Passive House – How Do You (Pre) Heat Yours?

To pre-heat or not to pre-heat, that is the question! And when it comes to airtight dwellings, it’s a pretty important question.

Local climate data dictates the requirement for pre-heating in Passive Houses – if the average temperature is unlikely to drop below 0°C/32°F during the year, you may not be obliged to include pre-heating into your ventilation strategy.

Let’s explore the alternatives, from not using a pre-heater at all through to the different types of pre-heaters you can choose from, and examine the likely outcomes from each option when the outside temperature drops below 0°C/32°F
Option one: Don’t use a pre-heater

Although in Passive House this isn’t always a choice, there are other construction standards where you may be wondering whether or not it’s necessary to preheat so I thought I’d start here.

Consider that a heat exchanger has stale, moist extract air constantly passing through it – this means water! Water and sub-zero air do not good bedfellows make for a heat exchanger! So, what you’re left with is a large block of ice creating a logjam for the air trying to pass through in all directions. If using a constant volume system, this puts pressure on the motors as they tried to ramp up to maintain the flow rates. This would be impossible to achieve so the resultant pressure drop and restriction to airflow mean only one thing – no ventilation. In an airtight property, the best possible results from this would be the discomfort to the occupants. The worst case isn’t worth thinking about!

So, in order to stop this from happening, the unit would simply switch off the supply air. This would negatively pressurize the property forcing it to draw in outside air from any possible source through the building fabric. The main issue with this, of course, is that this outside air will be neither filtered nor tempered which kind of goes against the point of having a heat recovery system in the first place. If you’re at the start of a long winter cold snap, you’re not going to feel any comfort benefit until it’s over.

Option two: Restrict the supply air and unbalance the system

Again, I’m not talking about Passive House here….

Some heat recovery ventilators (HRVs) will control the supply air, rather than switch it off completely. It is generally reduced in 1% increments until the heat exchange efficiency is back in sync. There is still an element of heat recovery happening but, due to the unbalanced system causing negative pressurization, top-up air is infiltrating the building fabric. This brings about a gradual temperature drop which, when identified by the sensors on the HRV, will prompt the unit to switch off the supply air to protect the heat exchanger – suddenly you’re back to option one!

Option three: Use a ground source air-to-air heat exchanger

This involves having a pipe under the ground where the temperature is pretty constant all year round – circa 8-12°C/46-54°F. Although a benefit in winter and summer, there will be times through the autumn and spring where this becomes a disadvantage. For example, the outside air is 15°C/59°F which, if taken straight into the HRV, would only need tempering by a few degrees to achieve the comfort temperature. By taking this air through the ground pipes first, the air temperature will lower meaning more work for the HRV. For this reason, this unit can also be used with a diverter valve with a sensor that monitors the outside temperature. Sensing a risky drop triggers the valve to redirect the intake air to the ground loop. For temperatures above zero, the intake air would be drawn in via the usual inlet. So far, so sensible. However, problems can arise if the pressure drop (from switching to ground source) isn’t accounted for at the time of commissioning the system. This would unbalance the system as the unit wouldn’t be set-up to compensate by speeding up the motors. On the other hand, should a constant volume HRV be installed, but again without the allowance for the pressure drop, the ramp-up of the motors could result in noise complaints.

Option four: Use a sub-soil earth-brine loop heat exchanger

Another pipe-under-the-ground scenario but this version uses a pump to move brine around a closed loop which takes on the temperature of the surrounding earth. One major advantage is that the intake air is always drawn in via the normal inlet which causes no fluctuations in pressure. The intake air is filtered and passed through a liquid to air heat exchanger to be pre-tempered before it enters the HRV. A major benefit from using this system is that the pump can be set to activate at specific low or high temperatures – not only can it pre-heat, it can also pre-cool! This difference also means that the unit won’t be active during the more clement months.

Option five: Choose an HRV with an integrated pre-heater

Space saving. Automatic. Simple! An integrated pre-heater uses inbuilt algorithms to know when to activate/deactivate so no occupant intervention is required.

Hopefully, the Shakespearian style question I (tenuously) posed at the start of this piece has been answered sufficiently to demystify the pre-heating debates we face as our discussions become more airtight.

Nicola Webster is Product Marketing Manager for Zehnder Group UK