Choosing MVHR - energy consumption angle

[Olf]

I'm slowly settling the unit I'm going to install, but recently found something cheaper - though (as expected) of worse spec. I struggled to meaningfully compare the numbers, but @Levo gave me inspiration.

 

So let me walk you through comparing MVHR units based on energy consumption

Assumptions:

Home temperature : 21 °C

Average annual outside temperature: 9.4 °C (https://www.statista.com/statistics/610134/mean-air-temperature-uk/)

House area 140 m² > 350 m³ of volume

Air exhange @0.4 ACH > 140 m³/h

Total heat lost (needed to calculate how much MVHR unit saves) with a bit of units juggling:

11.96 KJ/kg (https://www.engineeringtoolbox.com/heating-humid-air-d_693.html)

with air density 1.204 kg/m³ https://en.wikipedia.org/wiki/Density_of_air

9.93 kJ/m³ = 0.028 kWh/m³

So for my model house: 0.38 kWh

 

Now things get easier (as everyone is good at counting money)

Gas heating:  3.06p/kWh (my effective rate, taking both fuel and standing charge)

@0.38 kWh sent away > 1.18 p/h

Over whole year: 103.56 £

Very good unit with η=84% will allow to lose only 16% of that heat, worth £16.57

So-so unit with η=70% will waste £31.07

(efficiency in this case is defined as per Passiv House standard)

 

But there is another factor - electricity used to move the air

At 15.7 p/kWh good unit with specific electric power of 0.29 W*h/m³ will cost over year £55.84

Not so good unit with  0.5 W*h/m³ will cost £127.34

 

Let's say there is £1000 difference in price between them, in 12 years the cheaper unit's total cost of ownership (not including filters though, but I assume identical price) will exceed that of the good one.

 

The main takeaway is that specific power seems to have more importance (money wise) than heat recovery efficiency. That is independent of house size: in bigger house you'll have more air volume to exchange (and lose in the process), but proportionally more electricity to spin the fans.

I was a bit surprised, but this very good unit with 0.29 W*h/m³ @ 140 m³/h still ends up with with 40W per hour, or nearly 1 kWh daily use!

 

On top of that the better unit will (usually) reward with quieter operation, maybe better controls.

But you need to pay more £££ upfront.

[Bonner]

I am also looking at MVHR for my new build starting early next year. I haven’t done the maths in as much detail but my conclusion was that heat recovery savings are marginal. Moving the air is probably going to cost more in reality, once ducting and filtration losses come into play. I think most people enjoy the comfort/health benefits and that is what I will be looking from the system. Main selection criteria for me would be investment cost, running cost, noise/vibration and reliability/serviceability with heat recovery efficiency low down the list.

[scottishjohn]

on the assumption that you have no PV  then it will cost to run

I am guessing those that go for MVHR will also be fitting some solar PV --so cost in daytime to run system will free --sort of 

heat recovery if a well designed modern build  whilst a factor  -the cost to heat should be so low that the savings will be small anyway 

It is ventilation and clean air in cities where the advantages are   in my view 

my wife will still open the windows even if i fit one --and being in the country I have to agree with her  really that maybe its a cost + complication we do not need  maybe ?

same as bifold moving walls --lot of extra cost and complication -when you can just go out side to a covered area like a veranda around the house or a car porch type thing over the patio area .

uk is not Australia where they live outside as much as in 

cities and close proximity of neighbours I can see why some would -

maybe its me  just tight --  not into bling  just for the sake of it --and to me bifold doors are bling -end up with house full of bugs in evening if they are open anyway 

 

 

 

[joe90]

My take is a bit different, I only tend to use the MVHR in winter, windows open in summer, no draughts in winter and pre warmed/ filtered air in winter. No PV (spent the money on a classic car instead ?‍♂️) My bifolds are between house (kitchen diner and conservatory / lounge and conservatory) this means when it is warm in the conservatory, which is quite a lot of days, I open them and it heats the house, when the temp drops I close them. So far this autumn this has heated the house enough to not have the heating on at all. Yes I DIY installed the MVHR and got the unit dirt cheap on Ebay (I am tight as well). I firmly believe the small cost of running the MVHR is worth the lack of draughts/trickle vents and I am not too worried on payback.

[SteamyTea]

13 hours ago, Olf said:

That is independent of house size: in bigger house you'll have more air volume to exchange (and lose in the process), but proportionally more electricity to spin the fans.

It may make the numbers worse as the surface area to volume ratio is 6/a.

Basically large houses have a better kWh/(y.m²) number than smaller ones.

But then I think using the mean air temperature is wrong.   There will be no excess heating for a large proportion of the year.

I think this has to be modelled on a weekly basis, the MET Office probably has better data.

I use a daily mean air temperature of 10°C as the point where I need to turn on or off my heating, using your number of 9.4°C would mean I have the heating on all the time.

[DamonHD]

The notion of heating degree days and degreedays.net could be helpful.

 

Rgds

 

Damon

[AliG]

When looking at the costs and payback, something to remember is that you won't need any extractor fans to bathrooms, kitchens etc. This could run to a decent percentage of the MVHR cost.

 

The cost of these can add up, but also I find them annoyingly noisy and the MVHR much quieter. There is also the fresh air benefit and the lack of dust which is a surprisingly nice benefit.

 

I recently took a reading on my Dantherm MVHR and it seems that it runs to less than £20 per year, per unit. I run them on speed 1 for most of the time and find that entirely adequate, indeed running them on higher speeds in winter increases heating costs as it brings more cold air into the house. I change the filters once a year. The filters cost more than a year's electricity.

 

 

[scottishjohn]

41 minutes ago, AliG said:

When looking at the costs and payback, something to remember is that you won't need any extractor fans to bathrooms, kitchens etc. This could run to a decent percentage of the MVHR cost.

very good point 

[Olf]

11 hours ago, scottishjohn said:

I am guessing those that go for MVHR will also be fitting some solar PV --so cost in daytime to run system will free --sort of

 

It will never be free - but what figure you choose depends on your accountancy (purchase+installation price, assumed payback period etc).

Exactly as electric car is not 'zero emission' ...

 

11 hours ago, SteamyTea said:

Basically large houses have a better kWh/(y.m²) number than smaller ones.

But then I think using the mean air temperature is wrong.   There will be no excess heating for a large proportion of the year.

 

Correct, it is an oversimplification. But that means, that the heat recovery efficiency is of even less of importance from financial standpoint.

 

8 hours ago, AliG said:

When looking at the costs and payback, something to remember is that you won't need any extractor fans to bathrooms, kitchens etc. This could run to a decent percentage of the MVHR cost.

 

Of course! Coming form a house with rubbish air tightness and rubbish MVHR it was still in a different league to what I'm in at the moment - so the reason for these calcs is to have some objective reasoning behind my choice, as I'm allergic to marketing blurb. And hey - heat recovery efficiency is the first thing any MVHR manufacturer quotes, probably as psychologically anything closer to 100% oozes perfection, whereas W/m³ is some esoteric and not cool technicality.

 

I actually had a chat with PIV saleswoman, but when doing simple maths of all the bits to make such solution really work (central unit as advised + fans with partial heat recovery in bathrooms) it didn't make sense in my case.

[Adsibob]

On 07/11/2020 at 09:44, joe90 said:

My take is a bit different, I only tend to use the MVHR in winter, windows open in summer, no draughts in winter and pre warmed/ filtered air in winter. No PV (spent the money on a classic car instead ?‍♂️) My bifolds are between house (kitchen diner and conservatory / lounge and conservatory) this means when it is warm in the conservatory, which is quite a lot of days, I open them and it heats the house, when the temp drops I close them. So far this autumn this has heated the house enough to not have the heating on at all. Yes I DIY installed the MVHR and got the unit dirt cheap on Ebay (I am tight as well). I firmly believe the small cost of running the MVHR is worth the lack of draughts/trickle vents and I am not too worried on payback.

This is probably a stupid question, but given the UK is not really a country of binary weather, and much more a spectrum, how do you actually manage your MVHR in practice. Do you ever have days in spring or autumn that are neither hot nor cold and you end up forgetting to switch on your MVHR but also not opening enough windows? If so, do you get condensation on your windows or does condensation only happen when it is so cold outside that it is obvious that it is winter time?

[JamesP]

@Adsibob

Never turned it off.

Our MVHR runs 24/7, barely noticeable in the background unless on boost. You can set the supply, extract and boost speeds. Minimal condensation in the bathroom after having baths or long showers but clears quickly.

Probably down to airtightness, quality of windows (3G) and overall build quality.

Edited February 4, 2021 by JamesP

[Adsibob]

9 minutes ago, JamesP said:

@Adsibob

Never turned it off.

Our MVHR runs 24/7, barely noticeable in the background unless on boost. You can set the supply, extract and boost speeds. Minimal condensation in the bathroom after having baths or long showers but clears quickly.

Probably down to airtightness, quality of windows (3G) and overall build quality.

Thanks @JamesP, i wasn’t planning on ever turning it off either, hence my question to @joe90. Wanted to see how he manages it.

[joe90]

13 hours ago, Adsibob said:

This is probably a stupid question, but given the UK is not really a country of binary weather, and much more a spectrum, how do you actually manage your MVHR in practice. Do you ever have days in spring or autumn that are neither hot nor cold and you end up forgetting to switch on your MVHR but also not opening enough windows? If so, do you get condensation on your windows or does condensation only happen when it is so cold outside that it is obvious that it is winter time?

Not a stupid question at all, when I notice the weather is warm enough that windows are open along with doors (spring/summer) I turn it off fir the season, likewise in Autumn when it’s starting to chill it goes on for the winter.

Edited February 5, 2021 by joe90

[Olf]

As above, you might find it that in the summer open window ventilation works better - especially on hot high humidity days MVHR will boost for nothing.

At the end of the day what you what is to have fresh air with reduced heat loss, whatever the method. In the summer heat loss is no issue (unless running aircon, loss in the other direction) so if there are no other considerations (noise, smells, safety etc) open windows is perfectly fine

 

11 hours ago, Adsibob said:

do you get condensation on your windows or does condensation only happen when it is so cold outside that it is obvious that it is winter time?

 

Condensation forms when the temperature of a surface (any, but windows have the greatest chance due to relatively poor insulation properties) falls below dew point. That's it.

So with some fantastically insulating windows you may end up with no condensation, even if the air inside is 'stale'. 

 

 

[AliG]

2 hours ago, Olf said:

Condensation forms when the temperature of a surface (any, but windows have the greatest chance due to relatively poor insulation properties) falls below dew point. That's it.

So with some fantastically insulating windows you may end up with no condensation, even if the air inside is 'stale'. 

 

With 3G windows the condensation is usually on the outside which is a little surprising to see at first. The inside of 3G windows should be barely any colder than your walls which makes a considerable difference to interior comfort. Indeed it is almost impossible to tell what the outside temperature is whereas with poorly insulated windows you feel it as you walk past them.

[jack]

2 hours ago, Olf said:

As above, you might find it that in the summer open window ventilation works better - especially on hot high humidity days MVHR will boost for nothing.

At the end of the day what you what is to have fresh air with reduced heat loss, whatever the method. In the summer heat loss is no issue (unless running aircon, loss in the other direction) so if there are no other considerations (noise, smells, safety etc) open windows is perfectly fine

 

I don't think you need aircon to get the benefits of running MVHR in summer. On a hot day when it's cooler inside than out, opening windows is just going to let the heat in. You're better off keeping the windows shut and the MVHR on (ideally on as low as setting as possible while keeping the air fresh) to maintain the temperature difference as far as possible.

 

Once the temperature outside drops below the temperature inside, you can either open the windows (hopefully you have insect screens!) or turn on summer bypass if you have it.