Cylinder suitable for future ASHP

[JohnMo]

48 minutes ago, MikeSharp01 said:

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

Two shower rooms, one (downstairs) could be argued is for a tick in the box for accessibility, so really only one shower room, so would go with 2 bedroom and ensuite. So 180L, but would install a 200L or 210L slimline.

 

Is this a really big 2 bed house? Can't believe you need 6kW ASHP.

[MikeSharp01]

2 minutes ago, JohnMo said:

 

Is this a really big 2 bed house? Can't believe you need 6kW ASHP

We don't, we need about 1.2kW at -2°C OAT. But 6kW is the smallest I can find and the 6kW will do the DHW. 

[Nickfromwales]

17 minutes ago, MikeSharp01 said:

We don't, we need about 1.2kW at -2°C OAT. But 6kW is the smallest I can find and the 6kW will do the DHW. 

Don't go any smaller, or the DHW will take forever to heat up. I'd not go smaller than 250L, to allow you to strategize when you reheat.

[MikeSharp01]

48 minutes ago, Nickfromwales said:

I'd not go smaller than 250L, to allow you to strategize when you reheat.

We can't fit a 250l one in, back in 2016 the architect sized the utility for a combi boiler so no tank and now we have lost the boiler and gained a tank, we have literally mm to spare 550mm dia and 1500mm high is all we can fit into the space - hence me trying for a 200l Tempest - I will know in the next couple of weeks if the MSC umbrella company will accept 200l. I guess I can work out a heating strategy that will suit us. We shower first thing and can use an overnight smart tariff to do that with the immersion heater or have it heated from the PV during the day and pay the overnight loss in the morning or run the ASHP to do it when not charging the slab or, on the odd occasion, pay for full price leccy into the immersion. 

[Nickfromwales]

4 minutes ago, MikeSharp01 said:

We can't fit a 250l one in, back in 2016 the architect sized the utility for a combi boiler so no tank and now we have lost the boiler and gained a tank, we have literally mm to spare 550mm dia and 1500mm high is all we can fit into the space - hence me trying for a 200l Tempest - I will know in the next couple of weeks if the MSC umbrella company will accept 200l. I guess I can work out a heating strategy that will suit us. We shower first thing and can use an overnight smart tariff to do that with the immersion heater or have it heated from the PV during the day and pay the overnight loss in the morning or run the ASHP to do it when not charging the slab or, on the odd occasion, pay for full price leccy into the immersion. 

Ah, yes, sorry, I do recall.

 

I'll try and be more helpful....  

[JohnMo]

There are two of us in the house we have 210L and even having guest stay over there is no issues.  We just heat to 50, degs, sometimes once mostly twice a day if thermostat says yes. Never run out of hot water. 6kW will take about 45 mins to an hour to heat 210L.

 

6kW and small heat demand, thick screed I assume and batch charge floor. I would get a cheap tariff and charge cylinder with immersion. Or you could run out of time in cheap tariff time. 

 

But at 1.2kW coldest day demand, is a heat pump worth the effort, why not a Willis heater or two. 3kW x2  for 4.8 hrs, and the slab is heated. Then put a small cylinder in the space you have and heat to 70 degs.

 

2x Willis heaters £90 add a third for a hot swop £135. 140L direct cylinder £600. Add a 0.1 Deg hysterisis thermostat to control UFH charge.

[Gus Potter]

Quick pratical question. 

 

If you put your hot water cylinder in a modest cupboard with slatted shelves over the top to keep your linen dry and to get that "crispy warm towel effect" does it help prevent heat loss from the cylinder enough to make the excercise worthwhile?  

 

If you fancy a go at a fag packet heat loss calculation for a bit of fun!

 

The cylinder and cupboard insulation coupled with the air gap effect in the cupboard changes the numbers as they work in series. Easier to do a fag packet calc as there is little cold bridging that you get with say repeating timbers in a stud wall. 

 

Yes I know you will get convection effects occurring in the cupboard, and so on, but to give you a flavour and to anticipate this question here is a copy of the table from BS.. 6946. The cylinder insulation, air gap, and enclosure all work in series.  The question is how often do you open the cupboard, at what time and how many towels do you have? 

 

Seriously though. If you are tight for cylinder space then it's worth an ecclectic look. If you look at the table ( developed really for double glazing / secondary glazing) you can see that the horizontal resistance become constant once you exceed a gap of ~25mm. 

 

Or... if you are designer and have eh.. designed below the margin.. this might be a way of getting yourself out of trouble! When drowning you will grasp any straw!

 

 

 

 

 

 

 

Edited Thursday at 23:42 by Gus Potter

[MikeSharp01]

9 hours ago, JohnMo said:

But at 1.2kW coldest day demand, is a heat pump worth the effort, why not a Willis heater or two. 3kW x2  for 4.8 hrs, and the slab is heated.

Yes it is small and the Willis route has been examined and was my choice - following @TerryE's brilliant work at the outset BUT the SAP / EPC plays against you if you do because direct electric heating is frowned upon. We are set about with downsides so a smaller cylinder and an ASHP running at its lowest setting will be our way forward I think.

[JohnMo]

53 minutes ago, MikeSharp01 said:

ASHP

Do your research on modulation on any model you choose. Our 6kW gets down to about 4 kW only. Ours it turns out, is really just a derated larger unit. Looking at it's outside temp kW output is a clue. Ours puts out an almost flat 6kW at 30 degs OAT and -10.  A suitable sized compressor would loose kW output as it gets colder outside.

[Dave Jones]

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

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.

 

there is also the option of plate heat exchanger whilst more expensive gives a better COP.

[JohnMo]

8 minutes ago, Dave Jones said:

 

there is also the option of plate heat exchanger whilst more expensive gives a better COP.

But you need to run an additional pump as well. Not sure it's that much better CoP in reality. This morning at 16 degs outside I got a cop of 3.68 doing DHW heating and yesterday afternoon at 22 degs outside, I got 4.11. Ideal 210L slimline HP cylinder, which has a 3m² coil.

[Dave Jones]

1 minute ago, JohnMo said:

But you need to run an additional pump as well. Not sure it's that much better CoP in reality. This morning at 16 degs outside I got a cop of 3.68 doing DHW heating and yesterday afternoon at 22 degs outside, I got 4.11. Ideal 210L slimline HP cylinder, which has a 3m² coil.

 

not so, ,I have plate and the ASHP is the only pump in the loop. Agree the better COP is around 10% but it also gives the option of multiple tanks if capacity is needed and they are cheapish bog standard cylinders rather than coiled ones. they can also be in physically different locations in the house to suit space.

[JohnMo]

20 minutes ago, Dave Jones said:

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

That's interesting, how does the cylinder water get heated? So ASHP on one side of PHE that's using the circulation pump from ASHP, but happens on the cylinder side?

[Dave Jones]

the plate is on the ASHP side of the 3 port.

[JohnMo]

48 minutes ago, Dave Jones said:

the plate is on the ASHP side of the 3 port.

Understand that side, but I am interested in the cylinder side how water is moved about. Do you have link to make of cylinder?

[Dillsue]

3 hours ago, Dave Jones said:

the plate is on the ASHP side of the 3 port.

So PHEX I'd heating the DHW whenever the HP is running regardless of which way the 3 port is routing flow?? Intrigued to know how flow is achieved on the secondary side of the PHEX feeding the cylinder- gravity??

 

The coil we have available in our cylinder is undersized for our HP so looking at options to get HP energy into the cylinder without replacing it.

[JohnMo]

19 minutes ago, Dillsue said:

undersized for our HP so looking at options to get HP energy into the cylinder without replacing it

Something like this h the bigger the plate pack the better.

https://beetbg.com/products/copy-of-dhw-plate-loading-30xplate-kit?utm_source=copyToPasteBoard&utm_medium=product-links&utm_content=web

 

[JohnMo]

A video of a similar setup here

https://youtu.be/34slE6b6V_4?si=E3g0r9JgsCUmVTdf

[Dillsue]

40 minutes ago, JohnMo said:

Something like this h the bigger the plate pack the better.

https://beetbg.com/products/copy-of-dhw-plate-loading-30xplate-kit?utm_source=copyToPasteBoard&utm_medium=product-links&utm_content=web

 

Thanks for that. Hopefully Dave can share how he's "powering" the cylinder side of his PHEX without a pump.

[sharpener]

1 hour ago, Dillsue said:

The coil we have available in our cylinder is undersized for our HP so looking at options to get HP energy into the cylinder without replacing it.

 

You can improve the heat transfer on the outer surface of the coil by using a bronze pump to circulate the water in the tank while it is being heated. I did some experiments which showed a 2x improvement but in practice I don't get as much as that. You also need an NRV or you will have a parasitic path through the pump when it is off. Vaillant tech accepted this as part of the scheme to re-use the existing tank with a 12kW HP. Incidentally their smallest is 3.5kW - but that is just the 5kW with restrictive software.

 

I don't see how you get a better CoP with a PHE, there has to be a temperature drop across it (typical design figure is 5C) so the HP runs hotter for a given DHW temp therefore the CoP will be worse.

[TerryE]

9 hours ago, MikeSharp01 said:

Yes it is small and the Willis route has been examined and was my choice - following @TerryE's brilliant work at the outset BUT the SAP / EPC plays against you if you do because direct electric heating is frowned upon. We are set about with downsides so a smaller cylinder and an ASHP running at its lowest setting will be our way forward I think.

 

I understand the SAP / EPC issue, but in our case this also very flawed. We now use a double immersion heated 250 ltr OSO and our W/M and DW are in-appliance heated cold-fill and we almost always use ECO wash cycles time-shifted for cheapest Octopus Agile tariff rates. The OSO has under 1 kWh / day parasitic heat-loss and we (3 adults) use maybe 50 ltr at 50°C from our hot taps except on the occasional bath day. And again our OSO is heated at the cheapest Agile slots.  The bottom line is that our DHW typically costs less than £1 / day, so it is really hard to come up with a cost benefit justification for the complexity of using alternative means of heating the HW.  We don't currently have a ASHP but if we did I'd install a 5kW UHF only unit at a max O/P temp of 30°C direct into slab UFH at an SCoP of around 5.5.  

[Dillsue]

3 hours ago, sharpener said:

I don't see how you get a better CoP with a PHE, there has to be a temperature drop across it (typical design figure is 5C) so the HP runs hotter for a given DHW temp therefore the CoP will be worse.

Isnt that the same as heating an indirect cylinder via an internal coil?

[MikeSharp01]

11 hours ago, JohnMo said:

Do your research on modulation on any model you choose. Our 6kW gets down to about 4 kW only.

The one we are targeting says it has a range from 1.85-6kW  ( https://coolenergyshop.com/collections/pro-inverter-air-source-heat-pump/products/pro-inverter-air-source-heat-pump-ce-ih6plus  ) which seems like a good range, has all the integrations we need / want and can be sourced via the company's umbrella scheme. 

[JohnMo]

2 hours ago, MikeSharp01 said:

The one we are targeting says it has a range from 1.85-6kW  ( https://coolenergyshop.com/collections/pro-inverter-air-source-heat-pump/products/pro-inverter-air-source-heat-pump-ce-ih6plus  ) which seems like a good range, has all the integrations we need / want and can be sourced via the company's umbrella scheme. 

Have looked at them, seems a good range of modulation, very good price. Is it worth bothering with MCS rubbish to save a couple of grand. ASHP for the heating, then a small direct cylinder for your space available? Optimise the UFH to make the most of the heat pump?

[MikeSharp01]

9 hours ago, JohnMo said:

Is it worth bothering with MCS rubbish to save a couple of grand. ASHP for the heating, then a small direct cylinder for your space available?

We think so as long as we can meet the MCS requirements e.g. the size of the tank. As such we can always not bother with using ASHP to heat the water provided we meet those requirements to get the system across the line in the first place and then extend the capabilities of the tank through auxiliary immersion use to boost the water temp when we have guests. we will be cooling the slab and the MVHR feeds to the bedrooms. I notice today that there is talk of extending the grants to cover air conditioning but that will come too late for us. (https://inews.co.uk/news/politics/air-con-for-your-home-could-become-part-of-7-5k-heat-pump-grant-3799614)