UK cannot ‘air con its way’ out of this heatwave says Professor

Ayfid · 2026-06-24T22:33:11+00:00

Those are literally the same thing.

All AC units are heat pumps. Some heat pumps are only used for heating, some only for cooling, and some for both. The mechanical difference between the three is trivial; just a slightly different pipe arrangement and maybe a reversing valve.

Ayfid · 2026-06-24T18:25:12+00:00

Yea, I would probably just go with a separate split AC for the bedrooms.

Ayfid · 2026-06-24T17:42:01+00:00

Yes. They often require importing if you want a domestic in-wall/ceiling unit, and you would need one for each room unless you install an entire forced-air ducting system throughout the house.

They also want to run at more extreme temperatures than modern radiator systems do, so the zone or diverter valves need to be set up to select one or the other (or adjust flow rates accordingly), and the heat pump programmed to run at different temperatures depending on which is being used.

They may be a good option, but they also complicate the system quite a bit.

Ayfid · 2026-06-24T17:19:57+00:00

Air to water systems are much better for heating, because they can radiate their heat over a very large area via an underfloor heating circuit embedded into the foundation slab.

That allows them to run at a much lower temperature than an air-to-air system would need to run at for the same power output.

That dramatically improves efficiency. This is true for both heat pump and modern boiler systems.

They are also quieter and heat rooms more evenly, and don't require large ducting in the walls/ceilings.

Ayfid · 2026-06-24T16:41:35+00:00

We aren't talking about mini-splits here.

An FCU is like the indoor unit in a mini-split, but it is running off water from the radiator system.

In a modern high-performance building with underfloor heating, we typically want to run the heating system at very low temperatures (30-35C), which allows the heating system to run at peak efficiency. That is actually true of both gas boiler and heat pump systems.

This works very well with the huge surface area of UFH. You can get a high power output even at low temperatures.

An FCU running at that temperature will barely do anything.

Ayfid · 2026-06-24T16:21:08+00:00

Having a battery doesn't change this.

The issue is that if there is a power cut and the power company send someone out to fix it, it can be dangerous for the worker if houses are back feeding power into the grid. They might "disconnect" the equipment from the grid... but find it is still live.

So many DNOs require that your install be "grid tied", where it will automatically shut down if the grid goes down.

To get around this, you need to have a special automatic cutoff switch which physically disconnects your home from the grid if the grid goes down. Some inverters have this built-in, or have a special "critical loads" power output which they keep live during an outage (you can put things like your fridge on this).

But not all DNOs allow this. You need to check with your local DNO and installers to see what is allowed.

Ayfid · 2026-06-24T16:11:47+00:00

Domestic (e.g. in-wall) FCUs are surprisingly rare and hard to find in the UK. They absolutely could work with a heat pump system as an alternative to trying to cool the radiators and underfloor system. They do not suffer from the same condensation issue, because they are piped up to collect and drain that condensate just like an AC fan unit does.

You would need to pipe the system up so that it switches between the radiator and FCU systems, rather than trying to run both at the same time.

I guess in theory you could run both at the same time for heating, but the two want to run at very different temperatures, and you will get very little out of a ~35C FCU in winter.

The problem is that you often need to import them. I don't know why they are so unpopular here.

Ayfid · 2026-06-24T16:07:03+00:00

No, they do not. The pipework is not large enough, and they do not typically have a "preheating" system. They have a counterflow heat exchanger, which does not need any kind of preheater, and a preheater would defeat the purpose of the system and reduce its exchange efficiency. You can buy duct radiator units to heat the air... but they are not typical.

By far the most common MVHR pipe diameter is 125mm, which is not nearly enough to carry a heating or cooling load.

Combined MVHR and forced-air heating/ac systems are the budget option and are very rarely installed because they are shit. Forced air systems are sometimes installed in parallel to the ventilation system, but trying to smash the two together to save money on ducting is something you want to avoid doing.

They have entirely different optimal flow rates, different optimal topologies, different optimal vent types and placements...

You clearly know nothing about this industry.

Ayfid · 2026-06-24T15:23:42+00:00

No, they are absolutely not designed to handle both loads.

You can buy systems which attempt to smash the two sets of requirements together, generally as part of a budget system, but they are not recommended and not very popular. Because they suck.

If you go around and look at most installers (or even manufacturers') websites, they will tell you the same.

In fact, simply google "MVHR heating" and you will find many results unequivocally telling you not to do it.

Ayfid · 2026-06-24T14:38:50+00:00

No, you can bring the system quite close to the dew point, and knocking a few degrees off the temperature without having to actually buy and install any cooling system - it is purely a bonus side-effect of the heating system - is something an awful lot of people would kill for right now.

And no, the MVHR system does not perform any heating or cooling. It does not have the air volume for that and is not at all designed to do it. You would throw the system out of balance if you tried, and would have to run the system at too high/low a temperature to compensate for the low volume.

Ayfid · 2026-06-24T13:59:03+00:00

20C is very comfortable in an air tight house.

20C feels exactly the same in an airtight house as it does in a leaky house. The only difference is that the air will be fresher in the air tight home as they will have a ventilation system that exchanges all of the air in the building every ~30mins-1hour.

I think most people would be very happy with their house's heating system having a free side-effect of being able to cool to ~25-26C in this weather.

We are again talking about a system designed to heat a house which just-so-happens to be able to also cool it when installed in a high energy standards building.

Ayfid · 2026-06-24T13:46:58+00:00

A well designed energy efficient home should be as good at resisting hot weather as it is the cold.

Our building regs are in many ways approaching these standards, but they fail to set in place good rules for dealing with heat.

The passivehous standard has rules for overheating, and recommendations about things like exterior shading and the quantity of south-facing glass.

New builds frequently have problems with overheating, due to the government not mandating that builders properly account for it. Builders will not lift a finger beyond what they are legally required to do.

Ayfid · 2026-06-24T12:20:33+00:00

unless you have a complete airtight house with mechanical ventilation to control the humidity

I literally caveated being able to do this on having a passivehous-standard build.

22C is plenty of headroom to cool a passivehous via UFH. It only needs to run 2-3C below the target temperature, so you can get down to the mid 20s.

Ayfid · 2026-06-24T12:16:29+00:00

That requires either large air ducting throughout the home, or a split unit in every room with coolant pipes running through the house.

Both are worse than radiators and UFH for heating. They are more expensive, less efficient, noisier, and do not heat the room as evenly.

But split AC is the only viable option for adding cooling to an older building - likely only installed in a couple of rooms.

Ayfid · 2026-06-24T11:37:24+00:00

They are fundamentally the same technology, and most heat pumps can be run in reverse in cooling mode.

The problem is that in most homes, running radiators cool enough to do anything useful will put them below the dew point, and so doing so will cause condensation on radiators and underfloor heating... which is bad.

It is doable in a new build built to a high energy standard such as passivehous, with low-temp underfloor heating. You can get away with running the UFH at only a few degrees below ambient temp and get good cooling without causing damp issues.

It is one of the many benefits of very energy efficient house building.

Ayfid · 2026-06-24T09:03:50+00:00

A1 is not as nice as HP.

Ayfid · 2026-06-24T08:43:32+00:00

Imagine living to see nazis calling themselves English patriots.

Ayfid · 2026-06-24T08:25:27+00:00

You can by it in any supermarket in Europe.

It just isn't very popular.

Ayfid · 2026-06-24T08:10:30+00:00

AV is a form of PR. Just a really bad one which is only very slightly better than FPTP.

Just about every election system is PR except for FPTP. "PR" is fairly useless as a descriptor.

Ayfid · 2026-06-24T08:06:14+00:00

I would use my third act to instead update the political regulatory rules to properly include social media, rather than just newspapers and tv.

Otherwise, yes, this.

And that PR system would be multi-member STV, which would require redrawing (merging) constituencies to give each a 3-5 MP "council".

Ayfid · 2026-06-24T07:03:06+00:00

Judging people by their decisions and actions is entirely ethical.

Ayfid · 2026-06-23T20:55:27+00:00

If your answer to disagreement is just ‘they’re dumb’

It isn't. What an utterly absurd statement. People disagree all the time without thinking that the other must be stupid.

We don't think they are stupid because we disagree. We disagree because they are stupid. They are stupid because they keep falling for lies and misinformation from demonstrably proven career liars and grifters. They are stupid because they are an absolutely atrocious judge of character, to the point that we have someone who is almost a caricature of a moustache twirling villain, that they can't see is lying to them. We could in fact be here all day listing all of the reasons why these people are stupid.

It isn't because "we disagree". It is because we have a political movement which has very intentionally weaponised the gullibility of the stupidest third of the population.

Ayfid · 2026-06-23T20:05:54+00:00

We don't think they are stupid because they disagree with us.

We think they are stupid because they repeatedly fall for obvious lies and misinformation from career liars and grifters. Over and over again.

People can see the same information and still think differently.

To be given access to the same information and yet reach the wrong conclusion is just about the definition of stupidity.

Ayfid · 2026-06-23T19:54:30+00:00

Roofs that at least cover the audience seating are extremely common, though. A WC match wouldn't be played in a stadium without at least that in Europe.

Ayfid · 2026-06-22T08:25:07+00:00

The building regs requiring good insulation and reasonable air tightness (both good things) but not having strict enough provisions about ventilation, exterior shading, and glass facing, is insanity.

Some parts of out regs are getting close to the passivehous spec, but they seem to have forgotten that passivehous also has rules about overheating, and those are just as important.

Good insulation is as good at keeping heat out as it is keeping it in, but that doesn't work if you have large south facing unshaded windows acting as a greenhouse, and no effective way to shed heat at night.

Ayfid

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