Getting on with the first fix for the MVHR and I am getting to the point where I need to decide where the inlet and exhaust vents go in the roof. All I have from the supplier is that the extract should be higher than the inlet but no guidance on distance between them. The thread 'is wood wood' (or similar) has shown that how air travels around the outside of a building is not straightforward and what I don't need is feedback.

So are there any design rules of thumb?

I also need to site a gas boiler flue nearby. Question again is how near? Have the regulators caught up with this issue for MVHR? My suspicion is that they have not!

Putting them on different sides of the building can help to reduce fan power consumption if the inlet faces the prevailing wind & the outlet faces in the opposite direction. However, there is a risk that strong winds in the direction opposite to prevailing will overcome the fans' leading to underventilation.

So the general advice is to put them in the same part of the roof/wall where they will be subjected to the same wind pressure & separate them by >1.2 meters. The outlet will be slightly warmer than the inlet & the increased water vapour content will make it even more buoyant, so I would put the outlet higher than the inlet.

I don't think regulations have caught up with co-siting of MVHR inlets & boiler flues. The required separation of boiler flues from ventilation/window openings is something like 300-600mm. Given the additional risk with a whole house ventilation inlet, I would go for >1.2m separation & put the boiler flue highest of all as it should be the warmest/wettest.

David

I would say as far as possible away from the boiler flue (why do need one of them?) but the last time I talked to the gas council they said boiler flues were never a problem in relation to air intakes as the fumes mix with air too fast to be a danger!

References -

Well the height differential is just simple physiscs - if you can get a stack effect going on the fan power reduces - so that takes care of why the exhaust is ideally higher than the inlet, plus of course your initial post covers the type of air that's being discharged - ie more humid and warmer than inlet.

Seperation between inlet and outlet - you could take a look at BREEAM criteria, CIBSE TM's on Indoor Air quality and ASHRAE guidance

Seperation from flues - Part J and approved Documant J - Diagram 35 ? - which I think shows flues in realtion to Velux windows - the 2m dim is from there - but going from memory. An inlet to MVHR seems to equate to that openable velux window to my mind.

Regards

Barney

Posted By: barneyWell the height differential is just simple physiscs - if you can get a stack effect going on the fan power reduces - so that takes care of why the exhaust is ideally higher than the inlet, plus of course your initial post covers the type of air that's being discharged - ie more humid and warmer than inlet.

Isn't this potentially going to lead to a risk of higher airflow rates than designed? If so, then the increased air exchange rate will have an impact on the energy loss from the house. For very low energy houses, a large stack effect could cause a significant difference.

My supplier used to put exhaust above supply, presumably for the reasons you state. They have since changed their approach to go for side by side with a couple of metre separation.

Posted By: beelbeebubSnow

My inlet and outlet are about 18" above the tile line with a "mushroom" rain cap. During the last snowfall my inlet was just above the surface on the up slope side, if we'd had a couple more inches I'd have had to clear around it (luckily my inlet is next to a velux so no problem).

The exhaust kept clear by melting the snow around it.

This is a valid point and thanks for raising it. My inlet and exhaust will be through a wall abutting a (nearly) flat roof. I'll need to make sure that they are high enough to allow for snow depths. Getting up there to clear a vent in the snow will not be feasible (to avoid a "Nigel Pargetter" incident).

Thanks folks. When we are talking distances are these horizontal separation or direct measures?

I was going to put the inlet/outlet as far apart as possible but I now wondering if that is necessary. I should have also added that there are Velux windows in the vicinity but I am guessing that the regulators have no interest in these.

I also have a Soil Vent pipe to worry about but my intention is to take this down once we have completion and replace it with a durgo! It is totally superfluous.

As to the boiler flue this is for the Andrews Fastflo DHW heater to boost the solar water.

Snow, now that is interesting. The cowls I have are by Helios and are like a ridge tile with wire mesh. No option to raise them up. Will have to lean out the Velux and clear the snow. Does anyone do an electric heater to fit around it....

Found this document on the internet sometime ago. Dutch regulation of sighting of air inlet and out lets depending on position. Sorry document quality is not great as there is a limit on file size.

Posted By: davidfreeboroughYou will need one soil vent pipe to deal with positive sewer pressure caused by discharges from higher properties. However, this soil vent pipe doesn't need to be connected to your internal soil stacks, it can be totally outside your thermal envelope.

Yes, this is exactly what we are doing.