Boiler keeps short cycling with only UFH is on

[windsor-tg]

Good morning, wondered if anyone can advise please on a resolution?

My set up at home is that I have a 4-bed detached with 21 radiators (includes 4 towel rails), a 6m x 4m orangery/conservatory that was built last year with under-floor heating, and a Megaflo for storing hot water. The UFH contains two loops in a screeded floor, each tube loop in the floor to the manifold is approx 60m long.

Last year, I had all my radiators changed (unfortunately I did not have the system fully flushed) and the main pump upgraded to a Grundfos UPS2 25-80 pump. I control the hot water, central heating, and UFH through the Heatmiser app. The hot water, CH and UFH each have their own zone valve. I have attached a photo (UFH manifold) of my UFH manifold taken last October and a photo ('Zone valves labelled') of my airing cupboard where the megaflo, main pump and zone valves are located. Zone valves marked: 1 = HW, 2 = UFH, 3 = CH.

I currently have a Vaillant ecoTEC Plus 438 38kW system boiler which is 13 years old located in the garage. A few years ago, I sused to get an S.53 message displayed on the boiler. To eliminate this display message, I had to turn down the power of the boiler to 24kW (d0 = 24) so I have been running the heating & HW system with boiler capped at 24kW.

The problem I am having is that when I only have the UFH turned on, the boiler is continously/short cycling. When I turn on the central heating, the issue goes away. In discussion with the Vaillant technical team, their view is the reason why my boiler is cycling is because the flow rate through the boiler & system is not enough when only the UFH is operating. In other words, they are saying the boiler continues to cycle because the boiler is not able to modulate down far enough. For a 24kW boiler, the Vaillant technical person said the min flow rate needs to be 17.2L/min, and for a 30kW boiler, the flow rate needs to be minimum 21.5L/min. I have no idea to know what the flow rate is for my UFH.

There is mention from a couple of heating engineers about introducing a Low Loss Header.

Recently, my boiler is making the 'kettling' sound so I am happy to change the boiler but I also want to eliminate this short cycling boiler issue when I only have the UFH turned on.

Can anyone advise here?

 

[Timmy D]

Low loss header will help.

 

[Chuck]

A low-loss header (LLH) is the solution if the root cause is low flow rate through the UFH. If this is the cause the UFH won't be putting out enough heat to maintain the room temperature and this will get progressively more noticeable as the weather gets colder.

A LLH may or may not help if the root cause is low demand for heat from the UFH, i.e. it's below the boiler minimum because the weather is relatively warm or the house insulation is good. In this case, the cycling will get less frequent as the weather gets colder and the UFH will work as designed.

In the second case, the LLH will help to some extent by increasing the effective volume of water in the boiler-UFH loop, which will lengthen the cycling period. If the boiler's "anti-short cycle lock-out" state is being triggered an LLH may be enough to fix it. If not, the dead-cert cure in this case is to add a 'buffer store' to the heating circuit. People don't often do this because (a) cost, (b) space, and (c) modern boilers are designed to cycle (the amount of fuel it wastes is a tiny fraction of the annual total) when heat demand is low, and (d) it only happens in the autumn and spring when you barely need the heating on anyway.

Changing the maximum power setting on the boiler won't affect this aspect of the behaviour, which is related to the minimum power the boiler can deliver while running continuously. (This is governed by the burner design.) If you do change the boiler go for one, e.g. Vessmann, with a low minimum power.

 

[windsor-tg]

A low-loss header (LLH) is the solution if the root cause is low flow rate through the UFH. If this is the cause the UFH won't be putting out enough heat to maintain the room temperature and this will get progressively more noticeable as the weather gets colder.

A LLH may or may not help if the root cause is low demand for heat from the UFH, i.e. it's below the boiler minimum because the weather is relatively warm or the house insulation is good. In this case, the cycling will get less frequent as the weather gets colder and the UFH will work as designed.

In the second case, the LLH will help to some extent by increasing the effective volume of water in the boiler-UFH loop, which will lengthen the cycling period. If the boiler's "anti-short cycle lock-out" state is being triggered an LLH may be enough to fix it. If not, the dead-cert cure in this case is to add a 'buffer store' to the heating circuit. People don't often do this because (a) cost, (b) space, and (c) modern boilers are designed to cycle (the amount of fuel it wastes is a tiny fraction of the annual total) when heat demand is low, and (d) it only happens in the autumn and spring when you barely need the heating on anyway.

Changing the maximum power setting on the boiler won't affect this aspect of the behaviour, which is related to the minimum power the boiler can deliver while running continuously. (This is governed by the burner design.) If you do change the boiler go for one, e.g. Vessmann, with a low minimum power.

A low-loss header (LLH) is the solution if the root cause is low flow rate through the UFH. If this is the cause the UFH won't be putting out enough heat to maintain the room temperature and this will get progressively more noticeable as the weather gets colder.

A LLH may or may not help if the root cause is low demand for heat from the UFH, i.e. it's below the boiler minimum because the weather is relatively warm or the house insulation is good. In this case, the cycling will get less frequent as the weather gets colder and the UFH will work as designed.

In the second case, the LLH will help to some extent by increasing the effective volume of water in the boiler-UFH loop, which will lengthen the cycling period. If the boiler's "anti-short cycle lock-out" state is being triggered an LLH may be enough to fix it. If not, the dead-cert cure in this case is to add a 'buffer store' to the heating circuit. People don't often do this because (a) cost, (b) space, and (c) modern boilers are designed to cycle (the amount of fuel it wastes is a tiny fraction of the annual total) when heat demand is low, and (d) it only happens in the autumn and spring when you barely need the heating on anyway.

Changing the maximum power setting on the boiler won't affect this aspect of the behaviour, which is related to the minimum power the boiler can deliver while running continuously. (This is governed by the burner design.) If you do change the boiler go for one, e.g. Vessmann, with a low minimum power.

thanks @Chuck. As requested by another person, they wanted to see what the flow & return temps were showing on the boiler as well as the flow meters on the UFH manifold. Below are the readings (I also attached a file showing these readings incase the image below did not come through) . As well as taking the flow & return readings from the boiler, I also took readings from the UFH manifold. The blending valve on the UFH is set around 47 degrees.

I took the readings across 5 minute intervals. What I found is that when I had both, CH and UFH turned on, the boiler was happily firing (without any sound of kettling which there was earlier in the week). The flow meters on the UFH was showing 2.5 (range 0-5). When I turned off the UFH, the flow meters on the UFH went to 5 and these flow meters stayed on 5 when I turned on the UFH and turned off the CH.

After about 10 minutes of the UFH only being on, I could see the boiler short cyling (every 2 mins) and the boiler flow temperature would rocket from 45 to 72 degrees.

When I provided this information to the other person, their response was "Simple fix a 2 port buffer of around 25 to 50 L capacity (closer to 50L is better), across the flow and return loop to the boiler or across the flow and return from the UFH, where ever is easiest to install. No need for low loss headers or 4 port buffers and additional pump (s).

Parts list

• Two Tee pieces,
• A buffer or volumiser - these are cheap enough, you need to insulate yourself.
• Two isolation valves"

Would this potential setup work?

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[fowlerboi]

Your UFH loops are running at 5LPM of flow each?

 

[windsor-tg]

Your UFH loops are running at 5LPM of flow each?

The flow meters have been tinkered about of late by some heating engineers. You will see from the readings that when both, the CH and UFH are on, the UFH loops are each running at 2.5LPM. When the UFH then runs on its own, the flow bumps up to 5LPM. I do not know why.

Would this possibly make the boiler short cycle when only the UFH is on?

 

[fowlerboi]

John.g is the pump expert so hopefully he can chirp in on what pump setting you should be on to keep it more consistant

 

[windsor-tg]

thanks @fowlerboi. @John.g - what do you think?

 

[ShaunCorbs]

I suspect your boiler pump is interfering with the ufh pump hence why your getting more flow as you have a short / low demand system

You need to reset the flow meters back to spec / Heatloss as you could be damaging the floor / having higher return temps should be 7-10 dc difference between flow and return

 

[windsor-tg]

I suspect your boiler pump is interfering with the ufh pump hence why your getting more flow as you have a short / low demand system

You need to reset the flow meters back to spec / Heatloss as you could be damaging the floor / having higher return temps should be 7-10 dc difference between flow and return

Unfortunately, I don’t have any spec as the UFH was installed by the builder’s heating guy and there was no design.

Do I need to turn down the flow meters to 2.5L when I only have the UFH on? Will that reduce the boiler cycling?