Interesting. I’m surprised that sensor out of direct sunlight is affected by sunshine but you know your system. Likely this is on warmer days so maybe you are getting some defrost help on very cold days.
Thinking about it though, on any system the sunshine benefit may be insignificant compared with the frosting effect when the heat pump is running hard.
Hi @JRascagneres ,
I see your pain 
went though this recently.
My bet is your issue with low deltaT is comming from your buffer tank. Rule #1 is to equalize flow rates on primary and secondary circuits(between HP and buffer and between buffer and home).
You can check it by measuring deltaT in primary and secondary circuits and comparing them.
Regarding TRV I went with IMI Heimeier Eclipse. You mount them as TRV core in compatible IMI Heimeier TRVa, set the flow on them and they will keep it and your system balanced even when some other TRV will close.
Regards,
Peter
Yeah I need to add some tools to measure the delta T on the actual radiator circuit. Its just on the heat pump at the moment.
Hi,
I have a 5kW R290 FTC7 and my delta T is always 2°C ±0.5, it is as if the FTC7 is keeping it there because no matter what I do, I cant get it to increase. The only time it goes higher is when it is charging the hot water cylinder. I have the Ecodan cylinder and only one heating zone - completely open under floor heating with no room stats or anything to close off circuits.
Does the delta T need to be higher? Is the house warm enough?
My dT average over the past 24 hours is 2.2 with zero defrosts and a couple of HW runs. All nice and toasty.
My house is nice and toasty too. A higher delta T should increase the CoP of the heat pump and allow for lower flow temperatures, which is important when you have underfloor heating as there is a maximum floor temperature that cant be exceeded without damaging some finished floors.
I would be very interested if anyone had any insights into what the FTC7 logic is, at the moment I am controlling to a fixed flow temperature to try to understand the FTC7 logic better.
Mine is FTC7
That is why I thought my experience may help you, I am still figuring it out but as I understand it lowering the pump speeds to increase the dwell time (therefore heat transfer) is the name of the game. The minimum flow rate is 5L/min through the heat pump but be careful not to get too close to that as the heat pump will enter a fault condition if the flow rate drops below that.
I did lower mine down to about 10L. It was originally higher. The heating pump out of the buffer is already on its lowest setting - I’m not sure what it is as it’s just 1-2-3 options.
I could try lowering the main heat pump pump
Ok so I am making progress, the problem isn’t really the flowrate, the problem is short circuits. I have had to reduce the flow rate to the smallest rooms (and therefore the shortest circuits and the lowest heat loss) to virtually zero so that the biggest rooms (and therefore the longest circuits) get the flow.
Does anyone know if it is possible to get a flow regulating valve (FRV) that has a smaller range than 0-5 l/m? The smallest circuits are very difficult to balance when they are just a fraction above zero.
Hi Iain,
If your emitters are radiators, do you have the possibility to change their sizes to better work as a set and match the heat loss of each room?
Hi David,
I have underfloor heating throughout. I have figured it out now.
The highest I am able to get my delta T is 4, because the lowest flow rate my 5kW heat pump can supply is 9l/min, to achieve delta T 7 (which would significantly increase the CoP) I need a flow rate of 5l/min.
| Room | Area (m^2) | Delta T | Heatloss (W/m^2) | Power (W) | Q (l/s) | Q (l/min) |
|---|---|---|---|---|---|---|
| Livingroom | 16.44 | 4 | 44.75 | 735.69 | 0.04400 | 2.6 |
| Bathroom | 3.9 | 4 | 47.89 | 186.77 | 0.01117 | 0.7 |
| Hall | 5.5 | 4 | 39.47 | 217.09 | 0.01298 | 0.8 |
| Bedroom 1 | 14.43 | 4 | 39.91 | 575.90 | 0.03444 | 2.1 |
| Bedroom 2 | 8.62 | 4 | 30.53 | 263.17 | 0.01574 | 0.9 |
| Kitchen | 7.86 | 4 | 77.04 | 605.53 | 0.03622 | 2.2 |
| TOTAL | 56.75 | 2584.15 | 0.15455 | 9.3 |
| Room | Area (m^2) | Delta T | Heatloss (W/m^2) | Power (W) | Q (l/s) | Q (l/min) |
|---|---|---|---|---|---|---|
| Livingroom | 16.44 | 7 | 44.75 | 735.69 | 0.02514 | 1.5 |
| Bathroom | 3.9 | 7 | 47.89 | 186.77 | 0.00638 | 0.4 |
| Hall | 5.5 | 7 | 39.47 | 217.09 | 0.00742 | 0.4 |
| Bedroom 1 | 14.43 | 7 | 39.91 | 575.90 | 0.01968 | 1.2 |
| Bedroom 2 | 8.62 | 7 | 30.53 | 263.17 | 0.00899 | 0.5 |
| Kitchen | 7.86 | 7 | 77.04 | 605.53 | 0.02069 | 1.2 |
| TOTAL | 56.75 | 2584.15 | 0.08832 | 5.3 |
I would really stop stressing about this, getting your delta to 7 instead of 4 will not massively increase your CoP, or even make a measurable difference to it.
There’s an entirely different group of people who fixate on the idea that lowering their DT will increase efficiency, and they’re wrong too.
A delta T that is too high or too low is a problem, my understanding is that you want it between 3 and 7 (for UFH at least) and I couldn’t get mine to go above 2 because my installer hadn’t commissioned the system properly. At a delta T of 2, the flow velocities get up to ~0.8m/s on my highest flow rate circuit which is not good for noise or wear and my CoP was going as low as the low 2s and at best the mid 3s.
I have now managed to get the delta T to 3 or 4 which has reduced flow velocities and increased CoP to mid 3s or low 4s. I think the R290 units like a larger delta T compared to previous working fluids too.
Zero evidence that R290 heat pumps have any more of a preference for DT compared to other refrigerants.
I can’t speak to the changes you made previously, but I would suggest that the change in DT was not the cause of the increase in COP and going up to 7 will not give you a greater increase.
The product information sheet for the 5kW R290 gives the A-7/W35 performance for a return flow temperature of 30°C and therefore a delta T of 5K. Ecodan R290 5kW Monobloc Air Source Heat Pump Product Information Sheet - Document Library - Mitsubishi Electric
EDIT: As does the information sheet for the 5kW R32: Ecodan R32 5kW Monobloc Air Source Heat Pump Product Information Sheet - Document Library - Mitsubishi Electric
EDIT2: So I was wrong about R290 liking a larger delta T than previous refrigerants, but a Delta T of 5 does appear to be the sweet spot, and, anecdotally, my CoP tracks my delta T.
