System boiler modulation with low loss header LLH

[Sparkgap]

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.

 

[HRP123]

So I have both timers and thermostatic controls. In this case the UFH came on because the temperature dropped overnight - so it kicked in the heating.

 

[Sparkgap]

I would have thought only the room/s calling for heat shold be getting it, and looking at the photo it would appear that the boiler would come on, quickly heat up the small quantity of water in the LLH and local pipes and shut down. If it is coming on for longer periods and using excessive gas it could be due to unwanted circulation through other circuits, perhaps a motorised valve stuck open or one of the circuits off the manifold not shutting down (or even crossed wires!). I suppose you could wait until everything is timed off and cooled down then override one of the rooms on UFH and see what circuits get warm?

 

[HRP123]

Thanks for the reply! Check all the motorised valves / been watching all the pumps going on and off carefully - so far have not found anything untoward.

My only theory now is with the UFH that the effect of the mixing valve at 50 degrees is somehow distorting the temperature inside the header - such that the boiler thinks it needs to keep firing even though there is sufficient hot water flowing in the UFH circuit.

I'm experimenting range rating my boiler down and using my Nest thermostats to kind of do a hot water priority. In theory it shouldn't make much difference overall but it just means I can control the boiler not going nuts to heat a few circuits because of any distortions in the LLH.

 

[Phill K]

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 configuration

www.cibsejournal.com

 

[HRP123]

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 configuration

www.cibsejournal.com

Thanks for this. I was thinking in my novice mind that this pump flow issue could be an issue too.

The heating pumps are Grundfos Alpha 2s. The boiler pump is currently set to max constant pressure -

The UFH has been set to speed 1 of 3
CH to low constant pressure (I tried proportionate pressure but then it takes too long to heat the rads)
Towel rails to low constant pressure
HW to max constant pressure

I'm not sure how to check the flow rates of the pumps or circuits.

For example this weekend I noticed that for a couple of hours when just the CH circuit was calling for heat, the boiler was getting too hot and cycling. The same thing happens if only the UFH is calling for heat.

I have been trying to time the heating circuits to call for heat at the same time, which in combination with the low range rating (40% of 40kw) means the boiler only achieves a flow temperature of around 50 rather than 65/70. This lower flow temp isn't conceptually a bad thing I think (?) because the UFH mixes down to a lower flow temp anyway, and the rads can manage with 50 degrees too.

The problem is if the CH or Towel Rails also kick in, then there can be too much demand of on 16kw heat output currently from the boiler. Then the HW stays on for a really long time because it cannot heat the tanks to 50 degrees with a 50 degree flow temp.

 

[Phill K]

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

 

[HRP123]

Thanks - I don't quite understand what you mean - but there is a pump from the boiler to the LLH and a pump from each flow outlet on the LLH to the circuit. So one pump for UFH, one for CH, one for Towel rails and one for HW. There are no additional pumps on the UFH manifolds inside the house.

 

[HRP123]

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

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!

 

[HRP123]

I've also noticed that whenever the hot water cylinders come on, they also consume the full power of the boiler - even though in theory I think they should require less.