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Heat pump delta T too low - 2c

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(@derek-m)
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Posted by: @toodles

I have only had this Daikin EDLA 8 kW ASHP going for a few days and it is very mild at present; the installers left me with the pump running at 50 degrees C and the circulating pump on full speed. Within hours, I started reducing the set point and eventually left it at 35 degrees as this left the bathroom at a cosy 24.5 - 25 degrees but other rooms were far too warm!. I have gradually being shutting some of the lock shield valves whilst monitoring the radiator temperatures and am now much closer to desired temperatures. My aim is to allow the heat pump set on WC to run 24/7 with no or very little intervention from TRV’s. (As per Heat Geek advice)

I think I am getting close in some rooms but some lock shield valves are only perhaps 1/8th turn open. I observed a Delta T of approx 3 degrees on the largest unrestricted radiators as best I could measure it with an infra red thermometer measuring inlet and outlet points on these radiators. I have been reducing the circulating pump speed in small steps, now much quieter than when at full speed - and the Delta T was less than a degree at full speed.

The wind has increased noticeably this afternoon and this seems to have elevated the water temperature a little and the living room rose from approx 23 degrees to 24 degrees so I have closed the lock shield valve a little more for now. I don’t think mild weather is the best situation for setting up such systems - but it is at least a start! My hope is to have the system run steadily and just set TRV’s about 2 degrees above required temperature as a control.

I have carried out a cop measurement based on what the MMI control console reports to me and including the very over-the-top setting at commissioning, so far the cop is 3.22. I am wondering how low I might set the circulating pump, based on the Wilo on-line manual, I still have some latitude to slow it down further but don’t want to risk overdoing this at this early stage, it is a lot quieter if a TRV is starting to control the flow through a radiator than when at full speed and, as I said the Delta T seems better.

Any advice or comments would be much appreciated please. I appreciate that the installers should (if they were keen) be carrying out a lot of this initial setting up but I appreciate that I have more time to finesse the finer points myself! BTW, the heat loss calculation indicated 6.5 kW and the water heating is totally separate and supplied by a Sunamp Thermino ePV210 (I have 8.1 kWp of solar PV and a Tesla Gateway and Powerwall battery.) Regards, Toodles

As you have stated, the weather is quite mild at the moment, so assuming that the heat loss calculation of 6.5kW is reasonably accurate, you could expect the heat demand to be in the order of 3kW to 3.5kW at the present 11C or so.

I don't remember seeing any detailed operating data for Daikin ASHP's, but would assume that like many other makes, its minimum consistent output may be approximately 25% of the maximum output, so could be around 1.6kW. It may therefore be operating in a consistent manner.

To maximise overall efficiency, it is necessary to fully utilise the full capacity of the heat emitters, which means operating with any lockshield valves or TRV's as fully open as possible, but of course this could lead to some rooms being warmer than desired.

The first objective should be to lower the LWT, by adjusting the WC curve, so that the coldest room achieves the desired temperature with all radiator valves fully open. The heat pump should therefore be meeting the heat demand with the lowest LWT, and hence highest efficiency.

The temperature in the remaining rooms should be brought down to the desired value by adjusting the lockshield valves, with any TRV's fully open. TRV's should only be used to limit any temperature rise due to solar gain or human activity.

So basically, the system should be operated at the lowest acceptable LWT, with valves open to the point that meets the heat demand.

 


   
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Toodles
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@derek-m Thank you Derek, seems to be pretty well what is happening at the moment; I have lowered the LWT by a further degree to 34 and will let things settle down to see if I lose out on bathroom temperature (we like a warm bathroom!) once the sun starts to drop; the bathroom is a solar trap in the day anyway. 2-3 degrees of Delta T is the most I have seen yet and pump is getting down towards its’ lowest setting now. The radiators were replaced in in 8 out of 10 positions with larger capacity to suit the change to ASHP and a dual fuel heated towel rail was fitted above the new bathroom radiator - these radiators seem more than adequate and I don’t think cold winters will overtax them! The bathroom and one of the dining room radiators are running with fully open lock shield valves and fully open TRV’s at this stage - they are absolutely fine [though just 2-2.5 degrees Delta T] I don’t wish to be obsessive about this though! Regards, Toodles 

Toodles, 76 years young and hoping to see 100 and make some ROI on my renewable energy investment!


   
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(@derek-m)
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Posted by: @toodles

@derek-m Thank you Derek, seems to be pretty well what is happening at the moment; I have lowered the LWT by a further degree to 34 and will let things settle down to see if I lose out on bathroom temperature (we like a warm bathroom!) once the sun starts to drop; the bathroom is a solar trap in the day anyway. 2-3 degrees of Delta T is the most I have seen yet and pump is getting down towards its’ lowest setting now. The radiators were replaced in in 8 out of 10 positions with larger capacity to suit the change to ASHP and a dual fuel heated towel rail was fitted above the new bathroom radiator - these radiators seem more than adequate and I don’t think cold winters will overtax them! The bathroom and one of the dining room radiators are running with fully open lock shield valves and fully open TRV’s at this stage - they are absolutely fine [though just 2-2.5 degrees Delta T] I don’t wish to be obsessive about this though! Regards, Toodles 

The DeltaT is primarily dependent upon the water flow rate, the higher the flow rate, the lower the DeltaT.

I am not that familiar with the Daikin equipment, so I would suggest that you have a look through the manual and look for some way of lowering the water pump speed, or increasing the DeltaT, if the controller has any form of adjustable setting.

 


   
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Toodles
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@derek-m At present, the ashp has gone to sleep basking (as is the house) in this lovely sunshine and mildness ;-). I feel sure that when the temperature drops, it will start some warm water flowing again. The Daikin has been set up for heating only and as such did not require an indoor unit to accommodate all that didn’t go into the monobloc in the back garden. The circulation pump is a Wilo Yono Pico; this pump has settings for radiators, fixed speeds 1, 11 and 111 plus further settings for UFH pipes. The pump was set to radiator max speed when commissioned but I reduced this gradually so it is now near to the sloest it goes. I looked at the Wilo on-line manual and am attaching the relevant diagram. I am unsure as to the reference to ‘Head’ but assume this is either the highest radiator point above, or, perhaps below the pump head? In anticipation of further enlightenment, I remain intrigued. Regards, Toodles 

Toodles, 76 years young and hoping to see 100 and make some ROI on my renewable energy investment!


   
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Toodles
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I am not sure the diagram I attempted to attach left me so I’ll endeavour to do better this time. Toodles

24991C2A DDF0 418C B157 65AEDFE347F8

 

Toodles, 76 years young and hoping to see 100 and make some ROI on my renewable energy investment!


   
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(@derek-m)
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Posted by: @toodles

I am not sure the diagram I attempted to attach left me so I’ll endeavour to do better this time. Toodles

24991C2A DDF0 418C B157 65AEDFE347F8

 

If you have a vertical pipe approximately 10 metres in length and fill it with water, the pressure at the lower end of the pipe will be approximately 1 bar, this is known as the 'head' pressure. If a water pump is connected to the lower end of the pipe, it will need to produce a pressure of over 1 bar for water to flow out of the top end of the pipe. So the water pump has to do some work just to overcome the 'head' of water in the pipe.

With no water flowing, the pressure at different levels around the system will be the same. i.e. all pipework at a level of 5 metres will be at the same pressure. As water starts to flow around the system it will meet restrictions in the form of valves and even the pipework itself. To overcome these restrictions the pump has to work harder and increase its output pressure. The water pump of course has limitations, as to how much pressure it can produce, and therefore how much water it can push through a given restriction.

In particular with a heat pump, it is better to operate the system with the minimum restrictions to flow, hence opening TRV's fully and ensuring pipework of adequate size, but it is equally important to keep the required flow rate as low as possible.

With your particular system during milder weather conditions, it may be possible to operate the water pump at its lowest speed, provided all heat emitters receive an adequate supply of heat energy to meet demand. In cold weather conditions a higher pump speed may be required.

There is no hard and fast rule, it is very much dependent upon your own system layout and equipment.

 

 


   
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(@heacol)
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@derek-m  Correct but the best way to control flow is automatically with equipment that can do it for you. If your system does not have Delta T control, we would recommend removing the pump from the heat pump and install a Grundfos Magna 3 which will control it properly. We will only sell equipment that is designed and manufactured for best performance. If a heat pump does not have Delta T control, it tells me that the manufacturers do not know what they are doing.

Professional heat pump installer: Technical Director Ultimate Renewables Director at Heacol Ltd


   
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(@derek-m)
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Posted by: @heacol

@derek-m  Correct but the best way to control flow is automatically with equipment that can do it for you. If your system does not have Delta T control, we would recommend removing the pump from the heat pump and install a Grundfos Magna 3 which will control it properly. We will only sell equipment that is designed and manufactured for best performance. If a heat pump does not have Delta T control, it tells me that the manufacturers do not know what they are doing.

You are perfectly correct that there are many systems designed and installed by people who I would politely describe as 'idiots', and should have done the job correctly in the first place.

When offering help and advice I initially look at how to make the present system and equipment operate in an adequate manner, since I don't think that it is always well received, if one tries to convince people to rip out hundreds of pounds worth of equipment, that may have only recently been installed. By convincing people that their poorly performing system can be improved, with changes in settings and operating procedures, they are then more receptive to messages about further improvements that could be achieved by changing equipment to more appropriate types.

 


   
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Toodles
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@heacol When I magnify the symbol on the Wilo pump, I can see that it is Delta and on the range I am using, the automatic sensing is taking care of that aspect then! I am leaving the pump on its’ lowish setting (as indicated by the smallest house outline on the pump speed scale) and I would expect it to cope with the demand whilst mild; I’ll keep an eye (I actually only have one eye anyway!!!;-) ) on the radiator output and I think this should tell me if the cold weather takes it out of the range covered by this pump setting. Regards, Toodles

Toodles, 76 years young and hoping to see 100 and make some ROI on my renewable energy investment!


   
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Toodles
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Regarding the head; the only pipework in the system that is above the pump is approx. 0.5 metres of pipe from the pump’s output to the heat exchanger which is at most 30 cm. above the pump itself - all other pipework is below that level. The pump is situated in the airing cupboard and the lowest point in the pipework downstairs is approx. 3.5 metres lower. Oh and the heat pump has woken up now the temperature has dropped. Looks like next weekend’s weather may provide conditions for trying out the system at lower temperatures. Regards, Toodles

Toodles, 76 years young and hoping to see 100 and make some ROI on my renewable energy investment!


   
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(@derek-m)
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Posted by: @toodles

Regarding the head; the only pipework in the system that is above the pump is approx. 0.5 metres of pipe from the pump’s output to the heat exchanger which is at most 30 cm. above the pump itself - all other pipework is below that level. The pump is situated in the airing cupboard and the lowest point in the pipework downstairs is approx. 3.5 metres lower. Oh and the heat pump has woken up now the temperature has dropped. Looks like next weekend’s weather may provide conditions for trying out the system at lower temperatures. Regards, Toodles

I suppose that 'head' is relative, since the pump will have to work to 'pull' the water up the 3.5 metres.

If you don't have more than the specified number of radiators, I would assume that your water pump should perform adequately at the lowest setting.

 


   
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Toodles
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@derek-m Indeed, I agree with you entirely! This is the initial set up and although the installers have made a good neat job of the physical installation, they have suffered from a project manager’s ineptitude to the degree that after two weeks, some of the radiators had not been fitted due to non-arrival (Not ordered in time I suspect), finally all the radiators arrived but I am still awaiting the arrival of the Eddi relay card to control the dual fuel towel rail in summer. The electrician didn’t bother to check though I had written it all down for the project manager to pass on; he wired the towel rail to the secondary circuit on the Eddi - didn’t think to ask me why I had specified the relay card!

At one point, I enquired when the system was to be commissioned and the radiators balanced and was told they had balanced the radiators - there were still three missing at this point! Little things like lagging the pipework to and from the Sunamp Thermino had not been carried out and when I requested it be done, they asked me what needed lagging? There are still exit holes where they brought out power cables through walls that havent had repairs to them and with all the delays, they have been anxious to leave after a full days work so commissioning has not been given the attention it might have.

I am investigating the fitting of a Homely controller when a compatible version for Daikin is rolled-out and this will enable me to take greater advantage of Octopus Agile variable tariff rates as well as run the pump most efficiently. I don’t think I will be asking the installers to do this and I will be going directly to Daikin for annual servicing and fault insurance. One more visit from the electrician is booked for tomorrow when it is hope, he will complete his work. Regards, Toodles

Toodles, 76 years young and hoping to see 100 and make some ROI on my renewable energy investment!


   
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