The Boy That Cried PV Extension

[Mart]

Regarding coils, for a resistive tank, I was thinking 1kW at the top, and a 3kW at the bottom, to heat from cheap rate over night.

Happy to heat the tank during the day. As explained, if I'm already using the oil rads to heat from leccy, then being able to distribute that heat a bit more evenly via the rads (upstairs CH circuit?) seems to be the same energy to me. Plus the water tank being a large store of energy from cheap rate.

Bear in mind that we use about 100kWh of gas per month for DHW and oven. So that suggests to me less than 3kWh (per day) for DHW especially with boiler inefficiencies, as even in the winter now, the boiler is operating from cold. Wifey and I take quick showers, probably less than 5mins, and no bath, the room is too small.

So that HW tank (resistive or HP) could store 10kWh+ (I'm not sure, just guessing) of energy from cheap rate, so more than we need, but even if we have to top up during the day for heat, that's what we are doing already.

Hope that makes sense. Just trying to find out if the product description of small central heating system, is what I have upstairs. From there that would seem to answer/solve the upstairs issue, no need to get an immersion heater for the bathroom towel rad. I do share your concerns about a HPHW tank lasting, but that would help even more with daytime top up, as the COP would lower demand on the batteries.

[Mart]

Quick bump, just wondering if anyone knows if the description I mentioned of a HPHW tank also running a small central heating circuit is 'what I've got' with my upstairs 4 rad system? All small single panel rads.

I was going to post this on the recent UFH thread, where folk have mentioned the boiler upgrade scheme, but didn't want to mess that thread up ..... but, if I go with the HPHW tank, then I will be removing the gas boiler, so would I possibly qualify for the grant?

The bulk of the heavy lifting is already being done by the the 2 A2A units downstairs, but a HW tank will mean I can remove the combi boiler, so a HPHW tank, especially if providing a bit of heat as well as hot water, seems to fit the description for the grant, but I really can't tell. Any thoughts, don't worry if they are guesses, I will have a chat with a plumber soon, and can wait till the end of Autumn if necessary, no rush.

[Moxi]

I looked online at it looks like such a heat pump cylinder might be eligible subject to achieving the required minimum efficiency- I would suggest a discussion and agreement in principle with the bus grant provider before going forward though as it’s less than explicitly stated in the bus text.

Then there’s the question of if the technology is sufficiently refined to be worth the risk. Daikin seem to make it work but don’t seem to want to supply their tank to the uk ? Others do supply but don’t seem to have much interest from what I can discern.

Then there’s the thing that’s niggling me - ASHP run best low and slow, so a tiny compressor on top of the tank run it hard to put heat into the cylinder doesn’t seem to meet the ethos of low and slow ? Would a 4kW external system to give hot water and occasional upstairs radiator heat not be a better cost effective solution ?

That’s where my head is at the moment- I’m about a month and a half away from a full years gas boiler operational figures and it’s very clear our principle gas consumption is for DHW rather than heating. There doesnt seem to be much info on the www for homes needing that requirement presently.

Moxi

[Mart]

Thanks Moxi. There's also the noise of the fan, perhaps, to think about. I'm wondering how low a temp I could aim for, by going for such a large tank as 260lt, given our relatively low DHW needs. Which may fit with your lower use issue, but still probably a relatively high use for the heating months.

I think the oil rads are adding around 5kWh during the day, so the tank may be able to displace that from night rate storage, which would maybe save me about £1 a day, given that the oil rads will wind the TGM backwards, losing me a FiT unit for every 5kWh or so that goes through the DC side battery due to losses.

Hadn't really thought about the BUS grant, till the mention on the UFH thread. The main bulk of the displacement of the boiler will come from the A2A units, but they have already been done, and are currently excluded from the BUS, so it's long shot, and not essential either way (HP v's normal HW tank).

[Moxi]

Mart,

Have you seen this video before ?

I have only just come across it and thought it might be helpful for your ponderings - I know the early iHP's didnt do CH as well as DHW but I think mixergy (others do) now do a dual use unit ?. It gives an idea of operating noise and installation envelope plus running costs for DHW.

Having seen it through I cant help thinking that you would be almost better off either having a more standard ASHP install or just go the Resistive heating route for the DHW as Stinsey intimated previously, of course if you have a well insulated (acoustic) cupboard then it might still be of interest ?

Moxi

[Mart]

Thanks Moxi, it doesn't sound promising regarding the noise. The room is used as a gym, converted just in time for me to get long Covid and be too tired to use much, but even so, I'm pondering the noise issue. If I went resistive instead, then I might have to run new cables, but if that's not connected to rads, then the garage (storage room/spare room) is probably closer to the bathroom and kitchen DHW needs. And the CU in the garage has a spare slot, that was going to be for AC side batts.

Be nice to have a thermal store, rather than HW tank in the spare room, that could heat water and upstairs rads, all powered from a small outside HP unit. But I don't know where I could fit another ....... and with 2 already I'm pushing the PP issue a bit too far. Hope the revisions to the home heating rules in Spring bring some positive news for A2A units, but no idea if these can be used to heat a water tank. More revising needed.

Decisions, decisions.

[Mart]

And a small addendum, I'm getting some success adding more heat upstairs by removing the door closures, switching off the oil rads, and dialling up the the 2 A2A units. But the downside, is the rooms downstairs are a tad warm, the living room is now ~20.5C, up from 19.5C/20.0C before.

But it's all fun and games, just learning, and January was colder than average (for Cardiff), so I think a good test for future thoughts and plans.

Met Office Jan 2025 - Mean temperature 1991 - 2020 anomaly

[Looks like the link defaults to mean max temp, so you need to select meant temp, and tick the 3rd choice for 1991-2020 anomaly period.]

[Moxi]

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

[Moxi]

https://www.modernheat.co.uk/product/24 ... NRe4_P4Us8#

Typical of the types of HPHWC I was looking in to a while back - the more I looked at them though the more it began to feel like a compromise on many of the levels that really matter.

This tank does have a double outlet for heating and DHW but the efficiency looks decidedly modest and the sound pressure levels are "intrusive"

Moxi

[nowty]

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