MVHR external air intake – underground?

[Anonymous]

Hi all

My heat pump/heating/MVHR supplier says he doesn't think an underground air supply is a good idea but can't remember why!

Any thoughts?

One possible supplier of room-sealed wood fired range (yes I know it will roast our passivhaus) says an underground air supply to that is pointless too.

[Anonymous]

As long as you have vermin protection I can't see there being too many issues.

There are better solutions that opting for an underground supply to your wood fired range.

[Anonymous]

“There are better solutions that opting for an underground supply to your wood fired range.” – please expand….

[Mark Siddall]

It would appear that Kate has been spending to long with a certain mechanical engineer 😉 …..She is correct in suggesting that the thermodynamics of the heat exchanger means that the warmer air offers little benefit. The main benefit of earth tubes with MVHR is avoiding the need for frost protection with a high primary energy demand, however, in the UK the climate is not so demanding as to warrent the capital expenditure. (The cost benefit analysis that I have seen – for more ideal climates than the UK – suggests that because of earth tubes are not worthwhile.)

Technically cooling can be provided via the earth tube in the summer, but in the UK you may as well just open the windows (there are other posts on this forum that discuss the balance point temperature for when it is ideal to open/close windows.)

You can get some earth tubes coated with silver to reduce the risk of mould growth. You need to ensure that they are laid to falls, with a sump and suitable drainage. Also, there have been instances, apparently, whereby the water table has risen and filled the earth tube to one degree or another.

For the frost protection of the MVHR system a ground loop filled with glycol (and linked to a small pump and water to air heat exchanger) may be more appropriate than an earth tube – shorter lengths, less excavation, reduced cost, less risk of microbial gunk and less risk of flooding- ,however, it may still prove uneconomic (should that be an important consideration). It could also be used to achieve limited cooling in the summer (though opening the windows is still a better option.)

[Anonymous]

One possible supplier of room-sealed wood fired range (yes I know it will roast our passivhaus) says an underground air supply to that is pointless too.

Depending upon your wall & floor construction bringing the range air supply through the floor may allow less thermal bridging. For example, the ground below a solid floor generally stays warmer than the outside air & its often easier to build a round duct into a concrete slab as work proceeds with insulation fitted tightly around it than to do the same in a cavity wall.

However, I'd be concerned that the duct might flood & that you wouldn't notice until the house started filling with smoke (& carbon monoxide). It is difficult to perfectly seal around a stove door & an air supply duct blockage is likely to lead to poor draw & smoke in the room.

David

[Anonymous]

Thanks for all your comments. Work proceeds.

[Author]

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