@johnmo absolutely agree. However I've had conversations with heating engineers who categorically take a different opinion

Posted by: @bontwoody

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@johnmo absolutely agree. However I've had conversations with heating engineers who categorically take a different opinion

Sorry, missed something, what is the different opinion, they are taking?

Interesting discussion, and good to get the other view of the heat pumps ability to heat the DHW. I'm not againest using the immersion overnight and I will have a solar diverter to make us of any surplus electricty from the solar panles. One of the reasons for asking for a 300ltr tank is so we have plenty of hot water with out the HP having to fire up so often, with the lower temperature we have the DHW tank set at we find a 200ltr tank is getting cold after we have all had a shower.  The heat pump keeps and and we don't run out of hot water, but I think a larger tank will help with the morning rush.

We have a well insulated 60mm concrete screed that stays warm for a few hours in the annexe so even if the heat pump was heating the DHW for 2 hours I wouldn't expect the house to cool down noticably.

We are going to be fitting a wood burner so that could help on really cold days, although we do expect the house to stay warm without needing that extra heat.

The current installers offer Samsung or Vaillant, with Vaillant costing over £1k more but offering a 7 kW. They are open to fitting what I want and I think they can make the figures work for the grant with the smaller HP, but I don't want to insist on the smaller model without asking questions here to see what others think.

Posted by: @johnnyb

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They are open to fitting what I want and I think they can make the figures work

Panasonic are reported to be very good. They also have a flat output kW with variable outside temp. Quiet, good running algorithms so you almost never get short cycling, if I was buying again it would be my first port of call. Pretty cheap as well. So a 5kW version would be fine. They do cooling out the box as well according to the blurb.

Posted by: @Anonymous

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Posted by: @bontwoody

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@johnmo absolutely agree. However I've had conversations with heating engineers who categorically take a different opinion

Sorry, missed something, what is the different opinion, they are taking?

 

The point about needing a legionella cycle at all and how hot the water needs to be if you use one.

Its an interesting point that if people cannot agree on the correct temperature then where does the heating of the DHW stop and the legionella cycle begin. Hence my comment about the rule for all hot water requirements to be fulfilled by the heat pump.

BTW @johnnyb  don't mention the intention to install a wood burner as that would also be an issue with getting the grant. 

Posted by: @johnnyb

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The current installers offer Samsung or Vaillant, with Vaillant costing over £1k more but offering a 7 kW. They are open to fitting what I want and I think they can make the figures work for the grant with the smaller HP, but I don't want to insist on the smaller model without asking questions here to see what others think.

Check the detailed specs.  7kW Vaillant is actually 8kW + at your operating conditions so unlikely to be a better size match than the 8kW Samsung.  The 5kW Vaillant might well be spot on for you as the actual capacity is again greater than the 'sticker' under most conditions.

I have the 7kW for a 7kW house, nice machine, very well thought through and extremely flexible UI, only material complaint is that compressor gets buzzy at highest modulation frequency which occurs only during recovery from defrost at lowest temps.  I have just enough excess capacity at design oat to suppress this (in case it disturbs the neighbours) by running in low noise mode at night, and during the day it's not a problem.  Otherwise it's whisper quiet - it's located just 30cm from a window to the room where we spend most of our time.  There is a full review in 'articles' on this site.  The 5kW reputedly doesn't suffer from the buzzy compressor but I haven't verified this.

Posted by: @bontwoody

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Its an interesting point that if people cannot agree on the correct temperature then where does the heating of the DHW stop and the legionella cycle begin. Hence my comment about the rule for all hot water requirements to be fulfilled by the heat pump.

The regulations also include this paragraph:

4) For the purposes of [the paragraph requiring 100% coverage ] “hot water” means hot water which is intended for the personal use of the individuals who use or occupy the eligible property. and does not include hot water used for the purposes of—

(a)space heating,

(b)heating a swimming pool, or

(c)carrying out a process.

In addition the regs clearly permit any 'supplementary heater', including specifically an immersion heater, to be retained.  

I would suggest that a weekly legionella disinfectant cycle could well be classed as 'carrying out a process' because nobody needs hot water at 60C and it is clearly a distinct process ( this view would also answer the question 'when does hot water heating end and legionlla begin'). 

Of course the counter to this is that the water so processed is eventually used for personal use so one could argue that it is subjected to a process not used for a process.

Given that most of the heat pumps installed in the early days of the grant were simply incapable of doing a legionella cycle natively, a court that found that this invalidated the grant would be a bit perverse (but of course the law can sometimes be perverse!). 

It seems unlikely, for practical and political reasons, that the administrators of the BUS rules would initiate a claim against past recipients on this basis, unless the next government is opposed to green measures and decides to punish those who were supported by them.    The first of these is distinct possibility it seems, I can't comment on the second.

I suppose even now they might just initiate a claim against recipients that have a heat pump which is capable of native disinfection, but where the system design relies on the immersion heater to perform the process.  It still feels unlikely unless they give fair notice of future intent.

@jamespa Thanks for pointing out about the output difference. Maybe the 5kW Vaillant will be a better fit as it would have some head room.  I'll have a chat with the installers and see what they say.

@johnmo where did you find the Samsung spec?  The capacity table on the midsummer website has the 5kW R290 unit rated a fairly flat 4.95kW down to -10° up to 50° LWT.  The way it is listed they have either designed the system so it puts out a nearly flat output between -10 and 7° or they have massaged the figures. 

In Suffolk it is rarely below -5 for more than a few hours a day even on the coldest days.  We sometimes have a few days in a row where it is below 0° all day but not very often.

@jamespa That's interesting James. Two other things occurred to me after my last post.

The first is that all DHW cylinders include an immersion heater and currently there is no method of measuring how much heat is supplied by the heat pump or the immersion coil. 

The second which is pertinent to your comment that DHW never needs to be above 60C is to do with storage temperature as opposed to usage temperature. Storing water above 60C is useful in reducing the size of a cylinder fitted and/or reducing the number of cylinder charges needed in a set period. So for instance the OP may only need to charge the cylinder once at night at a higher temperature rather than twice at a lower temperature.

I appreciate this strategy isn't the best at minimising energy use with a cop of 1 but this could be much improved by doing an initial heating charge with the heat pump for an hour or so then topping up with the immersion. 

As for future government policies I hope the human race has more collective sense than to destroy its own environment (but I am by no means sure of this). 

Posted by: @bontwoody

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The second which is pertinent to your comment that DHW never needs to be above 60C is to do with storage temperature as opposed to usage temperature. Storing water above 60C is useful in reducing the size of a cylinder fitted and/or reducing the number of cylinder charges needed in a set period. So for instance the OP may only need to charge the cylinder once at night at a higher temperature rather than twice at a lower temperature.

Fair enough. 

With R290 heat pumps, which seems to be the direction if travel for reasons of gwp, this is possible natively at a cop of >1.  We just need to chill out a bit on the placement rules.

Posted by: @bontwoody

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As for future government policies I hope the human race has more collective sense than to destroy its own environment (but I am by no means sure if this).

Me too. 

Unfortunately collective sense is often influenced by the vested interests of the few, in this case oil companies and a substantial part of the media.  Both seem comfortable deliberately misleading people, and directly or indirectly influencing voting preferences in a way that is arguably more likely to benefit the vested interests than it is to benefit the voters in question.

@johnmo Thanks for the link.  That's the R32 HP, we are looking at the R290 which seems to be much closer to 5kW especially at higher LWTs. We have the R32 for the annexe so useful to compare the details.

Thanks all for your input, I can go back to the installers tomorrow armed with more info and the spec sheets to discuss the options with them.