february 18, 2010 12:09pm

PH supply air heating misunderstood?

Are we in danger of misunderstanding and misinterpreting the implementation of supply air heating in Passivhaus projects in the UK?
(Or is it me misinterpreting the writings of others?)

I think we may be confusing the heating potential of air supplied by an MVHR with the kind of supply air heating found in office buildings and the like via conventional HVAC systems. The supply air rates are very different.

The supply air volume in a PH is relatively small: 0.3-0.5 ach. There is virtually no infiltration via vents, doors, windows or building fabric.

For short term, rapid response heating:
Pump through enough hot air to raise the general indoor temperature by just 1°C within 1 hour and the relative humidity will drop to an unacceptable level. Most MVHRs would struggle to supply the volume required anyway.

At Y Foel we have nearly a full winter's practical experience with this. After many experiments and measurements I believe I have a reasonable handle on it.

Also, the 55°C scorched dust, smelly air problem may be a red herring. I am aware of several supply air heating methods available for installation in the ducting of PH ventilation systems:
Hot wire elements.
PTC (Positive Thermal Coefficient) elements.
Fluid based heat exchangers.

All 3 introduce a degree of air flow resistance.
To compensate and reduce the risk of unwanted pressure differentials the heating elements are either:
Significantly overdimensioned
Permit varying degrees of bypass circulation around rather than through the element.
or both.

I can only test what we have here of course, but I suspect that space restrictions and air supply rates will in practice determine results not too dissimilar to the following:

MVHR air exchange rate: 85m­³/h
PTC mounted immediately post MVHR consumes 500W
160mm ducting at MVHR:
Temperature at centre of duct 50mm above the PTC element: 64°C
Temperature at outer 20mm of duct 50mm above PTC element: 22°C
Average temperature of supply air as it exits 5 vents: 28.7°C

We have an F7 filter (as do most PH MVHR systems?) pre MVHR, as well as the built-in dust filters. How must dust is likely to hit the heating element? In our case virtually none. If it did the PTC element with a surface temperature much greater than 60°C would certainly cause scorching and odours.

How many of the above PH air supply heating units are actually capable of delivering 50°C supply air to rooms?

Posted By: MarkTiramani

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