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Author Topic: Expected efficiency of ASHP for domestic hot water?  (Read 1836 times)
DanielCoffey
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« on: May 28, 2019, 09:33:00 PM »

Hello folks - I would appreciate a "finger in the air" figure of the expected efficiency figure (consumed vs delivered) of an ASHP providing domestic hot water in the following situation.

Assuming correct installation, a clean machine and proper maintenance...

Model : Mitsubishi Ecodan 11kW
Age : 2 years
Domestic hot water target : 50C
Average outdoor temp : 12-16C

The situation is that our unit is not reporting the efficiency figures advertised and I am wondering if it is mis-reporting the actual results, is faulty or if I just have unreasonable expectations.

In the summer months when it is in hot water only mode, it is reporting an efficiency figure of 1.0 to 1.1.

In the winter when I have the underfloor heating on as well the efficiency increases to 2.30 to 2.40 due to the lower target temperature of the heating water but the outdoor temp is at or below freezing.

The engineer who serviced it lowered the target DHW temperature from 55C to 50C and it barely made any difference.

Thoughts?
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brackwell
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« Reply #1 on: May 28, 2019, 10:14:00 PM »

The efficiency figure (COP) can be found in the spec sheet for the product sometimes in the form of graphs otherwise figs at different ambient temp and outlet water temp. For 15C ambient/50C out i would expect it to be between 3 and 4 COP

Remember that a ASHP has to be used a lot different from a boiler in that it needs to be more or less permanetly on so that the water temp into the ASHP is only a few degC below your 50C.  The modern inverter ASHP reduces the energy input as the in/out temps close and the efficiency increases.  I think you maybe achieve very low COP if you use it like a boiler and expect it to heat cold water up to 50c in quick time and further some HPs also have leccy heating element to get the water to the desired temp if so programmed.

There are more knowledgeable people than i on here.

Ken
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marshman
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« Reply #2 on: May 29, 2019, 09:52:54 AM »

If the COP is really only 1 or 1.1 then something is obviously wrong. My own GSHP heating DHW only achieves a real world figure of around 2.5 (calculated from temp rise in DHW tank and electricity consumption of Heatpump. That is heating the tank to 55 deg C - at which point the Heatpump cuts out - with ground loop temps around 8 deg C)
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Not familiar with Ecodan units, but they do have a good reputation, Does your unit have an auxiliary heater for the DHW? If so I would suspect that this is coming on (not really any different to am immersion heater) and it will clobber the COP figure. As I recall on my GSHP the auxiliary heater was enabled by default, I have ensured it never comes on! Is there a "legionella cycle" set up, where the tank is heated up to above 60 deg C? if so how often is this triggered?

Finally how do you know the COP is only 1 or 1.1? and is it really this low?  Is it an accurate figure?

Roger
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DanielCoffey
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« Reply #3 on: May 29, 2019, 10:18:32 AM »

Thanks for the replies folks. It is really appreciated.

I got the figure from the unit's little display panel. It gives the kWh consumed and delivered which was why I was wondering if there was a simple software glitch causing a mis-reporting of the results. Today for the month to date it is showing 85kWh consumed and 89kWh delivered and we are on hot water only of course.

Yes there is a Legionella cycle and it is triggered once a fortnight I believe. The immersion icon certainly triggers for that cycle as I have seen it. I don't believe the immersion is triggering for the regular DHW warmups as the icon shows just the heat pump active (unless it enters a defrost in the winter when it is cold and damp of course).

The engineer was mystified too as the unit certainly appeared to be operating as expected three weeks ago. He dropped the target DHW temp from 55C to 50C but it made no real difference to the month's figures compared to last year.

I will ask them to feed the issue back to Mitsubishi.
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pdf27
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« Reply #4 on: May 29, 2019, 07:57:54 PM »

If the COP is genuinely ~1 then you're basically running in resistance mode only. Is the compressor actually running?
Also, the Ecodan PUHZ 11kW should have a 32A fuse - that means if it delivers more than 7.4kW of heat at a COP of 1 it'll blow the fuse. Since that's 75% of your designed heating capacity, it rather suggests that if this is really a problem then you should be struggling to keep warm on a cold day.

Have you got monthly-ish electricity bill data available? If the unit is showing 85 kwh used but you've only used 100 kWh for all causes that would be pretty strong evidence that the logging is faulty...
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andrewellis
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« Reply #5 on: May 29, 2019, 08:04:22 PM »

It is worth doing a manual calculation as Marshman did. Turn the heat pump off and empty your cyclinder completely of hot water. Take a reading of the temperature of the water in the tank and the electricity meter reading powering the heat pump. Then turn it on and wait for the whole cycle to complete.  Then take a top/bottom of tank reading and take an average. Work out power used from meter. The power required to heat the water will be
1.16 * litres in tank * (after temp in Celsius - before temp). Then work out cop from electric reading and manually determined power to heat tank. This will give you an idea of the cop being achieved. My NIBE heatpump has the ability to do cop calculations. However it doesn’t have all the kit fitted so all the numbers produced are garbage.

At this time of year I’d expect a better cop than my gshp if you have warm air.
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DanielCoffey
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« Reply #6 on: May 29, 2019, 08:30:02 PM »

The unit does have a 32A breaker, yes, and certainly seems to be operating when the fan is running because the air exiting the unit is definitely pretty chilly!

My non-ASHP electricity usage seems to be fairly consistent at about 300kWh each month plus whatever the ASHP itself reports as consumed. By this I mean that in the summer my metered usage is about 400kWh of which 100kWh is shown on the ASHP console. In the winter I peak at 750kWh a month with 400kWh on the ASHP console. A finger in the air is a steady 300-350kWh a month on non-heating electricity.

I don't have the means of taking direct readings from the tank I am afraid. I would have to rely on the average temperature reported on the ASHP console. The tank itself is 170L.

Would I get an adequate COP reading if I disabled the ASHP on its console, ran the hot tap till cold emerged, noted the ASHP console temp, took a meter reading, ran the ASHP till it terminated at 50C, took a new meter reading, checked the actual usage against the reported usage and then did the math? I know the background usage of the house would still be ticking under all that.
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andrewellis
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« Reply #7 on: May 29, 2019, 09:25:58 PM »

The readings from the heatpump are what I use. It just happens to have a top and bottom sensor. An average reading would give a reasonable ball park figure to see if you are 2.5+cop or 1.  Can you disable the house heating on yours?  The heat pumps do either the house or hot water but not both at the same time. Also, they can do for example 30 minutes of dhw and then 30m only heating. You might have a way of disabling the ashp above a certain oat .

For a family of 4 with showers and a bath maybe every other night we use about 150-180kwh a month for hot water with a gshp. Cop about 3 in the summer
« Last Edit: May 29, 2019, 09:28:05 PM by andrewellis » Logged

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DanielCoffey
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« Reply #8 on: May 29, 2019, 09:43:20 PM »

Yes, DHW and Underfloor are separate and the heating is off now anyway so this will be a clean test of the DHW only.

Rather than wait till tomorrow I decided to check it now. I ran the tank until the water at the tap was cold and read 12.0C on an IR thermometer. There would have been a little residual heat in the tank insulation but this will do for a ballpark. I took an actual meter reading and then put the DHW on manual. Ambient air at the moment is also 12C according to my MVHR sensors. I can confirm that the immersion is NOT running while the DHW is active.

Interestingly it looks like the unit is running the DHW side in "Normal" rather than "Eco". Any negatives to running it in Eco from now on? I do know the DHW timer has settings for a permitted time window, a required temp drop, a delay between successive operations and also a maximum run time. That last one is the one we might bump into on the Eco setting.

Anyway, the tank is coming back up as we speak. I'll report the figures when it shuts down.
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DanielCoffey
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« Reply #9 on: May 29, 2019, 10:23:40 PM »

OK, here we go...

Time start : 21:15
Meter : 13499.1kWh
Water temp : 12C

Time finish : 22:15
Meter : 13502.1kWh
Water temp : 51C

Ambient air : 12.2C
Tank size : 170L

Background household electricity usage in 1 hour is 10 kWh per day / 24 = 0.42kWh

So COP is 1.16 x 170L x (51C - 12C) / (3000W - 420W) = 2.98

Now that is MUCH better and more like it! I now strongly suspect that the unit is not reporting Energy Delivered properly since the manual mentions flow rates over different pumps and all that is set up in the Engineering side which is invisible to me.

Thanks for the guidance on how to find all this out! We can stop worrying about this now and just get the engineer to set up the reporting properly. They are due back anyway because this is logged as a fault under our Service Contract.
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andrewellis
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« Reply #10 on: May 30, 2019, 10:38:23 PM »

Cool, although rough and ready figures, they won't be far out.  Nice to know it's working for you.
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