Does MVHR affect heat loss & calculations much?

[ruggers]

26 minutes ago, joe90 said:

Yes I had to “treat” my west wall with waterproofer as it was sodden last winter 

Storm dry seems an excellent product for this.

[joe90]

2 minutes ago, ruggers said:

I think I'm with Joe on this one to a degree (No pun intended). The chilled wind could act as a wicking effect at drawing the heat away from the dwelling, especially if wet mortar between face bricks, I may be wrong because the inner and outer skins are independent, but we do know that poorly fitted PIR in a cavity can have the same effect as a radiator in reverse. Air passes between the gaps from poor install and draws heat away from the inner cavity wall. Effectively no wind should be able to get in other than tiny weep hole vents but its possibly just from the difference of temperatures.

👍

[joe90]

5 minutes ago, ruggers said:

Storm dry seems an excellent product for this.

Mine was very similar, dead easy to apply with a brush, did approx 90 sq m in half a day, can’t see it when finished, rain runs off it like glass 👍

[ruggers]

2 hours ago, SteamyTea said:

If the MVHR unit is 80% efficient (optimistic in reality)

I've spoke to the MVHR company today & they've confirmed with some cheaper units it's not always possible to know exactly whats being recovered, but with the likes of the Zehnder units with sensor monitoring, they have plenty of data to show some properties they've installed at having recovered heat at 1°C less than the indoor temperature at 20c.

47 minutes ago, Marvin said:

 

True. 

 

Using the data over the year 2020 we had an average temperature of 12.1 centergrade here.

Is this data on a website for set areas anywhere or is it your own data? Interested for my own, I'm sure it's 9°C local.

47 minutes ago, Marvin said:

The average temperature in the home was 21C (19 for about 8 hours over night and 22 the rest of the day.

We would require about 1157 Watts to heat up the same amount of air taken directly from outside, as apposed to, the 231 Watts of heating required and 32 Watts to run the MVHR. So a saving of about 926 Watts an hour. 

 

926 Watts by 24 hours by 365 days is 8111 kWh a year! 

In my humble opinion, you have to have some really really poor MVHR design for the running of the MVHR to cost more than the saving on heating. 

Interesting figures, good to see. Theres no doubt MVHR is of benefit, it's just making sure my calculations will reflect this. I'll check out the sheet Johnmo refers to as a comparison.

37 minutes ago, SteamyTea said:

There are evaporation losses from wet walls, these can be quite high, especially at night.

The latent heat of vaporisation is 2257 kJ.kg^-1. 

In layman's, is this something we should consider as a factor or is it minimal and not so noticeable?

[SteamyTea]

3 minutes ago, ruggers said:

recovered heat at 1°C less than the indoor temperature at 20c.

If it is 19°C outside, why would heat need to be recovered?

5 minutes ago, ruggers said:

1 hour ago, SteamyTea said:

 

In layman's, is this something we should consider as a factor or is it minimal and not so noticeable

Not sure, never modelled it.

[joth]

55 minutes ago, ruggers said:

@joth This is my concern, that the software only detects a change of defaults resulting in more air exchanges per hour so it estimates that i require more heat than needed because it doesn't factor in the heat recovery percentage. I will look into this in more detail because a tutorial video i recall a mention of 50% default for MVHR but an option to increase the %.  Is PHPP the passive house detailed calculation, do you have a passive house? I've included some attachments, the MVHR report is just one of the 3 companies who provided me with some details & a quote. 

 

 

Yes, we did PHPP and obtain PHI retrofit (Enerphit) certification. The houses 50Pa rated airtightness is 0.6 ACH.

Our MVHR supplies about 0.35 ACH. (Based on the MVHR app, that states 150m3/h flow rate right now, on a building of 440m3 volume)

 

Agree with @JohnMo's suggestion to use the heat loss calc from Boffin's corner if you can, but for our project we needed PHPP anyway (to obtain PHI certification) and the MCS Calculator anyway (to get the MCS cert on the ASHP install, to get RHI) so I found it better to spend my time on making those 2 tools work for me than add yet another into the mix.

 

 

 

Edited August 26, 2022 by joth

[ruggers]

1 hour ago, SteamyTea said:

If it is 19°C outside, why would heat need to be recovered?

It was 20 inside and the recovered heat was 19. Can't remember what they said the outdoor temperature was at the time. I do think 90% efficiency isn't uncommon though.

1 hour ago, joth said:

 

Yes, we did PHPP and obtain PHI retrofit (Enerphit) certification. The houses 50Pa rated airtightness is 0.6 ACH.

Our MVHR supplies about 0.3 ACH. (Based on the MVHR commissioning report, that states 196m3/h flow rate, on a building of 440m3 volume)

Do these figures mean your MVHR at 0.3 ACH is running at twice the capacity expected or half as good? I've not looked researched anything to do with ACH & how it's all calculated at this point only design, functions and performance.

[joth]

57 minutes ago, ruggers said:

Do these figures mean your MVHR at 0.3 ACH is running at twice the capacity expected or half as good? I've not looked researched anything to do with ACH & how it's all calculated at this point only design, functions and performance.

 

So to clarify there's no direct relationship between the ACH used to measure airtightness, and the ACH I quoted the MVHR is operating at.

The airtightness is measured at a contrived situation (50Pa pressure difference between indoors and outdoors). On any given day there may be zero difference, e.g. in very still conditions with no breeze; or there may be a hurricane blowing through. So the amount passive ventilation varies hour by hour but the active ventilation is  more or less constant. All we can say is, the house as a whole is receiving 0.3ACH minimum rate of ventilation, but much more if it's windy or the MVHR is on boost.

 

 

In terms of expectations: the design flow rate was 140m2/h and the measure actual flow rate is 150m3/h once everything was balanced out, so it's pretty close to expectations.

But what I think what you'll find a lot more interesting is the MVHR app's claim it has saved 6.75 MWh of heat/cooling energy since install Nov 2020.

 

 

 

[JohnMo]

4 hours ago, SteamyTea said:

There are evaporation losses from wet walls, these can be quite high, especially at night.

The latent heat of vaporisation is 2257 kJ.kg^-1. 

So where does this appear in any building loss heat calculations?  You are normally after the worst case heat loss, so if it's as significant as you seem to suggest, it should there?

[JohnMo]

Your MVHR is just a ventilation tool, which also recovers heat from the outgoing air and gives it the incoming air.  All houses need to be ventilated, an airtight house has to be ventilated by mechanical means.  Building regulations require a balance supply and extract.  Building standards normally require around 0.5 ACH (varies with UK country)

 

Your ventilation rate is 0.3 ACH, it nothing to do with with your air tightness test.  In addition to your MVHR flow rate, you will have a natural infiltration through the building fabric, which is not controlled, this will be small and is linked to your air test results and the prevailing wind speed and direction.

 

The rate of flow of the MVHR is based on a number of ACH, in your case 0.3 ACH.  Your SAP report will have a number it which represents natural infiltration, this should really be added to MVHR rate to get the worst case heat loss.

 

So your ventilation rate would be 0.3+ something, of which heat recovery would be 80-90% efficient.  So your effective ACH would be 0.3x0.2 = 0.06 ACH, with respect to heat loss.

 

Qv = 0.33 × n × V × ΔT watts.

=0.33x0.3x your volume X 24

=0.33x0.06x your volume X 24

 

The difference between the two figures is the benefit of MVHR.  Less the running cost for electricity and filters.  Which is generally lower the energy saved for heating.

 

[Adsibob]

10 hours ago, SteamyTea said:

… even if it reclaims an optimistic 80% of the energy in the expelled air.

 

 

Where did you get this 80% figure from. Most modern MVHRs achieve 90% plus.

E.g. The Brink Flair is marketed as “a highly efficient heat recovery unit with a thermal efficiency of more than 95%”. I think most Zehnder machines are similar.

[JohnMo]

Here you go, most in the low 80s, some in the 70s some in 90s.

 Those all passivhaus certified, other may not be as good, even if they are.

 

https://database.passivehouse.com/en/components/list/ventilation_small

[SteamyTea]

1 hour ago, Adsibob said:

Where did you get this 80% figure from. Most modern MVHRs achieve 90% plus

The temperature difference effects the efficiency. The greater the difference, the more energy transferred, but it may still only be 3 or 4 K above external calculation.

3 hours ago, JohnMo said:

So where does this appear in any building loss heat calculations?  You are normally after the worst case heat loss, so if it's as significant as you seem to suggest, it should there?

As I said, I have not worked it out, and it will change with wall type.

[Marvin]

4 hours ago, Adsibob said:

Where did you get this 80% figure from. Most modern MVHRs achieve 90% plus.

E.g. The Brink Flair is marketed as “a highly efficient heat recovery unit with a thermal efficiency of more than 95%”. I think most Zehnder machines are similar.

I think your right about the efficiency of the machine, but I'm not sure about the whole system efficency due to the location of the equipment and ducting.

I have adjusted my expectation after testing the actual air inlet temperature and comparing it to the calculated air temperature.

 

Individual results will be greatly dependant on if the MVHR and any ductwork is inside or outside of the thermal envelope.

 

[Adsibob]

2 minutes ago, Marvin said:

Individual results will be greatly dependant on if the MVHR and any ductwork is inside or outside of the thermal envelope.

 

True, but my understanding is that all manufacturers recommend the ducting and the machine are installed within the thermal envelope, so if you follow the manufacturer’s recommendation, you should be achieving the maximum efficiency.

[Marvin]

1 minute ago, Adsibob said:

True, but my understanding is that all manufacturers recommend the ducting and the machine are installed within the thermal envelope, so if you follow the manufacturer’s recommendation, you should be achieving the maximum efficiency.

Tricky to do on a bungalow like ours.

[ruggers]

17 hours ago, joth said:

But what I think what you'll find a lot more interesting is the MVHR app's claim it has saved 6.75 MWh of heat/cooling energy since install Nov 2020.

 

 

Quite impressive. What unit is that for? What are you heating with, gas, heat pump or other? If gas it seems like it would on average be saving you £20 per month at 7p per kw/h?

 

 

[ruggers]

16 hours ago, JohnMo said:

Qv = 0.33 × n × V × ΔT watts.

=0.33x0.3x your volume X 24

=0.33x0.06x your volume X 24

 

The difference between the two figures is the benefit of MVHR.  Less the running cost for electricity and filters.  Which is generally lower the energy saved for heating.

 

Thanks John, I think what has confused me was standard air changes for a none MVHR house vs a house with MVHR. Although airtight, I wasn't sure if having MVHR increases or decreases ACH. I was thinking it increases the ACH because theres a fan controlling it which will always be more than natural ventilation? As mentioned before, wondering if the heat loss software, although we may change the ACH for a house with MVHR, can the software detect how much heat isn't being recovered at this higher rate or is it just detecting a higher heat loss due to more ACH.

I've just found this (See attached) in an online manual for the software. So I've now selected MVHR at the default setting and all of the ACH figures per room have reduced, Is this suggesting I change the figure to 90 if the figure was 90% efficient? It seems a bit dramatic.

 

Without MVHR heat loss is 6.1kw

With default MVHR at 50 the heat loss is 4.5kw

With setting changed to 90 the heat loss would be 3.24. MVHR doesn't save that much energy so I must be doing something wrong.

[JohnMo]

From what was attached it makes to up the efficiency to 90%.

 

Looks like the individual room rates will need to changed to reflect the total ACH of 0.3.