Can the flow temperature be too low?

[JohnnyB]

I have recently had a 5kW Samsung gen 6 heat pump fitted, been running about 10 days, and I had a look at the energy used and the energy generated on Friday and I'm getting a COP of about 2 so something isn't right. I've contacted the plumbers who are going to take a look but I wanted to get a few thoughts from here on how I am running it.  The plumbers are known locally as being good and only fit heat pumps so they should be able to sort it but always good to have some knowledge before they try to explain things away.

 

It's a new self build with UFH and I have been running it 24/7 with the water outlet at the pump between 23 and 30 deg, regulated by the WC. The temperature on the floor is about 20-22 deg which keeps the house around 20 deg.

Is there a point where the flow temperature can be too low so the heat pump is cycling and not running efficiently?  Should I be using the thermostat in the house to switch the heating on and off with a higher flow temperature instead of running it low and constant?

Anything else I should be asking about?

[IGP]

I think Glyn Hudson found this exact thing, he starts talking about it around 8mins in the video 

 

 

 

 

[JohnMo]

1 hour ago, JohnnyB said:

is there a point where the flow temperature can be too low so the heat pump is cycling and not running efficiently?

Yes - look at cycle times.  I found the longer the cycle time the better the efficiency generally. Every floor and house is different.  I found a min on time of 10 mins was ok, but 20 mins was better.  I have recently set my WC curve to 30-35 and use a 0.1 hysteresis thermostat.  The heat pump runs generally at night and runs continuoisly for many hours.  It then generally remains off for the rest of the day. 

 

Also make sure you don't have any small zones calling for heat, this can drive down efficiency as you get lots of very short cyles. 

 

Heat pump running isn't that black and white, although many will make out it is - stick it on WC your in the best place - not always true as the video above demonstrates

[JohnnyB]

@IGP Thanks for the link, I watched the video then read he has updated to firmware so it works with a PWM pump and he can drop the flow temperature lower without effecting the efficiency.  I don't have a PWM pump but maybe my pump would be better on a lower speed?  Something to discuss with the plumber.

 

@JohnMo I only have the Samsung controIs and can't see details of what is running when and how long it is running for etc., is there an easy way to find out cycle times without an energy monitor like you have on your system?

We don't have zoning, there are no actuators on the UFH manifold. I've read lots of your comments on here so have been trying to keep it simple, although the plumbers have insisted we have to have a buffer for the warranty and one pump for the HP and another for the UFH pipework so it isn't as simple as you would recommend.

I think if I set mine to 35 deg we will be rather warm and our feet will cook, I haven't been above 30 at the water outlet yet and that was already quite warm, although if it only heated the slab to a lower temperature and then switched off we could probably control it that way.

 

[JohnMo]

48 minutes ago, JohnnyB said:

is there an easy way to find out cycle times without an energy monitor like you have on your system

Go outside with a cup of tea and seat, monitor what occurs over an hour. If it's cycling a lot, you will be done and dusted in 20 mins. If you have a smart meter, you should be able see increase in electric consumption and how long it lasts also, monitor flow temp on the controller etc

 

50 minutes ago, JohnnyB said:

plumbers have insisted we have to have a buffer for the warranty and one pump for the HP and another for the UFH pipework

That is just nonsense really. In that case make sure the flow rates are matched on all pumps and both pumps run together. Do you also have an UFH mixer? I assume not. You can check by measuring the flow into and out of buffer on the bottom ports, that need to be the same if both pumps are the same flow rate.

 

Your Samsung controller will be able to control various ways. Think one way is WC and it's built in thermostat working together.

[IGP]

From what I understand, the manufacturers actually don't when they are pushed by people who know what they're talking about. They include buffers / LLH's in the manufacturers instructions, but it's just excess stuff to prevent callbacks. 

1 hour ago, JohnnyB said:

although the plumbers have insisted we have to have a buffer for the warranty

 

[JohnnyB]

@JohnMo If only I had time to sit for an hour and see what the pump is doing, I'm trying to finish off the house! 

I don't have a smart meter indoor display, but the Samsung controller will show the power being it used by the HP. I'll have to watch that and see if it shows anything interesting.  It's a real shame they don't give access to proper data.

 

My pumps are identical and both set on the same setting, plumbers said that was necessary. They are mostly both on at the same time, apart from when the DHW cycle is on or occasionally one to the HP will be on without the other for a few minutes, the HP pump will occasionally start up without the thermostat calling for heat.

 

I've turned the flow temperature up and will turn it off when it gets to 4pm and see what happens to the indoor temperature. (I'm on Octopus Agile which isn't a great tariff for a cop of 2 at the moment!)

 

The plumbers told me the internal thermostat isn't great and that I needed an external one, but in the video @IGP linked to Glyn is using the internal thermostat and it sounds like it is working well for him so that could be an option.  Glyn had the flow temperatures basically the same in the WC settings as the COP dropped if the flow temperature dropped below 33 deg.

Edited Monday at 15:50 by JohnnyB

[JoeBano]

Since I replaced my radiators with fan coil units and dropping my flow temperature down to 32c it was

short cycling a lot. Managed to balance my pumps got better run time, turned my max flow temperature to 35c and now getting 4 hours run times. I don’t know why but my grant aerona3 doesn’t like super low temperatures.

 

[Beelbeebub]

If the flow round the HP is much higher than the flow around the UFH loop you will get the warm water "short circuiting" through LLH/Buffer. 

 

The fact your two identical pumps are set to the same may be a clue. The pressure drop around the HP to LLH loop will be low so the flow, higher for a given pump speed. 

 

The pressure drop around the UFH loops will be higher, just because there is alot more distance. So the same pump.setting will result in a lower flow. 

 

If the flow around the UFH is lower than the HP, you'll get the HP short cycling. 

 

Ideally you want the two flow rates to be identical but that's near impossible. I would err on the side of the UFH loop being a bit higher flow. This will reduce the temp around the loop below the level the HP produces, but shouldn't be a major problem with the low temps a UFH system uses and should minimise short cycling. 

[JohnMo]

54 minutes ago, JohnnyB said:

linked to Glyn is using the internal thermostat and it sounds like it is working well

He is running radiators - the hysterisis is different. If you need an external one Computherm Q20RF are nice, you can adjust the hysterisis down to 0.1 and do cooling. Hysterisis will affect over shoot and under shoot of room temperature.

 

Cycling is basically based on output compared to how easily the heat generated can be moved away. A low dT between room and water makes this more difficult than a bigger dT. With UFH it's how quickly the floor can absorb the heat and at the same release to the air.

[ringi]

If the heatpump is getting many short cycles and the UFH have the thermal mass to cope, what about a time switch that enables heating for 15 minutes per hour?  (Set min flow temperature to something like 30c, as most heatpumps don't seem to improve COP if lower.  Increase heating time per hour in very cold weather.)

 

(One of the reasons I am thinking of 100mm pipe spacing, so I can get heat from heatpump quickly into UFH thermal mass.)

Edited Monday at 22:18 by ringi

[G and J]

17 hours ago, JohnnyB said:

 If only I had time to sit for an hour and see what the pump is doing

A thought.  Stand an iPhone or iPad (or rather low rent android equivalent lol) next to your heat pump and set it to record on voice memos.   Go back an hour later and you can see a graph of the noise it produces.

 

I’ve just done a quick test using my voice (best I can do right now as we are a heat pump free zone) and this is what it looks like.