@andrewj Certainly mine cycles alot below 36C, it seems "happier" with the weather compensated minimum flow temperature at 36ºC or above. I think the really old manual also mentions this.
If you go back to generic basics, you must have correct pressure differential for as-designed compression, you need to avoid suction flooding, keep the inverter in its stable torque region. So if you go "too low" the pressure ratio becomes too small, and the compressor is basically pumping nothing.
Looking at some of my data, the pump really wants to maintain a set ΔT, so as you drop the minimum LWT really low the return water might only be 1–3 °C lower and destroy your COP.
At mild outdoor temps, your house may need <2 kW. So for my 9, the system must cycle and minimum flow temperature being too low makes it worse, because the compressor surges on start, return water spikes, unit trips off, repeat.
This is why dropping the minimum WC temp only increases cycling and reduces efficiency.
@swwils Thanks. I'm currently running with the Warm Weather flow temp set at 32c which, at a linear plot, drops the flow temp to 36.1 at an outside temp of 7c and 35c at 8.5c. Checking earlier in the week I saw a deltaT of 3.8 but that was with an OAT of 2.8c. I've been running with this value since the afternoon of 20th Nov: 20th Nov and 21st Nov where quite cold days and the COPs were 3.52 and 3.29; 22nd was warmer at 5.5c to 7.9c with a COP of 4.3. Today it looks like it will be a bit warmer still with the current temp of 7.9c and a COP so far of 4.34. At these temps and WC setting, the internal temp of the house is between 22c and 23c so comfortable and really what I'm aiming for.
I think what you are saying is that it is likely that as the temp heads up towards 12c, it is likely to cause problems and I'd be better with that set at 36c and adjusting the Cold Weather value? Certainly with the settings at 50c and 33c the house massively overheats and the only limit I have is by setting the thermostat but that causes comfort issues and also seems unnecessarily inefficient to heat the water just to let it drop off as the Cosy turns off. Hopefully I can get them to drop the minimum Cold Weather flow temp min value below 50c.
As Octopuses have 8 tentacles, surely this should be the very minimum number of programmes available? Quizzically, Toodles.
That has actually been mentioned to them before actually!!
@agentgeorge it does seem you are right. The best you can do is have a heating schedule to run from 19:00 to 04:00 at a normal temp and have your hot water scheduled to take advantage of the 10:00 - 00:00 slot. Or, I suppose, one of the other low price slots.
@andrewj I’ve noted when the hot water is scheduled, it stops the heating for the whole of the scheduled hour, even if the water has reached its required temp.
would be nice if it switched back to heating once the DHW was boosted
@andrewj my description of the issue needs a bit of clarification.
you can set the DHW for a cycle, I have it at 13:00-14:00 which is within a cosy cheap period and when it’s warmest outside.
I tried it at night; the HP used more energy to heat the DHW as it was colder and it had to work harder. It also made a lot more noise at 10pm going through a high temp heat cycle. Not the best for us or the neighbours.
So what I spotted was if the DHW reaches set temp, and the hour is not up, the heating tab still shows that the DHW is in use and no space heating occurs until the hour is up.
I got into the cosy hub settings and found the WC settings, you can put a number into the box, I tried 45 and it displayed the red warning, and changed it to 50.
I got into the cosy hub settings and found the WC settings, you can put a number into the box, I tried 45 and it displayed the red warning, and changed it to 50.
So a real Octopussy Man is required here 🙂
If you saw it with your own eyes I wouldn't attempt to dispute it! It isn't what I thought - I may ask the engineer who's visiting on Thursday if that's the case, if I remember. I hope that the maximum flow temperature isn't hard-coded to a value of 50, that would be a bit of an issue in stopping the house overheating - I'd have to go back to a thermostat.
@andrewj Octopus set my system up with a Cosy Pod thermostat. It’s 1.4m above floor in the kitchen diner which has a combined 3 longest UFH loops of the 9 loops.
We started with stats that turned the loops of when they were at desired temperature but quickly found the HP faulted with a minimum flow rate error.
The downstairs is now open circuit, controlled by the flow rate of each loop; a 100m loop has 2x flow rate of 50m loop and 3x flow rate of the 4 smaller 25m loops. This allows each area to heat up at the same rate/hr. This was recommended by UFH installer not 8pus.
The upstairs had resized radiators for the lower dT and has TRVs on the 3 bedrooms so they don’t overheat, bathroom TR had a TRV fitted by 8pus; they claimed the regs required TRVs to be fitted to all radiators.
Question to the Forum: Do we believe 8pus that there’s no requirement for 1 radiator to be always open as was the regs for a Gas Boiler system?
To summarise: I have radiators upstairs with TRVs set so bedrooms don’t exceed 19C, downstairs is a 9 loop UFH with Flow Rate set for each loop so all areas heat up at same rate. The Cosy Pod is set at 20.5C with a boost to 21C during the 8 Cosy Hours. I (and 8pus) have checked the Flow Rate in the Cosy Hub to see it doesn’t drop below the minimum of 12L/m for the Cosy6.
If you want to follow my approach, press the button on top of the Hub for around 10 seconds for Solid Blue (it happens after flashing blue).
Open web browser and type 192.168.1.1
This lets you view same settings as you can change on the App, e.g. WC settings.
If you delve too far you will be prompted for the passkey, it’s on the label on the Cosy Hub.
If you try to get into the System Initialisation part you will be prompted for a further code which only engineers can access by phoning 8pus.
Id like to know how other Cosy owners have their systems setup, and if anyone else manages to get into the Cosy Hub.
I believe it is building regs that requires a TRV on every radiator except the one that is in the room with the thermostat. Bit crazy really as these days moveable thermostats are very common but from a homeowners perspective, it just requires the TRV to be fully open in the same room as the thermostat. But still, Octopus have to fit them.
I can get into the CosyHub with the passkey, which effectively "gets you in" and you can see some data. Useful stuff seems to be stuck behind either a "manufacturer's password" or an "engineer's password" which aren't labelled on the hub.
I'm holding my temps steady although it's still a bit too warm upstairs - most rooms hovering around 23c; downstairs the heated areas are within 0.6c of each other apart from two rooms which are typically 2.2c cooler. These rooms have quite small radiators in and I think they might be a bit small for "consistent temps" but probably would meet their design parameters of 18c at -3.2c with a flow temp of 50c.
Bizarrely I'd like to have another cold snap to see how it holds up at outside temps < 4c. COP for yesterday was 4.62 and today so far it is 4.10 so I don't think anything is operating abnormally with the Cosy, particularly as it reached over 9c here yesterday.
The Cosy 9 has a minimum flow rate of 7L/m - there's a datasheet here: Cosy 9 Data Sheet
