Not enough hot water if UF heating is running at same time time as HW

[SJB060685]

If a slab is allowed to cool to say 10°c that’s an awful lot of mass that needs to be heated again before the return temperature starts to rise.
From a cold start a temperature gauge and timer would give a good indication to the time it takes for water to reach 45°c but remember even when the boiler flow is at 45°c the manifold will still be pulling all the boiler flow. Only when the boiler flow increases above UFH target flow temperature will the mixer start to throttle in. Use the heat balance equation and you will see.

 

[John.g]

It requires 15.7kwh to heat 300 litres of water from 15C to 60C, in two hours the boiler (if not range rated) would/could have produced 74kwh, so if the cylinder had reached 60C then the manifold loops would have emitted 58.3kwh or a heat output of > 29kw, the cylinder, apparently wasn't reaching this 60C so the loops output even greater or the cylinder coil is very modest or the boiler is range rated IMO, but we'll never know but the OP is happy now so doesn't really matter.

 

[SJB060685]

I don’t know the boiler output or UFH output but the problem with UFH is the amount of flow it requires due to smaller ΔT. If the slab or screed is cold or you start introducing some form of modulating controls etc then the UFH will pull more and more from the boiler. In a situation like this keeping the UFH constantly will solve the problem and ideally have the design flow temperature for the rads at 20°c higher than UFH return will keep the system flow nice and balanced.
If the UFH demand was irregular then realistically you would consider some sort of separation with a common flow sensor signaling the boiler to increase the flow temperature if needed.
Separation is not something I’m really familiar with though. I understand it and the problems it can cause if used when not needed but there are for more qualified people to discuss that out there than myself.

 

[exedon2]

Just wondering what kind of hysteresis blending valves operate to, presume not as tight control as say shower thermostatic valves etc. This will have a big effect on how much water is recirculated by the UFH TMV, presumably none as the TMV is coming up to temperature from cold so all the UFH water is supplied and then boosted by the manifold pump until almost up to the required UFH temperature, has anyone watched this temperature from cold and noticed any over or under shooting or any changes in the manifold flow rates?.

John over shooting temperatures when system not operated properly is very common.
We're polypipe registered underfloor contractors,regularly called to look at systems we haven't installed.
Most common complaint is temperature over shooting virtually alway caused by customer miss use/ poor set up by installer.
Run it 24/7 with max of 4 c back set and it will heat rooms perfectly (assuming enough heat output)
A lot of real problem ones are either diy or installed by screed team.
Pipe distances too great screed too thin pipework not pressurised or even tested before screed going down seen it all.

 

[SJB060685]

It’s also worth noting that the controls strategy for any hydronic heating system is important to get right at the design stage. If a dwelling has high occupancy and/or high internal gains like solar gain etc then you should consider a closed loop system with some sort of room influence. I’m an oil burner technician by trade so I’m not overly familiar with heating systems like an installer should be but I’m lucky enough to have been taken under the wing of some very knowledgeable people who do this day in day out. I have heard a lot of temperature overshoots in situations like UFH in conservatories with high solar gain and simple on/off room stats. In a situation like that you would require some sort of room influence which like an on/off stat is constantly aware of the internal temperature but also learns and calculates and adjust the boiler flow temperature to prevent any undesirable overshoots.
Although on/off controls work and have done for decades they really are primitive compared to modulating controls.

 

[John.g]

Edit: Rads returns should be 60C, with 70C boiler temp, (I copied wrong cell) but calcs are basically quite correct.

DHW sheet attached

Actually I see that there are rads in the above system so I just inputted a few numbers in my spreadsheet which may (or may not) be interesting.
Assuming UFH demand only of 10kw only requires a boiler circ pump flowrate of 4.48LPM@70C, this means the boiler will run with a dT of 32C, maybe problematic for some makes?. the only other remarkable is if the boiler temp is either reduced to 45C or the point that you are all making, that it has fired up from cold and hasn't become > 45C so the boiler pump circ rate would have to be 44.4LPM v/vs 28.4LPM with boiler temp at 70C (all 3 systems calling), hence the slow cyl warm up time or whatever?.