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Choosing a time for DHW heating?

DavidAlgarve · 2026-04-05T09:56:08Z

I find it difficult to decide on the ideal time for DHW heating both for winter and summer and often decide to try another set time, with no obvious conclusion. I am on a two part tariff with a 47% reduction at night and part of the weekend, so nightime is the usual first choice. However, with night time temperatures being lower than daytime (particularly in Winter) the efficiency of the ASHP is reduced, but to an unquantifiable degree. Then we have SolarPV with a longer time span in the summer so I could take advantage of that, but I only have a 4 kW inverter and my ASHP consumes almost 6kW peak on the DHW heating cycle, which the inverter can't deal with. It is an Ecodan 12kW ASHP and it is on "Eco" DHW setting. I also have a 4kW immersion heater, but with a residual 1-2kW this also takes consumption over the 4kW inverter power Finally we have the Melpump "Summer ECO" options, which can give significant savings, but has the limitation of only one operational period, so I cannot, e.g. set a midday time and an early morning top up. I would welcome other views.

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Air Source Heat Pumps (ASHPs)

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DavidAlgarve

(@davidalgarve)

Reputable Member Member

Joined: 2 years ago

Posts: 242

Topic starter 06/04/2026 9:23 pm

@grahamf They are not interested in small installations and they take advantage of the fact that there can be enough generation to still make it attractive.

There is another factor which I think smells somewhat.

My contract is with EDP, but when Solar PV came along they split off the entire distribution business to a new company, E-REDES and all questions of import/ export have to go through them even though there is no contractual connection.

I have a fight with them currently after a very unlikely bill in February and with a meter check tomorrow.

342sq m "Upside down" house in Algarve. Portugal
Mitsubishi PUHZ-120YUK 12kW ASHP
12 Solar Panels Growatt Inverter
2 x Growatt 7.5kW Batteries
Fronius EV Charger
Kia e- Niro 64kW

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JamesPa

(@jamespa)

Illustrious Member Moderator

Joined: 3 years ago

Posts: 4761

06/04/2026 9:43 pm

Posted by: @davidalgarve

↑

There is another factor which I think smells somewhat.

My contract is with EDP, but when Solar PV came along they split off the entire distribution business to a new company, E-REDES and all questions of import/ export have to go through them even though there is no contractual connection.

I have a fight with them currently after a very unlikely bill in February and with a meter check tomorrow.

Same here, you buy (or sell) electricity from/to one company (an electricity retailer), but its another company (the distribution network operator) that has to agree to solar/battery connection. However the regs oblige the DNO to agree to connections up to a certain amperage and oblige the larger retailers to pay for exported electricity (albeit at a lot less than 12p/kWh).

This seems to be a case where the UK got it right. Better not let certain sectors of the UK media know, it will spoil their fun.

This post was modified 3 weeks ago 2 times by JamesPa

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

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GrahamF

(@grahamf)

Estimable Member Member

Joined: 12 months ago

Posts: 122

20/04/2026 8:29 am

@davidalgarve very frustrating, but I get the impression that you may be making some progress?

Grant Aerona 290 15.5kW, Grant Smart Controller, 2 x 200l cylinders, hot water plate heat exchanger, Single zone open loop system with TRVs for bedrooms & one sunny living room, Weather compensation with set back by room thermostat based load compensation

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DavidAlgarve

(@davidalgarve)

Reputable Member Member

Joined: 2 years ago

Posts: 242

Topic starter 20/04/2026 9:55 am

@grahamf

Posted by: @grahamf

↑

very frustrating, but I get the impression that you may be making some progress?

Not really.

E-REDES did a very rudimentary meter check which proved nothing.

With help from MelPump I have obtained heat pump consumption from MelCloud over the exact period in question, (2,020kWh) and similarly extracted a figure from my heat pump Shelly EM3 (1,900kWh). It seems that I can't expect any better accuracy from either.

However, whichever I use of these takes me nearer to the billed figure of 2,554kWh and MelPump suggest that the greater consumption could be down to: a) colder and longer winter and b) the poor pipework mods to change the buffer tank to a volumiser, which resulted in a reduced available flow rate. (Almost wish I had never started on the buffer tank exercise!)

So I have more or less given up on this battle and dreading the new 2 month bill expected any time soon.

We are now on MelPump Summer settings and more solar being produced, so I am starting to plan doing the pipework improvements. I will have to use Plex again and I intensely dislike their sharp elbows, but there is not much copper availability here now. I am rooting through my parts boxes to find any larger radius screwed brass fittings.

Unfortunately, I have to have a hip replacement operation so don't know how all that will fit in!

342sq m "Upside down" house in Algarve. Portugal
Mitsubishi PUHZ-120YUK 12kW ASHP
12 Solar Panels Growatt Inverter
2 x Growatt 7.5kW Batteries
Fronius EV Charger
Kia e- Niro 64kW

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DavidAlgarve

(@davidalgarve)

Reputable Member Member

Joined: 2 years ago

Posts: 242

Topic starter 20/04/2026 10:23 am

@grahamf On the initial question of when to heat the DHW, I am trying an alternative approach with MelPump switching on at 15:00; 6Deg hysteresis and ending at 17:00, aimed at taking advantage of fully charged batteries.

However, I am not convinced because it still draws 6kW which my 4kW solar cannot provide so I am paying at peak rates for part of the consumption. Also, it is only just giving reasonable water temperature for 7:00am showers

342sq m "Upside down" house in Algarve. Portugal
Mitsubishi PUHZ-120YUK 12kW ASHP
12 Solar Panels Growatt Inverter
2 x Growatt 7.5kW Batteries
Fronius EV Charger
Kia e- Niro 64kW

ReplyQuote

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