I've seen all sorts of domestic MVHR units, from single room units and extractor fan replacements (in the region of £350) to whole house multi-room setups (£ lots).

Has no-one looked into any of this yet?

Some personal thoughts. They are quite expensive i think. 

If your home is very air tight and you want to improve air circulation without loosing too much heat then perhaps this is something to consider but i would have thought that is unlikely for the majority.

If you have an internal bathroom then again perhaps one of the stand alone units would be worth considering rather than a regular extractor to get rid of moisture but limit the heat loss.

I suspect the payback time is very long for the majority of houses. It would need to be a replacement for a fan you used an awful lot or to improve air circulation in a very air tight home. 

Hi @majordennisbloodnok

There are now available MVHR units that also incorporate an ASHP, but I have not delved into their capabilities too deeply.

Thanks, both. I might do some sums to confirm, but it’s not looking that hopeful.

OK, sums done.

I simply looked at a small-scale scenario; a replacement for an extractor fan for a bathroom.

Ventaxia Lo Carbon Tempra

It retails for £355.93 ex VAT or £418.22 inc. I worked on a scenario of three people in a household having an average of 4 showers each per week in a bathroom approximately 10m3. The Ventaxia unit is up to 78% efficient and I'm assuming an average air temperature difference inside to out of 9 degC. It doesn't much matter about air tightness because the air replacing that expelled would still have to be heated back up to temperature in a short period of time. I'm also assuming all showers are done during peak electricity rate and that the rate is what my tariff is currently (around 22p/kWh). That sidesteps the two big variables; the amount the tariff will increase in about 6 months' time and the fact I've a solar pv array/battery setup which dissociates time of use with time of consumption from grid (and produces significant free electricity, of course). 

Putting in all those assumptions and crunching the numbers suggests the unit would pay for itself in about 228 years.

Not exactly the world-changing revelation I might have hoped for....

@majordennisbloodnok its £245 here - so I've shaved about 100 years off your payback. That's me done for the next century 

EXTRACTORFANWORLD.CO.UK "https://www.extractorfanworld.co.uk/vent-axia-443310-lo-carbon-tempra-t-heat-recovery-unit--timer-1139-p.asp"

Vent Axia 443310 Lo Carbon Tempra T Heat Recovery Unit & Timer | Extractor Fan World

Shop for the Vent Axia 443310 Lo Carbon Tempra T Heat Recovery Unit & Timer. Order online from Extractor Fan World.

Posted by: @batalto

@majordennisbloodnok its £245 here - so I've shaved about 100 years off your payback. That's me done for the next century 

I'll have two, then, 'cos that'll double the saving.... 🤔 

Perhaps not.

Posted by: @majordennisbloodnok

Putting in all those assumptions and crunching the numbers suggests the unit would pay for itself in about 228 years.

Not exactly the world-changing revelation I might have hoped for....

Could you show the crunching, please, because that does seem surprisingly bad? I'm currently looking at heat-recovery ventilation for a space with an underpowered extractor fan.

I have 3 Kair heat recovery ventilators in 2 bathrooms and a toilet in the downstairs of our house. They work really well as they bring in a constant trickle of warm fresh air and keep the humidity at or below a set rate (50% in our house). I wasn't worried about the payback on them as comfort/performance was more important to me. 

The basic extractors we had before would leak cold air into the room when not in use. Even if you fitted a valve/filter to try to make it only allow air to flow one way it would rattle in the wind. With the new extractors I know the rooms will stay warm and there won't be any damp issues and I don't have to rely upon annoying trickle vents (which I hate).

For our bathrooms upstairs which vent vertically where these recovery units are harder to install I fitted normal extractors with an auto humidity sensor so that they properly clear the moisture after a shower as I've found that even with a 5 minute timer on an extractor your humidity levels in the bathroom will still be over 75% after a hot shower.

A good test for your existing extractors is place your hand over them when off and also when running in the winter and see how cold/drafty they are. 

Posted by: @mjr

Posted by: @majordennisbloodnok

Putting in all those assumptions and crunching the numbers suggests the unit would pay for itself in about 228 years.

Not exactly the world-changing revelation I might have hoped for....

Could you show the crunching, please, because that does seem surprisingly bad? I'm currently looking at heat-recovery ventilation for a space with an underpowered extractor fan.

OK, let's have a go.

Air has a specific heat capacity of 1.005, and 1m3 air weighs 1.205kg at 20degC. In fact, we keep our house at 19degC, which is close enough. Our bathroom has a volume of 11.6m3, meaning it would take 14.059 kj to raise the bathroom temperature by 1degC (actually 1deg Kelvin, but we won't split hairs).

Although the air being sucked in to replace that extracted comes from the rest of the house, that air in turn has to be replaced by air from outside the house. Averaging out temperature fluctuations, I'm assuming an outside air temperature over the year of 9degC, meaning we'd need to heat the bathroom's volume of air by 10degC if we were simply to extract one full room's worth (i.e. with a non-heat conserving extractor fan). That would need, therefore, 140.587kj or 0.0391 kWh. This is the amount of energy a normal extractor fan will lose us to replace the bathroom's air just once. If we assume for the three of us currently living here that we take 10 showers a week, and that we just need to completely change the air once to remove the problem moisture, that equates to an annual cost of the lost energy of £4.26 (our current tariff is 21p/kWh peak rate).

The MHVR I showed above is about 75% efficient, meaning it'll still lose a quarter of that energy anyway, meaning reclaiming £3.20 (I'm quoting from a spreadsheet I created for this, so there's a lot of rounded numbers I'm quoting here, but the spreadsheet isn't calculating on roundings of roundings).

Given the MHVR above was found at a price of £245 and a standard extractor fan is about £50, that means the heat recovery solution comes at a £195 premium. At £3.20 per year saving, that means we'd break even in near as makes no difference 70 years. I'll admit I forgot about the £50 I'd have had to spend anyway, hence the previous 100 year estimate.

Apart from this, it's pretty obvious I'm assuming I'll turn off the extractor as soon as all the air in the bathroom has been recycled. In practice, I'd expect it to run for longer, wasting more energy and creating bigger real-world savings. Nonetheless, 70 years is not good payback based on premium rate prices especially given most of the energy used to heat the replacement air is, in my house, being generated by our own solar array and, therefore, free.

Does that help?

How does a MHRV unit affect a heat loss calculation? I'm pretty sure most are only 50 - 80% efficient so some heat must be lost right? Even in completely airtight home?