Pipes

Boiler frequent cycling with underfloor heating - Common problem??

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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.

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Issue
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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Edited by Pipes

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What you need is a buffer tank. Someone will come along shortly I am sure to advice what and how to fit.

  • Like 1

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What temperature is the manifold mixer set to? If its very low then increasing it _might_ help a bit. Note there is usually a limit/safety stat on the manifold as well as the mixer. The safety stat is usually set to something like 50C or lower for some floor coverings that might be damaged by high temperatures (Karndean?). The mixer must be set lower than the safety stat.

 

Short cycling reduces efficiency, a bit like stop start driving a car. Its worse for an oil boiler than a gas boiler. I was concerned our oil boiler would short cycle and so intended to put a buffer tank between boiler and UFH. In the end we opted for a thermal store which buffers DHW as well as CH. Heat banks are just a variant on the theme.

 

More after Dr Who!

 

 

 

Edited by Temp

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@ProDave That's where my head is going at the moment but there seem to be a lot of systems without them working fine.

 

@Temp The mixer in the manifold is set to 40C in order to limit the temperature in the engineered wood flooring. I agree that increasing this will help the cycling and should be able to get away with a slight increase but probably not a lot. Wood suppliers advised a wood surface temperature of <=27C which we get close to if it's run for a while.

 

The UFH manifold is a JK Basic (see link below) with a max temp pump cutout at 55C. FYI they have a clever router that cut the UFH channels in the existing screed without raising the floor height.

https://www.jk-gb.com/downloads/jk-quick-start-manual.pdf

Edited by Pipes

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It sounds like your diagnosis is correct. Does the UFH appear to be working OK otherwise? I've heard of some builders making silly mistakes such as using airated screed which inadvertently insulates the floor loops a bit. That would cause return temperatures to be too high and shut down the mixer.

 

If you have to fit a buffer tank you can probably get away with a simple buffer tank (eg not one that splits the system). The main issue will be getting the sizing right. Some references suggest 10L per kW of boiler output, but they say that's not the minimum power but the "start up power" which presumably is higher. That would give a tank of perhaps 10*10kW=100L? Too small and it won't lengthen the cycling enough, too large is less an issue but it will leak more heat than necessary. 

 

 

 

 

Edited by Temp

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One possibility might be to fit smaller jets in the burner or even a smaller burner? Would need to talk to the boiler manufacturer. We fitted smaller jets in our oil boiler but oil boilers don't modulate like gas boilers, they run flat out all the time making cycling even more an issue. We did it because Grant said it improved efficiency a few % as well as reducing Max power. On a modulating gas boiler I don't know if  smaller jets reduce the minimum power or just the maximum? Reducing the max power could impact the DHW flow rate.

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53 minutes ago, Temp said:

One possibility might be to fit smaller jets in the burner or even a smaller burner? Would need to talk to the boiler manufacturer. We fitted smaller jets in our oil boiler but oil boilers don't modulate like gas boilers, they run flat out all the time making cycling even more an issue. We did it because Grant said it improved efficiency a few % as well as reducing Max power. On a modulating gas boiler I don't know if  smaller jets reduce the minimum power or just the maximum? Reducing the max power could impact the DHW flow rate.

You can’t if it’s a combi I as it’ll need it’s full kW rating to produce DHW ;) 

Producing heating actually requires a much smaller energy input vs firing up to produce DHW. 

5 hours ago, Pipes said:

Worcester Greenstar Highflow 440CDI combi boiler

That’s a 40kW high-flow ( that refers to hot water production not heating ) combi do you cannot do anything to reduce its max output capability or it’ll give warm not hot water. 

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I’m not a plumber but I’ve  the next size down greenstar 

and used a Gledhill 250L tank 

We are heating 150 m2 UFH Tsds to five beds 3 baths 

DE10D810-EC2C-4C14-9C7F-37EB89079377.jpeg

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8 minutes ago, nod said:

I’m not a plumber but I’ve  the next size down greenstar 

and used a Gledhill 250L tank 

We are heating 150 m2 UFH Tsds to five beds 3 baths 

DE10D810-EC2C-4C14-9C7F-37EB89079377.jpeg

The Gledhill provides your DHW though, so it’s a thermal store not a buffer, it just buffers as a by-product ;)  

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Very interested in this topic as seeing something similar in our house

We bought a property that had not had the heating fully completed. When we moved in i quickly got a heating guy in to finish the underfloor and set it up.

We are now dealing with the pain of this now. Boilers do not immediately coming on, cycling boiler is inefficient, and burns gas on initial heat up phase. I have tried everything to cure this with the only option of limiting the amount of time the boiler goes on and off. 

 

Reviewing online, some form of Energy Store is key. I am under the impression the size of the store should be rated to the heat requirement of your underfloor system. We are now looking into this, however also looking into the contributions for a Air Source Heat Pump rather than Gas boiler.

 

 

 

 

 

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Basically the boiler puts hot water into the buffer and UFH takes it out. The boiler ON time depends on how long it takes to fill the buffer tank. The boiler OFF time is determined by how fast the UFH empties the buffer. The two times added together give the cycle time.

 

If I've done my sums right a 29KW boiler would make about 0.35L/S or 20L/min with a 20C uplift. So it would take about 5min to raise a 100L buffer tank by 20C (ignoring any draw by the UFH at the same time).

 

To calculate how long the UFH would take to empty the buffer you need to know the power the UFH draws but we already know its less than the minimum the boiler can deliver (10KW?).  10kW is about  0.12L/S or 7.1L/min with a 20C drop between flow and return. So the UFH should take at least 100/7= 14min to empty the buffer. 

 

So the cycle time should be around 5min ON and 14min OFF. However the useable capacity of a 100L store probably isn't 100L. It probably depends on the position of any stats on the tank.

 

I'm sure things aren't quite that simple in practice so you should get the buffer tank sized by someone providing a warranty. Eg Heating Engineer. All I have is O-Level physics and some back of the envelope calculations.

 

 

 

 

Edited by Temp

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Thanks for the thoughts guys.

 

@Temp Yes the UFH works well otherwise, when the radiators are running as well and there's a consistent feed of hot flow to the UFH manifold it heats up reasonably quickly.

 

I agree with @Nickfromwales derating the boiler for UFH operation only (if it could be done) would limit direct hot water and potentially operation with both the radiators and the underfloor heating.

 

Seems that there aren't any nice quick fixes with setting changes (damn!) which still confuses me given that most UFH installers treat this as a standard setup.

 

As @Temp has suggested next steps are to select a buffer tank configuration and size it to see if it can be fitted anywhere in the house or whether there's still potential to draw through the highflows internal tank. Can anyone help with this or suggest a company that would be good to contact? I haven't found any companies who are proposing thermal stores for this particular application.

 

I got as far as looking to confirm the min power of the boiler and compare it with the power requirement from the UFH. 

  • From gas usage and the boiler display it gets down to around 11 kW during one of it's short on periods. 
  • Heated floor area is approximately 60m2 by 4 ~90m loops typically operating with 40C in and ~30C out. Multiplying the floor area by an approx heat requirement of 0.1kW/m2 gives a power requirement of ~6kW.

As another wild card are there any tricks that can be used with a low loss manifold?

 

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As you’re looking to be buffering kWh’s of heat and not looking to add volume per se,  then you really would benefit from a sizeable buffer tank, eg minimum 100L but preferably 200L tbh. 
A super insulated vessel is recommended unless it can be sited to utilise the waste ( latent ) heat loss beneficially eg in an airing cupboard or strategically positioned on the ground floor to contribute to local heat requirements. 
A low loss header will do very little to improve things as they’re normally utilised to attain hydraulic separation ( vs a dedicated buffer ). 
Your best bet would be to have the radiator zone valve always open whenever UFH is selected, and have one strategic radiator on manual valves as a bypass / heat loss radiator. You would again need to have that allocated where it would be advantageous / not a nuisance ( airing cupboard or bathroom towel rad(s) for eg ).

The boiler will quickly heat up a small buffer, and then you’re back into the flames from the frying pan. 

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Radiator circuit open option does sound like the easiest option. How would you close all other radiators so that you only get flow through the one? Smart thermostatic valves on all other radiators?

 

My initial thoughts were that it seems a wasteful to dump the extra heat being produced by the boiler into rooms that are unlikely to need it.

 

I'm sure there are significant limitations however I had a shot at initial buffer tank volume calcs using a similar method to @Temp

 

Screenshot 2020-01-07 at 11.52.13.png

Underfloor heating buffer tank initial calcs.xlsx

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TRV’s don’t need to be “smart” just £10 a pop and you’re cooking on gas 👌

There are open source intelligent / smart options available, and IIRC @DamonHD has such a system available. Send him a PM for details. 
I’d try that option first VS the buffer chop & change and see if the cycling reduces enough to not be problematic. One towel rad could make all the difference as you can include the volume in all the connective pipe work too.

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Sounds good, at the very least it will be a good idea to confirm the problem and give an indication of how much additional volume/heat loss will impact the cycling.

 

I was assuming that when operating the underfloor heating only (most of the day) I'd need to close the TRVs on the majority of radiators (instead of the overall radiator zone valve as current) in order to avoid upstairs being heated when it's not required then open them again when heating upstairs is called for again. Not sure how a standard £10 thermostatic valve will do that or am I missing something? @DamonHD's Radbot seems to be able to achieve something similar, instead of me telling the system when I'm not going to be upstairs Radbot would automatically detect that I'm not there but it's a similar cost to other smart TRVs (£50).

 

Anyone have any guidance on an acceptable boiler cycling frequency? Assume that's going to determine how many radiators I'd need to run with the underfloor heating or the size of a buffer tank.

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Just read this & my WB Greenstar 32 compact has similar problems, but on a wet screeded UFH system. Could I assume there is an air sensing thermostat somewhere? or are there more in different rooms. If there are you could check the differential (temp on / off). If it's 1 degree C & it's re-settable, set it to 2 C, that could be an easy fix. Alternatively if your room 'stat hasn't got an adjustable diff,' fit one that does, you can get one from Heatmiser for around £28.

DS1-Lv2 (1).pdf

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I have a Worcester Bosch Highflow 550cdi. You can reduce the heating output I, have a look at the following engineers guide (it covers highflow boilers but I can't find a newer one anywhere): 

 

https://www.heatingsparesltd.com/download/boiler_manuals/Worcester/Greenstar_CDI_Service_Booklet_For_Engineers.pdf

 

page 23, installer setting 1.A is "Max Output (Heating)" and the descriptor says "min adjustable output - 100%" nb. this is different to setting 2.b which is Max. Flow Temperature.

 

On my model the value can be changed between a minimum of 36 and a max of 76. I've reduced mine to 60 - I'm using with UFH throughout the house (no radiators on all 3 floors).

 

This setting only affects CH, not DHW (for which max is always available).

 

I can't find the modulation ratio for this boiler, but I'm hoping the above will make the system marginally more efficient than it is. This is reflected in my gas usage has gone down but it's hard  to know for sure when the weather temps change every day.

 

Hope this helps

valmiki

Swansea

 

 

 

 

 

Edited by valmiki

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I have a similar issue with Short Cycling. I addressed this in many ways but the most practical so far is to do away with the Boiler Thermostat and install an Elliwell unit. In short I can control both the upper temp the boiler turns off at and the lower temp it re-fires. It has made a significant improvement to the running of my system, more so that a few degrees increase in Boiler flow temp make a big difference in the house.Pic below.

Top unit is the Elliwell turns off at 60c and back on at 45c this also assists keeping the Boiler in Condensing mode with return temps never higher than 47C. 

Middle Unit Flow and return Temps

Bottom Unit is an Hour Meter.

Im about to invest in some Salus Auto Balancing Actuators.

 

Happy to answer any questions.

 

Ignore Temp readouts, I had the Bypass open and pump to max too check settings and parameters.

 

IMG_1393.jpg

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@Temp Wondering how you resolved your issues in the end as I’m for facing them too (albeit with just ufh, no radiators and a slightly larger boiler with higher min output) Thanks

 

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new member here,  i have  550 cdi, so gone through some troubleshooting before.

 

Would you be able to lower the flow temperature on the heating to a lower figure to help with the UFH issue, appreciate there will be a energy output loss to the Radiator circuit, but thought its worth a try

good luck

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Hello mate, there’s no way round it I’m  going to need a buffer tank, and I’m glad to say I’ve got a local wizard on the case 😉

Edited by valmiki

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On 30/10/2020 at 11:06, valmiki said:

@Temp Wondering how you resolved your issues in the end as I’m for facing them too (albeit with just ufh, no radiators and a slightly larger boiler with higher min output) Thanks

 

 

Ours is an oil boiler and we had cycling even with a 300L store. My solution isn't applicable to a gas boiler without a store.  What i did was turn up the boiler pump speed so it extracted enough power that the boiler never reaches the temperature  set on the boiler dial. That means the burner runs continuously until the store is satisfied. It also reduces the flow temperature.

 

If the minimum your boiler can output is more power than the load needs then the flow temperature  must increase until a sensor somewhere makes it cycle to reduce power output. To prevent it you either need to reduce power output somehow (Smaller jets? Smaller boiler) or increase the load (thermal store or buffer tank).

 

Another possibility  might be to link zones so that you always have at least two loops calling for heat. However you get less control that way. 

 

Edited by Temp

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Can see why this is a common problem @Pipes and one that could be heading my way too. I'm doing up the house; extension with a mix of solid concrete slab, plus suspended timber floors in an open plan area on the ground floor. Also have some ground floor rooms with rads. Upstairs rads too. The rads are on different zones.  I want to add some extra bathrooms, have no room for a system boiler + hot water tank so need to go for combie, a bit like Pipe's. Now you seem to get stuck as you get a big boiler to provide lots of hot water.. but when everything else shuts down apart from the UF this cycling thing crops up.

 

My understanding is that for the boiler to go into condensing mode you need a cool return flow and it wants to just chug away, which makes it difficult when you have sized combi for plenty hot water.

 

This may be blasphemy and offend the purists of UF heating.. but.. If you have UF heating that has an element of concrete slab then you have a good heat sink... which you may be able to use as the buffer. While you are supposed to keep the water pipe temperature constant maybe it's ok to allow the concrete temperature around the pipes vary a bit locally. So can this be the "buffer", heat sink... as after all, if the bit in the middle of the slab varies by 5 or 10 deg  will you notice this at the surface? Also, you do turn the heating off in the summer so some minor variation may be ok when in use.

 

How about trying to "trick" the mixer valve. The idea is that you let the return temperature drop a bit more so that when the system does call for hot flow from the boiler it can run for longer? Maybe you can experiment by just installing a couple of gate valves and manually operating them and see if the boiler settles down a bit, if so then buy the 2 port valves and stat, if not just blank off the pipes with stop ends and say "well at least I tried that and it has not cost me to much". I put some values on the diagram but I think they are a bit off.

 

See diagram below. I may well have missed a trick here, but no harm in getting some guidance.

image.thumb.png.046ebe04e20b412e453c7b51d1c9bc7e.png

 

 

Edited by Gus Potter
typos tidy up, but still same diagram

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