Heat recovery ventilation - is it a retrofit option?
Posted by Tim Pullen on 20 February 2012 at 9:21 am
Q: Is heat recovery something that can be retrofitted?
A: There are two things we need to know about heat recovery ventilation:
1. It does not work in a draughty house. In fact it needs a high level of air tightness to work at all.
2. It is big, bulky kit. Ducts tend to be 100mm to 150mm diameter and will not look pretty in any room.
Air tightness is critical. To give an order of magnitude, 2010 building regulations (for new build) require a maximum permeability of 10m3/hr/m2 @ 50 Pascals. A complex system of measurement the bones of which don’t really matter – it is the 10m3/hr bit that we need to hang on to. To put things in context, a unused chimney could equate to 50m3/hr.
For heat recovery ventilation to work at all we need an air tightness of less than 5m3/hr – twice as good as current building regulations. The probability is that houses built before 2006 will not meet that standard.
The reason is that mechanical ventilation systems (with or without heat recovery) draw air from warm rooms (kitchen, bathrooms) and inject air to cooler rooms (lounge, bedrooms). If the house is not reasonably air tight the effect of this is to draw in outside air. For heat recovery to work the amount of outside air drawn into warm rooms needs to be minimised or the cold air being drawn in has a bigger impact than the warm air being recovered.
As to ducting, it is big and bulky. Whether it is ugly or whether boxing-in is a viable option is a matter of personal choice but it is generally possible to physically install a system. There also needs to be space in the loft for the air handling unit and access through the roof for the air inlet and outlet, but again that is not generally a problem.
To make this work in a retrofit situation will mean first carrying out an air pressure test, find out where all the leaks are and seal them up. Then do the pressure test again to make sure the property is better than 5m3/hr. Then install the system, do all the boxing-in work and redecorate.
In a new-build 3 to 4 bedroom house a heat recovery ventilation system is likely to cost £3,000 to £4,000 fully installed. In a retrofit it is likely to be over £6,000, when all the extra work is considered. And the result may be a reduction in the heating bill of less than 5%.
So, yes it can be done, but that does not mean it should be done?
Picture: Evan Leeson
YouGen guide to ventilation and draughts
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