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Yep, confirmed - it's the return temperature being too low from the UFH system on startup. Just takes hours because the boiler won't run and am guessing is more setup for radiators Everything else is fine - not sure what I can do about this? Would a low loss header before the UFH system help...

Done more messing around - I think it's purely down to how long it takes to get the return temp up. Boiler only really starts firing when the return temp is ~25°C delta to the flow setpoint (roughly might be +/- couple dregrees either side). I've been testing running the system on the UFH only...

it's an OV system fed from a tank in the loft. Although I am ditching that next year and going straight off the mains with a pressure regulating valve.

It's similar, I think it still said 20-21W when just on the DHW & UFH (I'm a bit limited on my running modes from how the pipework is connected, I currently can only run the UFH with either the DHW or CH. I can't run the UFH by just itself). Well, tell a lie I could go round and close all the...

Will get tonight/tomorrow. It's pump speed 3 - so running flat out - I know it says 21W when running the UFH & CH together which is what i'm after. I wanted to use the max constant pressure setting but it's not fast enough for what I need.

This is what I have - 34W https://product-selection.grundfos.com/uk/products/alpha/alpha1/alpha1-15-5060-130-99415672?pumpsystemid=2520079087&tab=variant-specifications

Just the power - can probably work out flowrate from the datasheet if needed

I'll have another look tonight. You may be onto something there if the return temp sensor is out of calibration or something (the boiler is around 10 years old now). I'm going to turn it on later and before the boiler fires up, see if the flow & return temps are exactly the same. If the return...

Not sure, it shouldn't be. the blending valve on the UFH is brand new, I assume it is thermostatic. I've got it set to 45°C flow currently so as hot as I can get it really. I guess I can get the pump datasheet so find out what the flowrates are at the different speed settings, & set it to the...

No I'm looking at the flow and return temps. Return is always low and just taking ages to heat up. I did some more testing yesterday, the boiler fired for roughly 1 min - the flow temp got up to 65°C with the return temp at 25°C. The boiler then turned off (with the little waiting time symbol...

Thanks, but this didn't seem to make a difference. Again it's in it's start up mode, so anticycle time doesn't seem to make a difference. Once it's up to temperature it runs fine. I'm starting to think balance might be an issue, maybe I have the "primary" pump running too fast/slow compared to...

No so the system has a pump as i've shown above, then the manifold has a 4-port mixing valve with integral pump for the UFH. It has a standalone digital thermostat with a timeclock which enables the boiler, runs the pump and controls the 3-port valve - boiler then runs to its integral controls...

It runs fine. I thought a flow issue initially, but if the rads are up to temp and then I enable the UFH the entire system works fine. i've attached a sketch of the system, I don't think its connected wrong otherwise it wouldn't work once up to temp. Its more like the boiler needs to be told...

Boiler = Vaillant ecoTECplus418 Flow temp setpoint: 65°C Anti-cycle time= 20mins heat output: 18kW bit of background - have a small bungalow, boiler generally worked fine with just the radiators (7 off them). i've since done an extension and added UFH with a manifold which is where the problems...