The item I show is exactly like the condenser used in almost all water-heating heat pumps. (search images 'heat pump plate condenser'). The copper-aluminium heat-exchangers are for air, and typically Evaporators in ASHPs.

Sinmple answer.. dt of 3 to 5 ? Its actually hard to be specific, and may depend on the pattern the floor pipes are laid, and also on the temperature its designed to run at. So, if the dt is too much, then the flow to the floor loop is significantly warmer than the return back, so the floor area at the start of the loop is notably warmer than the end of the loop.... the floor feels uneven, and may, or man not be warm where you want it. If the flow is 45 and return 40 (dt5) then uneven-ness may not be too much, But if the flow were only say 30, and return 25 (dt5), then the unevenness could be significant. That said, uneveness may be more acceptable when its fairly mild out. The other thing to consider is this... you would emit about the same heat (kW) from a system with flow 45C, return 35C (dt10), as you would with a floor with flow 42C and return 38C (dt4). (mean water temp is the same) The COP should be a little better with the lower flow temperature, even though the return temp is higher. Finally, a spiral pipe design wont have the one-side-of-floor cold problem. Personally, i like the simplicity of having no mixer, but some people would advocate having a mixer and pump so that you can have the floor dt closer than the HP dt. I dont think the dt at the heat pump and for the floor are actually so fussy

Its a little hard to be specific here, but generally, heat pumps are operated for longer hours, at a lower temperature, so the kW output is less, hence dt less. e.g. 2hr 'blast' of 20kW boiler at dt10 = 28lit/sec. Same heat as 4hrs of 10kW heat pump with dt5, also 28lit/min. So, often, the pipework can be good enough. Circ pumps are much lower energy now, and 8m head is not uncommon now. Not that you want to operate at the top end of the pump. I suggest you could play with my simulator to look at each radiator. I also have a pressure drop simulator, but you could also search 'online pressure drop' for better pressure drop tools. Ecoforest have quite high pressure pumps within them, and these automatically adjust themselves to achieve dt of 5. I think the maximum pressure is around 10m head, and the power consumption quite low. In the days of old circ pumps, the pump wattage was a problem... less so now! heatpumps.co.uk/technical/flow-rate-and-pressure-drop-simulator/

@@johncantor4056 hi John, thanks for the reply. It is not considered good practice to exceed 1.5m/s in waterpipe, maximum should be 2m/s. In 22mm pipe this equates to 24-36 litres per minute. You can see why just adding high pressure pumps soon exceed this number even if it can be achieved. It is better, I believe, to fit several low pressure pumps to the system so that each can overcome the back pressure of that part of the system. However, it does not overcome the problem of high flow rate required for low delta systems. Could be the customer needs bigger bore pipes

I have never found problems with velocities well over 2m/sec, apart from excess pumping power, but this is less of a problem now that motors are more energy-efficient. I haven't had noise problems in pipe, but have done in valves where the velocity could easily be a lot higher. Anyhow, to the pump pressure issue... I cannot see any difference between say one circ pump operating at 6m head, and two (at different points in the circuit) operating at 3m head each.

@@johncantor4056 Thanks for an excellent tool John, its very interesting.

Hello Sam, I was trying to explain the situation where the flow-rate is low and hence the dt is large. The warm water entering the radiator rises to the top. The bottom of the radiator ends up being relatively cool. More heat is given off from the top warm bit of the radiator. If the flowrate were however very high, and the radiator quite even in surface temperature, then the radiant output is evenly spread, but now more heat is transferred to the air at the bottom of the radiator because the air is cooler here. Does that make sense?

@@johncantor4056 oh sorry I meant when you were holding up the heat exchanger

​@@SamDuke474 Ah.. OK. I did some graphs to help explain that... cannot find them!! I will try to seek them out. I need to tidy up my website!!

@SamDuke474 Sam, the graphs are at the bottom of the page here heatpumps.co.uk/cop-estimator/ Thanks for your initial question. You are the first to highlight this badly explained bit, so thanks for that. The point that I am trying to make is that with a condensing refrigerant, there is a constant temperature over the heat-exchanger’s refrigerant passageway (relating to the vapour pressure), but with a low water flow-rate, there is a large temperature variation along the water passageway. So, if the water flow-rate is low (hence dt [flow-return] is high), then most heat-transfer happens where the entering water is relatively cool. Little condensing happens at the top where the water temperature is close to the temperure of the refrigerant. (Left section of graphs ‘B’ & ‘C’). As a separate point, the vapour at the condenser inlet is superheated. It is many degrees higher, but this is a relatively small proportion of the total energy, and this vapour drops to ‘condensing temperature’ after passing down somewhere around 5 to 10% of the condenser. Anyhow, the point is that a fairly high flowrate (small dt) is generally desirable for best usage of the condensers surface. However, the CO2 Transcritical heat pump is different, an no condensing actually happens in the ‘condenser’. The CO2 refrigerant inlet experiences a gradient, and leaves the ‘condenser’ much cooler. So, with this refrigerant, a slow flow rate, and large rise in water temperure is desirable.

​@@johncantor4056 makes total sense thanks. I wasn't sure it was as you show in your graphs or whether the refridgerant would also get supercooled and effectively mean the 'condensing' would happen over a small distance, limiting opportunity for heat transfer and forcing slower refridgerant movement. This also makes the CO2 transcritical behaviour make sense too! As you say, it is effectively a high glide refrigerant. Do you know if natural refrigerants like propane have a similar glide? I had heard some reference to it... Looking at the phase-change and specific heat capacity charts of CO2 it seems you can actually dynamically target different dTs by changing the pressure of the CO2, and therefore the 'width' of the transcritical region. Along that note, I wondered whether in heat pumps the condensing temperature is actively controlled by injecting more/less refrigerant into the system? Thanks so much for the detailed response and explanations - really appreciate it - I haven't seen anyone have a serious go at explaining this. In fact I've seen people speculating dt of 5 is just some myth that's permeated the industry... If you ever wanted to work together on some of the above questions I would love to help out :)

Good question. IF the flowrate is the same (fixed-speed circ pump), then the flow-return dt reduces in proportion to heat output. Not seen evidence of any units happily dropping output more than around 30% in meduim conditions (lower on coldest day though... when you dont want low output!). IF the pump can modulate, then it could 'track' a dt of say 5, but at minimum compressor speed, the flowrate might be too low, so I think there would be a minimum speed 'capping'. I dont see much of a problem with low dt when at low speed, but possibly, some algorythms assume a certain dt, so very low dt could mess up the compressor speed control... possibly... I dont know. Another thing to consider is that any heat meter can loose accuracy when dt gets less than say 2 degrees... some even stop recording!, so it can be hard to proberly record heat output at minimum output. dt of 5 is I think a compromise. not wasting circ power, not too noisy, and the flowrate when dt is 5 is about right. If dt more, then average radiator temperature is significantly lower than the flow temperature... not ideal.

It also for the glide of the refriger gas they use like R410, propane or R32, different gas different glide of condensation different delta T

@@Bushtuckerman71 can you please elaborate? possibly linking where to find the theory. thanks

Nevermind, just found it on google!

Sorry, the simulator is only on my website

All I can think of is that you could try a compromise... lower the dt a little and see if you can record an improvement on running cost. There will usually be a region (maybe 4-7 dt) were the performance difference is not great. Circ pump power will increase if you speed the pump (unless you can open some flow restrictions to increse the flow) However, this exta may not be much with the latest pumps, and can often be outweighed by the compressor running cost saving.

​@@johncantor4056 Thanks for your answer. Everything started when I decided to install auto-balancing actuators, and I noticed I could not reach delta T of 7 degrees even if I opened all zones fully. I estimated a flow of 6 L/min through the manifold; that's why I think the pump runs at minimum speed, and I can run it faster. The difference between 1 and 2 speed levels is 55W, which makes 40 kWh for a month - not much. Also, with this low flow, the HP cycles a lot when there is low demand and weather outside is hot. Maybe with higher flow this would be better. But still I also thinks that I have to measure the difference.

@@nkitanov I have never tried auto-balencing valves.. I have never looked into them.. Have you adjusted them relating to the radiator size? I'm wondering if they could be an unecessary restriction?? Are some of them around the max setting? I should get one to try it.

@@johncantor4056 it's UFH, and the problem is that I do not know the lengths and specs, so tuning them blindly. That's why thinking of using auto balance valves, which have temp sensors of the flow and return flow, and by opening and closing the valves, they try to keep delta T of 7 degrees. Anyway, I checked the pump, and to my surprise, it's running at maximum speed. So, I won't be able to achieve better delta T without adding an extra pump and will run it like that. It's a bit strange that the pump even on max speed cannot reach the needed circulation.

@@nkitanov So, 7 degrees is quite a lot for UH. If say your flow temperature was 35C, then the return would be only 28C. this would be felt as quite a gradient in the room. (hot and cooler areas). Can you reduce the dt to 4 or 5 max? It sounds like the auto adjusters might be 'throttling' the flow in an attemp to get a dt of 7. In general, in a normal situation, at least one adjustable flow restrictor on the manifold would be fully open. Often almost all of them are fully open. It would only be the shorter loops that need restrictiong, (as you are trying to achieve).

Yes, I am talking the average temperaure, so if radiator inlet were say 36c and outlet is 30c, then the mean is near enough 33c. The difference in temperure between water inside and the paint on the surface is very small. If you do a rough calc of heat transfer through the 2mm of steel (or aluminium), its small enough to ignore. No doubt a lot of laminar flow inside the radiator, but its still a small temperature difference, particularly at low heat pump temperatures.