@mattc there's loads of experience with octopus on here and also on openenergymonitor forum, have a read around.

some basic questions to think about asking them , together with the answer that you want to hear 

• provide a comprehensive heat loss survey, taking into account all aspects of the property (insulation), and including current known heat loss in their sizing calculations (YES )
• calculate the heat loss of the house in mild weather, and minimum turn down of heat pump , as well as the max usage in cold weather (YES).
• Install a low loss header, a Buffer tank or a plate heat exchanger? (NO)
• Install TRV's (NO , except possibly in bedrooms)
• Install a bypass valve (NO)
• Install carefuly chosen system components (dirt filters, 3-way valves) with low pressure loss , to minimise system head loss and therefore maximise circulation pump efficiency (YES)
• Install an "single open loop" system design (YES)
• Balance the radiators (YES)
• upgrade radiators so that expected cold weather run temp is 40C or better (YES)
• Insulate internally and externally to all best practices (YES)
• provide a comprehensive system diagram (YES)
• fully explain all detailed parameters of the controls in the system handover (YES)
• setup the system with weather compensation for the lowest possible running temperatures in both cold and mild weather (YES)
• commission the system over a substantial run time (YES)
• check it repeatedly for performance over an extended period, in both cold and mild weather, tuning settings as needed (YES)

some of the above may seem common sense but you'd be surprised...

Posted by: @derek-m

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Ian,

I would suggest caution when encouraging people to go for smaller capacity heat pumps, since I feel most would be much happier having a heat pump that may be slightly oversized, but keeps their home warm, as against one that may turn out to be undersized and not keep their home warm.

I fully agree with your sentiment about getting the size correct, but don't forget DHW production and defrost cycles, as well as the fact that ambient temperatures below the design temperature do occur.

agree to a point, undersized and not warm is not good. but we all know the grief with oversized and short cycling.

mattc's 16hr gas data , from a day when it was -5, suggests that a 12kw is total overkill. We await the heat loss data of course, hopefully one done by a heat geek. I'm only trying to set the expectation that the initial sizes he was talking about , are almost certainly too large. not spec what he should actually be buying.

Thanks again for all the good advice.

One company did mention that they would need to look at the DHW pipes, which appear to be 28mm at both ends but he said it would be a problem if, for some reason, they drop to a lower size in between.

I think a buffer tank has been mentioned too.  Not sure about LLH and PHE (says he, frantically Googling acronyms).

To be fair, the sizes mentioned have all been speculative, since none of them have yet done a full survey, so recommendations may change once this is done.  Some have said that the system may come in cheaper once they get the results - but of course I will have to pay for their survey to know this.

I agree re size, our property is approx 2100 sq ex garages and we have a 11.2 kw  ecodan the property is L shaped well insulated dormer bungalow with vaulted ceilings , lots of glass, but feel its over sized compared to others on the site, does not modulate down well when temps go into double digits, we get loads of cycling Weather curve 44 @- 6 32@8. Keeps the house lovely @ -6 though so not complaining🙂 we on octopus tracker too so keeps costs down. 4932 kWh for the year just on heating and water, dont know cop obvs no energy stuff fitted.

Sounds like moaning, being negative, but honestly not negative about the ashp just the installers.

Posted by: @iancalderbank

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Posted by: @derek-m

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Ian,

I would suggest caution when encouraging people to go for smaller capacity heat pumps, since I feel most would be much happier having a heat pump that may be slightly oversized, but keeps their home warm, as against one that may turn out to be undersized and not keep their home warm.

I fully agree with your sentiment about getting the size correct, but don't forget DHW production and defrost cycles, as well as the fact that ambient temperatures below the design temperature do occur.

agree to a point, undersized and not warm is not good. but we all know the grief with oversized and short cycling.

mattc's 16hr gas data , from a day when it was -5, suggests that a 12kw is total overkill. We await the heat loss data of course, hopefully one done by a heat geek. I'm only trying to set the expectation that the initial sizes he was talking about , are almost certainly too large. not spec what he should actually be buying.

 

I don't disagree with you that 12kW would appear to be too large, I would usually suggest that a heat pumps capacity should to be 1.25 to 1.5 times the value of a reasonably accurate heat loss calculation, dependent upon the heat pump manufacturers data tables.

There are ways to overcome short cycling (6 times or more per hour) and also to reduce normal cycling, which is unavoidable in milder weather conditions, but overcoming an undersized heat pump, while possible, tends to be expensive energy bills.

Thanks for the comprehensive list Ian.

All of the rads already have TRVs (apart from one in hall where the main thermostat is located).  The advice I picked up from Heat Geek was to set them slightly above the main set temp so that they don't cause too much cycling/zoning, which I guess is as good as removing them.  It's going to take a while for habits to change after being told for years that these things would save energy and money.

Posted by: @mattc

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but of course I will have to pay for their survey to know this.

Therein lies the problem.  I would ask them questions about the system they insist on before paying for the survey.  Or just tell them you won't accept certain things and if they go away as a result, you know that you would be wasting your moneys.

@jamespa Good idea. 

One other thing that I haven't mentioned is that 5 of our radiators are on 10mm microbore plastic, while the rest are on 15mm copper - and we've noticed in the past that the rooms on microbore have been harder to keep warm in cold weather.  I've had different responses about this, some suggesting that the microbore 'might' have to be replaced, other saying that it won't be a problem, plus one suggestion that they might need a secondary pump to maintain an acceptable flow rate.

Avoid midea…. It will come from freedom and be fitted with a LLH or PHX that will cripple the efficiency.

We’re about the same size, 100m2 before we added the extension. 50mm cavity. Heat loss was 6.6 ish kW @ -4deg according to two bits of software.

Settled on a 9kW unit that modulates down to 4. It’s too big, but we aren’t freezing and it isn’t bankrupting me.

Posted by: @mattc

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@jamespa Good idea. 

One other thing that I haven't mentioned is that 5 of our radiators are on 10mm microbore plastic, while the rest are on 15mm copper - and we've noticed in the past that the rooms on microbore have been harder to keep warm in cold weather.  I've had different responses about this, some suggesting that the microbore 'might' have to be replaced, other saying that it won't be a problem, plus one suggestion that they might need a secondary pump to maintain an acceptable flow rate.

According to heat geek 10mm is good for 1.1kW at DT 5.   The heat geek figures are for copper I think, plastic will be a bit less (bit there again you can exceed 0.9m/s flow So of it's marginal id be tempted to suck it and see).  But yes of they are in large rooms you may well need to upgrade some drops.

https://www.heatgeek.com/does-my-pipework-need-upgrading-for-a-heat-pump-with-cheat-sheet/

Water Pump capacity is a separate matter.  Again it's true you may need to upgrade, but equally likely you won't.  The calcs need to be done

Posted by: @hughf

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Avoid midea…. It will come from freedom and be fitted with a LLH or PHX that will cripple the efficiency.

That's a shame.  I was beginning to veer towards Midea. What's the issue with freedom/LLH/PHX?  I obviously need to do some more research, but could you provide a quick summary?

Thanks

@mattc Midea is a good unit, but they'll only sell it to one of their installers, and they'll install it in a way that guarantees performance and minimises call-backs at the expense of operating costs for you.

Heat pumps should be installed direct, open loop for optimum efficiency. Hydraulic separation is to be avoided.