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Failed install (initial SCOP1.8), clueless installer compounded by poor support from Mitsubishi. Help needed desperately!

 
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Air Source Heat Pumps (ASHPs)
Last Post by Mars 3 months ago
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 Bart
(@bart)
Eminent Member Member
Joined: 11 months ago
Posts: 30
Topic starter 12/06/2025 6:08 am   [#2341]

Hi All,

My first post. I've learned a lot from reading this forum, wanted to validate my understanding and thought others might learn from my terrible experience.

Having completed a 5 month complaints process and made limited progress with the installer I've now had to escalate to the scheme provider/trade association. Given apparent collusion I'm not specifically sharing details (publicly) of the companies involved at this time.

Improvements

  1. reducing legionella cycle settings to minimum required (temp 60, scheduling & duration)
  2. reducing DHW to minimum required settings (temp 45, scheduling & duration)
  3. running in compensation curve setting to discover required water temps for desired internal temps for any given outside temp (45@-2 & 25@15)
  4. adjusting flow rate to maintain delta 5-8 (ie. gap between flow and return temps)
  5. Freeze stat function disabled as glycol had been added to the system

All the above increased COP to 2.7 on a perfect day ie. 30% (Thank you RHH!)

Problem

The system was short cycling

short cycling

I couldn't achieve internal temp above 19+ regardless of water temp

I couldn't achieve a delta 5+ even with pump running at 8l/min (manufacturers guidelines state optimal is 17l/min) and the pump wouldn't couldn't achieve 14+l/min.

This suggested:

  1. the ASHP was too large for my property AND/OR
  2. the radiators were too small
  3. the pump was insufficient
  4. something else?

1. A 6kwh system had been installed. My heat loss survey=3.6 (however u values, room sizes, layout were all wrong) and it was amended (post install) =3.1

2. The installer 1st requested additional payment for a bathroom radiator, then refused to fit it, then only complied after I refused to sign off completion. However the amended heat loss survey has subsequently found that the kitchen radiator is also too small (K3 needed).

3. As yet no reply at all regarding the pump that can't achieve optimal flow rate

4. Flow/return temps don't tally with flow/return temps at the radiators. Also flow temps never fall below 25 and return temps never fall below 20. 

"The Game's Afoot"

I queried the proximity of the design with the installer and manufacturer

thermostat proximity 20250428 171128 26.04.25

At the installer's insistence a sales manager from the manufacturer attended, later emailing: "all I can advise.... it's been installed as per our instructions" and sharing a document that suggests "initially setting min flow temp to 35" (erm I get it my flow temp is low but don't I get to decide how hot my house is?)

"You need a volumiser- Yep like a hole in the head"

In the interim 3 months the installer had decided to add a 32 litre volumiser in my loft (without my consent) on the longest radiator run in the house not on the main return (although I'd like it on flow for defrost cycles).

  • My entire system was less than 40 litres
  • Narrow delta continues
  • SCOP remains 2.3
  • Half of the heated volume of water is now located outside the insulated envelope of my house (wasted heat not included in SCOP)
  • Balancing radiators is a nightmare (7 vs 50 litres)

The same sales manager has defended this ridiculous decision sharing the turn down datasheet

turn down data

HELP?

  1. The manufacturer's home help technical team have yet to respond to my request for 5kw turn down data- can anyone help?
  2. I've argued this would appear to show that my ASHP is too large, not that I need a bucket of warm water in my loft- Anyone have any resources that might confirm this please? The missing K3 in the kitchen would add volume but not much.
  3. I've argued that a pump that can only achieve 14l/min given an ASHP that requires 17l/min optimal flow isn't compliant as it isn't fit for purpose. Anyone have any resources that might help please?
  4. I've argued that flow/return thermostats that don't record flow/return temperatures aren't compliant as they aren't fit for purpose. Anyone have any resources that might help please?

Thank you for reading.


This topic was modified 11 months ago by Mars
This topic was modified 3 months ago by Mars

   
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ASHP sizing heat loss survey volumiser
Mars
 Mars
(@editor)
Illustrious Member Admin
Joined: 5 years ago
Posts: 4549
12/06/2025 7:40 am  

Will reply later in detail. Who’s the manufacturer?


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 Bart
(@bart)
Eminent Member Member
Joined: 11 months ago
Posts: 30
Topic starter 12/06/2025 7:49 am  

@editor cheers.

 

Given the ongoing dispute I'll email



   
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Mars
 Mars
(@editor)
Illustrious Member Admin
Joined: 5 years ago
Posts: 4549
12/06/2025 7:54 am  

@bart great. editor@renewableheatinghub.co.uk.


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Subscribe and follow our YouTube channel!


   
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 ASHP-BOBBA
(@ashp-bobba)
Honorable Member Member Professional Installer
Joined: 1 year ago
Posts: 445
12/06/2025 8:21 am  

@bart Hi Bart,

Well done for the improvements, there is room to make these system run cop 3.6-4.5 so there must be some more tweaks that can be done, its super important to match the ASHP to the flow rate and emitters output which starts with design (also works in reverse and essential to match the emitters to the output of the ASHP. 

  1. The manufacturer's home help technical team have yet to respond to my request for 5kw turn down data- can anyone help?

Will need to know which manufacturer for the sheet, Mars no doubt will help you with this not to disclose at this point.

  1. I've argued this would appear to show that my ASHP is too large, not that I need a bucket of warm water in my loft- Anyone have any resources that might confirm this please? The missing K3 in the kitchen would add volume but not much.

Please share the extensive heat loss and design report the installer and designer would have completed.

  1. I've argued that a pump that can only achieve 14l/min given an ASHP that requires 17l/min optimal flow isn't compliant as it isn't fit for purpose. Anyone have any resources that might help please?

What is your pump model, where in the home (what floor) is it installed and how high is the highest radiator?

  1. I've argued that flow/return thermostats that don't record flow/return temperature
    Posted by: @bart
    ↑

    Freeze stat function disabled as glycol had been added to the systems aren't compliant as they aren't fit for purpose. Anyone have any resources that might help please?

Are you talking about sensors here? if so not all manufacturers record in the log what they say but the system will be reading them to maintain that perfect 5º(DT)


AAC Group Ltd covering the Kent Area for design, supply and installation of ASHP systems, service and maintenance, diagnostics and repairs.
Professional installer. Book a one-to-one consultation for pre- and post-installation advice, troubleshooting and system optimisation.


   
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 Bart
(@bart)
Eminent Member Member
Joined: 11 months ago
Posts: 30
Topic starter 12/06/2025 11:12 am  

@ashp-bobba Hi Bobba,

Thanks for your reply

I'm trying to find a way of sharing the latest heat loss survey (having removed all identifying info) and will share ASAP. My impression is that all ground floor rads are undersized as their 1st effort was an exercise in creative writing I've lost any faith in their data.

Pump is below:

pump

For metering I've relied on outputs from the ASHP manufacturer's controller. (low =8, med =11 high =14l/min) Its on the 1st floor same height as highest radiator. x3 heating loops which combine before the main ASHP feed and return.

1. kitchen rad

2. x2 bd2 and bathroom rads

3. goes up through the loft to feed the BD1 & lounge rads (which is where they've put the volumiser I believe on the return). 

 

Thermostat/ sensors. Yes hence my concern. They appear to track rad F/R temps above 25/20 OK, (compromised by the 50litre lounge/bd1 loop)

Today DHW heating cycle (roughly delta 5)

 

dhw heating cycle

 Thanks again



   
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 ASHP-BOBBA
(@ashp-bobba)
Honorable Member Member Professional Installer
Joined: 1 year ago
Posts: 445
12/06/2025 3:21 pm  

@bart Which county are you in? Kent by any chance? 

Send what you can and I will try to help.

KR

 


AAC Group Ltd covering the Kent Area for design, supply and installation of ASHP systems, service and maintenance, diagnostics and repairs.
Professional installer. Book a one-to-one consultation for pre- and post-installation advice, troubleshooting and system optimisation.


   
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 MPHB
(@mphb)
Trusted Member Member
Joined: 3 years ago
Posts: 31
13/06/2025 11:50 am  

This:

RHH

being uninsulated ensures that any reading out of this will be completely off (unless you're interested in room temperature).



   
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 ASHP-BOBBA
(@ashp-bobba)
Honorable Member Member Professional Installer
Joined: 1 year ago
Posts: 445
13/06/2025 11:55 am  

@mphb Which sensor is that supposed to be? I assume the primary flow or return?

All of the pipework should be insulated but yes especially that probe should be. 


AAC Group Ltd covering the Kent Area for design, supply and installation of ASHP systems, service and maintenance, diagnostics and repairs.
Professional installer. Book a one-to-one consultation for pre- and post-installation advice, troubleshooting and system optimisation.


   
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 MPHB
(@mphb)
Trusted Member Member
Joined: 3 years ago
Posts: 31
13/06/2025 12:03 pm  

That is something @bart would need to tell us. I have my probes firmly insulated and actually added thermal paste in order to get as close as possible to the temperature of the flow.

I cross my fingers that maybe it is this probe throwing the primary or return values off and so makes the ASHP work so inefficient.



   
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 ASHP-BOBBA
(@ashp-bobba)
Honorable Member Member Professional Installer
Joined: 1 year ago
Posts: 445
14/06/2025 6:21 pm  

@bart How you getting on with yours now? 


AAC Group Ltd covering the Kent Area for design, supply and installation of ASHP systems, service and maintenance, diagnostics and repairs.
Professional installer. Book a one-to-one consultation for pre- and post-installation advice, troubleshooting and system optimisation.


   
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 Bart
(@bart)
Eminent Member Member
Joined: 11 months ago
Posts: 30
Topic starter 15/06/2025 8:28 am  

Just by way of a brief update.

Thank you so much for the responses received here and privately (given newbie status I'm unable to respond by mail). After hours spent suffering in silence it has been overwhelming to receive such generous support. Your time is very much appreciated.

Advice received to date has confirmed:

  1. The system is oversized
  2. The addition of a volumiser is madness
  3. Things are compounded by the lack of a sufficient heat emitter in the kitchen and considering current performance a re evaluation of all rad sizing would seem appropriate
  4. Given required Dt for heat transmission from rads min cc flow temps need to be increased (I'm guessing overlay a temp schedule? to keep internal temps as required).

Thank you for also highlighting the uninsulated pipework and the positioning of the probes as a concern. By extension the uninsulated pipework in both lofts could also improve efficiency.

Given my escalation I'm not allowed to make any changes myself, hence my 'call to arms' regarding information at this stage. That said your advice to date has been invaluable in tapering down the 'apparent' misinformation/avoidant responses I've received as a result of my escalations. Thank you RHH

Having taken legal advice I'm unable to share my heat loss survey at this time. I'm afraid I'll have to update once the relevant bodies have completed their investigations.

Thanks to the powers that be for creating such a forum



   
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