Underfloor heating with boiler – common cycling problem??

4 Jan 2020
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United Kingdom
I seem to have what must be a common problem when using underfloor heating with a boiler however I haven’t managed to find a standard solution online. The boiler cycles at a high frequency due to the difference in boiler power compared to what the underfloor heating manifold will accept.

Setup (underfloor heating recently added by local plumber) – see image below
  • Worcester Greenstar Highflow 440CDI combi boiler (Central heating power of ~29kW down to ~7.6kW)
  • Radiators upstairs with separate thermostat and actuated valve on the CH flow.
  • Underfloor heating downstairs with separate thermostat and actuated valve on the CH flow. Manifold including recirculation pump and mixer valve to maintain the inlet manifold temperature at ~40C.

The Combi boiler cannot modulate low enough when running the underfloor heating on it’s own therefore it cycles frequently on and off. At a high level,
  1. The thermostatic valve on the underfloor heating manifold gets close to the required temperature and throttles flow from the boiler.
  2. Boiler flow then bypasses the manifold through the pressure relief circuit and causes the boiler flame to turn off.
  3. The flow temperature drops
  4. The thermostatic valve on the underfloor heating manifold then starts to open because it’s not getting heat, pulling in cool flow and opening more and more.
  5. The boiler restarts when it’s anti cycling timer or temperature limits have been reached and quickly exceeds the underfloor heating thermostatic control valve temperature so the process restarts.

If the anti cycle timer is put to it’s minimum of 1 minute, the boiler will run for 1 minute and turn off for one minute (the run time will further reduce when the underfloor heating gets up to temperature). At this frequency the flow temperature remains above the temperature setting of the underfloor heating thermostatic valve so the underfloor is happy.

I assume that this boiler operation isn’t efficient and I doubt it’s doing the boiler any favours?

When the radiators are operated concurrently with the underfloor heating the CH flow temperature remain stable and the system works nicely.

Variables currently available in the system
  • Boiler cycle time or temperature limit, currently 1 minute – If this is increased above 1 minute the underfloor manifold pulls in low temperature water as it cycles and therefore takes a long time to heat up.
  • CH flow temperature, currently 60C – If this is increased it takes a little longer for the boiler to achieve the temperature however an increase of 10C only added 25s to the cycle time (by the time the high temperatures are achieved the boiler is mostly short circuiting around the bypass).
  • Boiler pump speed, currently set to three – Assume a reduction would result in higher temperatures (lower flowrate with the same burner rate?) may also impact on the boiler/radiator operation?
  • Boiler pump operation modes – Don’t know enough
  • Underfloor heating pump speed, currently set to three – Don’t think this will have a significant affect.
  • Underfloor heating thermostatic valve, currently 40C – Increasing this temperature risks overheating the engineered wood flooring.
Next steps – help please!
  1. Is it a problem to leave the boiler cycling so frequently (1 min on then 1 min off)?
  2. Can any of the existing settings be adjusted to help?
  3. If current operation is a problem the only significant improvement I can think of is to add thermal mass between the boiler and underfloor heating manifold as follows (see image), any tips on these or is there a better alternative?
a. Add pressurised tank (are these referred to as heat banks/or thermal stores?) upstream of the underfloor heating bypass loop.
b. Draw the underfloor heating flow through the heat store in the boiler. This is the wild card option and I don’t like it because a connection would need to be made within the boiler (although it is accessible) and there may be an unforeseen impact on the hot water supply. I’ve mentioned it because it wouldn’t require another tank to locate, continually heat or pay for!​

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Cycling will indeed increase wear and tear, and gas consumption too. I'd recommend some hydraulic separation in the form of closely spaced tees by the UFH manifold. That should allow the loop to remain warm (ensure the pipework is well insulated) and the UFH to draw off the heat it needs
Thanks for the response sounds like a promising alternative, although I'm afraid I haven't fully understood. Could you add a little detail to the closely spaced tee arrangement?
I don't think closely spaced T's will make any difference.
Check your ufh flowrates. Should be about 2.5l per min.
Going by your drawing its a typical arrangement and works well. Plus I've installed a few in my time.
Many ufh systems have anti cycling controls built into the controls.
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What is the size of the UFH component, and what is its heating output? If it's lower than the boiler minimum output of (you state) 7.6kW then there is no doubt the system will cycle, but 1 minute on/ 1 minute off suggests another reason is to blame.

Do you have towel warmers on a by-pass loop, so that they heat with any heating load? If so temporarily turn them off.
Read the gas rate at the gas meter with the boiler firing, note the reading every 10 seconds and calculate the gas rate in m3/hour for each sample. Are they consistent with each other, and do they correspond with the gas rate for 7.6kW (about 0.75 m3/hr)? If so then the boiler is firing at its lowest rate, so your heating load is less than the boiler's minimum output. If the gas rate is much higher than this then you will need a registered gas fitter to set up the boiler for you.
I am not familiar with all the aspects of this boiler, but it may be that you can put the boiler in minimum output mode, which will make the gas rate measurement much simpler, especially if you put the rads on too while you measure (to stop the short cycling).

What results do you have?
Boiler power when running UFH only during a 2m45s 'on' period:


The boiler also has a minimum setting, when this is run it operates at 37% which is about 11.5kW (approximately what it was modulating down to).

Heated floor area is approximately 60m2 by 4 ~90m loops typically operating with 40C in and ~30C out. I'm unsure of how to calculate the required power for this but seems that logical that it's significantly below the 11.5kW min boiler output.

I guess my summary is that there doesn't appear to be any quick fix setting changes (shame I really liked that option!).

I assume that a form of thermal buffer will be required or does anyone know if a low loss manifold could help? If I'm going down the thermal buffer route I need to size the tank and see if there's anywhere I can put it or whether there's still the option of pulling flow through the one that's already fitted to the boiler (option 3a in original post). Anyone know how to do this or which companies would be able to help?
I have that boiler, I have UFH in Kitchen, & rads elsewhere. My boiler does not keep cycling, in fact Im amazed how little it come on & my whole system works well. My UFH is by www.underfloorheatingdiy.co.uk. I did not install this.
Interesting, do the UFH and radiators work independently (two port valves to each) or do they always work concurrently?
They are independent. When we moved into the property(7yrs ago). They worked together, I though this was wrong & contacted the supplier, he said there is a 2port valve so they should be independent. Turned out 2port valve was faulty & previous occupiers had jammed it open. Put new head on works really well. Last year I did change the programmer from a cheap one to a Honeywell which gives a better temp control.
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So there is potential! Few setup queries if you have time:
  • Do you have the pressure relief bypass highlighted below?
  • a>
  • What temp is the boiler central heating water and thermostatic valve on the UFH manifold set to?
  • Is there much pipework between the boiler and the UFH manifold? Mines very short, the manifold is ~2m away from the boiler.
Hi folks, thanks for everyone’s helpful replies and to the OP for the initial post.

I have had a similar issue with a condensing boiler cycling frequently, but a slightly different setup. I seem to have found a stable temporary solution that results in less cycling, but wanted to get your views as I’m not certain it’s the best way forward.

- 6x zones UFH only on engineered wood flooring (except bathroom). Small flat Approx 900sqft on one level. No towel rails or radiators. Having the zones now seems superfluous but here we are.
- Nu-heat calculated the heat requirement at 6KW which is just under the min output of my Vaillant EcoFit boiler.
- Nu-heat suggested flow temp is 55C but fear this may be too high to keep the floor surface temp to below 27C (and unfortunately no floor sensors were installed). Thinking of getting a laser temperature gun to check surface temps.
- Approx 6m of piping between the boiler and the manifold. Manifold has a thermostatic blending valve.
- No thermal store.

- Flat is fairly well insulated so I want to try run as low a flow temp as possible.
- Tried to set the boiler flow temp to 40C and blending valve at max but boiler still cycling.
- I increased the flow temp on the boiler to 45C with the blending valve at 3/4 max (marked 55 degrees) which gives about 40C flow temp on the manifold. Temperature differential seems ok at Approx 12-15 degrees C. Boiler is not cycling now and hovers around 38-45C flow temp with the return temp on the boiler showing about 10 degrees lower.

My question is whether I can safely have the blending valve at max or almost max and just control the flow temp using the boiler’s digital settings? Or is it preferable to have the blending valve throttling some of that hot water coming from the boiler?

At these temperatures the flat warms up comfortable but it’s only beginning December with outdoor temps around 6C now, so I imagine as it gets colder that flow temp will need to increase, and thus increasing the risk of my floor surface temp being too high.

I suggested leaving the blending valve at max and running a low flow temp as set on the boiler but the contractor who installed it nearly fell off his chair !

Any thoughts would be most appreciated. Thanks in advance.

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