HRV system - inlet position - thoughts please.

[marshman]

I am about to install a Heat recovery and ventilation unit. I had one at my last house and it was brilliant - controlled ventilation - minimal heatloss - no more condensation and fresh air in every room.

I have sourced a fan unit, sorted where to run the ducting, where to position the fan unit etc. My problem is this - where to position the fresh air intake? I have a woodburning stove - the only source of heating (underfloor) and hot water for the house. The flue is via a central chimney exiting in the centre of the roof. (I have taken account of the air feed to the stove). The chimney has an excellent "draw" and most of the time it is at least "breezy" outside so the smoke gets blown well away from the house. The problem is when the wind doesn't blow. There appears to be nowhere around the periphery of the roof that is guaranteed to be free from smoke from the chimney under windless conditions. When it is windy then at least one section of the roof is suceptable to smoke (downwind side).

I have thought of having several inlets and mechanically switching between them so that the best quality air is drawn in but this is complex and would mean checking and switching it over.

Also one central inlet actually in the loft space. This has two advantages - 1: drawing in warmer air from the loft in the winter increasing efficiency and 2: increasing loft ventilation.  The big disadvantage is in the summer the air will be very hot.

My final thought is to switch to extract only whenever the fire is lit.

Any thoughts anyone?  Is it possible to get filters for the inlet to remove any smoke (don't really fancy that as you don't always know what nasties are in what's being burnt on the fire).

Roger

[jwbellarby@gmail.com]

we have a similar setup, both inlet and outlet are on the side of the house at the gable ends, seems to work fine no problem with smoke etc. all ventilation system ive seen have filters so should stop any occasional smoke, they need filters to protect the heat exchanger if nothing else

James

[tz0c0s]

Hi Roger,
  Any chance of piping it underground from a distance away. You would gain some free heat & cooling from the ground?

Regards Andy

[djh]

Where is the outlet? Don't forget that you can get a reverse pressure difference if the outlet is far from the inlet. I'm not expert enough to know how important that is.

I believe it's quite normal to have intake and outlet pipes sticking vertically up out of the ground, perhaps a metre high. This might also be interesting:

http://en.wikipedia.org/wiki/Ground-coupled_heat_exchanger

[Angus]

Generally HRV units operate on a maximum 150Pa external resistance so there will be a maximum length of ducting you can install before the fan just wont push any futher. I would say that although it looks like the smoke is travelling everywhere, as long as you dont face the air inlet in the general direction of the stoves' terminal then the concentration of the smoke in the air in the vicinity of the air inlet will be small. Smoke particles are too fine to be erradicated by filtration - the higher spec the filter the higher the pressure drop on the fan and therefore the shorter the maximum duct run length possible.

A different tack would be to go back to the supplier of the stove and see if they have any guidance on the minimum proximity of the flue terminal to an air inlet - then just make sure you exceed it!

Otherwise there is always the building regulations for the proximity of flue locatons to air inlets. BS6461 gives guidance on the wind pressures around a house and the relative suction effects. This may be worth considering against your prevailing wind direction. There is also the possibility of modifying the flue terminal on the solid fuel fire to ensure taht the flue gases are dispersed away from the house.

Angus

[marshman]

Generally HRV units operate on a maximum 150Pa external resistance so there will be a maximum length of ducting you can install before the fan just wont push any futher. I would say that although it looks like the smoke is travelling everywhere, as long as you dont face the air inlet in the general direction of the stoves' terminal then the concentration of the smoke in the air in the vicinity of the air inlet will be small. Smoke particles are too fine to be erradicated by filtration - the higher spec the filter the higher the pressure drop on the fan and therefore the shorter the maximum duct run length possible.

A different tack would be to go back to the supplier of the stove and see if they have any guidance on the minimum proximity of the flue terminal to an air inlet - then just make sure you exceed it!

Otherwise there is always the building regulations for the proximity of flue locatons to air inlets. BS6461 gives guidance on the wind pressures around a house and the relative suction effects. This may be worth considering against your prevailing wind direction. There is also the possibility of modifying the flue terminal on the solid fuel fire to ensure taht the flue gases are dispersed away from the house.

Angus

Thanks for the replies and ideas.

The question was prompted by the fact that with the wind in certain directions my nose can smell the smoke when I am working in my garage which is around 30m away. With this in mind I didn't want the smell of the smoke to be pumped into the house by the HRV.

I live in an old farmhouse with a central chimney which is the flue for the wood burner so that is fixed. The construction and layout of the rooms limits my options in terms of inlet and outlet locations - the unit will have to loft mounted. Any thoughts of ground level inlets and outlets are a non-starter. The supplier of woodburner disappeared long ago - its been in for 26 years - and to be honest I'm not interested in what the regs say, I just don't want the smell of smoke in the house.

I will have to go ahead and install the unit and try the inlet in various locations.

Roger