My initial thinking is that the difference between the two isn't that great in terms of sizing but would be interested to see what the Grant owners think of their units. I'd lean more in favour of the 12kW based on what you've said above.

Have you considered undertaking the heat loss calculation yourself to see if the numbers stack up?

I can't comment on the calculations your installers have prepared, which I presume include for all ECO4 upgrades including internal wall insulation. There's clearly a big discrepancy between your estimated heat load and your installers estimates.

I own a Grant Aerona 10kW and it does seem to struggle to reach target leaving water temperature when the outdoor weather is causing frequent defrosts. I would approximate the capacity reduction to be around 25% of the nameplate during this time, i.e.: 7.5kW unit. Grant appear to over-state their unit COPs in their literature compared to the OEM Chofu data at identical performance conditions. The Grant is a reliable unit, as is the Samsung Gen 6 R32. I think both have moderately good SCOPs rather than class leading for their size. The Grant will modulate down to 20%, not sure how the Samsung compares, but it should modulate to 25% capacity before stating to cycle on and off.

My 10kW Aerona should really be a 13kW based on the MCS Heat Engineer calculation sizing, which came out at 10kW. For DNO supply reasons we had to drop to a 10kW unit, but I have a log burner as secondary heating. The 10kW modulates down brilliantly to under 3kW output, which for the ;large majority of the heating season means it chugs away all day very efficiently. Defrost cycles do tend de-rate the unit, and I hadn't fully appreciated to what extent that happens with Grant / Chofu products. Without the log burner I would be resorting to direct electric supplementary heating. There is a 3kW immersion heater in the Grant low loss header, but despite several attempts at setting the parameters up, I've never succeeded in getting it to work, ever. I think it may have been incorrectly wired up as the controller calls for supplementary heating but the immersion never energises!

Posted by: @allyfish

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I own a Grant Aerona 10kW and it does seem to struggle to reach target leaving water temperature when the outdoor weather is causing frequent defrosts. I would approximate the capacity reduction to be around 25% of the nameplate during this time, i.e.: 7.5kW unit. Grant appear to over-state their unit COPs in their literature compared to the OEM Chofu data at identical performance conditions.

just trying to assist the OP: are you saying that at -3C, its effectively a 7.5kw machine? what's the target WT?

Posted by: @allyfish

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not sure how the Samsung compares, but it should modulate to 25% capacity before stating to cycle on and off.

the samsung 12 is effectively a 16 with its top-end de-rated, unfortunately this means the bottom end is pretty much the same as the 16, i.e best you'll get is 4kw in a well setup system. whereas you are saying the grant will modulate down to 2kw right?

Posted by: @iancalderbank

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just trying to assist the OP: are you saying that at -3C, its effectively a 7.5kw machine? what's the target WT?

Yes, at -3degC target LWT should be 45degC, but tends to never get above 40degC, leaving a shortfall of heating duty and requiring supplementary secondary heating to achieve desired indoor set point. Part of this is due to the sizing of my ASHP, 10kW, where it should really be a 13kW. But the larger part is due to the frequency of defrosts, which Grant units have a propensity for - sometimes up to every 45 minutes - twice as often as other OEMs, and the amount of energy that removes from the hydronic circuit each time vs the time the ASHP requires to replace that lost energy.

Interesting the Samsung 12kW being a de-rated 16kW, thanks. 4kW minimum turn down is quite chunky,  and the unit will cycle frequently especially if the actual heat loss in the OPs property is towards their estimate. The grant will chug along at 2-2.5kW minimum turn down, and 600W-700W power consumed, which is very affordable in the milder heating seasons.

With my refrigeration engineer's hat on, the way A2W ASHPs defrost isn't clever. It's a by-product of most domestic heat pumps being designed as reversible refrigeration units to provide hydronic heating and cooling function, but under UK MCS only used for heating. Losing heat energy from the hydronic circuit to use for defrost isn't ideal. It's contrary to current thinking on the most efficient way to design hydronic circuits using minimum system components, getting rid of buffer tanks, headers, etc and minimising the hydronic volume. The minimum system volume is usually determined by the defrost function, a poor approach. Passive or hot gas defrost methods could be used, or stored energy from condenser superheat. There's other ways to defrost an evaporator without removing energy from the hydronic circuit.

@allyfish thanks. yes the whole way defrosts work is an issue. the next HP I buy whenever that is, will hopefully have a better engineered way of doing defrost!

@tobyg I think we talked about this on your original samsung thread and mars has re-echo'd, have you attempted to do your own heat loss calc? I know you got somewhere between 3 and 5.5 kw based on the empirical gas methods, but going through the room by room maths yourself is not impossible, there are tools (and people on this forum) to help.

there's a bunch of discussion about grant defrosts on another thread (look for one with  "defrost" in the title) and them not handling it well, particularly if the target WT is high, it basically can't get heat into the house at anywhere close to its rated number.  It sounds like to use  the Grant, you would need your system to be designed for a lower WT than what has been mentioned, for it to be stable in defrost conditions. Probably no more than 40 at -3C.  If this isn't taken into account, you might be cold in winter.  Which basically means bigger radiators. Do you have proposed radiator specs yet?

however it is clear from allyfish's info, the minimum turn-down of the grant is a lot better than the samsung, so you'll be able to run it more stably in "low and slow" mode when its milder.

Wow! I have been so impressed by the thoughtfulness and support here, which just keeps coming. I really appreciate it.

To respond to some of your points, and make sure I’ve understood the important ones:

Yes, the installers’ calculations allow for the new insulation.

You’re saying that the Grant 10kW modulates down to under 3kW, while the more powerful Samsung struggles to get below 4kW – so the Grant should be substantially (?) more efficient in mild weather, i.e. most of the time.

However, in very cold weather the Grant struggles and doesn’t perform at its specified power of 10Kw. @allyfish, you’ve said in our case it’s more like 7.5Kw, partly due to it being undersized for your property, but even if it was absolutely spot on, it would still be compromised because of the way it handles defrosts. This affects the leaving water temperature (that’s to the radiators?), so central heating is actually cooler just when you want it at its maximum. To gauge how much this would be a problem for us you’re advising we do a heat calc. (I’ve avoided this so far because I didn’t want to get into a back seat driver dynamic with the installers, and on an initial scan heat punk didn’t seem user-friendly, but I now will have another look at this.)

The way to mitigate a lower WT is bigger radiators. All our radiators are being resized to be bigger, but as I understand it for a 50c flow temperature; I think we’ll struggle to negotiate for larger radiators at this stage, and in some rooms they’d be hard to squeeze in.

One of our main concerns is the domestic hot water system, and whether performance could be compromised with the less powerful Grant compared to the Samsung (as you say, there’s ways of dealing with below par central heating, but not hot water)? And in relation to this I think you’re saying, @allyfish, that the immersion heater set up with your Grant hasn’t functioned – do you think that would be a more general concern?

My other question was whether there’s much in it in in terms of how user-friendly they are / quality of aftercare (I can see there’s a certain amount of learning required , but I’m not a natural techie!)?

If I’ve misunderstood anything @iancalderbank and @allyfish, please correct me.

Again, this is all super helpful – thank you.

@tobyg a target flow temperature of 50C is not low, in a heat pump system. its a high flow temperature. If your system is being sized for 50C flow at -3C, that is not going to be efficient, and is at high risk of suffering defrost pain in cold/damp weather. 40C at -3C is a much more efficient target, this will enable you to run somewhere in the mid-30's when its mild. radiators is the easiest bit of this to upsize, unless you have a super-difficult room situation. Fan coil units can really help, higher output for smaller size at the expense of a modicum of fan noise.

hot water is a completely different issue. the water temperature that you get out of your taps is determined by whats in your stored water cylinder. NOT the water temp that the HP runs at when in central heating mode. The HP goes into "hot water mode" to heat up the cylinder, and sends the hottest possible water that it can into the hot water cylinder's heat-exchanger coil. This coil transfers heat to the water that you actually use. This is not a combi boiler situation. a 12kw heat pump will do it (in theory) 20% faster than a 10kw, but they'll both get it done. in practice other system constraints may be in play (flow rate, capacity of coil in hot water cylinder) and they might well both be the same in the real world. All that matters is can the heat pump produce water that is hotter than what you need in your taps, in order to heat up a cylinder, and can it reheat a cold cylinder in a reasonable amount of time, which they both can.

the immersion @allyfish referred to is a backup resistance heater in the central heating circuit. Not one in a hot water cylinder.

you don't have to backseat drive your installers. just do your heat calcs,  @derek-m  is a bit of an expert on these. then publish what you've got. also publish your installers radiator sizing.

EDIT: Ian's covered most of this, he's got quicker fingers! But my thoughts as per his....

I think you've got a great understanding @tobyg of the topic. To clarify, for Grant, the low loss header includes a supplementary 3kW immersion heater, this is in the central heating circuit between primary and secondary flows. It can be configured for use as an emergency direct electric heater, say if the ASHP fails, as a supplement to defrost (but in my view that's literally throwing money outdoors) or as a supplementary 3kW heater over and above the ASHP rating (that doesn't quite hold true as the ASHP can modulate down to compensate for the added 3kW of immersion heating, so you might not see 3kW of supplementary heat input). As it happens mine has never worked out of the box, I think due to being incorrectly terminated or not terminated somewhere, but it's of limited benefit in my view however it operates.

The 3kW immersion in the LLH doesn't contribute to DHW production. A decent system not water tank should only need charging once a day if you use moderate amounts of hot water. The Grant can supply water at up to 60degC, and it will do this in DHW production, even when it's cold outside. That gets you a tank of hot water at up to 55degC within 60-75 minutes for a 250l cylinder. The return water temperature rises fast enough to assist the leaving water temperature to rise to target set point. During this time heating is not available, but that's true of most ASHPs, you can't have CH and DHW simultaneously as the ASHP simply hasn't got the power to do this.

The LWT below set point issue is with the space heating, especially if running the system from a 'cold start' (People get very opinionated about whether to turn the CH off overnight or leave it on - personally I turn ours off from 9pm-5am on the orders of my wife!) There isn't a set back function on the Grant controller, you can't knock the LWT down 2 or 3 degrees overnight, which is a shame. If you could I would do that. If you turn a master room control thermostat down you start cycling the ASHP on and off, and that's not good or efficient control.

When my CH system first starts in the morning the return water temperature is much lower than set point, and in my experience the Grant units seldom operate at a delta T of much more than 6 or 7 degC on the primary side, meaning that the leaving water temperature is a slow rise up to set point, because it's pinned to the return water temperature which only rises slowly as space heating load demand starts to reduce, and if repeated defrost cycles keep knocking the LWT down, it never makes it to set point Bear in mind my 10kW unit is a little undersized, so it's not designed for pre-heating the house from cold. That's where having a slightly larger ASHP can benefit you, but on the flip side the minimum turn of a slightly larger unit down could be a little less efficient in the milder seasons.

I would say (and I think it has been noted by others) 50degC is a high design temperature for an ASHP. That will knock the SCOP down and the running costs up. 45degC is really as high as you want to be to get a reasonable efficiency and running costs aligned with or slightly lower than gas or oil. If there's a way to increase emitter size, K3s for K2s (Yes, they are a bit bulky), then look at what can be done. Standard radiators are relatively cheap to change out, I had 12 changed when I converted from oil, and the cost was very reasonable.

Posted by: @tobyg

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My other question was whether there’s much in it in in terms of how user-friendly they are / quality of aftercare (I can see there’s a certain amount of learning required , but I’m not a natural techie!)?

on this , although you say you aren't a natural techie, try downloading and reading the manuals for the user-facing controller for each. For samsung, its this. I don't think either has a reputation for being that user friendly to be honest. however both units are supported by a 3rd party control system called Homely , custom developed in the UK for heat pumps, which seems to be a good way to bring a really user-friendly control system into the picture, have a read of their stuff, there are write ups by users of it on this forum.

@allyfish 

Sizing the heat pump is crucial, but perhaps it depends on how the house temperature is controlled during the day and what you are trying to optimise, efficiency or cost. If you want the best efficiency over the whole year, the Grant 10KW that modulates down to 2KW will be the machine to choose, as it will operate well in the Autumn and Spring months. But do use temperature compensation to keep the rad flow temps as low as you can get away with and size them according to each room’s heat losses. In the very few periods it gets really cold, you may have to run the Grant for 24 hours a day, to keep the house warm.

I have had a Samsung Gen6 12KW heatpump installed for about 18 months and this is the second winter. If I was running the heat pump 24/7, as was recommended by the installers, it is way oversized, as the house heat losses are around 4.8KW/Hr at -2C outside.

The Gen6 12KW Samsung won’t go below 4KW and cycling could happen as a result. It also gets very noisy when modulating down this low. But if you want to have the lowest running costs, the Samsung might be a better bet combined with a suitable variable rate tariff.

I am on Octopus’s Cosy Tarriff, which gives 6 hours of low cost electricity each day, and throughout the winter, the heatpump is on for those 6 hours, early morning and afternoon. And if it’s cold, it has to come on in the evening (after 7pm) for another 3-4 hours. The great thing is that the house is warmest when I get up, and again when I get home. So when it’s really cold, having the 12KW heatpump is an advantage when using low cost, off peak electricity, it is able to increase the house by about 1C per hour. The house then stays at around 19C +/-2C throughout the day and night.

So by having an oversized heatpump, running 3 times a day for 3-4 hours each time (in Winter), I may have a bit more variation in house temperature throughout the day, but my running costs are fairly low. Except for the peak times 4-7pm, when the heatpump is definitely off, I just use Set Back temperatures throughout the day and night, and let the system take care of things. And the assurance that if it gets very cold, and it has been -7 to -8C overnight and below freezing for days on end, the 12KW Samsung has been able to cope, by just running it for bit longer, at a greater cost for those days.

Regardless of which you choose, make sure the radiators in the ‘living’ rooms and bathrooms are sized adequately so they reach and maintain the desired target temperature, whilst keeping the heat pump flow temp as low as you can for a given outside ambient temperature. I have tweaked the Samsung temperature compensation curves twice and have been able to improve the SCOP by around 10% and then a further 5%.

@allyfish and @iancalderbank, thanks again. I'm now concentrating on trying to do a heat calculation; I've tried again with heatpunk and am finding it a nightmare to navigate. I'll DM @derek-m and see if he can suggest anything more user friendly.