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Ecodan appears to be ignoring Temperature Compensation

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(@mcwatson1974)
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@derek-m

20231128 091507

Current outside temp according to MELCLOUD is 5c.

Acknowledge the low end is very low but if I increase it higher to enable a higher flow temp, rooms don't warm up particularly but KwH usage gets hammered. And I don't understand why the flow temp gets affected by the hot water tank heating up overnight, thought it would be unrelated.


   
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(@derek-m)
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Posted by: @mcwatson1974

@derek-m

20231128 091507

Current outside temp according to MELCLOUD is 5c.

Acknowledge the low end is very low but if I increase it higher to enable a higher flow temp, rooms don't warm up particularly but KwH usage gets hammered. And I don't understand why the flow temp gets affected by the hot water tank heating up overnight, thought it would be unrelated.

The WC curve needs to match the heat loss of your home. What is the heat loss calculation?

When heating the hot water cylinder, the heat pump increases the LWT to above 50C, otherwise it would not be able to heat the DHW to the required temperature. With your present WC curve, the heat pump will be supplying water in the mid 20's to your UFH, which will not be sufficient to warm your home. You need to set your WC curve to more appropriate values, something like 45C LWT @ -5C OAT and 25C LWT @ 20C OAT.

You can then fine tune the WC curve to match the heat loss for your home, but this can be a lengthy process.

 


   
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(@mcwatson1974)
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Posted by: @derek-m

What is the heat loss calculation?

Heat loss calculations below.

image

OK, will adjust the weather comp settings. Maybe I'm being too impatient for them to kick in but thought half a day or more would be enough, after that I've been reverting as watching the electric bill going north with no temp gain.


   
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(@harriup)
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Posted by: @mcwatson1974

I then reverted to Weather Comp. House seems to warm up better when I run at 30c or under which makes no sense to me but I do use less KwH but ASHP does cycle much more per hour. Still struggle though to get the house to the desired 21c that I'm striving for when it drops below 5c outside. This is an example of an hour of cycling.

-- Attachment is not available --

But the bit that is confusing me the most. Why does the UFH behaviour seem to be affected so much by my heating of the Hot Water. Currently got my HW coming on at 3am whilst on cheap tariff and heating to 52c which then usually lasts me the day. But the flow temp of the UFH system seems to 'follow' the heating of the HW. This graph is 3am to 4am.

-- Attachment is not available --

And then once the HW has achieved the 53c, the flow temp then starts reducing. But all this time, the house isn't warming up, infact room temps start falling. This graph is 4am to 5am.

-- Attachment is not available --

There is no cycling at all from 3am until around 9am or 10am when the flow temps have all reduced to a particular level, and then all the cycling kicks in. Personally, my objective is to get rooms to 21c and not use a zillion KwH in doing so over reducing cycling. 

-- Attachment is not available --

My instinct suggests I'm running my UFH temp too low at sub 30c, but if I whack it over that and closer to 35c or 40c, I simply use loads more KwH but without the benefit of warmer rooms.

Generally I would say you need to work out how to get your system heating the house as you want it, then work out how to do it more efficiently, then work out how to reduce costs by compromising how well you heat the house. If you try to do all three at the same time you won't know what is doing what.

That first and fourth graphs show how consistent the return temp stays with UFH, it cannot drop below the temperature of the slab. It also shows how trying to have a flow temp barely above the temperature of the slab causes the pump to cycle manically because it even at its lowest power it needs to raise the temperature of the water by two or three degrees, but it can't without going above the WC set flow temp.

The second graph shows the flow temperature produced by the pump when it is in the HW cycle - the temperature is measured before the water is diverted to the either the tank of CH loop, so it is not specifically CH loop temperature. As discussed earlier in the thread when the HW cycle is on the CH should be off - so yes, there will be some cooling of the house during this, though UFH should continue to have some heat to supply.

The third graph continues to show the temperatures of the circulating water, the remaining heat is being delivered into the slab but until the return flow falls to a temperature below the target temperature defined by the WC curve the heat pump will not restart because it is not being asked to add heat.

 

 

Mitsubishi EcoDan 8.5 kW ASHP - radiators on a single loop
210l Mitsubishi solar tank
Solar thermal
3.94kW of PV


   
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(@mcwatson1974)
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I think I'm making progress finally!

Taking Derek's advice on adjusting my WC settings, I set it to 45C LWT @ -5C OAT and 25C LWT @ 20C OAT and the rooms all heated up so I can only assume the setting I had before were too low for the heat loss of the house.

Flow Temp seems to be around the 33c to 35c area depending on the outside temp and the rooms are all sitting at around 22c so have been gradually lowering the offset on the FTC controller till the rooms get to the desired 21-21.5c temp.

Weirdly, when I had the system set on fixed flow temp and set it at 45c, the rooms wouldn't heat up at all but just used loads of electricity. Not sure why but also not that bothered now that the rooms are where they need to be.

I've noticed that my cycling has reduced to be about 1 or 2 an hour rather than the 5 or 6.

Unfortunately I'm still using loads of KwH but being its around -2c to 2c over recent days, can't be helped I guess. COP is pants as well, 2.88 yesterday.

Below is the flow temps, peaking during the night when the hot water gets warmed up.

image

Showing that its only cycling the once or twice an hour.

image

Just a shame it uses so much electricity to achieve, 40 KwH just on heating alone yesterday. Still got a little lowering of flow temps I reckon to get the rooms to drop the half a degree or so. Be nice also if the ASHP got the flow temp to the desired temp and then had a break for a bit.

image

Thanks everyone for your input and assistance. ASHP settings are a minefield.


   
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(@derek-m)
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@mcwatson1974

I think that you will find they are defrost cycles due to the cold and damp weather.

Energy consumption should reduce approximately 10% for each 1C lower you set the IAT.

It may be more efficient to heat the DHW during the mid afternoon period when OAT should be higher, unless of course you are taking advantage of a cheaper overnight tariff.


   
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(@mcwatson1974)
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Topic starter  

Posted by: @derek-m

It may be more efficient to heat the DHW during the mid afternoon period when OAT should be higher, unless of course you are taking advantage of a cheaper overnight tariff.

I get a cheaper tariff between 12.30am and 4.30am, is 8.9p per KwH versus the usual 31p so try to do as much overnight as I can.


   
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