My ASHP Journey in Bonnie Scotland.

Reed_Richards

matt_drummer said:

With a heat pump there are many situations where you can consume more electricity to produce less heat.

Here is an example.

If heating the whole house for a day @20c requires 50kWh of heat and you do that at an efficiency of 400% then you will consume 12.5kWh of electricity.

If you turned off some of the radiators and now only needed 40kWh of heat but your efficiency drooped to 250% then you would have consumed 16kWh of electricity.

In my made up example you have consumed more electricity to produce less heat.

Zoning, aggressive set backs, short cycling and high flow temperatures can all result in outcomes like this.

I think you need to be more specific about why the decreased heat requirement brought about such a big drop in efficiency.  Otherwise it seems as if you are just plucking imaginary figures out of the air.   

If you are talking about short cycling then you could say the same for a gas boiler, possibly more so, but I very much doubt that the effect would be as great as you suggest.  And any decent heating controller will block short cycling from ever happening.

And where are these high flow temperatures coming from?  Typically the heat pump is constrained to a maximum flow temperature, as is a gas boiler and an oil boiler.  If you are using Load Compensation to raise the temperature of the house quickly after an "aggressive" setback then that might well override and boost the maximum water temperature setting in both a gas boiler and a heat pump, giving you a period of reduced efficiency, although you benefit in comfort.  I can't do Load Compensation so for me this never happens.  

Likewise zoning, which could have the same consequence as turning off some of the radiators, should be about convenience.  I have one zone for the bathrooms and another zone for the rest of the house and that's very convenient, allowing me to have warmed towels off the heated towel rails even in summer.  The heat pump isn't working hard when it's only heating that bathrooms zone in summer, maybe just running for 5 minutes every twenty for a while.  I don't see this as inefficient except insofar as I could have cold damp towels instead and save money.          

matt_drummer

Really I don't want to argue with you,

I said my example was made up, just to illustrate what could happen. My DHW example is real.

What is the SCOP of your heat pump, you've had it a while?

I am having problems with my heat pump, its minimum output is more heat than my house requires in these temperatures and I can't run it at really low flow temperatures as my radiators cannot deliver the heat produced at those flow .temperatures even though the house is warm enough, the efficiency is too low so it is cheaper to run at higher flow temperatures at the moment. I suspected this would happen as my calculation of required radiator sizes was bigger than I have if I want to run at those low flow temperatures. My installer didn't do a great job but I already knew that.

Can you explain, how do you use a heat pump that is capable of heating your entire house to heat only a towel rail?

What is the minimum heat output of your heat pump and what is the power output of your towel radiator?

I would assume that the heat pump generates more heat at its lowest output than your towel radiator can handle, where does the excess heat produced go?


I have learnt a lot in the last couple of weeks from some knowledgeable people and I am passing it on. I am no expert and never said I was.

But please stop criticising me, I have had enough of it.

When you want to say something please leave my name out of it and any reference to what I have said.

I will extend you the same courtesy.

Reed_Richards

I have had a heat pump for almost 3 years now.  During that time I have read about heat pumps and watched how mine works.  I am fortunate that my inverter measures and stores data for the power consumption of my house so I can see when the heat pump has been on and roughly roughly the peak power it ran at.  The heat pump has its own meter so I can see exactly how much electricity is consumed.  But I have no means of measuring the heat output so I don't know the day-to-day COP and I can only estimate the SCOP by comparing my electricity consumption with the oil consumption of my old oil boiler; it is somewhere in the region of 3, which is what I was promised.

Any question on this forum is likely to receive answers from people who have differing opinions.  If somebody posts something that I am in complete agreement with I usually tick the "Thanks" option and leave it at that.  If I disagree in some respects I may post my own opinion.  I don't regard having a difference of opinion as personal criticism.  I suppose in some respects it is, but that's how the forum works.  The OP of the topic can then read the various comments and form their own opinion as to which ones to believe.

With regard to heat pumps I do believe quite strongly that there is far too much mystique surrounding them.  I am happy to answer questions about my own experiences with a heat pump.  The best place to ask would probably be to post in the topic I created here: https://forums.moneysavingexpert.com/discussion/6240076/i-bought-a-heat-pump/p1 
   

     

matt_drummer

Nevertheless, please just avoid the temptation to quote me or reference any of my posts.

You are quite entitled to your opinion and I respect that.

Your heat pump and the way it performs may suit you but your assertions are based on, possibly, informed guesses at best, you have no actual data to know how your heat pump is performing, it's just a hunch and a guess. It might even be able to do better if you knew what was going on.

I have a billing grade heat meter fitted., I know exactly what my heat pump is doing and it does not behave how logic might make you think.

Please just avoid me in future, comment on topics you like to, even if I have but please cut out the constant criticism of me. Answering questions is fine, knocking me at every opportunity isn't.

My opinion is as valid as yours and I am no longer interested in whether you agree with it or not.

Just say what you want to say about your thoughts, leave mine out of it please.

cannugec5

I am very happy with the settings as I put them yesterday.
 The house is warm, our bedroom was comfortable (yes, window remains open throughout the year, with the exception of gale force winds and above). The TRV in our room remains off, but the towel radiator in the en suite  is switched on. I was a little concerned the bedroom might influence that but of course I was being foolish.
The Hive indicates  a steady temperature with little cycling. The set back overnight showed a slow decline and similar rise this morning. It was cosy when we got up. 

I kept an eye on the hot water tank and it took 75 minutes to get back to temperature at 2pm. So I might in the future cut back the heating time from the current 2 hours. 

I don’t know how to calculate COP from the controller- in fact from the other forum I think it is not an easy thing/ nigh on impossible with a Grant. I know the stated COP on the documentation but am aware this has not a lot of bearing on real life use. Similarly without a working Smart meter it is challenging to accurately measure energy usage. We are unable to access any of the split rate tariffs until we are smart. 

Speaking of towel radiators.  Against our installer’s advise I requested towel radiators that are considered dual fuel ( yeh, electric ASHP and um electric!). When the ASHP is off in the summer, we can, if we wish flick a switch to simply heat the towels for a wee while. He didn’t recommend it and we have made sure the switches are not going to be switched on by mistake by being placed accessible but obscure. I might never use them, but I didn’t want to regret not putting them in. 

Reed_Richards

cannugec5 said:

I don’t know how to calculate COP from the controller- in fact from the other forum I think it is not an easy thing/ nigh on impossible with a Grant. 

It may simply not be possible.  My LG heat pump does not record enough data to do this and others have paid extra for special equipment to allow them to measure COP.  Although I would love to know my day-to-day COP, this will be strongly influenced by the outside temperature, which also changes hour-to-hour and deconvolving that from changes you make internally could be tricky.

cannugec5 said:

Speaking of towel radiators.  Against our installer’s advise I requested towel radiators that are considered dual fuel ( yeh, electric ASHP and um electric!). When the ASHP is off in the summer, we can, if we wish flick a switch to simply heat the towels for a wee while. He didn’t recommend it and we have made sure the switches are not going to be switched on by mistake by being placed accessible but obscure. I might never use them, but I didn’t want to regret not putting them in. 

That doesn't seem a bad idea to me.  It might well be as good a way as any to give yourself warm towels in summer, particularly if you buy-in to the philosophy that you shouldn't use zones.

Anyway, what's most important is that you have achieved some settings that give you the warmth you want.
  

Strummer22

All of the discussion about short-cycling etc is presumably less applicable for systems with a buffer tank? My GSHP has generally been running at minimum load recently, and when the buffer tank reaches the set point it turns off until the buffer tank temperature drops below the delta T (currently 4⁰C below the set point). How long this takes depends on a few factors but I would guess is 10 - 15 mins. This probably occurs once every hour, i.e. heat pump is on for 50 mins and then off for 10-15 mins, then turns back on.

Is this the kind of short cycling I should be concerned about in terms of efficiency? October's COP is 5.8 so far but this is the most efficient time of year with warm ground and low flow temps so perhaps could be improved even further... 

Reed_Richards

Strummer22 said:

Is this the kind of short cycling I should be concerned about in terms of efficiency? 

If your heat pump runs in cycles of 50 minutes on, 10-15 minutes off then there is nothing remotely short about those cycles.  Short cycling is really when a heat pump or boiler fails to reach its normal operating state.  In the case of a boiler it hasn't reached temperature so combustion is less efficient and perhaps more soot is formed than usual.  In the case of a heat pump, a short cycle would be where most of the hot water only reaches the feed pipes and does not make it beyond the buffer tank.  Short must mean no more than a very few minutes in every case.

I don't know so much about GSHPs but ASHPs don't necessarily have a huge dynamic range of power outputs.  If it's mild out they can run at minimum power but still cycle.       

MikeFl

cannugec5 said:

I don’t know how to calculate COP from the controller- in fact from the other forum I think it is not an easy thing/ nigh on impossible with a Grant. I know the stated COP on the documentation but am aware this has not a lot of bearing on real life use. Similarly without a working Smart meter it is challenging to accurately measure energy usage. We are unable to access any of the split rate tariffs until we are smart. 

I have some crude COP calculations for my Grant, but as you suggest, it's not a straight-forward task.
For 'power consumption' there is a value of 'current consumption' available, in units of 100W, so over a period this can be used to find "energy in". This is, I believe, the energy consumed by the external unit, but will not include any secondary pump (which you'll probably have for a Grant install - they suggest a LLH/Buffer tank for all installations).
For 'power produced' I use the formula Grant mention which is Heat output (kW) = Temperature differential (K) * Specific heat capacity of water (4.2kJ/kgK) * Flow Rate. Flow rate is generally fixed and readable on the flow regulator, and the controller shows flow and return temperatures (to find the differential).  
From these two figures you can find your COP.
I calculate this, not because I feel this is necessarily accurate, but I hope it's consistent, enabling me to see if the adjustments I'm making to the set-up are helping or hurting performance.

I believe some controllers (e.g. Daikin) just show COP on the screen for users to read. Imagine. 

Current COP is around 3. Nowhere near Grant's stated values, but they seem to inflate the values given in the Chofu documentation, which seem more reasonable at just over 4.

Strummer22

MikeFl said:

cannugec5 said:

I don’t know how to calculate COP from the controller- in fact from the other forum I think it is not an easy thing/ nigh on impossible with a Grant. I know the stated COP on the documentation but am aware this has not a lot of bearing on real life use. Similarly without a working Smart meter it is challenging to accurately measure energy usage. We are unable to access any of the split rate tariffs until we are smart. 

I believe some controllers (e.g. Daikin) just show COP on the screen for users to read. Imagine. 

My EcoForest model shows the COP as instantaneous, monthly and annual values. Wonders never cease!