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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

More information:

YouGen guide to ventilation and draughts

Can I install a MVHR system in a Victorian house?

Do I need MVHR?

Find an installer

About the author: Tim Pullen is eco-editor for Homebuilding & Renovating magazine, author of Simply Sustainable Homes and founder of sustainable property consultancy WeatherWorks.

If you have a question about anything in the above blog, please ask it in the comments section below.

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6 comments - read them below or add one

Kim R

Kim RComment left on: 18 November 2014 at 10:37 am

From my experience;

1: installation in a bungalow is likely to be simpler as all rooms can be connected via the loft so the ducting is easier.

2: even modern plastic windows can be remarkably drafty.

3: most of the rest of the drafts can be tracked down and blocked diy. Be thorough I found a lot of draft throu the back of some electrical sockets and other cable entry points.

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Eco Andrew

Eco AndrewComment left on: 5 November 2014 at 10:47 am

Has anyone installed this FreshR ductless HRV system?  It uses a fine copper wire heat exchanger.  From its claims it seems to get over the disadvantages of retrofit described by Tim here (ductless, easy/cheap to install, cost to buy £1600).  It is installed on an exterior wall with exhaust and intake ducts going through the wall. To work for the whole house there would need to be gaps under/over doors.  For a large house you would probably need two (bathroom and kitchen?).

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sunnyComment left on: 18 June 2012 at 11:46 am

This is a great site. We've just had heat recovery ducts installed. They are large, however the heat that is retained is excellent and warms other parts of our serviced offices.

We're going to be looking into solar now as we have a 45degree pitched roof and have some great sunlight between 10 and 4 most day, even in winter.

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HiltingburyComment left on: 20 February 2012 at 6:50 pm

Firstly may I say how pleased I am to see a blog entry on HRV. I do think, though, that Tim is too pessimistic.

We have a 1959 house which we have recently extended with three extra rooms. We did not want to put various vents (e.g. trickle vents) in the new part so that has been well sealed. As a result we opted for an HRV system and decided to do both the new and old (less well sealed) parts of the house. The company we purchased it from had an excellent 3D drawing package and even though they used 150mm rigid steel pipes you would be hard put to find them. They go down through fitted wardrobes and along between the joists. One even goes under the bath to get to the downstairs cloak room. So retrofitting was not a real problem provided it is given enough thought.

We have tried to draught proof the older part of the house as well as we can, particularly the suspended floors. We can't do much about the working fireplace though, so in no way are we fully airtight. However it all seems to work very well. Subjectively the house feels airier (you can certainly tell when the HRV is switched off) and the house feels of a more even temperature. The system works well transferring excess heat from the kitchen and shower rooms into the other rooms of the house. This evenness of temperature is not something the sellers of these systems major on but it is certainly something we have noticed (and we have been living here for 30 years)

As to efficiency it is difficult to tell. Again subjectively, with it -4C outside we were seeing only a 3C drop between the extract and supply air temperatures. e.g house temp 20, inlet fresh air 17.2.

Of course Tim might argue we are spending too much on the heating because of the lack if airtightness. I wish I could be specific but we had a new boiler, solar hot water, a thermal store and wet underfloor heating in the new parts at the same time. All I can say is that our gas consumption for heating, averaged over a whole year is just 45% of what it was before, and we have a rather bigger house to heat, and last winter was very cold too!

I also agree with Kate de's comments too.

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Kate_deComment left on: 20 February 2012 at 12:22 pm

While the reduction in heating bills may not be spectacular (will leave that calculation to the experts) it is worth noting that MVHR allows you to increase ventilation rates (to what they should be) without losing heat or suffering draughts.

There is a lot of underventilation in UK homes in winter, and this can have worrying health implications - even in well-insulated homes - because of high humidity levels. The ventilation unit can also filter out pollen & air pollution, and allows fresh air without outdoors noise. So the decision is not just about energy, also comfort and health.

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nnw49Comment left on: 20 February 2012 at 10:59 am


 What about the single room heat recovery replacements for extractors - like the Vent-Axia HR-25 type units?(I'm sure there are others like it...)

 Can a bathroom/wet room be well enough sealed to make using one worthwhile?

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