Well, I agonized over the double glazing decision, and I am wondering if I should have spent my time thinking about the heat recovery system, which I just went with almost without thinking….
We’ve struggled with poor heat distribution and ventilation in our existing home, with a freezing downstairs and boiling upstairs, so I thought this would be great. It extracts air from some of the rooms (chosen to be the warmer ones, with heat sources: kitchen, bathroom, lounge (with the new wood-burning stove) and Ilsa’s glass workshop with its kiln and blowtorches…) and pumps it out of the house, while bringing in fresh air and transferring the warmth from the stale to the fresh with a heat exchange coil like in a fridge.
Footprint were keen that I should do it, because the house will be moving towards airtightness, as the chimneys are sealed and the double glazing installed. But it wont be truly airtight – sash windows cant be as good as those well-sealed Scandinavian jobs in this respect. So something must be done about ventilation. But since making the decision, I’ve seen results from Charles Ainger analysing his retrofit home. He found that the electricity used to power the heat recovery system actually made the carbon emissions slightly worse for him - it was saving fuel, but in his case it was wood pellets with low emissions. In our case it is gas, which is not as low carbon as wood pellets, but is a lower carbon source (currently) than grid electricity. Of course, I could say that our electricity is 100% renewable from good energy, but that is kind of cheating….
I asked Footprint and Vaillant (the manufacturers) for some info around the expected power consumption of the fan, but what they gave me was pretty incomprehensible and didnt answer my question. I know footprint have chased it up, but I still dont really know.
So that leaves me with a big question mark on this one. Should we have instead given up on near-airtightness and installed some kind of passive ventilation system like that used by George Marshall in his low-carbon ‘Yellow House’? Or will the energy used to power the fans be more than offset by the savings in gas? Certainly, the original refit west survey suggested this would be the case; I guess we will never know for sure.
On the positive side, though, I expect it will allow the wood stove in the lounge to spread its heat around the house more effectively. I also suspect that having the heat more evenly distributed will allow us to comfortably keep the thermostat lower in the winter.
Hi Paul,
We are planning to put in a mechanical ventilation with heat recovery (MVHR) system, our house will be a lot more air-tight than yours (more modern house; no sash windows; solid ground floors). It will be some while before it’s done, there’s some building work to do first and we’re waiting for quotes on that at the moment. I’ll measure the electricity that it uses but it will be hard to determine how much gas it saves – we’ll have a new boiler, under-floor heating and a lot of other changes to the house at the same time so no point in comparing previous gas bills.
Where can I read about Charles Ainger’s experiences?
Gavin (“Bishopston: three bed semi”)
This is from Charles, mentioned above: My paper was/is still draft, and not generally available, but here’s my latest calcs/comments for our house, to help answer you and Gavin.
Insulation + MVHR:
saves 15% (4000kWh) of heating energy (& cost). This saves 780kg CO2/yr with the old gas boiler (OGB), but only 90kg/yr once the wood pellet boiler is in (WPB).
But the MVHR fan runs 24hrs/365days; uses extra 885kWh mains electricity/yr = + 475kgCO2/yr.
So net kgCO2/yr effect = -305 (v. the old gas boiler) – only 40%, net, of the saved gas Co2; but + 385 (v. Wood pellet boiler) – an increase.
But – the MVHR has been worth it for comfort – evens up room temperatures, totally prevents condensation and mildew;
It may help us close down our Wood Pellet Boiler for longer in spring and autumn, so reducing more CO2 than I’ve calculated – and we are now switching it off when summer windows are open and other ventilation available, to reduce kWh and CO2
We are glad we did it, overall.
For Chris’s case, retaining gas, it’s likely to be just CO2 beneficial or neutral; but if you make sure that your sitting room with the wood burning stove is extracted from by the MVHR [sitting rooms are normally blown into?] it could as you say make a much more effective use of the wood burner stove heat throughout the house, too.
You also need to watch like a hawk the actual fan wattage settings they set up when installing the MVHR – they tend to go for robustness rather than lowest possible watts.
If your friend has access to the SAP software, it should be possible to use it to calculate the extra fans kW and CO2, and the gas energy and CO2 saved by the ?80%? – heat recoveryof the MVHR?
I think it would be reasonable to expect every quote for MVHR systems to provide this calculation, for your home..
Thanks Charles. The sitting room will indeed be an extraction room which will hopefully improve things overall.
Thanks Charles.
We will have a new efficient gas boiler and a large but well insulated house, so we may use less gas than your old boiler. We’ll run our MHVR during the months when we use our central heating, probably 5 months per year, the rest of the time we’ll have windows open to give us the ventilation we need.
I’m off to EcoBuild on Wednesday where I’ll be quizzing manufacturers on the efficiency and power consumption of their MHVR products before I make a decision. Recommendations or further tips wecome!