Cylinder suitable for future ASHP

[KOS]

We need to put in a new hot water cylinder in the very near future. After that we’re hoping to go through the process to get a grant and install an A2W heat pump.

 

Ideally we want to choose a cylinder that won’t limit our options. What should we look out for? Any particular brands that are good or bad for compatibility (or any other reason)? Any and all advice would be very much appreciated

[jack]

Welcome to BuildHub.

 

A couple of points (I'm sure there are others):

• ASHPs don't heat water as hot as gas boilers do, so you'll want a larger tank to deliver the same amount of water to things like showers.
• Make sure you get a tank with a heat pump coil, which is longer than a standard coil for use with, e.g., a gas boiler. A longer coil allows for better transfer of heat into the water.

[JohnMo]

Ideally look for a 3m² or larger coil in the cylinder. Telford are reported as good go to Cylinder2Go website. I have a 210L slimline made by Ideal which is good also (3 bed house), lots to choose from. Personally I would avoid pre-plumbed, lots of stuff you don't need. Also don't waste any money getting a cylinder with built-in buffer.

 

You need to do a search on cylinder sizing, it's based on number of bedrooms and it's an MCS guide.

 

The heat pump installers will look for 28mm feed pipe to the cylinder coil. But they will do away with the S or Y plan heating system also, replaced with a 3 port diverter valve.

 

But a heat pump cylinder will be great with a boiler and give really quick reheat times.

Edited Monday at 13:02 by JohnMo

[sharpener]

Gledhill have reportedly had quality probs with leaking fittings and loose/rattly internals.

 

OSO have a good reputation (and make Vaillant cyls under contract). Mine dates from 1995 and has needed only a replacement pressure reduction cartridge since then. The HP installers re-used it, as changing it out was going to involve substantial building work.

 

Ideal are rebadged cyls made by another major mfr (I forget who) but are actually cheaper to buy from Ideal.

 

IIRC the recommended size is 45 l x (# of bedrooms + 1). This is designed to give you 1 shower p/p.  Should be plenty as with an HP you can reheat cheaply several times a day (depending on tariff e.g. 3 x with Octopus Cosy).

[JohnMo]

28 minutes ago, sharpener said:

Ideal are rebadged cyls made by another major mfr

Gledhill - no issues with mine and paid exactly half the price with the Ideal badge on it, from the same plumbing merchant that sells both. May be the difference is mine is a slimline so coil is shaped differently.

[Nickfromwales]

On 07/07/2025 at 12:53, KOS said:

We need to put in a new hot water cylinder in the very near future. After that we’re hoping to go through the process to get a grant and install an A2W heat pump.

 

Ideally we want to choose a cylinder that won’t limit our options. What should we look out for? Any particular brands that are good or bad for compatibility (or any other reason)? Any and all advice would be very much appreciated

Go to Trevor at cylinders2go (Telford) and mention my username and the forum, as he's given favourable rates to many other members here over the years  

[KOS]

@jack @JohnMo @sharpener

 

Thanks all for your help, we really appreciate it.

So, apart from size, brand reputation, energy rating and coil size are they all basically the same as each other? No ‘smart’ features or bells and whistles or connections or anything that we need to look out for? Just get the cheapest that ticks the boxes and it should work with whatever ASHP you end up choosing?

 

Thanks @Nickfromwales I'll get in touch with him

 

[Nickfromwales]

1 hour ago, KOS said:

@jack @JohnMo @sharpener

 

Thanks all for your help, we really appreciate it.

So, apart from size, brand reputation, energy rating and coil size are they all basically the same as each other? No ‘smart’ features or bells and whistles or connections or anything that we need to look out for? Just get the cheapest that ticks the boxes and it should work with whatever ASHP you end up choosing?

 

Thanks @Nickfromwales I'll get in touch with him

 

Just needs to be a stainless cylinder afaic, as this will outlive both of us, whereas copper will not. 100% avoid glass-lined cylinders (you can tell when it's one, as these have electrically powered sacrificial anodes). These often rot from the inside out, and are utter crap.

[Dillsue]

1 hour ago, KOS said:

So, apart from size, brand reputation, energy rating and coil size are they all basically the same as each other? No ‘smart’ features or bells and whistles or connections or anything that we need to look out for? Just get the cheapest that ticks the boxes and it should work with whatever ASHP you end up choosing?

 

If you're only going to heat DHW with a HP then you'll likely only need a pocket for a temperature probe. If you've got or might get solar PV then a second immersion lower down the cylinder allows you to divert excess solar into the cylinder when the solar is intermittent

[ReedRichards]

On 08/07/2025 at 18:08, Dillsue said:

If you've got or might get solar PV then a second immersion lower down the cylinder allows you to divert excess solar into the cylinder when the solar is intermittent

 

But at the moment you can get a better rate for exporting your solar electricity and heating your water with the heat pump (with a CoP of 2 or more).  And it's even cheaper if you can use a night-rate/EV tariff when heating your water with the ASHP.

[Dillsue]

1 hour ago, ReedRichards said:

 

But at the moment you can get a better rate for exporting your solar electricity and heating your water with the heat pump (with a CoP of 2 or more).  And it's even cheaper if you can use a night-rate/EV tariff when heating your water with the ASHP.

It gets quite complicated and eveyones circumstances are different!!

 

You don't automatically get paid for export as you need MCS or jump through a few hoops to get a non MCS system accepted for export. Either way there's ££s to pay. Without export payments then diverting to DHW could be beneficial.

 

I specifically said intermittent PV as you can't run a heat pump on intermittent PV without a significant draw off the grid.

 

Heating DHW overnight is fine if a TOU tariff works for you. We're on Octopus Go so we get 5 hours cheap overnight but pay a premium unit rate the rest of the time. We want to run weather compensation on the HP I'm putting in just now so for 19 hours a day we'll be paying premium rates to run the HP.

 

If we went for Octopus Cosy then we'd lose the heavily discounted EV charging period and only be able to run the heat pump intermittently on cheaper eleccy. Again we'd be paying a premium outside the cosy periods and a big premium to cook tea!

 

Buying a cylinder with a lower immersion boss defers making complex decisions now and gives flexibility in the future

[Nickfromwales]

1 hour ago, Dillsue said:

I specifically said intermittent PV as you can't run a heat pump on intermittent PV without a significant draw off the grid.

So few people get this.....

 

IMHO excess (potential export) should go to an immersion via a suitable diversion controller so it can be fed in proportionately, if you don't have a battery.

 

If you have a battery then the whole game changes, and using the HP (routinely and strategically) from self-generated electricity becomes a 'thing'.

 

Batch heating an insulated raft foundation is best, done overnight and in pockets of off-peak TOU windows, but folk with thinner emitters (screed over insulation or rads) will suffer a bit less efficiency here due to near zero 'storage heating', such is life; you have to pay something for heating so this isn't exactly news.

 

As far as DHW goes, I am an advocate of putting in a bigger cylinder so it can be heated from an HP at a cooler (better COP) flow temp, and still have huge additional heat energy capacity for when it's winter and the PV has go ta-ta. Then you can 'super' heat it via the immersion to say 75-80oC, once overnight, and then that will have enough to get you through the day (assuming at least one more off-peak window during the day, which you'd then use to top up again if so necessary). 

 

Advice for this will change according to the available tariffs of course, so it is quite difficult to plan to cover all eventualities as some are variables. Go large is the one that is the wisest choice, for future-proofing, AFAIC.

 

 

[JohnMo]

1 hour ago, Dillsue said:

You don't automatically get paid for export

And if you don't have a smart meter for whatever reason, you may only get a pathetic 4p per kWh even after paying for a MCS install.

[SimonD]

3 hours ago, Nickfromwales said:

Then you can 'super' heat it via the immersion to say 75-80oC, once overnight,

 

Yes, absolutely. And this will change a bit now with the r290 heatpumps working 'fairly' well up to 75C, so on an Octupus intelligent go rate, it becomes cheap as chips to batch charge on a reduced cop and then top up a few degrees with immersion. I think the cylinders should have a low level pocket for a thermostat so the call for heat to the heatpump is optimised according the stored temp around the coil. 

 

I've currently got a 200l thermal store knocking around and wondering how I can keep this in parallel to a heatpump unvented, potentially using the thermal store as a heat bank for the heatpump as we don't have a massive slab.

 

But like with all these things, just got to get the calculator out and crunch the figures!

 

On 08/07/2025 at 15:26, Nickfromwales said:

Go to Trevor at cylinders2go (Telford) and mention my username and the forum, as he's given favourable rates to many other members here over the years  

 

+ 1 I've been ordering from here for the past few months for all my installation jobs.

[Nickfromwales]

21 minutes ago, SimonD said:

I've currently got a 200l thermal store knocking around and wondering how I can keep this in parallel to a heatpump unvented, potentially using the thermal store as a heat bank for the heatpump as we don't have a massive slab.

I promise you it'll be a waste of time and effort, as banking for space heating needs to be on a MUCH bigger scale/volume, like 1000L-1500L minimum, to make any sense.

 

[JohnMo]

19 minutes ago, SimonD said:

currently got a 200l thermal store knocking around and wondering how I can keep this in parallel to a heatpump unvented, potentially using the thermal store as a heat bank for the heatpump as we don't have a massive slab.

200L with a useful dT of about 10 is only 2.3kWh. Not really worth the effort.

 

Batch charging with a heat pump sized for the property heat loss, is ok when it's 7 Deg, but will still overrun any cheap periods to fully charge a floor. A heat pump double the correct size will still need 12 hrs running on a design day to charge the floor. It's actually quite economical really as the heat pump hasn't got enough power to flow a high temperature. Did quite a few tests, floor temp would drop to around 20 to 21 during the off period, then when heat pump started it slowly increased flow temp, stating at about 25, after 7 hrs it was at 32 degs. Using a 6kW heat pump against a 100mm screed UFH.

[Dillsue]

52 minutes ago, SimonD said:

 

Yes, absolutely. And this will change a bit now with the r290 heatpumps working 'fairly' well up to 75C, so on an Octupus intelligent go rate, it becomes cheap as chips to batch charge on a reduced cop and then top up a few degrees with immersion. 

Doesn't the intelligent version of Go need Octopus to control your EV charger to get the very low unit rates? How would that work if you're adding your own, presumably uncontrolled, loads such as night time HP running?

[SimonD]

9 minutes ago, Dillsue said:

Doesn't the intelligent version of Go need Octopus to control your EV charger to get the very low unit rates? How would that work if you're adding your own, presumably uncontrolled, loads such as night time HP running?

 

Intelligent go smarlty controls ev charging 24 hours a day but gives you a window of cheap rate leccy every night to use at your own pleasure - the window is actually slightly longer at night than with the dumb version go.

[Nickfromwales]

Yup. A good time to charge the domestic battery, as well as the one on 4 wheels, and the DHW, and batch heat the slab, and run the washing machine and TD.....doubt they give 2 hoots tbf.

[SteamyTea]

On 08/07/2025 at 15:26, Nickfromwales said:

On 07/07/2025 at 12:53, KOS said:

Go to Trevor at cylinders2go (Telford) and mention my username and the forum

Did me a good deal and it was delivered in a couple of days.

[Nickfromwales]

3 minutes ago, SteamyTea said:

Did me a good deal and it was delivered in a couple of days.

That's strange, I specifically remember telling him you lived in Belgium. 

[SteamyTea]

1 hour ago, Nickfromwales said:

That's strange, I specifically remember telling him you lived in Belgium. 

Belgium is closer to Trevor than I am.

[MikeSharp01]

On 07/07/2025 at 13:38, JohnMo said:

You need to do a search on cylinder sizing, it's based on number of bedrooms and it's an MCS guide.

Had a look at that (https://mcscertified.com/wp-content/uploads/2019/08/Domestic_HW_cyl_selection_guide.pdf) just left me confused, as in nowhere near covering all the combinations. Ours is a 2 bedroom place with one bathroom but also has a wet room downstairs - does not have a bath so is not technically a bathroom and if you take the US interpretation of a bathroom the downstairs WC would also be one but has no shower or bath. We need a 6kW ASHP so which cylinder do I choose, 180l? or 210l would a 200l be OK - I guess its up to the MCS crew to sort it out - Telford don't do a 210l they do a 200l and 250l Tempest with 3m² coil. It cannot be beyond the whit to have an upper and lower bound equation with all the factors.

[sharpener]

41 minutes ago, MikeSharp01 said:

Ours is a 2 bedroom place with one bathroom but also has a wet room downstairs

 

IMO 200 l would be plenty for this situation, assuming the wet room does not imply any great medical/disability requirements.

[MikeSharp01]

Just now, sharpener said:

assuming the wet room does not imply any great medical/disability requirements

No, not as yet but we have put it in just in case we need it as we get older.