To MVHR or not MVHR, that is the question.

[Dave and Helen]

Hi,

 

We're just about to start our build and had been planning to install an MVHR system. Our built is both a conversion and a renovation of an old Severn Trent pumphouse and water tank, located in a rural setting in Derbyshire.

 

Our interest in investing in MVHR has to date been based on a need to make sure we remove any damp from the converted water tank. Whilst we're not looking to create a passivhaus, we are looking to install as much insulation as we can to the fabric of the house as efficient as possible. We understand the 'sales blurb' about clean air and extracting heat from the vented air, although there seems to be some mixed messages on this last point. 

 

The layout of the house design has been challenging for our architect, with a need to combine the existing pumphouse and the water tank into a single dwelling. 

 

So having debated this with my partner, is it worth the expense, is this more equipment to maintain and potentially go wrong, we'd be very interested to hear feedback, from those who've invested in MVHR, as to whether they would do it again. 

 

Look forward to hearing comments for and against. 

 

Dave C

 

Edited June 3, 2022 by Dave and Helen
My spelling!

[Nickfromwales]

Hi. 
 

If you do not tackle ventilation heat loss, you can have a shedload of insulation and it will feel like you have 25% of what you installed. 
Fabric heat loss is relatively inconsequential, but ventilation heat loss is dire, so to build to a good standard, and then drill holes in the shell for extractor fans, and then fit trickle gents to every window would be barking mad IMO. 

 

With the house sealed and an MVHR installed, you can recover ambient heat, instead of pumping it out to the clouds, and reduce your space heating requirements. That then reduces the energy bills forever plus reduces ( scales down ) the heating system.

 

All of these decisions have ‘knock-on’ effects, so you absolutely should approach this with a “fabric first” ethos, or it will be downhill all the way back to a swelling which slightly improved upon our fore UK BRegs. 
 

Get a plan as to how you will make the dwelling(s) airtight, eg as one airtight envelope, and reassess from there.  If you cannot get under 1-2 ACH ( target 1 or less ) then MVHR is a waste of time anyways ( as natural infiltration will surpass the airflow rates of the MVHR ). 

[JohnMo]

Some reading

 

https://www.open.edu/openlearn/nature-environment/energy-buildings/content-section-2.3#:~:text=The key factor in determining,fresh cold air from outside.

[PeterW]

Wouldn’t have without - plus that concrete box will need it and it won’t have the ventilation flow of a normal structure. It’s not “something else to go wrong “ as it’s basically a fan in a box and not much more besides !!

[Iceverge]

If fitting the ducting is too much hassle you can get decentralised systems. 

 

They're not as efficient, quiet or cheap however. 

[Rishard]

Now this might put the cat amongst the pigeons, And I will state I am no expert nor have I mvhr installed, yet it is certainly on my plans for our near passive build house. I heard a podcast discussing just this point. To mvhr or not? The conversation was between the host and a ventilation supplier/manufacturer who also produces and supplies mvhr systems. He referred to a study on 270 European passive houses, half with mvhr and half with on demand ventilation. The study looked at the indoor air quality, heating load on heating system and primary energy to run the ventilation system. They drew on indoor air quality. Mvhr was top for heating load on heating system with a 6% saving on energy. However when they looked at energy usage to power the ventilation systems mvhr required 5x more primary energy. 
 

As I said I’m no expert and I imagine you guys will have some lived experience of running mvhr and be able to talk of its benefits. Would be interesting to hear views on this. I will link the podcast with the time segment for your own listening. I’m sure it’s been discussed plenty but I found it surprising. I also haven’t done any greater reading on the subject. 
 

https://www.buildingsustainabilitypodcast.com/ventilation-and-timber-futurebuild-2022-vince-house-matt-stevenson-bs077/
 

report discussion @ 9:00 

[Dave and Helen]

Thanks for all your feedback on this subject.

 

Rishard, I've listened to the podcast you provided a link to, which made interesting listening.

 

For us, we do have a need to extract moisture from the tank and increase the air quality, this is primary reason for looking at MVHR. If there is some energy saving to be made in recovering even a small amount of heat from the extracted air, then this would be a bonus. The energy used to run the MVHR system will hopefully come from our PV and electrical storage, so hopefully the running cost would be virtually zero. 

[Temp]

We installed MVHR even though our house isn't as air tight as recommended. I've no idea if it saves us money and it does use slightly more electricity than we expected. Would I fit one again? 100% yes. None of our previous houses had such a system and all had condensation issues of some sort. Our first few nights in the new house felt like we were camping out under canvas the air was so fresh when waking up. Towels dry quickly in the bathroom so haven't needed the heated towel rail. We dry all clothes on a rack in guest bathroom, don't use tumble dryer or outside line. We also don't have to dust surfaced do frequently.

 

Your experience may vary but we think it's one of the best decisions we made.

 

Just make sure the filters are easy to access and ideally washable as we find they need cleaning every few months. You should see all the muck and bugs they keep from coming in.

 

[Adsibob]

I’ve only had mine for a couple of weeks, but I definitely don’t regret it. It was expensive, but it will keep the house dry and smelling great, which is impressive for a building that has had damp problems in the past and is almost 100 years old.

Edited June 6, 2022 by Adsibob

[SteamyTea]

On 05/06/2022 at 19:26, Rishard said:

Mvhr was top for heating load on heating system with a 6% saving on energy. However when they looked at energy usage to power the ventilation systems mvhr required 5x more primary energy. 

Two different ways of measurement there.

Comparing delivered energy with primary energy is a bit naughty.

 

To David and Helan

Why would you not try and get an airtight building while it is a shell? Beat the architect with a stick until he works out a decent method of detailing that can be constructed.

[jack]

On 03/06/2022 at 12:05, Dave and Helen said:

So having debated this with my partner, is it worth the expense, is this more equipment to maintain and potentially go wrong, we'd be very interested to hear feedback, from those who've invested in MVHR, as to whether they would do it again. 

 

I've lived in a house with MVHR for over 6 years, and my answers are:

 

Yes, it's worth the expense.

 

Yes, it's more equipment to maintain and potentially go wrong.

 

Yes, I'd do it again. Knowing what I know now, there's no way would I ever choose to build a house and not include it.

 

It probably costs a couple of hundred quid a year in energy use and filters, but the improved air quality is more than worth it even if I don't make up that amount in reclaimed heat (I've never done the calculations to estimate what energy savings I might make).

[SteamyTea]

14 minutes ago, jack said:

I've never done the calculations to estimate what energy savings I might make

The energy transfer is greater when there is a larger temperature difference between OAT and IAT.

So when you want the maximum transferred, you get it. 

[jack]

10 minutes ago, SteamyTea said:

The energy transfer is greater when there is a larger temperature difference between OAT and IAT.

So when you want the maximum transferred, you get it. 

 

Sure, but my point was that I've never bothered quantifying it.

 

It seems a pretty difficult thing to model accurately, given the almost constantly changing outside air temperature, and that's before you get into second order effects like heat recovery via condensation at the heat exchanger in very cold weather (which itself is at least partly recovering heat absorbed from the environment when the moisture initially vapourised).

[MJNewton]

On 05/06/2022 at 19:26, Rishard said:

However when they looked at energy usage to power the ventilation systems mvhr required 5x more primary energy.

 

5x times what though? It might well be a relatively low cost either way. (Apologies if this is all explained in the recording; I haven't had chance to listen)

 

For example, our MVHR system cost us £24 (120kWh @20p/kWh) to power for the last year which I consider to be negligible given the benefits it provides.

Edited June 7, 2022 by MJNewton

[JohnMo]

So the comparison is between demand control ventilation and MVHR within 149 passivhaus buildings.

 

Heating, MVHR used 6% less energy to heat the properties on average than demand based ventilation.

 

But MVHR used 5x the energy to power the ventilation system.  For the same standard of air quality in the properties.

 

A similar outcome in this other report also.

Atamate_SDAR+Paper+2019+(1).pdf

Edited June 7, 2022 by JohnMo
File didn't upload correctly

[SBMS]

2 hours ago, JohnMo said:

So the comparison is between demand control ventilation and MVHR within 149 passivhaus buildings.

 

Heating, MVHR used 6% less energy to heat the properties on average than demand based ventilation.

 

But MVHR used 5x the energy to power the ventilation system.  For the same standard of air quality in the properties.

 

A similar outcome in this other report also.

Atamate_SDAR+Paper+2019+(1).pdf 2.47 MB · 38 downloads

I might be misunderstanding but is the report saying that although MVHR ‘saved’ 6% of heating energy, 5x of this energy saved was utilised in powering the system?

 

Ie if it cost £1000 to heat on average and 6% equates to £60 saved; are they saying it cost 5x the savings to power the system (ie in my example £300)?

 

If not I’m not sure how the two things can be compared. 

Edited June 7, 2022 by SBMS

[JohnMo]

12 minutes ago, SBMS said:

I might be misunderstanding but is the report saying that although MVHR ‘saved’ 6% of heating energy, 5x of this energy saved was utilised in powering the system?

 

Ie if it cost £1000 to heat on average and 6% equates to £60 saved; are they saying it cost 5x the savings to power the system (ie in my example £300)?

 

If not I’m not sure how the two things can be compared. 

Responding to the pod cast linked to by Rishard. To answer your question, I am not saying that.

 

The podcast, stated MVHR used 6% less energy to heat the properties, when compared to demand based ventilation (due to heat recovery).  So the heating system heat input only.

 

 

But MVHR used 5x the energy (electricity) to power the ventilation system (running the ventilation fans, controls etc) compared to demand based ventilation.

[SBMS]

1 hour ago, JohnMo said:

Responding to the pod cast linked to by Rishard. To answer your question, I am not saying that.

 

The podcast, stated MVHR used 6% less energy to heat the properties, when compared to demand based ventilation (due to heat recovery).  So the heating system heat input only.

 

 

But MVHR used 5x the energy (electricity) to power the ventilation system (running the ventilation fans, controls etc) compared to demand based ventilation.

I get you. It’s difficult to draw any actual conclusions, for comparing overall effectiveness I mean.  If the electricity usage of DCV was 10kWh then 50kWh for MVHR is negligible for say, a 6% energy saving on a 10,000 kWh annual heating demand. 

[Marvin]

Hi @Dave and Helen

 

We have a 100m2 renovated 1970s bungalow with PV and MVHR.  The MVHR runs at 19 to 26 watts.

We like it. 

 

Would have it again if we moved.  Like our ASHP, these things work differently form traditional setups.

 

We have good insulation, good airtightness, MVHR, ASHP,PV and EV all helping to keep the running costs down.

 

However, as @Nickfromwales said no good airtightness, on good MVHR.

 

GOOD LUCK 

 

M

 

 

[Dave and Helen]

Thanks for all your great comments and feedback... the calculations lose me a bit, but the benefits are clear and it's great to hear positive feedback on the other aspects of MVHR, the ability to remove damp and condensation, which as mentioned was our primary reason to invest in this technology due to the potential issues with converting the water tank. 

 

Do any of you have any comments as regards radial or branched systems. I was lead to believe a radial system would be better at keeping noise propagation down, but as there would be more ducting to install, I guess the costs would be higher. 

 

Dave C 

[JohnMo]

Not sure there are real cost differences, radial is certainly easier to install because the diameter tends to be smaller.  Branch systems need plenty of attenuators to stop cross talk.

[SBMS]

3 hours ago, Dave and Helen said:

Thanks for all your great comments and feedback... the calculations lose me a bit, but the benefits are clear and it's great to hear positive feedback on the other aspects of MVHR, the ability to remove damp and condensation, which as mentioned was our primary reason to invest in this technology due to the potential issues with converting the water tank. 

 

Do any of you have any comments as regards radial or branched systems. I was lead to believe a radial system would be better at keeping noise propagation down, but as there would be more ducting to install, I guess the costs would be higher. 

 

Dave C 

Not necessarily. Radial will eliminate crosstalk but may be noisier due to a smaller ducting radius (more air resistance - noisier). We went through the same dilemma when picking our MVHR design. We ended up opting for branch with rigid metal spiral ducting. We’ve got crosstalk silencers as well. One of the things that you have to be careful of with semi rigid radial is making the bends too acute and introducing resistance and noise. Rigid ducting have pre manufactured bends which maintain the designed air flow. If you

dont have web joists you could struggle with a branch system as they use higher diameter ducts. If youre self installing, radial might be easier. I don’t think one is better than the other I think the things that impact your decision are:

 

- joist size

- ducting routes

- self design or pro design

- self install or pro install

If you’re going radial you would have a better chance of self designing it, but you’d still need to calculate ducting runs, air pressure and extraction rates (and need to certify it meets building regs). If a branch system I would get it designed as you have to simulate intake along the branches and main trunks to determine ducting size. And you’d have to be sure about where your crosstalk emerges and silence those (usually bedrooms). 

 

[MJNewton]

We've got 75mm semi-rigid ducting and it is absolutely silent in use, even on boost. I wouldn't consider using anything else. 

[joth]

Ours was installed a bit over a year ago and here's the stats from its built in data logging.

249 kWh consumed running the ASHP

6724 kWh energy saved through heat recovery.

 

So about £700 saved over one year and a bit (assuming our ASHP COP 3 would have provided the heat otherwise)

 

 

[JohnMo]

You must have a quite large house I assume. 

 

You also need to factor in the cost of filters and service costs to get the total savings and a percentage of the hardware and install costs, based on expected life span, say 20 years.