The Boy That Cried PV Extension

[Moxi]

The lower COP values of these units will be constrained by the air ducting, when compared with having a split system.

Yes it always surprises me how much air these units require, 8" in and out was quoted for a mono-block A2A unit I looked at last year and 6.4" in and out for the HPHWC. Those are big holes for any house but for the older stone properties theres plenty of scope for things to go wrong!

Moxi

[Stinsy]

I'm sure I've said it before, but I'll repeat: I don't see the point of these DHW cylinders with integrated HP. They're complicated and expensive things with lots of moving parts that will inevitably fail. They have perilously little benefit over a simple resistive cylinder and ToU tariff.

Here is my gas bill for January:



I paid £15.17 for 111.6kWh of gas. That works out as 13.6p/kWh. A resistive cylinder would therefore save me a fortune, particularly when you consider that my 30-year-old combi is probably woefully inefficient when heating DHW. Even assuming an 80% efficiency then that's be 89.3kWh which at 7p/kWh would be £6.25. So a resistive cylinder would save me c. £107 a year.

If on the other hand I had a HP cylinder with a CoP of 2.5 then I'd only need 35.7kWh of electric to create the 89.3kWh of heat. That'd save me a further £3.75 a month.

How much are those HP cylinders again?

[nowty]

The lower COP values of these units will be constrained by the air ducting, when compared with having a split system.

Yes it always surprises me how much air these units require, 8" in and out was quoted for a mono-block A2A unit I looked at last year and 6.4" in and out for the HPHWC. Those are big holes for any house but for the older stone properties theres plenty of scope for things to go wrong!

Moxi

And for completeness I should have also added to my sentence with "or a monobloc situated outside".

[Mart]

Hi Mart,

The noise of that unit - albeit on boost and just starting up - was a bit of an eye opener for me, the manufacturers literature for many of the units of this type that I had looked at previously seemed to be much quieter but maybe thats "test" condition figures and the best of a range ?

The CoP range he had in his tables looked about right but again it wasnt unusual to see HPHWC with a CoP range running from 2.6 to 3.5 on the data sheets so again you appear to pay an efficiency penalty for the HPHWC or maybe that's because its measured primarily heating DHW ?

Moxi

I think I could insulate the noise quite well in the cupboard, especially as it's the far side of the room away from our bedroom. But it would be on a party wall with the other half of the semi-detached, and if anything, that's concerning me more. New neighbours are lovely, and tbf sound doesn't carry well through the party wall, but a nightime humming could be horrendous for whoever is in that room. [Edit - The room on the other side of the wall.]


Showing my ignorance again, but are there HW tanks (resistive) with a heat exchanger*, that could help with a small top up of the upstairs rads? Given we probably only need ~2kWh of DHW per day, then a 2 or 3kW element would provide ~12 to 18kWh of heat storage, but I assume the tank doesn't hold that much (I really know very little). But tapping that as a storage heater, even if just 4 or 5 kWh per day, could save me around a pound in FiT subsidies by avoiding using the battery, and subsequent losses.

Just spitballing really, and trying to think ahead. My sparkie friend neighbour is currently on holiday, and I'm looking after their cats and hedgehog again. But with them this time is his plumber friend who lives round the corner. So he said he would ask about a HW tank for me.

Wifey already eyeing up leccy ovens, so we can ditch gas. But that's another problem, as it will need a 30A spur. First World problems!

*Doh! Just had a thought, am I right to assume that a HW tank heated from a boiler, does this via a coil / heat exchanger, so therefore, yes, tanks often come with a heat exchanger?

[Countrypaul]

Not all electric ovens require such a large supply, some only equire a 13A supply.

[Moxi]

Stinsey those hpHWC are from £1500 to £2650 and as you say they are marginal choices at best ( being really polite)

Mart you can get a twin coil heat stores that would do the hot water and the radiators you would need to work out the DHW requirements for a day and the radiators output to size the store and then determine its charge time using the 3kW immersion element only 100% efficient but only faults being immersion element every now and again and the pump for the radiators might fail but easy diy and cheap as chips relatively. I think you could also use an indirect cylinder too where DHW is maintained in the main body of the tank and the indirect heating coil is used to run the rads - I m expecting one of our resident plumbing experts to correct me on that one though as there could be regulations to stop that, you may also find that the immersion isn’t in an optimal position on these types of cylinder making them less attractive as options.

But if you consider that route then why not leave the radiators and stick with the oil rads and then get under sink hot water systems?

Moxi

[Stinsy]

Stinsey those hpHWC are from £1500 to £2650 and as you say they are marginal choices at best ( being really polite)

Mart you can get a twin coil heat stores that would do the hot water and the radiators you would need to work out the DHW requirements for a day and the radiators output to size the store and then determine its charge time using the 3kW immersion element only 100% efficient but only faults being immersion element every now and again and the pump for the radiators might fail but easy diy and cheap as chips relatively. I think you could also use an indirect cylinder too where DHW is maintained in the main body of the tank and the indirect heating coil is used to run the rads - I m expecting one of our resident plumbing experts to correct me on that one though as there could be regulations to stop that, you may also find that the immersion isn’t in an optimal position on these types of cylinder making them less attractive as options.

But if you consider that route then why not leave the radiators and stick with the oil rads and then get under sink hot water systems?

Moxi

+1 for all this!

However swapping an oil-filled for a SH should be a consideration.

ToU and resistive beats the CoP of a HP in most situations once you consider the cost of installation.

[Mart]

Not all electric ovens require such a large supply, some only equire a 13A supply.

Wifey want s a double oven, to replace the double gas ...... but it may be negotiable, especially if wiring is a problem. So many thanks.

[Mart]

Thanks Moxi and Stinsy. I won't give up on the HPHW tank yet, but it looks close to dead in the water. One final thought is to go up, into the loft, which is a converted room. The tank could then sit up there, with the cold exhaust cooling the PV panels ....... yeah, I may be going a bit mad now!

Back to reality, and resistive heating, I do like the idea of being able to use a HW tank as a heat battery to time shift some energy. That will help the batts, on the worst days, but obviously still lots to ponder for the best combination. And I suppose with resistive heating I can go hotter (without a COP loss), so store a bit more energy.

Didn't expect to be considering a complete shift away from gas this year. Funny how things work out ....... sorry, may work out, let's not get ahead of myself.