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MCS - BS EN 12831-1:2017 – Heat Loss Survey

 
Air Source Heat Pumps (ASHPs)
Last Post by JamesPa 6 days ago
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trebor12345
 trebor12345
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Topic starter 05/02/2026 2:38 pm  

I have now received 2 quotes and heat loss calculations for my replacement heat pump from installers that are MCS accredited.  Whilst I am happy with the heat loss calculations as I have performed my own and have the SAP report for the building. What concerns me is that both installers did not take account of the property having a MVHR unit and the result of a Air Permeability Test.  As I understand, MCS standard (MIS 3005-D) requires the heat loss calculations to be performed to BS EN 12831-1:2017, as I have MVHR and the result of a Air Permeability Test the installers can not use the Simple method described in the standard i.e 0.5, 1.0, 1.5, etc, ACH.  They are required to perform a calculation based on the performance characteristic of the MVHR unit and take account of the Air Permeability result.  The 2 methods of calculation have a significant effect on the actual air changes per hour, simple method 0.5 ACH (2000 watts heat loss), result with MVHR and Air Permeability 0.2 ACH (900 watts heat loss).

I have raised my concerns with the installers, but a unhelpful reply.

So when it comes to certifying the installed system, they will be untruthful that a heat loss calculation has been performed to the MCS standard and BS EN 12831-1:2017.

Its fair to say that because of my circumstances, replacing a 18 month old heat pump, am I being over cautious that this could be indentified by MCS and certification would not be granted?


Hitachi Yutaki SCombi Heat Pump
(Indoor Unit ) RWD-3.0RW1E-220S-K
(Outdoor Unit) RAS-3WHVRP1

2024 build bungalow
Southern england
179 m2
High level of insulation
Underfloor heating
All 12 circuits are fully open all the time
1 thermostat in family room
7KW heat pump
50 litre buffer tank (4 port)
3.6KW solar panels
Energy used by heating 2527 KWh - 7527 KWh (SCOP 3.5 approx)


   
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JamesPa
 JamesPa
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Posts: 4206
05/02/2026 3:10 pm  

The change in calculation methodology (which mandated  BS EN 12831-1:2017 as opposed to the simpler calculation) were first published early in 2025 and became mandatory on 2nd Jun 2025.  There is a useful but not authoritative summary here and the definitive document is here

If the contractors have not complied with the new MIS-3005 then the installation does not meet MCS standards.  If the difference that the calculation makes matters to you then I would definitely insist in writing and, if they cant explain their reasoning, refuse to pay for the survey (if it was chargeable), look elsewhere and consider copying their response to MCS as evidence that they are not complying with their certification requirements. 

If it doesn't matter to you then you could take the risk, which is likely rather small given the poor record that MCS has of enforcement, but why should you unless you have very good reason to trust these particular installers.  If they are 'unhelpful' at this stage what will they be like later?

The new methodology is said to result in more realistic ACH figures, whereas the previous one frequently overestimated.


This post was modified 7 days ago 6 times by JamesPa

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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trebor12345
 trebor12345
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Topic starter 05/02/2026 5:33 pm  

Posted by: @jamespa
↑

If it doesn't matter to you then you could take the risk, which is likely rather small given the poor record that MCS has of enforcement, but why should you unless you have very good reason to trust these particular installers.  If they are 'unhelpful' at this stage what will they be like later?

JamesPa So the risk of MCS picking this up is small, which is good to hear.

The prime contractor has a good track record, so I am I little surprised at them not complying with the regulations.  in terms of my specific requirements (personal and technical) this company ticks all the boxes.  They are in fact my only option.

 


Hitachi Yutaki SCombi Heat Pump
(Indoor Unit ) RWD-3.0RW1E-220S-K
(Outdoor Unit) RAS-3WHVRP1

2024 build bungalow
Southern england
179 m2
High level of insulation
Underfloor heating
All 12 circuits are fully open all the time
1 thermostat in family room
7KW heat pump
50 litre buffer tank (4 port)
3.6KW solar panels
Energy used by heating 2527 KWh - 7527 KWh (SCOP 3.5 approx)


   
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JamesPa
 JamesPa
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Illustrious Member Moderator
Joined: 3 years ago
Posts: 4206
05/02/2026 6:36 pm  

Posted by: @trebor12345
↑

Posted by: @jamespa
↑

If it doesn't matter to you then you could take the risk, which is likely rather small given the poor record that MCS has of enforcement, but why should you unless you have very good reason to trust these particular installers.  If they are 'unhelpful' at this stage what will they be like later?

JamesPa So the risk of MCS picking this up is small, which is good to hear.

The prime contractor has a good track record, so I am I little surprised at them not complying with the regulations.  in terms of my specific requirements (personal and technical) this company ticks all the boxes.  They are in fact my only option.

 

I think its small, but off course it could happen so please dont rely on it.  They do a sample check of some kind on installers and my system was checked as part of this process.  However I think you can opt out (I had to give permission).  Obviously the installer is unlikely to say anything.  Are you claiming BUS? I guess not and if not then no risk there!

My guess is they havent yet updated their software with the new regs!

 


This post was modified 6 days ago by JamesPa

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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trebor12345
 trebor12345
(@trebor12345)
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Posts: 125
Topic starter 05/02/2026 8:59 pm  

Posted by: @jamespa
↑

Are you claiming BUS? I guess not and if not then no risk there!

You are correct no BUS.  Are you saying that the MCS certification is only an issue if there was a BUS?

 


Hitachi Yutaki SCombi Heat Pump
(Indoor Unit ) RWD-3.0RW1E-220S-K
(Outdoor Unit) RAS-3WHVRP1

2024 build bungalow
Southern england
179 m2
High level of insulation
Underfloor heating
All 12 circuits are fully open all the time
1 thermostat in family room
7KW heat pump
50 litre buffer tank (4 port)
3.6KW solar panels
Energy used by heating 2527 KWh - 7527 KWh (SCOP 3.5 approx)


   
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1
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Mars
 Mars
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Joined: 5 years ago
Posts: 4195
05/02/2026 10:36 pm  

@trebor12345, in my opinion the MCS certificate is functionally worthless to a homeowner beyond unlocking the £7.5k BUS grant. That’s it. Strip everything else away and that’s the only tangible benefit.

MCS likes to present itself as a quality gatekeeper and a consumer protection scheme, but it’s neither. When installations go wrong (and we see plenty of catastrophic ones) MCS and its associated consumer codes are nowhere to be seen in any meaningful sense. Endless forms, endless process, zero accountability, zero outcomes. I’ve yet to witness MCS actually step in, take control of a situation and force a bad installer to properly put things right.

The truth is that MCS exists to protect the scheme, not the homeowner. It polices paperwork, not workmanship. You can be MCS certified and still deliver an utterly incompetent, badly designed, badly commissioned system, and there are countless examples of exactly that.

Homeowners are paying for that certificate either directly or through an umbrella scheme, so if you go with an MCS installer that registers the job with MCS you will be paying for that. 


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JamesPa
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Joined: 3 years ago
Posts: 4206
05/02/2026 10:39 pm  

Posted by: @trebor12345
↑

Posted by: @jamespa
↑

Are you claiming BUS? I guess not and if not then no risk there!

You are correct no BUS.  Are you saying that the MCS certification is only an issue if there was a BUS?

 

Since May 2025 it no longer a requirement in England to benefit from permitted development rights (ie planning consent without having to make a planning application), the only MCS related requirement for planning is to meet the MCS noise spec.  Not sure about Scotland, Wales and Northern Ireland, they may be behind England in changing the legislation.

There is talk that some solicitors/conveyancers might ask for it when you move house, but I would think that its unlikely to be a deal breaker unless the person in question is really risk averse and doesn't much want the house.

Other than that I don't think that absence of MCS is an issue.  

 

 

 


This post was modified 6 days ago 3 times by JamesPa

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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