COP of MVHR

[Iceverge]

As we all know the efficiency of ASHPs is measured in COP or coefficient of performance. A good ASHP might have a COP of 4.

 

 

For a little light pre bedtime reading last week I was perusing the manual of my MVHR. unit . I happened upon a table. 

 

 

Further reading led me to this. 

 

 

 

Note that at zero degrees, nostril freezing cold, the MVHR unit recovers 1257W of heat whilst only expending 30W of electrical energy. 

 

This corresponds to a COP of about 42. 

 

Yup FORTY TWO.  

 

 

 

  

[MikeSharp01]

Hmmmmmm .. Interesting spot and one wonders if these two factoids are conflatable in the way you have. One, the latter, refers, if I have it right, to the power consumption of the fan while the other has no input power of the rest of the unit while recovering the 1257 W and I guess it will be approximately 300-420W. If my estimations are correct then the fan around 10% of the power usage of the unit at this recovery and you are right 420W / 10 = 42 so that bit is right .

 

[Iceverge]

The energy trade off improves the colder it gets too. 

 

Unlike ASHPs or combustion devices. 

 

From a energy efficiency hierarchy it's £ much better spent in my view. 

 

 

[JohnMo]

Comparing with a humidity controlled MEV and humidity controlled inlet devices, so it only just ventilates enough, how does it compare?

 

Trouble with MVHR is it runs 24/7/365 at a rate of 0.3 to 0.5 ach. Ventilation is whole living space, need it or not. If you didn't need the ventilation on, although recovering lost heat, it recovering need less lost heat.

[Iceverge]

5 hours ago, JohnMo said:

Comparing with a humidity controlled MEV and humidity controlled inlet devices, so it only just ventilates enough, how does it compare?

 

Trouble with MVHR is it runs 24/7/365 at a rate of 0.3 to 0.5 ach. Ventilation is whole living space, need it or not. If you didn't need the ventilation on, although recovering lost heat, it recovering need less lost heat.

 

I think I did the sums when building our house. It's what definitively swung it to MVHR for me. 

 

Take a typical house, 4 occupants, 0.3ACH as per passivhaus and a volume 400m3. 

 

Average temperature annually were I live is about 10deg so a dT of 10deg indoor (20deg) to outdoor.

 

 

First the MVHR house. 

 

0.3ACH *400m3 = 0.033m3/second (33l/sec)

X

density of air @1.2kg/m3 = 0.04kg/sec

X

specific heat capacity of air ~1Kj/kg/K X dT of 10  = 400W of heat lost by ventilation losses. 

 

but we have MVHR  so 

 

400W * 85% heat recovery = only 60W of heat energy needs to be replaced plus the fans at say 40W so  ventilation takes = 100W. 

 

 

 

Next the dMEV house. 

 

Each house occupant produces an average of 2.5l of vapour that needs to be removed so 10l/day in the house or 0.11g/sec. 

 

Air at 20 deg can carry about 18g/m3 of moisture while at 9deg it carries only 9g/m3.  Taking a 1m3 block of air at 9 deg and heating it to 20deg means we could absorb 9g of moisture. 

 

That means that only about 13l/sec of air need to be heated and moved through the house to keep the humidity under control. With no heat recovery that's 157w plus the (smaller) fans of 13w 

 

= 170 W for the dMEV. 

 

 

 

From the above we can see that dMEV even at 1/3 the airflow of the MVHR uses 1.7 times the energy. 

 

If you used the MVHR a 24 hr period would use 100W * 24hr = 2.4kWh

 

The dMEV would use the same energy in 14hrs so may well be better in a house that was only occupied 60% of the time. 

 

 

Most of us however don't run MVHR at the occupied demand rate 24/7. We run it slower and use the internal air volume to buffer air quality during times of occupation say 0.1 or 0.2 ACH. In reality MVHR always wins in this regard, as does dMVHR.   

 

 

Of course this disregards 

1. Filter cost
2. Install cost
3. Noise
4. Building disruption
5. Particle filtration
6. Maintenance costs 
7. Comfort
8. Cost savings by having a reduced heating system

 

 

dMEV has a place for:

 

1. Budget constrained buildings
2. Retrofit
3. Very intermittently occupied places (churches, village halls)
4. High variations in fresh air demand. ( sports team dressing rooms etc)
5. Unreliable maintenance practices ( filters!!!) 
6. Install skills and design knowledge for MVHR is absent. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[JohnMo]

11 minutes ago, Iceverge said:

 

I think I did the sums when building our house. It's what definitively swung it to MVHR for me. 

 

Take a typical house, 4 occupants, 0.3ACH as per passivhaus and a volume 400m3. 

 

Average temperature annually were I live is about 10deg so a dT of 10deg indoor (20deg) to outdoor.

 

 

First the MVHR house. 

 

0.3ACH *400m3 = 0.033m3/second (33l/sec)

X

density of air @1.2kg/m3 = 0.04kg/sec

X

specific heat capacity of air ~1Kj/kg/K X dT of 10  = 400W of heat lost by ventilation losses. 

 

but we have MVHR  so 

 

400W * 85% heat recovery = only 60W of heat energy needs to be replaced plus the fans at say 40W so  ventilation takes = 100W. 

 

 

 

Next the dMEV house. 

 

Each house occupant produces an average of 2.5l of vapour that needs to be removed so 10l/day in the house or 0.11g/sec. 

 

Air at 20 deg can carry about 18g/m3 of moisture while at 9deg it carries only 9g/m3.  Taking a 1m3 block of air at 9 deg and heating it to 20deg means we could absorb 9g of moisture. 

 

That means that only about 13l/sec of air need to be heated and moved through the house to keep the humidity under control. With no heat recovery that's 157w plus the (smaller) fans of 13w 

 

= 170 W for the dMEV. 

 

 

 

From the above we can see that dMEV even at 1/3 the airflow of the MVHR uses 1.7 times the energy. 

 

If you used the MVHR a 24 hr period would use 100W * 24hr = 2.4kWh

 

The dMEV would use the same energy in 14hrs so may well be better in a house that was only occupied 60% of the time. 

 

 

Most of us however don't run MVHR at the occupied demand rate 24/7. We run it slower and use the internal air volume to buffer air quality during times of occupation say 0.1 or 0.2 ACH. In reality MVHR always wins in this regard, as does dMVHR.   

 

 

Of course this disregards 

1. Filter cost
2. Install cost
3. Noise
4. Building disruption
5. Particle filtration
6. Maintenance costs 
7. Comfort
8. Cost savings by having a reduced heating system

 

 

dMEV has a place for:

 

1. Budget constrained buildings
2. Retrofit
3. Very intermittently occupied places (churches, village halls)
4. High variations in fresh air demand. ( sports team dressing rooms etc)
5. Unreliable maintenance practices ( filters!!!) 
6. Install skills and design knowledge for MVHR is absent.

That was a good answer.

 

It does demonstrate that following building regs ventilation flow rates does over ventilate, by quite a large margin.

 

I think I need to turn my down my flow rates further. Currently at 0.3 ACH.

[Iceverge]

I find that I can smell or taste the humidity levels in a room now that I've been paying attention to it for a few years. 

 

I walked into my parents kitchen earlier and instantly recognised it as North of 70% RH. It felt close and stuffy.

 

I try to keep the MVHR dialled down as much as possible but in recent months I've felt that 36% fan speed isn't enough and dialled it up to 41%.

 

I think it is a factor of less windows being opened due to winter. Mrs at home on maternity, more babies clothes being dried inside. More moisture being dragged in on boots and coats etc.

 

A dMVHR would be the ideal solution. 

 

 

 

 

 

[JohnMo]

My only gripe about MVHR is that you have a base flow rate and it can only upwards with a boost. When it does boost, it boosts across the whole house. There doesn't seem to be an automated lowering of duty point when humidity levels are low.

 

Have seen on MEV systems they can have modulating extract terminals based humidity, the MEV unit is constant pressure controlled, so if the extract terminals close in because no one is home, making humidity, the unit almost stops. If the house is full of people showering ect the unit speed is increased as the terminals start to open.

 

No idea why MVHR doesn't feature the same sort of logic, it's just a matter of running supply fan to match the extract duty, to keep the system in balance.