What Boiler to prevent short cycling?

[windsor-tg]

@SimonD - just to run some further tests, I reduced the speed of the main pump to speed (1) from speed (3) (photo attached) and took the following readings with just the UFH on.

 

[SimonD]

 

3 hours ago, windsor-tg said:

Are the readings still strange?

 

Not as strange as they appeared before! A couple of observations. In your earlier measurements the differential flow/return temperature in your ufh was almost 20 degrees C and your latest readings show up to about 15 degrees C. This is too much should be between 5 - 10. Also notice that your ufh return temp does not go much above 30 (now bear in mind it sometimes takes a long time for ufh return temps to stabilise so you may need to run the system for longer to see what eventually happens). You also have a constantly low boiler return temperature that barely reaches 45 degrees. (do keep in the back of your mind the max return temp when the whole system is open too as that stays quite low)

 

To go back to my initial observations, my first place to look is flow rate and more specifically a low flow rate because the heat is not going through the system. It's also indicated by the rapid rise in heat exchanger (flow temperature) causing the boiler to switch off.

 

Given your current big circulating pump which should in theory be able to cope with up to 8m head and 50l/min flow there's something wrong, particularly given you've got the pump set to maximum.

 

10 minutes ago, windsor-tg said:

I reduced the speed of the main pump to speed (1) from speed (3)

 

I was going to suggest you try this just to see what happens.

 

Because you know that this behaviour doesn't happen (am I correct here?) when you've got your rad zone open, the problem isn't in your boiler heat exchanger.

 

First port of call for me would be to investigate a flow restriction within the ufh zone (e.g. from and including the motorised zone valve), so either the primary pipework from boiler to ufh and or the ufh pipework. Only once this is confirmed would I look at alternative solutions.

 

You may have already said this but where is the ufh connected to your primary pipework? Is it by the boiler, pump, or somewhere else and if somewhere else, how far away from the boiler?

 

I am slightly curious about the differences you're seeing with ufh zone flow rates....

 

 

[Adsibob]

4 hours ago, windsor-tg said:

Yes, I've heard that a Veismman cam modulate down a lot further but none of the geating engineers I have spoken to have suggested a Veissman

Not everybody is certified with Veissman to give you the guarantee. Search for an engineer that is. 

[Diydad]

I have a viesmann 222f. Best boiler I have ever owned.

[SimonD]

11 hours ago, Adsibob said:

Veissman

 

Having fixed and serviced a few of them now, I'd like to provide an alternative perspective. I'm still trying to get a replacement flue terminal for a customer because the standard unit that was supposed to clip on to the existing one (according to Viessmann's info) has a completely different design so won't go on. According to the Viessmann flue info what I have doesn't exist:

 

Go to about 8:15

 

 

 

I am now personally seriously looking a ATAG Boilers to start installing them. Last year I took apart and then removed one and the engineering in it was very impressive. They're now offering 18 year warranties and they have built in weather compensation that can be configured for mixed radiator and ufh heating systems (with options to have ufh (main zone) + radiators or radiators (main zone) + ufh. The system can be set for weather comp only or weather comp plus room

 

They're also Openterm.

Edited September 17, 2023 by SimonD

[Crunchynut]

I don’t disagree with any of the advice given to fix the root cause, but boy is it complicated and expensive!

 

This might fix the symptoms if not the cause.

 

https://forum.buildhub.org.uk/topic/34283-cheap-way-to-prevent-boiler-cycling/

 

 

 

Edited September 17, 2023 by Crunchynut

[windsor-tg]

18 hours ago, SimonD said:

 

 

Not as strange as they appeared before! A couple of observations. In your earlier measurements the differential flow/return temperature in your ufh was almost 20 degrees C and your latest readings show up to about 15 degrees C. This is too much should be between 5 - 10. Also notice that your ufh return temp does not go much above 30 (now bear in mind it sometimes takes a long time for ufh return temps to stabilise so you may need to run the system for longer to see what eventually happens). You also have a constantly low boiler return temperature that barely reaches 45 degrees. (do keep in the back of your mind the max return temp when the whole system is open too as that stays quite low)

 

To go back to my initial observations, my first place to look is flow rate and more specifically a low flow rate because the heat is not going through the system. It's also indicated by the rapid rise in heat exchanger (flow temperature) causing the boiler to switch off.

 

Given your current big circulating pump which should in theory be able to cope with up to 8m head and 50l/min flow there's something wrong, particularly given you've got the pump set to maximum.

 

 

I was going to suggest you try this just to see what happens.

 

Because you know that this behaviour doesn't happen (am I correct here?) when you've got your rad zone open, the problem isn't in your boiler heat exchanger.

 

First port of call for me would be to investigate a flow restriction within the ufh zone (e.g. from and including the motorised zone valve), so either the primary pipework from boiler to ufh and or the ufh pipework. Only once this is confirmed would I look at alternative solutions.

 

You may have already said this but where is the ufh connected to your primary pipework? Is it by the boiler, pump, or somewhere else and if somewhere else, how far away from the boiler?

 

I am slightly curious about the differences you're seeing with ufh zone flow rates....

 

 

@SimonD - thanks for this. Yes, you are correct in that I do not get this behaviour/issue when the rad zone is open. I do possibly think the heat exchanger on the boiler is partially blocked as the boiler was making a kettling sound earlier in the week. 

 

Regarding the pipework on the UFH, I have attached some photos of the pipework in the airing cupboard in the landing where the main pump, 3 zone valves (HW, CH, UFH) and the UFH pipes (2 white plastic pipes) come up from the UFH manifold which is located in the room downstairs. The flow & return pipes come up from the boiler in the garage up to the airing cupboard (photo 'pipework around pump' and labelled '1' and '2'). 

 

To your question "where is the UFH connected to the primary pipework" - photos 'Airing Cupboard 8 & 9' shows the UFH zone valve connected to pipework from the CH pipework and then connected to one of the UFH white pipes. The other white pipe is connected to one of the existing pipes which is connected to the pipe going to the boiler. A few of the heating engineers who have visited have said the pipework is correctly installed. 

 

The flow & return 28mm pipework runs from the boiler to the airing cupboard is 6-7m in length. The distance of the UFH pipes from the airing cupboard to the UFH manifold is around 8.5m

 

Let me know if it would help for more photos. 

 

I did some further readings today and results are below. General observation is that when the UFH is only on, boiler continually cycling. When CH turned on, majority radiators luke warm with rads furthest away cold. Only when the boiler flow temp went up 60+, did the radiators start heating up. Not sure if this is due to the main pump speed being turned down to speed (1). When the main pump speed was (3), all the rads definitely get heated pretty quickly (with the furthest rads taking longer). 

 

When both, the CH and UFH were on, and the boiler flow temp got into the 60s, I noticed the higher temps on the UFH manifold readings. Are the readings still strange? Anything I can do to determine if there is a flow restriction in the UFH?

 

 

[JohnMo]

5 hours ago, SimonD said:

am now personally seriously looking a ATAG Boilers to start installing them. Last year I took apart and then removed one and the engineering in it was very impressive. They're now offering 18 year warranties and they have built in weather compensation that can be configured for mixed radiator and ufh heating systems (with options to have ufh (main zone) + radiators or radiators (main zone) + ufh. The system can be set for weather comp only or weather comp plus room

 

They're also Openterm

I have an Atag boiler, they are quite impressive.  They are advertised as opentherm, but like a lot of companies they are not fully opentherm, you may need to use one of their controllers to get all the features.

[SimonD]

3 hours ago, windsor-tg said:

Let me know if it would help for more photos

 

Would you mind running your ch on its own with ufh off. You can run it in existing setup with pump in 1 and then later with pump in 3. I'd like to see flow and return temps over time. It would also be interesting to see behaviour with boiler range rated back to max output. If you can set flow temp to 70 that would be great too.

 

You have 21 rads? What are their sizes and are they single/double etc.

Edited September 17, 2023 by SimonD

[windsor-tg]

2 hours ago, SimonD said:

 

Would you mind running your ch on its own with ufh off. You can run it in existing setup with pump in 1 and then later with pump in 3. I'd like to see flow and return temps over time. It would also be interesting to see behaviour with boiler range rated back to max output. If you can set flow temp to 70 that would be great too.

 

You have 21 rads? What are their sizes and are they single/double etc.

Thanks @SimonD. I will look to run these tests tomorrow. Just to confirm, do you want me to just run the CH on its own, and

1. set the boiler flow temp to 70?
2. initially on main pump speed (1) for say 15 mins (providing readings in 5 minute intervals)?
3. change the main pump speed to (3) and run for 15 minutes providing readings in 5 minute intervals?

Yes, I have 21 radiators of which 4 of these are towel radiators. Here are the sizes, etc:

[SimonD]

1 hour ago, windsor-tg said:

Thanks @SimonD. I will look to run these tests tomorrow. Just to confirm, do you want me to just run the CH on its own, and

1. set the boiler flow temp to 70?
2. initially on main pump speed (1) for say 15 mins (providing readings in 5 minute intervals)?
3. change the main pump speed to (3) and run for 15 minutes providing readings in 5 minute intervals?

 

 

Yes, please, but run the system for a good hour to 1.5 hours on each pump setting to really let it stabilise.

 

 

[windsor-tg]

Will do @SimonD. Just as a matter of interest, since I am having issues with just the UFH, by running these tests on just the CH, what will this possibly prove?

[SimonD]

10 hours ago, windsor-tg said:

Will do @SimonD. Just as a matter of interest, since I am having issues with just the UFH, by running these tests on just the CH, what will this possibly prove?

 

At the moment I suspect you're not just having issues with the UFH but have a more fundamental system problem. This is because in all your tests so far the boiler return temperature is too low whether or not your system is on rads, ufh or both. It's just that the problem is made much more obvious with just the ufh running. So running these test is going to tell more about the system without ufh distraction.

[windsor-tg]

thanks @SimonD. I will look to get these tests run later today and will report back. 

 

From the photos I sent of the pipework yesterday along with the rad sizes, did that help? 

[windsor-tg]

@SimonD - so I ran the system by first increasing the boiler temp to 70 as requested and taking readings generally at 10 minute intervals unless shown below otherwise. Observations from this were 1) the pipe between the manual gate valve and automatic bypass valve was very hot (pipe shown on the attached photo and circled in yellow) 2) I could hear the sound of water circulating through the pipework in the airing cupboard 3) after about 50 minutes of the system running, the boiler started cycling every 3-4 mins and flow temp rocketing from 45 to 60-71. It may have been doing this earlier but I was not standing by the boiler all the time. The main pump continued to run. 

 

After 80 minutes of the system running, I then increased the pump speed to 3. Immediately I saw the boiler stopped cycling and the flow temp remained at 70 with the boiler still firing. Some radiators far hotter than others which I assume is due to the TRVs. Radiators furthest away getting much hotter when speed turned to (3). Overall, it appeared the boiler was running better with the main pump speed at 3 with only the CH on

 

When I turned off the system, I did notice the return temp rising from 62 to 69. dropping down to 62 and then rising up to 67. Not sure if this has something to do with the pump overrun when the system shutsdown. 

 

Not sure what the next step is now 😕. 

 

[John Carroll]

2 hours ago, windsor-tg said:

@SimonD - so I ran the system by first increasing the boiler temp to 70 as requested and taking readings generally at 10 minute intervals unless shown below otherwise. Observations from this were 1) the pipe between the manual gate valve and automatic bypass valve was very hot (pipe shown on the attached photo and circled in yellow) 2) I could hear the sound of water circulating through the pipework in the airing cupboard 3) after about 50 minutes of the system running, the boiler started cycling every 3-4 mins and flow temp rocketing from 45 to 60-71. It may have been doing this earlier but I was not standing by the boiler all the time. The main pump continued to run. 

 

After 80 minutes of the system running, I then increased the pump speed to 3. Immediately I saw the boiler stopped cycling and the flow temp remained at 70 with the boiler still firing. Some radiators far hotter than others which I assume is due to the TRVs. Radiators furthest away getting much hotter when speed turned to (3). Overall, it appeared the boiler was running better with the main pump speed at 3 with only the CH on

 

When I turned off the system, I did notice the return temp rising from 62 to 69. dropping down to 62 and then rising up to 67. Not sure if this has something to do with the pump overrun when the system shutsdown. 

 

Not sure what the next step is now 😕. 

 

 

You could run on CH only, as above, pump on speed 3 (8M) and when the temperatures steady out just close the manual by pass and see what effect this has on the temperatures, particularly, the return. If the boiler remains running just keep reducing d.0? output in say 0.5kw steps until the boiler flow temperature just starts to fall, its quite easy then to calculate the actual system flowrate.

[windsor-tg]

13 minutes ago, John Carroll said:

 

You could run on CH only, as above, pump on speed 3 (8M) and when the temperatures steady out just close the manual by pass and see what effect this has on the temperatures, particularly, the return. If the boiler remains running just keep reducing d.0? output in say 0.5kw steps until the boiler flow temperature just starts to fall, its quite easy then to calculate the actual system flowrate.

thanks @John Carroll. I am not sure what this will achieve by doing this? I can only decrease d.0 in 1kW slices. @SimonD wanted these results to see if I have a more fundamental system problem so I think I will wait until he has seen the results. 

[John Carroll]

 

Oh yes, wait for @SimonD.

 

When you were running on pump speed1 (6M) the boiler, just before cycling, had flow/return temps of 69/44, a dT of 25C, that boiler apparently has a minimum output of ~ 6.5kw so, by calculation, the flowrate is only, 6.5*860/60/25, 3.73LPM, which, with all those rads just doesn't add up except some big restriction somewhere, you then increased the pump head to 8M and no more cycling, the flowrate should have increased by just over 15% due to the increased head so it still looks as if the heat demand is quite low, maybe, when you get the opportunity, start with d.0 at 7kw, the boiler flow temp should then surely start falling and just increase d.0 (1kw steps) until the flow temp increases to its target temp of ~ 70C, this will then tell the flowrate almost exactly. (by calculation)

Edited September 18, 2023 by John Carroll

[SimonD]

Thanks @John Carroll interesting observations.

 

@windsor-tg This is how my thinking is developing, although remember it's just conjector right now and hopefully not totally wide of the mark

 

. As @John Carrollsuggests, the results when running on no1 point towards a restriction somewhere as does the bypass getting hot as this is designed to bypass when there's too much restriction in the heating circuit.

 

What I'm thinking is that it's the size of your circulator that is causing the main problem. This is because not only does it deal with up to 8m head, it also has a high flow rate - so about 2.25m3/h in 1 and 3.25m3/h in 3. Your radiator heating circuit is about 24kW without including the towel rails, so lets say 30kW for arguments sake in total. To transfer this amount of heat around your heating system at 70C with a target temperature differential of 20C (so your return at 50C) requires a flow rate of about 1.3m3/h so whilst your pump is more than capable of providing this, it is oversized for your needs and causing a problem within the boiler. The Vaillant 438, which I believe is the model you have has the attached pressure loss curve. As you'll see from this curve, at 1.3m3/h (or 1300l/hour as per the Vaillant chart) you have a pressure loss of only 2.5m head. However, if you take 2.25m3/h and 3.35m3/h you get 2250l/hour and 3350l/hour respectively. These figures bring the pressure loss across the boiler to above 4m head and off the chart. I suspect that when you're in no1 on your pump, the total system so close to the 6m head that you get the high differential temperatures and when it's in no 3 it's enough to cope with the boiler pressure loss plus the rest of the system - but just. This would also be why you're seeing the bypass heating up as there's probably a lot of fluid going through there due to the high resistance within the system.

 

I suspect that the reason it's so obvious when you're only running your ufh is that because the ufh manifold blends flow & returns, this is increasing back pressure too much within the whole system.

 

In terms of heating circulator, something like the Grundfos Alpha 25-80 would have possibly been better because it has a high head capacity but much lower flow rates - perfect in the region of 1-1.5m3/h

 

Circulator sizing is an important part of system design but appears to be commonly overlooked.

 

Unfortunately, the only way you can test this is probably by installing a lower flow rate circulator pump and then see if it works any better.

 

Overall I don't think the solution to your heating system will be cheap as it probably needs balancing as well as a few components. If you do decide to go for a new boiler, get a system boiler as that has the pump built in which will be variable output, get modulating controls, and probably ask the installer to connect the ufh to the primary heating circuit using close coupled tees.

Edited September 18, 2023 by SimonD

[windsor-tg]

20 minutes ago, SimonD said:

Thanks @John Carroll interesting observations.

 

@windsor-tg This is how my thinking is developing, although remember it's just conjector right now and hopefully not totally wide of the mark

 

. As @John Carrollsuggests, the results when running on no1 point towards a restriction somewhere as does the bypass getting hot as this is designed to bypass when there's too much restriction in the heating circuit.

 

What I'm thinking is that it's the size of your circulator that is causing the main problem. This is because not only does it deal with up to 8m head, it also has a high flow rate - so about 2.25m3/h in 1 and 3.25m3/h in 3. Your radiator heating circuit is about 24kW without including the towel rails, so lets say 30kW for arguments sake in total. To transfer this amount of heat around your heating system at 70C with a target temperature differential of 20C (so your return at 50C) requires a flow rate of about 1.3m3/h so whilst your pump is more than capable of providing this, it is oversized for your needs and causing a problem within the boiler. The Vaillant 438, which I believe is the model you have has the attached pressure loss curve. As you'll see from this curve, at 1.3m3/h (or 1300l/hour as per the Vaillant chart) you have a pressure loss of only 2.5m head. However, if you take 2.25m3/h and 3.35m3/h you get 2250l/hour and 3350l/hour respectively. These figures bring the pressure loss across the boiler to above 4m head and off the chart. I suspect that when you're in no1 on your pump, the total system so close to the 6m head that you get the high differential temperatures and when it's in no 3 it's enough to cope with the boiler pressure loss plus the rest of the system - but just. This would also be why you're seeing the bypass heating up as there's probably a lot of fluid going through there due to the high resistance within the system.

 

I suspect that the reason it's so obvious when you're only running your ufh is that because the ufh manifold blends flow & returns, this is increasing back pressure too much within the whole system.

 

In terms of heating circulator, something like the Grundfos Alpha 25-80 would have possibly been better because it has a high head capacity but much lower flow rates - perfect in the region of 1-1.5m3/h

 

Circulator sizing is an important part of system design but appears to be commonly overlooked.

 

Unfortunately, the only way you can test this is probably by installing a lower flow rate circulator pump and then see if it works any better.

 

Overall I don't think the solution to your heating system will be cheap as it probably needs balancing as well as a few components. If you do decide to go for a new boiler, get a system boiler as that has the pump built in which will be variable output, get modulating controls, and probably ask the installer to connect the ufh to the primary heating circuit using close coupled tees.

Many thanks @SimonD and @John Carroll. I don't have too much of a problem in changing the boiler as it is 13 years old. I know I could get a few more years out of it but seeing that it was making a 'kettling' sound earlier last week, there is probably a chance that the heat exchanger on the boiler is partially blocked. 

 

Where you say 'get modulating controls', what controls will these replace and/or add? Also, where you say the heating system will probably need balancing, is that adjusting the lockshields on the individual rads to try and get all the rads to heat up equally? What other components would be needed?

 

Regarding connecting the UFH to the primary heating circuit using close coupled tees - would this all be done in the airing cupboard where the circulating pump, and all 3 zone valves are located? 

 

Is there anything to be changed with the setings on the UFH pump, blending valve or flow meters? 

 

What will the closed coupled tees potentially achieve that a low loss header or buffer cylinder will not? 

 

Finally, I have a heating engineer coming back on Wednesday to check out my system. Is there anything specific you think is worth him checking out that may help in validating the problem (e.g. detecting a blockage in the system) and/or possibly resolving the problem?

 

 

[windsor-tg]

@SimonD - if you are able to clarify my previous message, that will be greatly appreciated

[SimonD]

On 18/09/2023 at 22:03, windsor-tg said:

Many thanks @SimonD and @John Carroll. I don't have too much of a problem in changing the boiler as it is 13 years old. I know I could get a few more years out of it but seeing that it was making a 'kettling' sound earlier last week, there is probably a chance that the heat exchanger on the boiler is partially blocked. 

 

Where you say 'get modulating controls', what controls will these replace and/or add? Also, where you say the heating system will probably need balancing, is that adjusting the lockshields on the individual rads to try and get all the rads to heat up equally? What other components would be needed?

 

Regarding connecting the UFH to the primary heating circuit using close coupled tees - would this all be done in the airing cupboard where the circulating pump, and all 3 zone valves are located? 

 

Is there anything to be changed with the setings on the UFH pump, blending valve or flow meters? 

 

What will the closed coupled tees potentially achieve that a low loss header or buffer cylinder will not? 

 

Finally, I have a heating engineer coming back on Wednesday to check out my system. Is there anything specific you think is worth him checking out that may help in validating the problem (e.g. detecting a blockage in the system) and/or possibly resolving the problem?

 

 

 

When you install a new boiler, you need to add new controllers because the overall efficiency of the boiler is calculated using a combination of boiler/controller.The controller could be programmer & thermostat and/or weather compensation.These need to be modulating and not on/off (e.g. relay) to achieve the best energy efficencies. There are still loads of installers putting in new on/off controls. And even if they're sold as smart controls,like the Hive, they don't necessarily provide boiler modulation.

 

Balancing is adjusting the lockshield valves to get all rads warming up and just as importantly making sure you have the right temperature differential across the system.

 

The close coupled tees may be installed in the airing cupboard off the primary pipework, but it depends on the overall pipework layout. There are many ways to provide hydraulic separation and close coupled tees do the same thing but obviously sometimes the other options may be more suitable - without seeing your system it's difficult to say exactly.

 

I think you should let him have a look and maybe report back on the findings.

[windsor-tg]

I wanted to follow up about the Vaillant controls. I am looking at having a Vaillant ecotec plus 625 system boiler installed. I currently have the Heatmiser system installed to control my HW, CH and UFH. I can control each of these on the thermostat/control device, as well as remotely on the Heatmiser app.

 

Can anyone advise which Vaillant controls I would need (incuding the wiring centre) which would replace my Heatmiser setup? All the Heatmiser devices are hardwired back to the wiring centre. 

[marshian]

Interesting thread - really like the calcs on LPM based on kWh, Flow and Return temps - might have to pick 

 

It's worth reading the installation manual if you have it - Vaillant Boilers are probably as fussy as Glow Worm of similar era (I have a Glow worm so speak from a little experience although not as a gas engineer)

 

They have a min flow rate (in the case of mine range rated down to 10 kWh it needs 10 litres per min  - at 24 kWh they need 17 LPM and at 30 kWh over 20 LPM - if your system can't supply that flow rate at the relevant kWh output then they short cycle or throw faults

 

If your flow temps rise rapidly the boiler hasn't got enough flow or isn't modulating down fast enough (it's more likely to be the former)

 

They also really don't like the difference between flow and return temps to be over 20 deg C so if you wind all the lockshields in to get better delta T's at the rads and then exceed 20 deg C difference at the boiler they will throw faults

 

I range rate mine to 16 kWh for CH in winter and in summer when it's just HW down to 10 kWh - it still puts in more than 10 kWh on restarts but it quickly modulates down to 10 kWh which is still way more than the HW tank coil can take out so on a 117 litre tank it'll run for 10 mins, cool for 6-7 min and then run again for 4 mins before cooling again for 6-7 mins (total HW time 28 mins to get the tank to target temp

 

I've increased the pump over-run time from factory 4 to 8 mins (to try and maximise the cool down time or the boiler will hit target temp of 68 deg way too fast

 

I think your UFH circuit is too restrictive to allow the boiler to be happy - you could range rate it down to min and it might help but it's still going to be trying to shove 6 kWh into the UFH circuit and if the flow is as calculated at 3-4 Litres per min it's not going to be a happy bunny

 

JM2pW

[JohnMo]

3 minutes ago, marshian said:

rated down to 10 kWh

Should read kW not kWh

 

5 minutes ago, marshian said:

think your UFH circuit is too restrictive to allow the boiler to be happy - you could range rate it down to min and it might help but it's still going to be trying to shove 6 kWh into the UFH circuit and if the flow is as calculated at 3-4 Litres per min it's not going to be a happy bunny

I said that at the start of the thread, and basically got ignored.