Cylinder suitable for future ASHP

[HughF]

OSO (Vaillant) slimline for me, it was £500 on eBay 😂

 

1m2 coil, 9kW pump - reheat times are perfectly acceptable and the coil can take all the heat pump can throw at it.

[SteamyTea]

13 minutes ago, HughF said:

reheat times are perfectly acceptable and the coil can take all the heat pump can throw at it.

That may be more to do with the temperature differences, heating is not linear, it starts with rapid energy transfer, then the rate if transfer reduces as the stored water temperature rises.

[HughF]

Just now, SteamyTea said:

That may be more to do with the temperature differences, heating is not linear, it starts with rapid energy transfer, then the rate if transfer reduces as the stored water temperature rises.

Coil is right at the bottom, I’ve yet to see it ramp either the pump or the compressor down during a dhw charge.

[SteamyTea]

3 minutes ago, HughF said:

Coil is right at the bottom

Generally the best place for them.

 

It would be an interesting project looking at reheat time with different deltaTs, and how it affects the CoP of the systems.

[MikeSharp01]

2 minutes ago, SteamyTea said:

It would be an interesting project looking at reheat time with different deltaTs, and how it affects the CoP of the systems.

Someone like @JohnMo must have done that somewhere on here?

[JohnMo]

27 minutes ago, MikeSharp01 said:

Someone like @JohnMo must have done that somewhere on here?

Heat geek have and have a video out there somewhere. To get best CoP you need the biggest coil you can get in the cylinder. Then your flow rate through coil has to be slow, the idea is the return temperature needs to be as close to the bottom of the cylinder temperature as possible. You then prolong the heat pump increasing flow temperature for as long as possible. Heating time extends but so does CoP. Talking high 4s early 5s for DHW CoP. 

 

My ASHP is factory set for DHW and there are no settings to change. So cannot replicate the above.

[SteamyTea]

54 minutes ago, MikeSharp01 said:

Someone like @JohnMo must have done that somewhere on here?

In electrical engineering, to get maximum power transfer, you use Jacobi's Law.

To muddy the waters, there is also maximum power efficiency.

 

PV inverts have MPPT built in, I wonder how difficult, or even if worthwhile, to have a similar system in HP.

There are a lot more variables, OAT, IAT, WT, run times, input costs etc, but it would be a good add on to basic weather compensation.

[sharpener]

11 hours ago, MikeSharp01 said:

Someone like @JohnMo must have done that somewhere on here?

 

@JamesPa has a spreadsheet which I used to good effect when I was doing my system design. But it was a bit optimistic, never managed to quite work out why.

[Marvin]

One of the best savings is achieved by insulating the hot water tank within an inch of its life not just the 2 inches of foam around it.

 

[Nickfromwales]

1 hour ago, Marvin said:

One of the best savings is achieved by insulating the hot water tank within an inch of its life not just the 2 inches of foam around it.

 

Most modern UVC's have very low standing losses, and we are advocates here of a bit of oversizing so the stored temp can be around 50-55⁰C, further reducing losses.

 

Good advice if it's a thermal store for DHW though.

[Dave Jones]

Glad to see interest in the plate heat exchanger! There is a circulation pump 'behind' the plate to stir it all up. schematic of our system below.

 

Also worth noting that non-coiled cylinders are upto 30% more efficient than coiled ones. It all adds up right.

 

[Dave Jones]

excuse the clutter can just see it at the bottom

 

[Dave Jones]

9 hours ago, Nickfromwales said:

Most modern UVC's have very low standing losses, and we are advocates here of a bit of oversizing so the stored temp can be around 50-55⁰C, further reducing losses.

 

Good advice if it's a thermal store for DHW though.

 

52C max from the heat pump worked out best CoP for me. Free overnight electric and excess solar drives it to 80C via immersion.

[Dillsue]

On 11/07/2025 at 08:43, Dave Jones said:

 

not so, ,I have plate and the ASHP is the only pump in the loop. 

There's a circulation pump for the secondary/cylinder side of your PHEX in the schematic you've not long posted????

[Nickfromwales]

26 minutes ago, Dillsue said:

There's a circulation pump for the secondary/cylinder side of your PHEX in the schematic you've not long posted????

Bronze too by the look of it. 

[sharpener]

1 hour ago, Nickfromwales said:

Bronze too by the look of it. 

 

So someone knew what they were doing...

[TerryE]

On 12/07/2025 at 06:57, MikeSharp01 said:

I notice today that there is talk of extending the grants to cover air conditioning but that will come too late for us.

 

I remember Jeremy talking about how nearly all ASHPs are actually Aircon enabled internally but this function is disabled in the UK to qualify for MCS grants. IIRC, in his case he had to flip a DIP switch to re-enable. In other cases, there's a buried set up option to re-enable.

[JamesPa]

1 hour ago, TerryE said:

I remember Jeremy talking about how nearly all ASHPs are actually Aircon enabled internally but this function is disabled in the UK to qualify for MCS grants. 

 

It is not, and the the best of my knowledge never has been, a requirement of either the BUS grant or planning permitted development that the heat pump be incapable of cooling

 

Until recently it was a requirement under permitted development rules that it should not be used for cooling (which is not the same as incapable of cooling).  However that changed (in England) at the end of May.  Now the only permitted development rule related to cooling use is that it it must be used solely for cooling.

 

 

 

 

Edited Tuesday at 12:54 by JamesPa

[JohnMo]

1 hour ago, TerryE said:

remember Jeremy talking about how nearly all ASHPs are actually Aircon enabled internally but this function is disabled in the UK to qualify for MCS grants

All heat pumps are cooling (not aircon) enabled in the software, but some manufacturers choose not to enable or make difficult to enable for the end user. Vaillant for example requires a hardware but added.

 

The cooling thing predates bus scheme, the scheme prior to, or even prior to that required a heat pump not to be enabled to allow cooling, so was heat only devise. Although it uses a cooling function during defrost.

[JamesPa]

2 hours ago, JohnMo said:

The cooling thing predates bus scheme, the scheme prior to, or even prior to that required a heat pump not to be enabled to allow cooling, so was heat only devise. Although it uses a cooling function during defrost.

OK so its a legacy thing then.  Its a pity the misinformation is still around.

[SimonD]

5 hours ago, JamesPa said:

Now the only permitted development rule related to cooling use is that it it must be used solely for cooling.

Class G 3 as of May 2025:

(aa)the air source heat pump is not solely used for the purpose of cooling;

[JamesPa]

1 hour ago, SimonD said:

Class G 3 as of May 2025:

(aa)the air source heat pump is not solely used for the purpose of cooling;

Sorry, I meant to write not be used solely for cooling, my finger trouble!  Hopefully I didn't cause too much confusion.

 

The point is that it is now legitimate for a heat pump installed in England under PD rules to be used for cooling, provided it is not used solely for cooling.  Previously it was legitimate under PD rules for it to be capable of cooling albeit not used for cooling.

Edited Tuesday at 19:54 by JamesPa

[SimonD]

16 hours ago, JamesPa said:

my finger trouble!

 

Don't worry, you're not the only one. My fingers don't always have a great connection with the brain and sometimes my brain doesn't work to begin with 😁

16 hours ago, JamesPa said:

The point is that it is now legitimate for a heat pump installed in England under PD rules to be used for cooling

 

Finally....... took them long enough to catch up, didn't it.