How is it controlled? And why does it need to come on in the middle of the night? Just rehashed a local house with 2 rad zones, 2 UFH zones and HWS. Boiler only fires and pumps only come on when timeclocks AND zone/cylinder thermostats are calling.
Welcome to the forum. Although you can post in any forum, the USA forum is here in case of local regs or laws
Follow along with the video below to see how to install our site as a web app on your home screen.
Note: This feature may not be available in some browsers.
Thanks for this. I was thinking in my novice mind that this pump flow issue could be an issue too.First rule of LLW design. “Thou shall not use multi tapping headers” Sorry but they are a PIN. And if systems arent perfectly matched and flow rates set correctly will leave you wide open to parasitic flow across the ports.
First thing I’d be checking is flow rate of your primary pump, Vs flow rate of your secondaries.
Something is pulling that flow water across the header rather than it circulating around it, so boiler then is ramping up as it’s running on its thermistor readings to control the gas rate. If the heat is going around the system rather than round the header it’s going to take an awfully long time before the temps are coming back to the boiler hot enough to start the modulation process
![]()
Guide to low loss headers - CIBSE Journal
Achieving hydraulic separation through well-designed low loss headers is key for efficient heating and cooling systems. David Palmer and Baxi Heating’s Ryan Kirkwood provide detailed design guidance, including a recommended system configurationwww.cibsejournal.com
I had the UF heating only come on last night for an hour. It consumed the full power that the boiler is rated up to currently (approx 20kw) for the hour that it came on.Is there a pump from the header sending water to the underfloor pump? Technically there should be as the underfloor pump is just that. Only for circulating round the underfloor
Might the underfloor is your source of leeching.
Valves are opening up, pumps shunting water and the boiler pump is then forcing the flow across the header rather than circulate it around it as there is no separation of systems occurring
Post number of loops, length of each loop, the flow/return flow of each loop, the manifold flow/return temps or any info you have, for example if you have 9 loops with a flowrate of 2.0LPM/loop and with manifold flow/return temps of 45C/37C, dT of 8C then the UFH requires 9X2x60X8/860, = 10.04kw say 10kw and the boiler output will be exactly the same, I thought we looked at this previously?I had the UF heating only come on last night for an hour. It consumed the full power that the boiler is rated up to currently (approx 20kw) for the hour that it came on.
Can anyone help suggest what is the best back of the envelope way to calculate what the UF circuit usage should in theory be? I have an idea of the metres of pipework and pipe centres + floor area overall. Thanks!
Yes thanks we did look at this together - but what I am trying to work out is what you mentioned in your last line - if you calculate the heat demand is 10kw, this should in theory translate to the boiler needing to burn around 10kw (lets say a little more because its not 100% efficient).Post number of loops, length of each loop, the flow/return flow of each loop, the manifold flow/return temps or any info you have, for example if you have 9 loops with a flowrate of 2.0LPM/loop and with manifold flow/return temps of 45C/37C, dT of 8C then the UFH requires 9X2x60X8/860, = 10.04kw say 10kw and the boiler output will be exactly the same, I thought we looked at this previously?
The boiler output will always match the UFH heat demand, if the UFH heat demand is lower than the minimum boiler output then the boiler burner will cut out at the boiler flowtemp setpoint+5C and will cycle on/off to match the heat demand, simple as that, the fuel input is the boiler output/its efficiency.
Can you post a few details of the UFH as requested above?
Sorry I made a mistake above and am having problems editing the post. This is the schematic attached.The boiler output will always match the UFH heat demand, if the UFH heat demand is lower than the minimum boiler output then the boiler burner will cut out at the boiler flowtemp setpoint+5C and will cycle on/off to match the heat demand, simple as that, the fuel input is the boiler output/its efficiency.
Can you post a few details of the UFH as requested above?
Yes, 10kwh! It is the zone called Family Room which I tested - so 3 loops. The boiler flow was 70 degrees, I didn't measure the actual boiler flow / return in this instance but generally from the flow tends to be measured at 5 degrees less than the boiler temp and the LLH return tends to be 10 degrees less - i.e. at 70 on screen it would be 65/55.Gas consumption 10.69kw/30mins???, so gas consumption 21.38kwh per hour??.
What is the boiler flow temperature setpoint and what is the actual boiler flow temperature, also can you see the boiler return temperature??.
Also how many loops are open?, I am calling a loop as like one of the 8 above.
Yes, 10kwh! It is the zone called Family Room which I tested - so 3 loops. The boiler flow was 70 degrees, I didn't measure the boiler return in this instance but generally from the LLH the return tends to be 10 degrees less usually.
Still not clear if its 10.69kwh/30mins or 10.69kwh/hour.
If 10.69kwh/hour then loops emittance 10.69X85%, say 9kw, which means each loop emitting 3kw which equals a flowrate of 4.2LPM at a dT of 10.2C (53.8-43.6), I'm a bi surprised that a dT of 10.2C at that flowrate but thats what your info shows.
If the heat emitted is 18kw , 6kw/loop, then the flowrate is 8.4LPM/loop, very unlikely IMO. I normally read of loop dTs of ~ 8C with a flowrate of 2.5LPM so a loop would typically emit 1.4kw.
We recommend City Plumbing Supplies, BES, and Plumbing Superstore for all plumbing supplies.