What energy rating is your HW cylinder?

[Iceverge]

We're direct electric here for everything. 

 

From my calcs/experience any space heating is better off done by a heat pump unless your annual demand is less than about 1MWh. (Electricity price dependant) @Spacey73you are already there with your A2A units. 

 

DHW is much more marginal given the lower COP achieved once you get to tap water temps. 

 

If you've already got an ASHP for space heating it's the cheapest solution. However the ASHP capital for DHW alone will take decades to pay back. 

 

Then it's much of a muchness between adding PV and a divert to the UVC or an ESHP. It really depends on where you are beginning from. For us I plan to add PV at some stage to the garage as the UVC has years of life left hopefully. If we needed to replace the UVC anyway an ESHP would be best. 

 

The Sunamp argument is separate. They're good for high temp heat sources, if you're really tight on space and need to avoid G3 servicing requirements and need to minimise heat loss requirements. I think an ideal application may be as a replacement to an electrical boiler or conbi in a tight cupboard to self use PV. 

 

For most situations a Combi, or an UVC or thermal store will be cheaper. 

 

However, who really cares, it's your money after all. This perpetual accountancy would have us all taking cold showers and eating spam direct from the tin. 

 

 

 

 

 

 

[SteamyTea]

11 minutes ago, Iceverge said:

eating spam direct from the tin. 

Lentils, more kWh for your one English Pound.

[markocosic]

1 hour ago, Iceverge said:

DHW is much more marginal given the lower COP achieved once you get to tap water temps. 

 

If you've already got an ASHP for space heating it's the cheapest solution. However the ASHP capital for DHW alone will take decades to pay back. 

 

Why do you say that the COP for hot water is lower @Iceverge?

 

You're lifting from incoming at 10-20C up to 45C. (or should be)

 

The COP lifting from 10-20 will be stonking. The COP from 35-45 less so. The energy weighted COP should be well over 3 though - unless there's a stuffup with how it's implemented.

 

This chap knows what he's doing for example. Summertime COP averaging out at 4.7 for ASHPs doing pure hot water preparation. (Initial COP reheating from cold (19 external 32 delivery to tank) was just north of 10.

 

 

 

 

Big coils or loading via plate heat exchanger in "fully stirred reactor" heating mode (heat up the whole tank gradually) not "fully plug flow" heating mode. (what mixergy tanks do and how phase change units like sunamp behave)

 

Discharge should always be plug flow.

 

Delta capital for dedicated all in one heat pump tank vs unvented tank is probably <£1k. Obviously doesn't apply if replacing what you already have.

 

[Iceverge]

https://www.researchgate.net/publication/276499152_The_Efficiency_Analysis_of_the_Exhaust_Air_Heat_Pump_System

 

Reckons the winter COP will be about 3 and summer about 3.4 . 

 

 

 

Heating far superior. COP of 7!!!!

 

The case for heat pumps for DHW isn't as clear as space heating. 

 

Quick googling shows a Dimplex Edel 270l for £2752 + ducting and a Telford 300l UVC for £599 and 3kW of Solar PV for about €3000. Assuming self install as in this case. 

 

I made a brief table for 3 electricity prices and 3 scenarios. UVC only, ESHP, UVC+3kW PV ( all self used on DHW) 

 

 

 

The numbers tell the story. Obviously it really depends on your DHW usage and electricity price. 

 

Edited September 25, 2022 by Iceverge

[joe90]

My ASHP only heats to one temp and I set that to 48, blended down to 25 fir UFH via buffer.

[Iceverge]

1 minute ago, joe90 said:

My ASHP only heats to one temp and I set that to 48, blended down to 25 fir UFH via buffer.

 

Would it be possible to get it to heat to 25 for the UFH and 48 for the DHW separately? I'd imagine you'd have a lower electricity bill? 

[markocosic]

1 hour ago, Iceverge said:

The numbers tell the story. Obviously it really depends on your DHW usage and electricity price. 

 

 

And the share of bought in vs generated electricity.

 

I think you're generous with PV on two counts

 

- Probably generates less than 1000 kWh per kW(p) per year

 

- Seasonality is all wrong so you'll waste loads in summer and not have enough in winter (when the water also needs more heating - try 5-20C variation)

 

Try a seasonal calc using pvgis and variable temperature and I think you'll conclude that you need a lot more than 3kW(p) to supply that kind of demand.

 

 

 

 

[Iceverge]

 

 

I've re done my table with a 4.6kWh system (real prices from easyPV) and upped the amount of bought in electricity.  PVGIS shows 4141kWh of production. With a large DHW tank it should be possible to use 3/4 of that I reckon. (Does anyone actually have any experience?) 

 

The ESHP wins out here although it's close. However, I'm unconvinced it will last as long as the PV which guarantees the panels to 25 years. If you had to replace the ESHP at any stage it'd be more expensive. Also the PV can be used to displace lots of peak electricity elsewhere in the house too which isn't in the above calcs.

 

The simplicity of it and the lack of any moving parts get my juices going too!!! 

 

 

 

 

 

[joe90]

10 hours ago, Iceverge said:

 

Would it be possible to get it to heat to 25 for the UFH and 48 for the DHW separately? I'd imagine you'd have a lower electricity bill? 

No, it only has one temp, our Jeremy had the same unit and confirmed it only worked that way 🤷‍♂️

[Spacey73]

If I only had a terraced roof as an option for PV or a small garden then yes, heat pump for HW supply would make sense efficiency and all that (high upfront cost espy with installation).

 

But I’m in a unique situation where I can add more solar than I could possibly use all year round. I’m not needing to worry about electricity availability, panels are cheap as are 48v mppt inverters to charge battery.

 

Roughly 12kw solar at the moment, easy to add more and about £2k per 4kw module (including fixings). I’m sure a mini nuclear realtor would be even more efficient energy transfer for HW but at some point cost, servicing and complexity (moving parts mostly) are just not worth the efficiency gains.

 

Plopping in a dedicated south facing 4Kw array (already have 2 x 500v open voltage mppt spare) just for HW does make sense to me. 
 

so many out buildings facing the right way at a good angle seems a waste not to use it, ground mount ‘fence’ system doing really well performance wise.

 

I already have 2 heatpumps, really don’t want a 3rd

Edited September 26, 2022 by Spacey73

[Nickfromwales]

Just remember the SA PCM58 ‘melts’ at 58oC, but SA recommended a much higher input temp to get a decent recovery rate eg kW absorbed vs time to satisfy the stat with the SA fukllt recharged. 

A low temp heat pump will give 65oC at full wallop, with poor CoP, and still recharge the SA quite slowly. In winter you risk freezing of the HP when running high temps for extended periods.

A true high temp HP will be a split, with a noisy, obtrusive indoor unit chattering away ( fine if you can lose it somewhere where it isn’t a nuisance ) but having seen installs with these fitted I am not a fan of them vs a monoblock. 

A monoblock will heat an UVC perfectly well with a good CoP, but that will not be the same with a SA. These are hugely inefficient ( overall ) when being heated by an external heat source. SA are best being heated via the in-built immersion heater.

 

*Then comes the caveat that if the immersion heater fails, the ENTIRE SA has to be removed ( at your expense ) shipped to SA ( at your expense ) repaired at their leisure ( whilst you have ZERO DHW at home ), and then shipped back and re-installed by you ( at your expense ).

Won’t be a saving on the comparative energy efficiency then, and will take about another 2 decades to give you a return on the already huge period for RoI. UVC from Telford ( and some others ) = lifetime. *And that’s just if the immersion fails.   

[Spacey73]

On 25/09/2022 at 16:31, Nickfromwales said:

Do you have planning permission for the >9m2 ground mounted array?

Do you have a hybrid inverter / DNO permission to add to the PV system?

Run a large Solar Farm as a business. House power is separate and off grid DIY without export. A mere daisy in a large field of daisies 

Edited October 24, 2022 by Spacey73

[Nickfromwales]

On 25/10/2022 at 01:43, Spacey73 said:

Run a large Solar Farm as a business. House power is separate and off grid DIY without export. A mere daisy in a large field of daisies 

Cool. Can I run an extension lead from there back to my house? You’ll barely notice it 

[SteamyTea]

16 minutes ago, Nickfromwales said:

Cool. Can I run an extension lead from there back to my house? You’ll barely notice it 

[Nickfromwales]

1 hour ago, SteamyTea said:

Jesus. 
 

At least he will be meeting Jesus soon. 
 

 

 

 

 

 

Disclaimer. Jesus does not actually exist. 

[SteamyTea]

There is a song called '30 thousand pounds of bananas' 

Has a line in it.

"That's when he lost his head, not to mention an arm and a leg' 

 

There is a really important message in the song.

 

 

 

 

 

[joe90]

That’s an idea, overhead supply and a couple of jump leads 😵‍💫

[SteamyTea]

14 minutes ago, joe90 said:

That’s an idea, overhead supply and a couple of jump leads 😵‍💫

And some people on here pay to get the pole taken away and bury the cable.

As much as I dislike the pole and cables out the front of my house, I could, either a couple of broomsticks, reach them.

May be useful oneday.

[Nickfromwales]

1 hour ago, SteamyTea said:

And some people on here pay to get the pole taken away and bury the cable.

As much as I dislike the pole and cables out the front of my house, I could, either a couple of broomsticks, reach them.

May be useful oneday.

I have many “doomsday” plans for staying warm and well lit. 👀.