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. 
 

[JohnnyB]

Thanks for all the input. I'm going to have to make time to see is the HP is cycling, watching it for 10-15 minutes it seems to be fine, I assume the fan would stop if it is shutting down. 

I'm also trying to read up on heat pumps and heating, exactly what I didn't want to do and why I used plumbers who are supposed to know what they are doing but there are so many things you are saying that I don't really understand. I'm trying to build 2 houses and thought this was one thing I could leave to others to sort easily, didn't realise HPs need to know so much just to run them efficiently!

I had a message yesterday from the plumber and they have another unit like mine that is getting a similar COP which the client has full monitoring on. They have emailed Samsung for advice and he is going to try to get out to me before Christmas to take a look and see if he can see anything obvious.

 

I have been looking for more info and it seems that the video linked to above, and ringi, aren't the only people saying really low flow temperatures don't equal a good cop. If I set the flow temperature so it's leaving the HP at 30-35deg I would probably have the heating on 2 or 3 times per day to warm the floor up, but that isn't as comfortable as having the flow temperature at 23-25 deg and the floor at 20-22, which kept downstairs at a fairly constant 20deg. The higher temp has helped the COP, it was 2.8 yesterday, but it was off all night and has taken a long time to get the floor back up to temperature this morning.  The problem of a well insulated house and floor, it doesn't need much heat.

Maybe I can set the system so it uses the floor temperature as the set point instead of the room temperature, put water in at 35deg but switch off when the floor gets to 25deg and come back on when it cools down to 22 or something like that. Should be slow heat and cool down times for the floor. I have 60mm screed with 2 layers of 100mm PIR insulation beneath and it seems to hold it's heat for several hours.

 

On 16/12/2024 at 16:14, Beelbeebub said:

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. 

By LLH are you referring to my buffer tank and pump?  What you are saying is the opposite of what I thought I was understanding when reading on the renewable heating hub forum, that the main pump should be running faster than the secondary pump.

I have noticed this valve is set to a low pressure and the water seems to be passing along the shortest distance.  I was told this is to allow water through if the valve doesn't open and the pump starts, but I can't see why I even need the valve there as the pump only starts if it is open and it doesn't divert the water, just stops it flowing.  I do have a diverter valve on the main F&R the switches the flow between DHW and heating.

As mentioned before I didn't want the buffer in there but was told I had to have it.  If they can't sort this sufficiently I will insist it is removed. I really can't see the need for it when I can't close the loops, there is no zoning and I have a lot more than 50 ltrs in my pipework without the buffer, they have fitted 32mm flow and return to the HP and the buffer, my DHW cylinder and buffer are 3-4 mtrs apart because of space issue with 32mm pipe between them, must be about 15mtrs of 32mm pipe

 

[JohnMo]

32 minutes ago, JohnnyB said:

mentioned before I didn't want the buffer in there but was told I had to have it. 

Interesting, reading another post kno idea which) but it had this Samsung drawing. So you have two options buffer or volumiser. See attached. Convert to volumiser?

 

No idea why you need a buffer and min flow bypass valve.

 

quick-start-guide-samsung-gen-6-kits.pdf

[JohnnyB]

I don't think I need a volumiser either, there is more than 30 ltrs of water in my UFH pipes and another 10 ltrs in my F&R pipes, plus everything else.

Would changing it for a volumiser make much difference to the efficiency? 

I'm starting to understand there's probably a lot of things in my system that's not needed and it could have been a lot simpler

[marshian]

1 hour ago, JohnnyB said:

I don't think I need a volumiser either, there is more than 30 ltrs of water in my UFH pipes and another 10 ltrs in my F&R pipes, plus everything else.

Would changing it for a volumiser make much difference to the efficiency? 

I'm starting to understand there's probably a lot of things in my system that's not needed and it could have been a lot simpler

That’s not a huge circuit IMO

 

On the bypass valve (my system is boiler rather than HP) but I have it set to only open when the circuit pressure is driven up to the point where just 1 rad is in circuit (from memory that’s about 0.3 bar on the scale) and that stops me dead heading the pump and keeps the flow rate up enough for the boiler to fire once flow temp has dropped enough

[JohnMo]

4 hours ago, JohnnyB said:

don't think I need a volumiser either, there is more than 30 ltrs of water in my UFH pipes and another 10 ltrs in my F&R pipes, plus everything else.

I think without actual doing calculations - another 50L would be good. It will make the on and off times longer. Less cycling equals longer equipment life.