperature of the inlet air to the supply air.

(Simplifying assumptions include: the air flows are balanced, condensation can be ignored, heat capacities of the airs at different temperatures are the same.)

E.g., outside/inlet air at 0 °C, room/extract air at 20 °C. If the temperature difference across the heat exchanger is 2 °C then the exhaust air will be at 2 °C and the supply air at 18 °C.

If, however, you pre-warm the inlet air to 10 °C then the MVHR heat exchanger will be transferring less energy so its temperature difference will be less. Suppose the temperature difference is now only 1 °C. Then the exhaust air will be at 11 °C and the supply air at 19 °C (instead of 18 °C). So for your 10 °C increase in the inlet air temperature you've increased the supply air by only 1 °C and the exhaust air by 9 °C throwing 90% of the heat from the flue air out. Better than 100%, I suppose, but still …

Don't cool the flue too much or it won't draw.

Agree with others about need to keep flue temp up, to avoid resinous and acidic gases condensing on inside of flue, and to keep flue drawing well. Also keep flue straight so easy to sweep.

Having said that, if this is a thought experiment, how about using heat from the flue to pre-heat the external air supply before it goes into the stove? This is how twin wall balanced flue works for gas boiler s.

I see why you might think this is worth it, but we have what you're planning - air tight, MVHR, UFH, WBS with external air feed and our main problem is that the WBS make the rooms they're in too hot to sit in! They're a great focal point, but we haven't been able to run them at full-bore for any appreciable period of time yet.

I think I would look at a better way of moving the hot air from the room with the WBS. As written extensively on here, the MVHR is unlikely to redistribute this heat effectively enough via any nearby exhaust air points to make it worth doing and I'd be inclined to think the same would apply with any chimney / flue interface. It might sound a good idea, but is it worth it for the complexity it will bring?

If we were to do our project again, I would consider either trying to integrate a WBS water boiler into our DHW tank (the UFH is run directly off the heat pump) or, as an Oz-dwelling friend suggested, using a big ceiling fan! Their particular part of Oz has a climate that needs both stoves and fans and they find the fan in the winter distributes the stove heat through the house very well...