Use Ventilation Exhaust Air for Space Heating and Hot Water

The Passive House standard has two magic numbers related to the heating and cooling requirements for buildings — 15kWh is the maximum allowed heating and cooling demand per square meter per year and 10W maximum allowed instantaneous heating or cooling load per square meter. And the buildings need to match just one of these parameters.

What is 10W per square meter?

It signifies the incredibly low heating or cooling power that is needed to keep a passive house comfortable. It signifies how little heat is lost through the building envelope and that is happens really slowly. For example, a 100m2 house requires just 1000W or 1kW during the coldest winter nights or the hottest summer days to keep the indoor temperature steady.

It is easy to see how boiling water for tea, cooking food and running other household appliances could generate enough heat that is eventually absorbed by the building to keep the additional heating need even lower. That’s why it’s called “passive” house because it requires so little addition energy to what is already generated by the people just living in it.

Ventilation with Heating and Cooling

Ventilation is the only active process in a passive house as it needs to happen in a way that doesn’t transport any heat out of the house. The magic numbers for this is 30m3 of fresh air per person per hour at a 90% heat recovery efficiency.

What if we used this air to transport heat around the house — add heat during the winter and remove it during the summer? Would it be possible to transport those 10Wh of heat per square meter every hour?

Energy in Air

The heat capacity of air is around 1000 joules or 0.278Wh per kg per degree and its density is around 1.2kg per m3 which means that 1m3 of air is able to transport 0.278Wh/kg × 1.2kg = 0.33Wh of energy per each degree in temperature change. Compare that to the same volumetric amount of water which has 4 times the heat capacity, 830 times the density and is able to carry 1151Wh energy per degree difference.

A four person household needs 4 × 30m3= 120m3 of fresh air per hour which could transport 120 × 0.33Wh = 40Wh of energy per hour per degree increase. So in order to add or remove 1000Wh of heat by transporting 120m3 of air every hour we need a 1000Wh ÷ 40Wh = 25C degree hotter or colder air compared to the room temperature. Doubling the amount of air to 240m3 would require only a 12.5C degree difference.

This proves that in theory it is possible the heat or cool the whole house just by using the air that is already used for ventilation. Not only that — we can use a heat pump to extract even more heat from the air leaving the home.

Heating Air and Water

The ideal way to do this is by placing a heat pump on the exhaust side of the ventilation after the cross-flow heat exchanger to extract even more energy from the outgoing air — like a traditional air-source heat pump but with the evaporator placed inside the exhaust line of the ventilation unit and the condenser in the supply line (in the heating mode). It would also recover all the energy left over by the cross-flow heat exchanger!

There are several products (filter by “Compact heat pump unit”) on the market that do this:

Some of them have additional routes for the incoming air so that it can be used only for heating water and doesn’t need to be moved through the house when the ventilation needs are low.

They all look pretty much like this:

Nilan Compact P
Nilan Compact P with ventilation and heat recovery at the top and hot water tank at the bottom.

and they all work like this:

Nilan Compact P working diagram
The outgoing air at 22C first heats up the incoming air through a cross-flow heat exchanger and then passes by an even colder heat pump evaporator which on the condenser side heats up the air going to the rooms to 38C.

As you can imaging, the same system can also be used in hot climates to both cool the incoming ventilation air and heat up the water just by reversing a few valves.

Some of these units have support for ground-source heat pump additions which can further increase and optimize the available heating and cooling loads at amazing efficiencies.

Interestingly, the Nilan Compact P datasheet has the follow graph:

Nilan Compact P heating power

which confirms that a heat pump is able to extract 2200W of heat from 220m3 of exhaust air every hour. The ventilation loss shown just below the heat output is confusing because my understanding is that the heat pump is placed after the cross-flow heat exchanger so it shouldn’t impact the heat recovery efficiency.

These units cost somewhere in the range of $12,000 to $25,000 and they reduce the utility room down to this one device which still needs to be hidden behind a well insulated walls due to noise, but the savings on piping and installation work should easily justify the cost.

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