John Carroll

Are you saying the pipe marked "flow"is the hot supply to the mixer?, thought that might be the manifold cold return to the boiler, if the flow is as shown then the Mot. valve is on the return?.

Cycling doesn't have a huge effect on efficiency, also you don't see too many complaining about the constant cycling with systems like Evohome which will still cycle a boiler even when the system demand is greater than the minimum (gas) boiler output.

The DU144 gives excellent control over different by pass requirements especially where high pump heads are required. Even with pump heads of 6M or more a broad range are bypass flows are available, but the DU145 (angled type) has completely different flow characteristics (bottom flowchart) so basically useless with a high pump head requirement, ie pump head of 6M with ABV set to index 6 gives a minimum flowrate of ~ 800 LPH, 13.3LPM forcing the installation of a manual (throttling) by pass valve in line with this ABV if bypass requirements are less than this type of ABV, these characteristics seems true of all ABV manufacturers but I certainly wasn't aware of it until I read of someone posting on here who required a high bypass flow of > 14LPM, 840LPH using a 8M pump and I thought it was a faulty ABV as the straight type will provide this at a index setting of 6 (0.6) or less but I was looking at the wrong type of ABV, the DU144 instead of the installed DU145.

I would suggest that you fit isolating valves as AAVs are notorious for getting blocked, at least you can remove and clean them or replace them then.

If you are getting 3C dT then UFH output is ~1.1kw, have you a link to the mixing valve, it seems to have a adjustable bypass (if like the one below) there is only ~ 0.7LPM of hot water at 45C mixing with 4.3LPM of UFH return water at 23C so requires very tight control, you would require 1.0LPM mixing with 4.0LPM to get 33C mixed temperature.

A dT of 2C with a assumed total flow of 5.0LPM = UFH output of 0.7kw.

Have you tried turning up the mixing valve setting until you achieve 33C, it should react fairly rapidly.

What temperature is the HW (from the HP) going to the mixer?, if its 45C then it should give a mixed flow of 33C if the index on the TMV is accurate, once the slab is heated up then you might expect a return of 25C/27C, a dT of maybe 6C to 8C, if the flowmeters are flowing 2.5LPM each then the UFH input will be 2.1 to 2.8kw.

What temperature are you running the UFH (flow) at and what total flow rate through the two loops and return temperature do you expect?.

So, if it states on a stat that the differential is 0.5C does that differ from a stat that states its hysteresis is 0.5C or does it mean the same thing? it's interesting because I have a simple digital stat set to its lowest differential setting of 0.3C, as soon as the stat switches off, you then have to increase its SP by 0.3C to get it to switch in again, it will then switch off at its SP temperature but after a few cycles it will switch OFF/ON at SP +/-1.5C. Its FantiCosmi stat.

Are you sure the Wunda stat is working correctly? A setpoint temp of 22.0C with a switching differential of 0.5C should mean a switch on at 21.5C and switch off at 22C.

Could you please post the above with Tout extending to its max, say ~ 60/65C. Thanks. John

True, but generally the emitter would/should be matched however roughly to the expected heat loss, I think/hope the OP is only talking about the commissioning process, he is hardly going to oversize by a factor of ~ 5.

You left hand manifold has 11 loops, at a dT of 8C and a flowrate of say 2.5LPM will output 15.3kw so the willis at 3kw will do OK for initial heat up/drying out especially since you can set it's thermostat but hasn't a hope of supplying the required UFH output when drying out or whatever when completed.

Yes, but achieves exactly the same objective, to give you the required hot manifold UFH temperate, I know these controllers as tapstats. There is no loss of heat, you have a lower flowrate with a higher dT on the heat pump side and vice versa on the UFH side, both balance. The HP will run with reduced efficiency if its required temp of 48C is required elsewhere like for HW production but your floor covering dictates a max water temp of 36C.

Yes, it's the thermostatic mixing valve,TMV, generally contains a plastic piston/shuttle which can be removed.

Can you remove the shuttle, clean it up, apply a bit of silicone grease and see how it goes.

What way is the arrow on the manifold pump pointing, is it up or down?

Oh yes, wait for @SimonD. When you were running on pump speed1 (6M) the boiler, just before cycling, had flow/return temps of 69/44, a dT of 25C, that boiler apparently has a minimum output of ~ 6.5kw so, by calculation, the flowrate is only, 6.5*860/60/25, 3.73LPM, which, with all those rads just doesn't add up except some big restriction somewhere, you then increased the pump head to 8M and no more cycling, the flowrate should have increased by just over 15% due to the increased head so it still looks as if the heat demand is quite low, maybe, when you get the opportunity, start with d.0 at 7kw, the boiler flow temp should then surely start falling and just increase d.0 (1kw steps) until the flow temp increases to its target temp of ~ 70C, this will then tell the flowrate almost exactly. (by calculation)

You could run on CH only, as above, pump on speed 3 (8M) and when the temperatures steady out just close the manual by pass and see what effect this has on the temperatures, particularly, the return. If the boiler remains running just keep reducing d.0? output in say 0.5kw steps until the boiler flow temperature just starts to fall, its quite easy then to calculate the actual system flowrate.