It will work, yes but it might be expensive and you will almost certainly end up using peak rate electricity to heat your home in cold weather.

Electricity is 100% efficient at heating, off peak can be 1/4 the price of peak rate electricity but there will be some losses from the store 30 to 50%

A heat pump can be 350% efficient with no losses. What do you have against heat pumps?

Posted By: georgiegirlAlternatives that don't involve heat pumps?

Just to throw something else into the mix, we are all electric and passivhaus like djh above. However we are using a thermal battery to absorb our excess PV. Specifically SunAmp, though I think there may now be others available.

This uses a phase-change salt material to store the heat which is more 'energy-dense' than water, so sits in a smaller footprint than the equivalent water tank would need to be.

Generally it has been a mixed-bag for us. Being early adopters we bought direct from the manufacturer and thus have suffered both from a lack of post-sales customer support and the fact that one battery is, in retrospect, undersized and doesn't deliver enough hot water for our needs.

Neither issue is something that would affect you necessarily, but it helps to be informed! Now we've had it for 5 years, my only other observation would be that it should work well with a PV battery because you can spread out the charging profile through the day.

Generally the 'guilt-free' hot water in summer is a real benefit and I'm happy we went for this route rather than a direct electric 'zip' heater for example.

Posted By: djh

Posted By: georgiegirldue to having an annexe with a shared meter I don't know what our kWh heat demand actually is.

It would probably be fairly simple to have a sub-meter put in to measure the annex's consumption. Or even a CT clamped around the cable (Current Transformer) would give a good idea.

I'm thinking that during the summer a diverter could take excess from our PV array to heat the top of the store for evening showers. Central heating won't be required so the bottom immersions aren't needed. When its cold enough to switch the heating on the bottom immersions would be activated and heat the tank at the cheap rate. Does this sound like it could work? Alternatives that don't involve heat pumps?

We have a very similar arrangement. Our house is a passivhaus so has a low space heating demand met by direct electric heating. We heat our hot water using a Gledhill thermal store with two immersions. The bottom one is connected to a diverter that diverts spare energy from our 4 kW PV system. It is set to a high temperature to maximise storage capability. The top immersion is fed from the mains through a timeswitch and is set to a lower 60°C temperature. That provides enough hot water in the winter for our two showers a day plus all other domestic needs. It doesn't use any power in the summer because the water is already above 60°C because of the solar diversion.

I haven't been able to justify the cost of installing and maintaining a heat pump yet, given our low heat demand. Losses from the thermal store in winter aren't really losses at all since they are picked up by our MVHR extract and recycled into the air supply, avoiding some of the space heating demand. During summer the losses are from free PV power and the heat is dumped by the MVHR summer bypass so it isn't a problem then either. WE use an E7 tariff at present.

What is best for you will depend on exactly what your space and water heating demands are.

Great to hear that you have a similar set up, but interesting that you have the immersions functioning they other way around. I've spoken to a couple of thermal store suppliers and they are suggesting that I will need a very large store in order to compensate for the store not being heated by a 24kw boiler. However, as we have also got approx 70m2 of underfloor heating being installed thatthe volume can be reduced as heat will be stored in the slab.

Posted By: georgiegirlHow do you provide heating?

Sorry, I hadn't picked that up from your first post. We're in a passivhaus so heating is minimal and provided by 2x 600W electric panel heaters.

My understanding of the SunAmp is that it was designed as a 'drop-in' alternative to a hot water cylinder, so yes you'd need something else to do the 'boiler' part. This could be an ASHP, but I realise you're trying to avoid this route.

**Edited to add, the SunAmps do have a heating element so you can run them as water heaters like an immersion tank, especially on an off-peak tariff. We actually do this each night on our smaller unit to ensure that we've got enough for showers in the morning.

However, I'm not sure how efficient it would be to use it in this way for space-heating.

Posted By: georgiegirlThanks, nothing against heat pumps, but I understand they can't work with a thermal store and that's ideally what I want.

I'm inclined to think the opposite, but I seem to be in a minority.

Key thing seems to be that the coil inside the tank that does the water heating, has to be large enough to heat the DHW without the tank itself being excessively hot.

Other than that, and for wet heating systems, a TS seems to be an ideal solution as the flow return for the HP is then independent of the heat demand of the house, it is just to do with the heat demand of the TS. This should then give long runs for the HP.

There does need to be good stratification and the ability to just heat the bottom of the tank so keeping the flow temps from the HP down at times.

.

What is 'a very large store'? And why does it need to be larger if you don't have a 24 kW boiler?

Go back to basics. What is your heat loss and DHW consumption? Put in another meter and find out.

I don't understand it enough hence why I am hear asking questions of those that know more than me.

One of the chaps that I spoke to from a manufacturers said that with the immersion providing 3kw that it would take 8 times longer to heat the store compared to a 24kW boiler. At least that's how I understood what he was saying.

Someone at a different company said that a 300l store would only be sufficient for a 2 bedroom flat.

Simplifying/generalising my comments below, as it takes a detailed conversation and background building data to design these systems properly, but maybe there'll be something useful for you...

t's not "usual" to have a thermal store, as it's often a means to decouple when you can input energy from when you use that energy (or different types of energy inputs). An ASHP can pretty much run on demand, so "usually" you would not run it and store the heat, to be used later. People then conflate that, into stating that it cannot be done. I must be carrying out miracles, cause I've done it a number of times.

You really need to have an idea of the heat loss for your home (in the post-refurb state) so you can understand if the rate of heat you can put in will match the rate of heat you are losing. That's a capacity/sizing/power question. That would determine the size of the thermal store, as you need to be able to store the heat when it's cheap, and release that over the expensive period. With Eco7, there's not that big a diff between night and day rate. Similarly with Eco10 (if you can get that). The flexible Octopus type tariffs can have quite low off peak rates, but you'd again need to run some numbers, as the onpeak tariffs are huge.

Then you need to understand over 12 months, the quantity of energy (let's say in kWh) so you can decide if off-peak "direct" elect (immersion/panel heater) will be affordable when multiplied by the number of kWh of energy consumed.

You also need to play in to all these figures, how PV and batteries can supplement. This is not NASA stuff, but it's not finger in the air stuff either.

None of that is news to you I suspect, but my point is that rushing in to a decision, or asking a bunch of people who don't have the data to make an informed decision, may not deliver the optimal solution.

I think I understand:
- located SE England (likely rarely below 0oC and decent sunshine hours)
- reasonable insulation
- ground floor UFH
- existing gas boiler to be relocated, so maybe get rid now, and use immersions for heating & DHW
- have 6kW PV and 5kW batteries

On the little info I have, (I design these systems), you might consider the following, if you do not gather/calc the heat loss info, or annual energy consumption data.

1. instead of spending money on a thermal store, which I imagine would be £4k installed, add to your batteries. Store energy as elect in the battery, not as hot water made from elect. Import on a flexi tariff at the lowest times.
2. use a small inline elect heater to heat water that would feed the UFH and the DHW cylinder (just like your gas boiler does at the moment). Maybe 3-4kW would suffice for a few £hundred (easy to upgrade if it's not enough, though if it's not big enough you're going to have hefty elect bills).
- design for the installation of a small ASHP in the future (space for external unit, a small buffer tank internally, and ducts to allow the insulated pipes to run through your ext wall to get from the ASHP ext unit to the buffer tank location...BUT do not install the ASHP

That should minimise your spend now, allow usage of lower rate elect times by providing storage of energy, allow you to test out the direct electric option, and if it proves very costly to run, you could flip to an ASHP very easily. You'll also know what size of ASHP would be required, as you'll have lived with the heating system.

Posted By: georgiegirlI don't have time to put in another meter as building work is imminent.

Alarm bells are ringing. Doing building work when you don't know basic details nor exactly what you are aiming for is a recipe for disaster. Wasted money and poor results perhaps.

I agree with what Green Paddy said.

Posted By: WillInAberdeenNo point metering your existing consumption patterns if you're just about to insulate.

There is a point in being able to monitor energy consumption going forward, and there is a point in measuring current consumption so as to be able to calculate the difference from a baseline. Starting from data is better than starting from theoretical calculations.

Installing a meter is something an electrician can do in a hour or so. Installing a CT clamp is a DIY job.