@techgeek, I think that's a very fair challenge and it probably deserves more attention than I gave it in the original article, which I did write a while ago.

It's almost counterintuitive that a heat pump might experience more defrost cycles at -2C than at -10C, but your explanation is correct. Those damp, near freezing conditions can be particularly troublesome because there's still plenty of moisture available to accumulate on the evaporator. As temperatures fall further, the air generally contains considerably less water vapour, so frost accumulation can actually become less of a problem. That's not to say defrosting disappears altogether, because humidity, evaporator temperature and local conditions still factor into things, but it does suggest Damon's original statement was probably too categorical.

And I think this helps sharpen the discussion we've been having with @susannasaunders. The challenge of prolonged extreme cold isn't necessarily that heat pumps become trapped in endless defrost cycles, but whether they can continue delivering sufficient heat as the building's demand increases and the machine's available output and efficiency potentially decline. I also continue to have massive issues with manufacture's labels and published outputs that don't deliver heat at temps below 0. 

Your experience at -7C is particularly interesting because it challenges the assumption that colder conditions automatically mean greater operational difficulties. I'd be interested to know whether you noticed any meaningful deterioration in efficiency or heating performance during those days, despite the absence of defrost cycles. I know our bodged heat pump system failed miserably when we had a prolonged -7C spell 4 or 5 winters ago, while consuming over 100kWh of electricity every day.

It's also a useful reminder that heat pump behaviour isn't always as intuitive as we might assume, and that real world events can add important nuance to theoretical concerns and assumptions. I'm glad that this conversation is challenging my original article! Very interesting indeed.

I really wouldn't worry too much about the heating at -20
At a sustained -20 a large number of UK permeable brick and stone houses with water in the bricks will simply fall down, our water supply will fail and lots of people will die. Heating is not going to be the top issue.
 

@etchedpixels, jovial thought for the day 🤣

@susannasaunders 

Posted by: @susannasaunders

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But what happens in a -10C or even -20C winter, which has a once per decade probability (10%), in the event that the AMOC continues to shut down over the next decade? In this situation, I presume that the heat pump will be significantly under sized and will not keep your house warm! How can this issue be managed with a heat pump without 'over sizing' it?

@editor 

Posted by: @editor

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Your experience at -7C is particularly interesting because it challenges the assumption that colder conditions automatically mean greater operational difficulties. I'd be interested to know whether you noticed any meaningful deterioration in efficiency or heating performance during those days, despite the absence of defrost cycles.

@editor Thank You Mars!

You Raise some very valid points to my post and I appreciate the feedback!

"The difficulty is that homeowners are rarely shown what those margins actually are, so they have little idea whether their proposed system would cope comfortably at -10C or begin struggling almost immediately."

This is a key point that installers and system designers need to be doing! What is the impact going to be around the heating capacity envelop of a given system design, it's likely increased costs at a range of operational temperatures [outside the design envelop], additional wear and tear [e.g. increased flow rates increases wear within the pipework] and the expected drop in efficiency as flow temperatures are ramped up to increase heat output. This would be a key consideration I would certainly want to understand! Under what conditions would the system cease to keep my home at the desired 22-23C and what steps can be included to mitigate these conditions, both with the heating system and any secondary heating systems. The discussions I have had with a couple of installers to date have been careful NOT to go anywhere near this territory.

"I'm not qualified to judge which climate model will ultimately prove most accurate, and I don't think homeowners should have to become climate scientists to make sensible decisions about their heating."

I'm most certainly NOT qualified to judge such issues either! But I can at least be aware of these issues and try to draw some broad opinions about where our climate MIGHT go... As for the Manufacturers/Installers - it's a s**t sandwich they are very much trying to dodge atm and hoping that the customer simply won't think to ask.

"There are heat pump systems capable of operating in a cascade arrangement, where two or more units work together. Mitsubishi Electric has offered this with Ecodan, and Panasonic has cascade-control systems for its Aquarea range. So the underlying concept certainly exists. The limitation is that adding another heat pump isn't remotely as straightforward as adding another battery module." ...

Thanks for this info! I'll look into these offerings to see what they are about. And yes, I appreciate that adding flexibility into any system design adds cost, complexity and potential redundancy. All issues that the industry would rather duck out of... It's not going to be as easy as sticking another battery or DCEV charger on top of the stack. But it's a direction of travel the industry should be thinking about imo.

"As crazy as it sounds, the hybrid boiler approach still makes most sense to me. The heat pump does 95% of the winter work, and if we have a crazy winter spell of low temps, the boiler (gas or oil) happily takes over. But in the current grant language, to avail of BUS, the boiler has to go."

And therein lies another problem we have with the current government initiatives... It's all black and white thinking [most to avoid cheating and manipulation of the system aka just add a A2A aircon system for free]. But also to benefit the rich that can afford a complete system swop too... You need a significant pot of money to take advantage of the BUS schemes 'as is', which might become mute anyway if they are scrapped in the forthcoming budget.

"Another possibility, depending on the property's layout, might be an air-to-air heat pump serving particular rooms. That could provide additional heating during exceptional cold weather while also delivering something increasingly valuable during our hotter summers... cooling. Given your experiences in Somerset this year, I suspect that dual benefit might be worth considering." 

This is very much the direction I am heading towards given the summer we have just endured! If you had asked me last year when we have the solar PV and batteries installed I'd have said go with A2W and be done. This year I'm very much looking at a combined A2W for heating and DHW and an A2A for cooling and *additional* heating capacity! The points made about the available additional range of operational temperatures discussed above again comes into play, along with the potential impact upon running costs etc. We specifically bought a new build in 2024 with a south facing roof [in hindsight I wish we had bought an East/West roof but that's another story] for the purposes of solar PV and future electrification with a heat pump to help mitigate the heat losses during colder winters. The house purchase was one of the worse decisions of my life [because of the property developers!] but it is certainly warm and well insulated!

"In fact, I'd also propose a fourth... commission a properly considered heat pump design that includes a documented assessment of what happens outside the normal operating assumptions, alongside an affordable plan for future adaptation.

...This wouldn't eliminate every uncertainty, but it would give you something considerably more reassuring than a sales proposal promising an attractive SCOP and little explanation of what happens when conditions depart from expectations."

That is indeed exactly the plan I have for myself! It should be the default for all ASHP installations imo. At the moment though, I don't think we are going to have the money [circa £5-7k cash plus the BUS grant] so it might be kicked into the long grass anyway. But a plan and a vision of where the industry is going would be helpful.

"That doesn't automatically mean they are acting irresponsibly IMO, but I do think the industry has been too narrowly focused on demonstrating compliance with current design requirements rather than discussing longer term adaptability with the people paying for these systems."

Agreed. System 'Adaptability' has an uncomfortable *smell* of eating away profitability so the industry is probably reticent to open this particular pandoras box (especially in a 'young' industry in the UK) and is instead turning a blind eye to it. That's my opinion anyway.

"I would also argue that any future changes in our understanding of the climate should be reflected in national design guidance, rather than expecting individual installers to interpret AMOC research. The industry needs a credible mechanism for reviewing those assumptions as evidence develops. We had a rough summer this year; now let's see what this winter brings."

This is where I think the government should be doing more - providing national guidance on adaptability both for our current housing stock and for future electrification and progression in the housing market more broadly. We should have an independent government body to help advise the heating (and cooling) industry about likely future housing needs and strategies. But, in the UK, we are lucky if we can even get the property developers to put up some houses with the bare minimum [G98/3.68 kW cap] amount of solar panels! The MCS certification process is I suppose a step in the right direction but it's barely a minimum and does nothing to address the issues I raise in this thread!

"Where I think your contribution is particularly valuable is that you're approaching this as somebody who actively wants to install a heat pump, not somebody searching for reasons to reject the technology. You're asking for greater confidence before committing your money, and I don't believe that is an unreasonable expectation. I think this is sensible and this is what I'm all homeowners to deliberate."

Indeed! This is where the industry needs to go! Lets have a frank and honest discussion with the manufacturers and installers AND the government... Tell us clearly and precisely what variance we are likely to have to deal with and advise us on the best mitigation strategies rather than offer us Black and White solutions that paper over the real issues that we face as we go forwards with electrification!

As for myself, yes I have obtained a number of basic quotations but without a proper heat loss evaluation as of yet as these cost money and only provide refunds of this expenditure with the installer that you go with. So I could spend £150-250 on a heat loss evaluation and then find I can't get that knocked off the install bill because I've gone with a different installer... Another cart before the horse issue. Coupled with the 1st April VAT deadline etc... Irksome!

I'm starting to work through my own heat loss calculations [or at least hoping too! Help!].  I have two years worth of heating data for this house already and that is also helping to guide my thinking on heating needs. I've been looking into degree days https://www.degreedays.net I plan to take a look at Heatpunk over the next month or two as well. https://heatpunk.co.uk What happens in the pending budget though will also impact what we do. I suspect the the BUS scheme will be raided by BS Burnham [I'm a Green party voter] to pay for the increased military expenditure...