Crofter

I'd like to add a simple wall mounted panel heater to our holiday cottage, so that we can keep it at a reasonable temperature when nobody is there, using the off peak overnight rate. At the moment we leave the heated towel rail on when the weather is cold, which is probably not the most cost effective method. I was thinking of just buying a hardwired wifi relay (e.g. a 25A one from Shelly) but then I came across heaters with Wifi functionality built in. That's quite appealing because it would allow people in the cottage to use the heater as well. I'm a bit worried about the longevity of any built in 'smart' technology though. Having owned an early Smart TV which after a couple of years was no longer supported, I feel that it's easy for the app to be updated and suddenly older tech no longer works. Maybe this could happen to the Shelly relay too, but at least in that case I only have to replace that one small component. Any thoughts? And any recommendations?

This seems like a good time to point out that the Scandinavian countries seem to get on just fine heat pumps too.

Sadly you are correct. A house has to be in budget, in the right location. For a lot of us anything more than that is a bonus. It's not like buying a car where you can shop around all over the country.

I've never been a huge fan of subsidies. In fact other than getting cavity wall insulation in my previous house, I've never claimed a grant or subsidy for anything, ever. Which is pretty unusual when you own a croft! Make electricity cheaper, put those costs on to fossil fuels. That's all they have to do. Oh, and get rid of standing charges. Spread those costs so that those using the most electricity pay a bigger share.

And the biggest ticket item is the triple lock. But pensioners actually get out and vote, so there's no way any realistic government is going to touch them.

On the ceiling, I used 100mm PIR. I marked the rafter line in places that wouldn't be covered by the PIR, then ran a straight edge down to draw that line on to the PIR. It helped that I was using paper faced, not foil. I can't remember quite how I managed to drive the screws in square, possibly I used a block of wood or something to act as a set square. You only need to be super accurate in one plane, you can be pointing a bit too far up or down the line and still hit the rafter. The insulation washers are essential. I think I was using Pz3 screws, at these lengths that what you'll find. I was fortunate and got most of mine on clearance from Screwfix. If you do mess up and miss the rafter, just try again. And if you end up leaving a hole, you can foam it up. My battens went directly over the same line. Really, the first set of screws just need to hold the weight of the PIR for a short while until the battens go on. I used 25mm battens which is minimal really in terms of creating a service void. It worked, but it made the wiring pretty fiddly. The back boxes I was using were designed for a deeper cavity and I had to trim the lugs on them to make them fit. That got a bit tedious after a while!

I used very long screws to hold the PIR to the rafters directly. No battens attached to the rafters. I used insulation washers, which are big plastic things, to spread the load and stop the heads pulling through. I then installed a polythene vapour barrier, with battens following the line of the rafters, and even longer screws. The PB then went on top of those battens.

I know diverters aren't the right answer for everybody. My interest in this was actually sparked by seeing a cheap used diverter for sale locally. The unusual thing about my use case is that, being a holiday house, I'm not in a position to load shift. Guests are going to use electricity when they want to, not when the sun is shining.

I looked up the Eddi and the ability to power two different devices sequentially looks absolutely perfect. Is this a common thing on diverters? What should I be considering when choosing the heater? As it happens I never removed the old storage heaters when I put in my A2A, so it would be tempting just to wire up the one that's in the hallway, which would add a nice bit of background warmth to the whole house. But I don't know what wattage is appropriate for this, e.g. should it be sized to take account of the array output?

My heat pump is air-to-air, the water is simply heated via immersion in a UVC. So the array is just going to boost that when there's enough sun. There are two immersion elements on the tank, I'm not sure how best to set everything up to take best advantage of the diverter. I guess that ideally I would have the immersion powered by the diverter set to a higher temperature, and the one powered by the grid would be set lower as a backup. But of course one of them is half way up the tank. My thinking with the panel heater would be simply to dump excess power, but I'd really want it to operate only from the solar. A very crude way of doing that would be to simply fit a timer, so that it is always off overnight. But it would be far better to set it up to only come on when the diverter had already fully boosted the UVC. I'm not sure how to do that.

That's a very similar application to mine, great to hear that it's working out so well for you. Using a panel heater as a dump load is an excellent idea- and I presume that its own thermostat is enough to prevent overheating the property? My understanding (and I could be wrong) is that a grid tied inverter feeds in a higher voltage than the grid, so that the house draws preferentially from it- and any surplus just gets exported, whether you are getting paid for that or not. So e.g. I could leave a panel heater switched on and it would take from either the grid or the panels depending on how sunny it is. Obviously not a very clever way to do things, but I'm just trying to understand how the system will work.

That's basically what I was suggesting, and how I built my own house. Except I used 2x1 battens on top of the PIR. Make sure you mark the line of the rafters before you cover them up.

This might not be possible, but another thought: In the same way that the PV diverter heats the water automatically, could a system be put together which would switch on the heat pump if certain conditions were met? (Sufficient solar input, tank topped off, temperature of house below X°). It's a Panasonic A2A unit, installed last year. I'd imagine that such a task would, if even possible, require a high degree of geekiness involving things like Pi or Arduino etc, not that I really know anything about those. Maybe it would be better to consider adding a battery instead?

If the goal is to add extra insulation, could you use rigid insulation boards that go across the rafters? This would also reduce thermal bridging.

I thought I'd need a second inverter because the extra three panels will see quite different conditions. Maybe I'm overthinking it. Tbh they don't add a huge amount, it would be easier just to stick to the main array of six.