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Discuss UFH heat starvation? in the Plumbing Forum | Plumbing Advice area at PlumbersForums.net

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Hello guys.

I am having a number of issues with my Central Heating system. It was commissioned in July 2019 whereby the heating was not turning on (via the room stats) due to the summer weather understandbly. I only noticed these problems in the winter when my heating expectations were not being met. So the problems I am experiencing are likely to have been there from day 1 and not developed over time.

The system...
Intergas Xclusive 30 modulating condensing boiler. I believe the default output and modulating pump settings are in play. 75C is the CH water temperature
DHW served through the boiler to the kitchen tap only
CH served via a S-Plan to...
1. the unvented hot water cyclinder - serves the bathrooms and WC
2. the downstairs UFH manifold with mixer set at 45C with variable pump
3. the upstaires UFH manifold with mixer set at 55C with variable pump

Experiences
The downstairs UFH is on screed and doesnt drop in temperature much
The Hot water cyclinder seems to be working fine - i.e we are never short of hot water in the bathrooms/wc
The upstairs UFH drops in temperature by 3-4C before required to come back up to 19-20C when the room stats call for heat.
The upstairs UFH never/rarely gets to 55C at the mixer, the result is that I am lucky if I reach 19-20C in some rooms.
The downstairs UFH behaves the same also I believe but is less noticed as the temperature differetials are much lower and the rooms seem to hover around 18,19,20 downstairs.

Problem
When all three CH zones are calling for heat, the upstairs UFH (and possibly the downstairs UFH too) are not getting heat from the boiler resulting in the manifold never reaching the set 55C.

When I stop the Call for Heat on the Hot water cyclinder and the downstairs UFH, the upstairs UFH starts to warm up and reached 55C.

I have done a number of tests where, from cold, I call for heat from only the upstairs manifold and this results in the UFH getting up to the 55C temperature. So I am relatively sure that it is some sort of a starvation problem?

I think that this might be happening to the downstairs UFH manifold too but I havent 100% confirmed this as downstairs screed seems to stay warm and we dont notice a 'lack of heating' downstairs for me to worry about.

Also, if I turn off just the Hot water call for heat and leave on the upstairs and downstairs UFH call for heat then again, the upstairs UFH mandifold fails to get to 55C. So I am not sure if there is an inherent priority in play favouring the hot water cylinder, then the downstairs UFH and then the upstairs UFH.

Any ideas on whats going wrong here?

Note: The system was tested by the installer i.e upstairs UFH and the downstatis UFH and then unvented hot water cylinder, but may have been tested individually hense signed off as working
 
Any photographs of your install ? 55° c should be enough for underfloor heating intergas boilers internal pump can be adjusted but we really need to know how your system is installed, do you have pumped manifolds for upstairs and downstairs zones ? Its hard to advise you unless we know what you have really . Regards kop
 
Here are pictures of the downstairs and upstairs manifolds.
87329B98-8C37-4B7F-85CE-EA9D645AC57B.jpeg

35A27A32-A5A8-42DE-9FC2-CBF29B6B9B83.jpeg
 
Could be a simple case of balancing the systems.

Do you mean a type of balancing other then adjustment of the flow rates through the underfloor heating loops?

Would this other balancing would be done on the HW cylinder, upstairs UFH and downstairs UFH piping?
 
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When commissioning any new system it needs to be properly balanced, radiators, UFH and maybe the HW coil. I'm not saying this is the problem but from what you describe it's what I'd approach first
 
On a system that large i'd be setting it up as HW priority where the HW gets reheated first before anything else then other zones can heat after this. The main reason being some coils in the cylinders can be rated up to 24kw and can start to act as a big bypass thus nothing reheating properly.

Does you HW switch off when it reacts temperature?

Also how are you running the underfloor heating, on timed or 24/7 using the stats? You should run it constantly on using the stats to the temperature you require.
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Your other issue too is that the pump on the upstairs UFH is set to a lower speed than the downstairs manifold. These need to be the same, as the downstairs is probably starving the upstairs manifold as the pump is pulling more flow through.
 
Stagger your hot water times also tbh ufh in winter should be on 24/7 and just run it on the stats
 
All looks ok there keep pump speeds the same on both manifolds reheat of the cylinder program different to your main heating as advised above , you could try adding a balancing valve on the cylinder coil and get the auto bypass set correctly it could be bypassing when it shouldn't. Kop
 
Many thanks for the responses.

The pumps on the manifolds are variable pumps. As more actuators open, the pump ramps up the speed. I read somewhere that UFH manifolds should not have variable pumps installed. Is there truth to this?

"Does you HW switch off when it reacts temperature? " - I have yet to prove this but I think it does. Yesterday I had the HW cylinder calling for heat for 1 hour. After 55 minutes in, the cylinder circuit was still calling for heat. I will monitor this to see if there is an underlying issue there and if it does actually eventually turn off when up to temperature.

I have staggered the timings for the HW cylinder, upstairs and downstairs UFH manifolds. Things seem a lot more consistent now and I think I can live with it. I spoke to my plumber and informed him of all my findings - he suggested that a low loss header may help. Any thoughts on this?

Now that the manifolds get hot flow from the boiler due to having staggered the timings, I have seen that they cause the boiler to cycle. Cycle time is set to 5 minutes (default) on the boiler. I can see that the boiler shutdowns and starts up after 5 minutes while the manifold is still calling for heat and pumping away. While the boiler is cycling, the manifold's water temperature is lowering and is pumping cooled water through the loops. I believe this is a common problem(?) and a thermal store might be the only solution to keep the boiler running efficiently i.e. condensing properly and not cycling?
 
A low loss header is a means of hydraulic separation between primary and sub circuits. If the boiler parameters regarding flow rate and temperature are not met this can cause a number of problems in the entire system, not just localised to the heat source. Fitting a low loss header will enable you to control the flow rate and temperature through the boiler, whilst maintaining sufficient flow rates and temperature through the rest of the sub circuits. It is one possibility you could consider, alternatively is a thermal store. This option gives the boiler a large load to work against which will help with boiler cycling, at the same time supplying sufficient power and flow to the sub circuits. Both low loss headers and thermal stores need to be sized correctly, something I am not familiar with as my understanding is basic if I'm honest.
With regards to modulating pumps on the UFH I'm under the understanding they should be fixed rate and on the maximum setting.
 
Many thanks for the responses.

The pumps on the manifolds are variable pumps. As more actuators open, the pump ramps up the speed. I read somewhere that UFH manifolds should not have variable pumps installed. Is there truth to this?

"Does you HW switch off when it reacts temperature? " - I have yet to prove this but I think it does. Yesterday I had the HW cylinder calling for heat for 1 hour. After 55 minutes in, the cylinder circuit was still calling for heat. I will monitor this to see if there is an underlying issue there and if it does actually eventually turn off when up to temperature.

I have staggered the timings for the HW cylinder, upstairs and downstairs UFH manifolds. Things seem a lot more consistent now and I think I can live with it. I spoke to my plumber and informed him of all my findings - he suggested that a low loss header may help. Any thoughts on this?

Now that the manifolds get hot flow from the boiler due to having staggered the timings, I have seen that they cause the boiler to cycle. Cycle time is set to 5 minutes (default) on the boiler. I can see that the boiler shutdowns and starts up after 5 minutes while the manifold is still calling for heat and pumping away. While the boiler is cycling, the manifold's water temperature is lowering and is pumping cooled water through the loops. I believe this is a common problem(?) and a thermal store might be the only solution to keep the boiler running efficiently i.e. condensing properly and not cycling?
You need to set your pump speeds to constant full speed rather than variable on the GrunFos pumps on the manifolds. You say you've staggered your times for the UFH, as said previously you need to run this on 24/7 and use the stats if possible, and run both upstairs and downstairs at the same time.

Setting the speeds of the UFH pumps on constant full you'll probably find your cycling problem is less of an issue, if at all.
 
UPDATE:

I have set both UFH pumps to 'constant pressure'. Previously they were left at the factory setting of 'proportional pressure' and adaptive speed. This was based on recommendation from the pump manual based on having a UFH system. I think this was suggested by EvilDrPorkChop above too.

I also set the speed to max based on feedback above.

I still get boiler cycling however. I have tried setting the modulating settings of the boiler to the lowest setting available (3kW) (15% of full CH power of 19.4kW) (min flow rate of 85l/hr) but i suspect my automatic bypass lowest setting 0.1 bar does not allow bypass of flow that low when the UFH manifold mixer valve closes even at this low level of modulation. I will explore a low loss header and/or thermal store to resolve this I think.

The UFH is constantly running. I have set 'set back' temps on the room stats for upstairs and downstairs depending on night and day occupancy on those floors/rooms.

Also, I have now concluded '"Does you HW switch off when it reacts temperature? " and found that the HW cylinder call for heat was not turning off and was caused the boiler to cycle due to no temp loss at the return flow. I checked the wiring and all looked fine. The stat was set to 65C but wasn't cutting out. When I lowered the stat to near 60C, the stat opened and call for heat was stopped.

It seems that the starvation at the UFH manifolds was because the boiler was continuously cycling when my HW cylinder heat-up schedule was around the times the UFH wanted heat due to the HW Cylinder stat not cutting out.

I suspect I can now have the HW Cylinder heating up when UFH are calling for heat as now the HW Cylinder call for heat will be closed when water temperature is reached.
 

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