@johnnyb

I think potentially you may be right that 8kW will be too large for your heating although the 5kw may struggle with DHW.

Can I ask if you have an EV and can access cheap overnight electricity? 

I dont heat my DHW using my heat pump. My Mixergy tank is heated via immersion at 7p a unit overnight. I could buy a heat exchanger to use the heat pump but there is no cost incentive to do so. I even think I gain a bit more because I don't have to heat the whole tank because the stratification in mixergy tanks is so good.

Just a left field option you could consider. 

@bontwoody We do have an EV but I've been on Agile for years as it has always worked out cheapest.  I do intend to get batteries at some point and then I think an overnight will work out cheaper, but there are much more important things to buy at the moment so they will be put for for a while.

Are you saying you only heat the water overnight and using an imersion heater not the heat pump?  With the HP being at least 2.5-3 times more efficient I thought it would be cheapest to use that.

@johnnyb Hi. Yes I just use the immersion overnight. Although the heat pump could probably get a DHW COP of 3.5 as the price per unit is 3.5 x less overnight it works out at about the same price as using the heat pump during the day.

You could argue that it would be even cheaper to use the heatpump overnight but we are talking about saving pennies really and for me the cost of fitting a heat exchanger would make the payback time decades.

It has just occurred to me that your potential DHW problem will only be an issue during the coldest days so you could just use an immersion then to boost it and save the additional costs of fitting an 8kw heat pump and pipework. 

@johnnyb  very much in tune with the thoughts above and would advise against oversizing.  The problem may be if you want the BUS grant as that requires full coverage of dhw and space heating (actually that's true for retrofit, not sure exact rules for new build - you might to check). 

Even if the BUS grant does require full coverage some creative (possibly more realistic) loss accounting may rectify that.  Another fudge is an 'backup heater'. Several manufacturers supply these.  It might get you out of a hole if installer is uncooperative.

You could perhaps consider the Mitsubishi R290 8kW which has 2 compressors, a 6kW variable and a 2kW fixed.  Controls more flexible than Samsung but probably more expensive.  Of course I understand you may want to stick to a single brand for simplicity.

Another check you might do is to find out the actual capacity of the 5kW Samsung at design conditions.  Sticker capacity and actual can differ very significantly and in either direction.  You can then work out the available energy for dhw.  So for example if the actual capacity is in fact 5kW and the house loss is 4.3kW that leaves 0.7*24= 16kWh, enough to heat 360l of water from 8C to 48C.  Anyway, so what if you have to switch the immersion on for the handful of days in the 

My strongest advice is not to be bullied by your installer unless you know that they are one of the best, which case they will be able to explain their reasoning logically and bullying won't enter the equation.  In the end it's your house!

Posted by: @johnnyb

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4.3kW at -2.5°C, design temperature of 43°,

I have to agree with the installer. A 300L cylinder with a 5kW heat pump will take a while to heat, circa an hour each time, maybe longer when at design outside temp. Looked through the Joule brochure and the give a random number generator for reheat times anything from 10 to 110 mins. My 6kW (at your design temp) and 210L is about 40 mins. It then goes into defrost. So a realistic figure could be over an hour 

A 5kW Samsung at -2 and 45 degs flow but out 4.58kW. Two heats of the cylinder per day you then only have 22 hrs (or less) to supply 24 hrs of heating. 4.3 x 24 is 103.2kWh, divide by 22 is 4.7kW. Factor in defrosts at design temp, it's all too close for my comfort. Last thing you need is an immersion running when it's cold outside.

Maybe choose a different brand, with slightly better cold weather performance.

Posted by: @Anonymous

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A 5kW Samsung at -2 and 45 degs flow but out 4.58kW.

@johnmo well done for looking that up!  I don't think I ever managed to find the detailed Samsung specs, but it's been a while since I looked!

If that's the case (and I have no reason to doubt it) and those are the operating conditions I would agree unless @johnnyb  you want to take the risk on undersizing.  

If you definitely want to go with Samsung (which I can understand given you already have one) then it seems you may either have to accept an oversized model or accept the risk/ and or add in a backup heater if your installer will play ball.

Posted by: @Anonymous

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Last thing you need is an immersion running when it's cold outside.

Im not sure I follow why you are opposed to using an immersion heater for the very cold days? If the price using off peak energy works out to the same or cheaper than the heat pump using standard rate?

Posted by: @bontwoody

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not sure I follow why you are opposed to using an immersion heater for the very cold days

I am not against immersion use, it gets used every day on mine, but that is besides the point.

The grant rules, state the heat pump has to do all the heating and hot water, not the immersion. Not my rules, the rules to get £7500.

Don't get the grant, fill your boots. I would just push for a different heat pump. One that does 5 and bit kW at -2. Or the mitsubishi one referenced above with two compressors.

Posted by: @Anonymous

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. One that does 5 and bit kW at -2. Or the mitsubishi one referenced above with two compressors.

It's interesting that you make this point, suggesting you think (I'm not disagreeing btw) that the gaps between heat pump capacities in the range of any one manufacturer may be just a bit too large at present. 

The practice, used by many, of 'filling in' with software limited models doesn't help (other than in respect of noise spec)  and maybe suggests that the heat pump manufacturers, or at least their marketing departments, also think the gaps are too large!

@johnmo That's fair enough. Although I'm not sure how that rule squares with having to use an immersion for legionella in addition to the heat pump.

Posted by: @bontwoody

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Although I'm not sure how that rule squares with having to use an immersion for legionella in addition to the heat pump

Different discussion really. With a propane heat pump you most possibly don't need to use the immersion. But do you need to do legionella cycles at all? Do a risk assessment.