Impact of MVHR heat recovery on heat demand cals

[Babak]

Hi, have a question about incorporating the impact of MVHR when calculating heat loss.

 

Background: New ICF build near completion. My Heat pump installer has calculated I need 8kw of heat at -1/21. However, after incorporating the 90% efficiency of MVHR into heat loss calculations, this drops to 6.5kw. This was done by reducing heat loss through ventilation to 10%. This also synchs with my UFH room by room heat demand calcs. Theoretical efficiency of the MVHR unit  is over 95%, and my u-values are conservative, eg I have double bubble-wrap foil under roof but have ignored its additional R value. Same with impact of plasterboards, render etc. So fairly confident about the stated heat losses not being more.

 

Issue: My heat pump installer INSISTS that I should ignore the MVHR because it could fail and if that happens in the middle of -1 degree winter I need to have additional capacity for heat.  He wants to install a bigger system to suit the 8kw demand. The change to the bigger unit means my outside unit will double in size, which I absolutely do not want unless I really have to.

 

My arguments has covered the following, none of which has convinced him:

- extra cost of bigger system (£1,000) will take years to be recovered through multiple failures of MVHR just when outside is really cold

- even ignoring the upfront costs, having to supplement heating with electric radiators/fans for a couple of weeks every few years (IF the MVHR fails), is not actually that huge.

- MCS has been non-comittal on this and says it is a design issue for the installer to decide

- What happens if the heat pump fails? What happens if there is no power for 10 days? Why do we need to assume the MVHR fails? But not anything else?

 

What am I missing here? I realise I am a little on the borderline with the 6kw system which only has a couple of extra 100 watts of power at -1 outside, but I am quite prepared to supplement that with normal electric heating when I have to. MCS have not objected, and it is my risk to take.

 

Any advice? Is there a rule about excluding the impact of MVHR when calculating heat losses? It is just so incredibly odd that my installer wants to disregard the heat recovery impact when working out ventilation heat losses, which sure.y goes against the spirit of MCS.

 

Thank you! 

 

 

 

 

 

 

 

Edited August 10, 2022 by Babak

[Kelvin]

Why is he so convinced the MVHR will fail? There’s more to go wrong with the ASHP and it’s various pumps, valves, and actuators. 

[JohnMo]

If your MVHR fails on the coldest day your heat demand would go down not up. 

 

You are in a nearly airtight house otherwise you wouldn't have MVHR.  Think logically, MVHR provides the ventilation, it recovers heat from the outgoing air and transfers to incoming air with some losses.  If there is no outgoing or incoming air, because your MVHR has failed, then there is no losses, so heat demand reduces.

 

Your installer is less than dim, possibly has no real idea what an airtight house is, or what the MVHR does

[Babak]

2 minutes ago, Kelvin said:

Why is he so convinced the MVHR will fail? There’s more to go wrong with the ASHP and it’s various pumps, valves, and actuators. 

I wish I knew. I think his line of thought is that should the MVHR unit fail, I should still have all of my heat demand met through the primary heating system. 
 

But thank you, I will also argue with him that the ASHP is more likely to fail.

[Babak]

4 minutes ago, JohnMo said:

If your MVHR fails on the coldest day your heat demand would go down not up. 

 

You are in a nearly airtight house otherwise you wouldn't have MVHR.  Think logically, MVHR provides the ventilation, it recovers heat from the outgoing air and transfers to incoming air with some losses.  If there is no outgoing or incoming air, because your MVHR has failed, then there is no losses, so heat demand reduces.

 

Your installer is less than dim, possibly has no real idea what an airtight house is, or what the MVHR does

LOL, yes certainly looks like he is not comfortable with this additional factor. We did talk about the failure scenario, when we need to open the windows.  But yes even then it is unlikely the ventilation losses would be as much as he would have as default in his calcs, which probably assumes trickle vent as well as through the fabric losses.

[Kelvin]

7 minutes ago, Babak said:

I wish I knew. I think his line of thought is that should the MVHR unit fail, I should still have all of my heat demand met through the primary heating system. 
 

But thank you, I will also argue with him that the ASHP is more likely to fail.

Put a jumper on until it’s fixed. We were in our last place for 7 years. In that time we had a pump fail, two valves fail, and one of the manifold valves fail. 

[ProDave]

Why is your MVHR supplier specifying your heating system?  I would expect that to be a different person.

 

What does your SAP say?

[JohnMo]

The other way to look at the heat loss.

 

Say you have an airtightness of 2m3/m2 at 50Pa., Your infiltration rate is 0.1358.

 

So inputting that in to a ventilation heat loss calculation

 

Qv = 0.33 × n × V × ΔT watts.

 

So say your volume is 300m3

 

0.33 x 0.1358 x 300 x 22 = 295W

 

So at an airtightness of 2 if your house has a volume of 300m3 your ventilation heat loss without MVHR is 300W.  If your house is is bigger small just change the 300 figure.

 

Rules of thumb do not apply

 

 

Edited August 10, 2022 by JohnMo
Spelling

[Babak]

1 minute ago, ProDave said:

Why is your MVHR supplier specifying your heating system?  I would expect that to be a different person.

 

What does your SAP say?

it’s the ASHP installer who is specifying the heat demand, not the MVHR supplier.

 

SAP efficiency 95%. I need to get hold of the calculations, yes. They are not room by room though, if I remember correctly.

[JohnMo]

To comply with MCS regs he has to do a proper heat loss calculation.  This is a contractual document.  He cannot put his interpretation on things, he should not use rules of thumb etc.

 

I believe to comply with MCS installer rules he is to quote you before and work commences and get the bus grant

 

If he's a lone contractor tell him what you want or he is off the job. If he's from a bigger company, get his boss to site and ask him to show you where in his installation regs it states you can apply rules of thumb and "what if" to heat loss calculations.

 

 

Edited August 10, 2022 by JohnMo
Repeat word

[Babak]

20 minutes ago, JohnMo said:

To comply with MCS regs he has to do a proper heat loss calculation.  This is a contractual document.  He cannot put his interpretation on things, he should not use rules of thumb etc.

 

I believe to comply with MCS installer rules he is to quote you before and work commences and get the bus grant

 

If he's a lone contractor tell him what you want or he is off the job. If he's from a bigger company, get his boss to site and ask him to show you where in his installation regs it states you can apply rules of thumb and "what if" to heat loss calculations.

 

 

 

It is  medium sized company and he uses a software for heat loss calculations. The issue is that he wants to exclude the impact of heat exchanged in the ventilation system, yet include the heat lost through outgoing air. I actually think he is genuinely interested in protecting me, but just does not quite get it.
 

This is what he wrote to the MCS and the MCS reply (he changed numbers as I have for anonymity). He says he has consulted with three other installers as suggested by MCS below, but I reckon t(e installers are also unaware of the difference between MVHR and central ventilation.

 

Query to MCS:

I am thinking of buying an Air Source Heat Pump (ASHP) and having Mechanical Ventilation and Heat Recovery (MVHR) installed. The heat loss calculation for the house confirms use of a 16 kW heat pump without MVHR but reduces this to 12kW with MVHR. My concern is that if I have the smaller 12 kW ASHP installed and the MVHR breaks down or stops working for some reason then the heat pump will not be powerful enough to satisfy the heat demand for the house. Is there any MCS guidance about this to help me choose the correct size of heat pump when using MVHR ?

 

MCS reply:

What you are asking is to do with design guidance, and unfortunately our technical team are unable to provide this. So we would recommend that you speak to at least three contractors and see what their response is. 

 

I can refer you to our best practice guide, which I have attached. There is a very small section on MVHRs on page 13. But the best route would be to speak to some of our installers. 

 

If you have any other questions please don’t hesitate to ask.

 

Kind Regards

[Nickfromwales]

1 hour ago, Babak said:

What happens if the heat pump fails?

Internal backup immersion kicks in.

 

1 hour ago, Babak said:

What happens if there is no power for 10 days?

Same as the neighbours, put a jumper on, buy some candles. 

[Nickfromwales]

1 hour ago, Babak said:

Any advice? Is there a rule about excluding the impact of MVHR when calculating heat losses? It is just so incredibly odd that my installer wants to disregard the heat recovery impact when working out ventilation heat losses, which sure.y goes against the spirit of MCS.

MVHR has a fan. It goes around for a VERY long time before starting to wear out. That is usually notable, and an annual service will identify this early on, ergo it is a known and predictable failure which you can actively mitigate against.

[Babak]

5 minutes ago, Nickfromwales said:

Internal backup immersion kicks in.

 

Same as the neighbours, put a jumper on, buy some candles. 

So taking the above approach, when the MVHR fails, a couple of windows open. If it gets too cold, a couple of electric heaters kick in.

 

Now try convincing my installer of it. TBH timelines are a bit tight (as with any new self build) and I have too much invested in the make and the installer, otherwise would just move on….

Edited August 10, 2022 by Babak

[Nickfromwales]

1 minute ago, Babak said:

So taking the above approach, when the MVHR fails, a couple of windows open. If it gets too cold, a couple of electric heater kick in.

 

Now try convincing my installer of it. TBH timelines are (as with any new self build) and I have too much invested in the make and the installer, otherwise would just move on….

Just grab the bull by the horns, and tell your installer the "problem" is yours, not theirs. Conversation over, nobody gets hurt  

[JohnMo]

Where's does the 12 or even 16k come from, the original post said 6 and 8kW. Sounds a big heat pump for an ICF new build.  Unless the house is huge.

 

My house, 190m2 single storey, lots of glazing and a 70m perimeter, but at -5 I have a heat demand just over 3kW.

 

What does your SAP EPC, say your heat demand is?

[Adsibob]

1 hour ago, JohnMo said:

If your MVHR fails on the coldest day your heat demand would go down not up. 

 

You are in a nearly airtight house otherwise you wouldn't have MVHR.  Think logically, MVHR provides the ventilation, it recovers heat from the outgoing air and transfers to incoming air with some losses.  If there is no outgoing or incoming air, because your MVHR has failed, then there is no losses, so heat demand reduces.

 

Your installer is less than dim, possibly has no real idea what an airtight house is, or what the MVHR does

+1.

i suggest you find a different installer, preferably one with a brain.

[Babak]

18 minutes ago, JohnMo said:

Where's does the 12 or even 16k come from, the original post said 6 and 8kW. Sounds a big heat pump for an ICF new build.  Unless the house is huge.

 

My house, 190m2 single storey, lots of glazing and a 70m perimeter, but at -5 I have a heat demand just over 3kW.

 

What does your SAP EPC, say your heat demand is?

He has  changed the numbers for anonymity. He is also saying it is for ‘him’, even though he is the installer. The actual numbers are just over 6 and just over 8kw, which are just below the two heat pump models output at -1.  When you go from the smaller model to the more powerful model, the external unit gets an additional fan and becomes almost twice as big. Also a no no for us!

 

Fyi this is the gentleman’s comment as he forwarded the MCS reply to me. I suspect his  other installers have not actually installed in a house with MVHR.  

 

Dear  Xxxxxx

 

Please find forwarded the reply that I received from MCS regarding my question about using heat pumps with MVHR.

I followed their recommendation and spoke to three accredited contractors, including Joe from Xxxxxx, who all said that when working out heat losses of buildings to size a heat pump they do not include the affect of MVHR.

I personally agree with this approach particularly when using a modern intelligent modulating heat pump model like we are proposing for you.

 

Best wish

[Babak]

…. But there is a positive to all this….when looking through the forum for similar issues I came across a discussion of switching the heat pump to chilling mode. Looks like it is a little more complicated that I first thought, so can plan for it better…. 🙂

[Kelvin]

55 minutes ago, Babak said:

So taking the above approach, when the MVHR fails, a couple of windows open. If it gets too cold, a couple of electric heaters kick in.

 

Now try convincing my installer of it. TBH timelines are a bit tight (as with any new self build) and I have too much invested in the make and the installer, otherwise would just move on….

No electric heaters just wrap up. I wasn’t kidding when I said put a jumper on. If I can survive a winter in this old farmhouse rental with what seems like zero insulation you can survive a broken MVHR in a modern well insulated house 😂 

[JohnMo]

There are issues with oversizing.

 

The modulation range of the heat of pump, usually about 2.5:1  This is not an issue when at the lowest temperature outdoors, but becomes apparent when the outside air temp is in the 5 to 10 degC range.  At 11 degC outside your heat demand is half your design case, so about 3kW.  If your heat pump cannot modulate to that level, you then need a buffer added to the system.

[Babak]

4 minutes ago, Adsibob said:

+1.

i suggest you find a different installer, preferably one with a brain.

To be fair to all installers, common sense is only common amongst those who have been through the problem and solved it already. It is surprisingly uncommon otherwise. 

to illustrate, I had similar issues with UFH designers and kept moving on after the first conversation. They mostly could not understand the low heat losses and wanted to stick with statistical heat loss factors for UK new build UK housing… I actually registered as an installer with one of the larger suppliers and got access to their online tool for working out heat requirements and pipe spacing based on floor and fabric types. 

Also had issues when looking for SE (found a really good one though - no prior experience of my ICF and roof, but with open mind). And ground works guys .…  Majority just wanted to stick with how they done things, and only a few willing to listen and do some thinking /research (also found a really good groundwork’s guy!) 

 

 

[jack]

10 hours ago, Babak said:

to illustrate, I had similar issues with UFH designers and kept moving on after the first conversation. They mostly could not understand the low heat losses and wanted to stick with statistical heat loss factors for UK new build UK housing…

 

Our plumber recommended least a 12 kW ASHP for our house. Even after I showed him the heat calculations, he was still concerned that the house temperature would drop to uncomfortable levels when recharging the DHW on cold winter days. 

 

The 5 kW ASHP we installed has been perfectly adequate for nearly 7 years now.

[SteamyTea]

Has DHW been taken into accont woth the ASHP sizing?

 

Being a bit oversize does not do any harm.

 

What will the actual percentage heat recovery be in the temperature difference ranges between 10K and 22K, extremes only last hours and can be a bit misleading.

 

MVHR does not replace 'built in' air leakage, it adds to it, but recovers some power.

Edited August 11, 2022 by SteamyTea

[Nick Laslett]

I feel your pain. Until the house has been finished and you have gone through a full year’s weather, it is very hard to model in advance how it will perform. Even the HeatGeek YouTuber casually mentions in one of his detailed ASHP videos that you will have to adjust the weather compensation when the cold weather arrives to get it right for your dwelling, if you install the ASHP in summer. 
 

After reading many threads here on the topic, I feel that short cycling is the main factor you want to engineer our with good design. I’m sure the heat loss tools are good, but you will find a big difference between the PHPP model and the defaults in SAP. It is a lot of effort to model the window junction heat losses accurately. 
 

My understanding is that constant short cycling will wear you ASHP quickest. 
 

I wonder where your installer stands on the Legionnaires weekly boost program?