Beelbeebub

I have to say, the wooden baths I've seen seem to be a little thicker sided, though how much is necessary and how much is looks I don't know

Very cool! Is the bath overflowing continuously (ie constant water input) or is it just that it is designed to be tolerant of overflowing? I know wood baths shouldn't dry out or they start leaking so will you be keeping it full all the time?

As your storage volume goes up, the required input power of your heating device trends to whatever the daily power requirement is. Eg a 1000l tank might only need a 2kw heater as it can produce 48kwh a day. The smaller the tank the closer your input power needs to be to the maximum instantaneous power requirement. A combi boiler might need to be 28kw to produce a decent flow. Unless the OP planned to use more than 300l in a sitting, then pretty much any HP should do.

How big is this bath?

Here it is Daikin multi+ 120l tanks so might need 2!

I'm sure I saw some manufacturer makes an invented cylinder heated by a refrigerant loop that you hook up as one of the head units to a mumtisplit. The cylinder has the necessary electronics in to interface with the outdoor unit. So get a 2 or 3 head 5kw multi split and have one head as the cylinder and the other 2 as heating or cooking as required. Probably cost less than the a2w setup.

Sorry I can't help, I haven't got around to trying one yet. Mulling over this or just biting the bullet and going for a full HP job.

The other option, which we've discussed here before, would be a small HG cylinder this no G3 with a 9kw instantaneous heater after it. Ideally the 9kw unit would be modulating so you can bring whatever the HP delivers (say 3kw on a cold night) plus another 9kw. As it's an instantaneous heater no G3 either. And the instantaneous heater is only needed occasionally so most of the time you get the high cop of a HP.

Yeah, the inclusion of an immersion heater would come with a small chance of overheating (but very small now the units all have built in non resetting cutouts as well as the usual thermostat and presumably the actual control relay as well). But if there was no immersion in the system then zero chance of overheat.

For what it's worth, my large TS seems to (generally) cool equally from top to bottom ie I lose 5C from top and bottom as I draw off water. Of I only stored water at 45C I wouldn't get much out *However*, the coil area of the store isn't huge as it was designed for a 65-75C store temp and IIRC it's distributed towards the top of the volume. I can imagine, if you had a very big coil, running from top to bottom, you might get closer to a bottom up draw off. You would still have the issue that, as you got towards the end of the store capacity, your effective coil area would fall and you wouldn't be able to access the last bit of restored energy. I agree with the assesment that this is more of a work around the UVC regs than anything else. Which does beg the question, why couldn't we just have the regs amended. As has been pointed out before, a UVC hooked up solely to a HP, especially without an electrical booster, has zero chance of overheating. If the regs just carved out an exception for that circumstance then we could fit small "under counter" UVCs that would potentially perform a little bit better. Edit & slightly off topic - in the same way if the regulations on grants for A2A were changed to allow them to collect grants as long as they were firmware locked off from cooling below (say) 30C it would massively help uptake

If the regs allow an immersion heater in the buffer/store without bringing the whole weight of G3 into things that would be fantastic. Another 3kw on top of the HP would probably make it near indefinite for showers and taps. My thermal store set up is similar albeit with a very big tank for solar and solid inouts. The gas boiler is maybe 15kw (might be 12, not sure) and that acts more or less as a combi for showers. The trick is having enough stored capacity to provide hot water whilst the system is "ramping up". I've found it I make the storage volume too small (I have a home brewed virtual thermostat on the tank that allow me to set how much volume to store) there is a dip in shower temp before the boiler manages to catch up and hit steady state. If I get the volume right then it effectively works as a combi

Just had a look, it appears as @JamesPa says to be a small thermal store with a big DHW coil. I'm guessing they are relying on the "it's a HP so can't boil" approach to avoid the G3 regs. The big issue I can see, is that it relies on the HP filling the store as you draw down to extend the volume of water you can draw before running cold. This might be drastically curtailed when you start hitting defrost conditions.

Split units are basically a "normal" air to air unit outside (including the compressor) and then inside, instead of an air to air room unit, an air to water room unit with a conventional central hearing pump, plate heater exchanger, some valve and other gubbins. I believe Panasonic are confusing things with "hyro split" which is a mono block outside unit and an inside unit with some pumps etc in. The main characteristic of split systems is the fluid that goes through the wall is refrigerant not water. As such you need an F-gas installer. I don't believe there are any R290 systems as r290 is a bit of a grey area in Europe and america so the big players are holding back For what it's worth, most of the effort in the UK seems to be towards monoblock as they don't require fgas certs to install. I know other countries are going down the split route as f-gas trained installers are. More common due to higher air con usage.

Why not swap out the 15mm pipes? As a thought, switching to 22mm speed fit would get you a boost in flow/reduction in velocity with) potentially) easier installation. Yes 22mm plastic isn't as big a bore as 22mm copper but it is still bigger than 15mm copper.

I found massive turbulence corrosion in a 15mm cold water copper pipe running around 8lpm albeit after about 9 years 24/7 and just downstream of a restrictor that would have generated significant turbulence. The pipe was paper thin and full of pin holes for about 40mm downstream of the restriction.

I think the instantaneous Cop figures are pretty solid (not that many manufacturers provide these). The issue comes that the outside temp and inside temp demanded are continuously varying so rather than getting one point on the graph, you wander around it sometimes getting good cop and sometimes not. The Scop tries to capture this, but is (like the car mpg) only ever an approximation based on idealised standard conditions. Part of the problem in the UK is that our elec/gas unit price ratio is around 3.5 to 1. In order to achive price parity your installation needs to be fairly well done. Typical industry practice (sloppy heat calculation, buffers, zones etc) does not reliably hit this performance - hence tales of disappointment. If the ratio were (say) 2.75 to 1 then many more installs would better that and we would have far fewer "my heatpump costs more to run" stories for the daily mail and telegraph to print. If we had a ratio closer to 2 to 1 then people woukd be falling over themselves for Heatpumps as even a fairly sloppy install coukd save you money and a good one woukd save significant sums.

Is it to protect against any crud in the heating system? Are their units particularly susceptible to dirty water? Most manufacturers seem happy with a good flush and then filters.

Is the heat exchanger also being used to isolate a glycol circuit for the HP?

My 2bobs worth.... Is your HP set to weather compensated? You don't mention it. That will help. Not keeping you warm in the cold periods: you may need to up the flow temp for the cold periods. Weather compensation will ensure that the flow temp is lower during the milder periods. The plate heat exchanger won't help. It means your HP will need to run a few degrees hotter than it migbt otherwise. You mention it was installed to allow a boiler to also be used (it may also be to allow glycol to be used in the HP side) Is it possible to get the system replumbed so you run the HP direct and the boiler is connected via the heat exchanger? Given you have an (oil?) boiler backup - you could use that for the few very cold periods when you estimate it's cheaper to run or the HP struggles. Running the boiler a few times a year will help keep it working well and you mihbt as well use it of you've paid for it.!😁

As pointed out before, if there is a fault with the diverter valve setup so that there is flow through the tank coil during heating mode, the hotter water in the tank will be cooled by the water passing through that coil at the cooler heating flow temp. Symptoms would be Your tank swiftly loses temp only when the heating flow is on. The return flow out of the tank coil (usually the lower pipe) is warmer than the feed flow in (usually the upper pipe) If your controller is set to hot water priority it will start heating the house, then notice the tank has cooled, switch to heating the tank, then go back to heating the house, then notice the tank had cooled.... And so on. Try setting your hot water to timed. At this point the above won't happen outside of the hot water times but you would quickly see the tank temp drop to the heating flow temp and then stay there. If there is not a diverter fault then another possibility is the tank is losing temp naturally. Because the prove is at the bottom it can only "see" the temp there. As the tank loses temp naturally the cooler water sinks to the bottom. Your probe will "see" cool water but be unable to know the water above is still hot. My tank can be half full of 65C water and the bottom be at 20C (I have multiple sensors). Again, if your hot water us set 24/7 it wouldn't take much demand or cooling to trigger another heating cycle even though there is ample hot water left

Just a thought. A return water temp of 28C implies either your slab is at 28C - that is pretty warm for a floor slab, what are your floor finishes? My bare concrete slab hits maybe 24C for a 20C air temp though we are very well insulated. If you have wood or carpet then your slab temp. May be higher but 28C is the recommended max floor temp for UFH so I would be surprised. The other option is that the flow is too fast for the water to give up enough heat. Have you checked the flow rates? I believe the Grant installation kit has a flow adjuster with a sight glass in it so you should have an idea. The Aerona Heatpumps come from the factory with the pump configured for maximum. Grant say this is almost always too high and usually needs to be set back to med or low. If your builders aren't very knowledgeable about the HPs, they may have missed that step. Combined with the fact the bedroom zones are mostly off, the flow in the other zones will be higher than may have been set anyway.

That's a fairly unique set of circumstances. Basically an extended power cut and temperatures in the "mushy glycol" zone. I'd imagine, if you arrived back to an ongoing power cut you would want to conserve your battery power for things like Internet, phone charging, lights etc. and off using your precious power to run the heating pump and using your aga as the heat source. The only downside of glycol is the expense, which can be countered by not using giant buffers and the mess issue*. The efficency and viscosity in normal operating conditions seem to be very close to water. *as someone who regularly has to deal with small leaks from heating systems and small spills of water when fixing them, this is the biggest drawback for me.

I notice the first pic says Inlet backup heater water temp 43C ... Refrigerant temp. 37C I may be wrong but this would imply some of your heat is being generated by the backup heater?

Glycol/Antifreeze valves are very much for the chance combination of rate events (though power cuts are often correlated with very cold weather). I understand the reluctance for glycol especially if you have a huge system volume, but I do think the case against it has been overstated - especially regarding efficency. For me the biggest "downer" is the potential mess from leaks, either during use or spillages during maintance, but then again spilling water from a cruddy open vented system is pretty nasty anyway.

But we're only interested in freeze protection if there is no power. If there is power then the fluid temp won't ever be below zero so the increced viscosity at very low temps isn't an issue.