Is Mixergy cylinder a good choice for heat pump install?

Hi Zarch, firstly I am non technical. I am looking for a simple system. I am due to have a Daikin altherma 3 8kw ASHP installed. The whole thing is a complete replacement of my existing system, pipes, rads, cylinder etc, complete , total.

They have quoted a mixergy cylinder. However having watched the recent Heat Geek video on the mixergy I was concerned about 2 things.

  1. Efficiency incl integration
  2. Doubling up of maintenance costs and conflicts of responsibility.

I approached Daikin, who were frankly unhelpful. From the installer , in response to my query they said

“Basically the manufactures state with the normal unvented cylinders they best work with a water pressure working rate of 1.5 and a flow rate of 22 litres per minute.

From our survey, your water pressures came out at a 5.0 standing pressure, 1.2 working pressure and a flow rate 0f 15 litres per minute.

The mixergy cylinders manufacturers state they work better for lower working pressure under 1.5 all the way down to 1. Hence why we have assigned the mixergy cylinder with your heat pump. We believe following our guidelines and the manufacturers this will be the best for your individual circumstance.”

As I said I’m non technical, does this mean anything and is it a valid reason for putting in a mixergy.

I also have a supplementary question. My existing cylinder is only 117litres and has proved sufficient. The mixergy proposed is a 180litre. Is it normal to put larger cylinders in these days and doesn’t that impact efficiency?



Hi Terry,

Not related to your tank directly but here is my advice.

Buy a normal tank that is probably cheaper than the Mixergy.

Spend the extra on the 8kW Daikin.

The 4, 6 and 8 are all the same at the bottom end of performance and you’ll lose nothing but at least have the extra capacity if you ever need it.

Better still, have a look at the heatpumpmonitor website and maybe choose a heat pump from a manufacturer with a proven better performance.

Maybe a 5kW Vaillant is a better choice?


Hi Terry

I’m inclined to agree with Matt.
The Mixergy has a lot going for it, around smart tariffs, dual fuel and if you’re on a renewable journey and taking the cylinder with you. But you don’t really need any of that with a heat pump.

I am in the middle of proving that the Mixergy performs just as well as a standard cylinder. There is nothing wrong with a Mixergy at all. it’s just got lots of bells and whistles than you may never use.

So I think you’d be able to get a good cylinder like the Newark for less money than a Mixergy.

I’ve not seen anything talking about pressures and performance. new one on me.

The flow rate into the house has nothing to do with the flow rate you can achieve with the heat pump and cylinder.

Who is the installer? Have you approached someone like a Heat Geek?
Have they done room by room sizing and calculated all the pipe sizing?
If you’re having the whole thing redone, will be this a complete open loop system? no buffers, no low loss headers, no zoning, just one big open body of water for the heat pump to work with.

Take a look on this forum about issues with Daikin. Not saying yours will be bad, but worth knowing about issues.

One of the reasons to go bigger cylinder is that the recharge times can be much slower on a heat pump than a gas boiler. Think gas boiler at 32kW versus heat pump at 5kW.
Size of cylinder all depends on you daily usage and requirements. No point going small cylinder if you’re a family of 4 and you have big water demand. the heat pump would spend all its time reheating water. And it might not do it quick enough to meet the demand.

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Thanks both Matt and Mick. BG is the installer.
“will be this a complete open loop system? no buffers, no low loss headers, no zoning, just one big open body of water for the heat pump to work with.”
Honestly I don’t know, but it’s a question I’ll go back to BG with. I have no intention of having any zoning, which I think helps. If I want different rooms colder I’ll just turn off the rads. I have asked about the vaillant before but didn’t get an answer. Not sure if BG only supply Daikin. Interestingly all three quotes I got specified Daikin!

I have so many anxieties , but am trying to work through them , because sadly BG is unhelpful as was Octopus as was an independent Heat Geek guy. They all seem to think you should already understand everything. Clearly I understand very little. Feeling a bit like I’m opening Pandora’s box!

Another thing to check on is the design temperature each installer has quoted at. Ideally you’d want the system designed around a lower flow temperature, say running at 40C at the calculated heat loss, typically between -2C&-3C depending on where you live. If they have designed at 50C who have smaller radiators and won’t achieve the best efficiency. Feel free to share the heat loss calculations and radiator schedules you have been given and we can provide feedback.

If there isn’t much difference in the quotes I’d pick the heat geek every time.

The 8kw Daikin does have a much better operational range for a typical house than the bigger version so of us have, so it’s not a bad choice. The hot water cylinders do need to be bigger, because with heat pumps you heat domestic hot water to a lower temperature than with a gas boiler, so it’s not diluted as much at the tap and you’ll use more heated water.

Hope that helps provide some reassurance.

Hi Terry,

I understand your anxiety and it is justified, that said you have done the sensible thing and are researching before you commit!

This forum is a great resource with many knowledgeable people, another is the facebook heatpump UK community here Redirecting... which is also worth joining.

My advice would be take your time and get comfortable with the installer, details of the design and how that relates to your usage before you press ahead. A well designed system will save you money and provide a very comfortable house whilst reducing your carbon footprint. A poorly designed system will cost you a fortune, cause you unneccessary stress and not necessarily suit your heating/hot water needs.

In addition to getting the right installer who can answer all of your questions and provide a properly designed installation I would say the next priority for me would be selecting a heat pump manufacturer that can a) readily service the system throughout its life and b) provide controls that don’t require you to spend your life becoming a technical expert on heatpumps in order to achieve decent levels of efficiency and comfort.

Good luck and ask lots of questions online and offline and don’t be afraid to push for a comprehensive answer.

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This is a “liability minimisation” exercise by BG.

They want to cover their backside against as many claims against them as possible. (Octopus etc are similar)

There is nothing wrong with having a regular unvented cylinder on a mains supply that can’t deliver 20 litres per minute at 1 bar.

There are many reasons for using a regular invented cylinder too (works well, simple, reliable, future proof etc)

Unfortunately the cylinder vendors don’t explicitly state that it ok to use these when the mains flowrate is below 20 litres per minute at 1 bar.

Nor do Daikin explicitly state that their hot water solution is suitable.

The only folks that DO explicitly state that it’s ok to install on supplies running less than 20 litres per minute at 1 bar are Mixergy.

That’s therefore all that BG will offer you; so that if they install the mixergy unit and you complain about the flow rate they can point at the manufacturer documentation and say that everything is ok.

Are there downsides?


It costs more, it isn’t simple and will take up more space/take more effort to get working well, and it will be less reliable by virtue of having more parts to go wrong on you/at best can only work as well as a regular cylinder and at worst can work less well.

Are there upsides?


If the plate scales up it is more practical to clean it than it is to clean the coil on a cylinder. That’s pretty much the only possible upside.

But…if you want a BG install…it’s the only option on the table unless you can improve your incoming cold water supply.

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To clarify - it is complete and utter nonsense about mixergy cylinders “working better” than any other unvented cylinder at lower flow rates/pressures.

All cylinders are a pipe coming into a tank and a pipe coming out. The flow rates and pressure drops on the mains water side will be absolutely identical.

The only difference is that mixergy don’t put a warning in their installation manual to be careful installing these on suppliees running mess than 20 litres/minute at 1 bar in order to avoid disappointment on the hot water flow rates that result.

(i.e. there’s no ammunition for a consumer to complain about the flow rate when converting from an old gravity vented setup to an invented setup)

How many bathrooms? What are your current flow rates for hot water and for cold water with the outlets full open?

If they are already under 12 litres/minute you won’t care. If they are above this then you might want to consider upgrading the incoming mains before going unvented OR retaining the vented/gravity fed setup with a booster for showers where required.

You can also feed cold water to an attic tank then pressure boost ALL the water from there down to the house and run an unvented cylinder that way. (becoming a pressure boosted setup with a break tank) That’s how you’d improve cold water pressure without digging up /upgrading the mains.

A decent plumber can advise on all these options. They’re not heat pump specific. BG are just covering their backside with insisting on the mixergy.


Thanks for all the input. I’ve been trawling through the documentation I have and I seem to be missing ( possibly as in never received) some data. Hope to update you on this.
In response to Dan, not as sensible as you think. I did do a lot of research, for as much good as it did. But I entered into a contract ages ago. Beyond all my other concerns I am still awaiting planning permission after some 9 months! Hence I have some time to get further granularity.

What stopping getting simple DWH cylinders with a oversized coil installed before the likes of BG quotes, then choosing the option of not having them “upgrading” cylinder?

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My whole Mixergy heat pump journey has lead to a monster DHW performance analysis and blog post over on Energy Stats.

Hope people find this useful.