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Discuss Short Cycling Boiler in the Plumbing Forum | Plumbing Advice area at PlumbersForums.net

Messages
1
Hi,

First time poster. I've been banging my head against a wall this past month on google searches and soliciting advice of a couple of plumbers as well as the boiler supplier.

I got a De Dietrich 28-33kw Combi boiler installed just over a year ago, with UFH downstairs (3 zones) and radiators upstairs (4 upstairs in total). I started to notice the following problems at the start of last month.

Boiler cycling in and out - Initially I thought this was more so on the UFH circuit, the boiler would fire up and would soon shoot up away over the set temperature on the boiler and cut out, before cooling down, restarting and repeating the process. I've since seen that this is happening on the upstairs radiators also, and the cycling is more frequent.

Whistling Noise coming from UFH TRV - I established that I had a whistling noise coming from the TRV on the manifold, and that if i turned the TRV up/down in temperature it would temporarily stop the whistling before it would start again once it reached the set point temperature. This was also causing more frequent cycling. To stop the whistling, I removed the TRV completely and reduced the output temperature on the boiler to 55c, so the temperature going through the manifold now is between 45c -50c.

From studying the behavior of the boiler closely, with the TRV off, the boiler will cycle in and out for the first few minutes, but as it reaches it's set temperature the burner light stays on the set temperature remains consistent.

Also, with more load and demand being called for i.e. a couple of UFH zones on at once or a UFH zone and rads on, the burner light will stay on and temperature remain consistent.

The provider of the boiler says it is not a boiler issue and boiler doing it's job (although they didn't do that thorough of a check), other plumbers that I have had out seem to think that installing a bypass might solve it, however other plumbers don't. The flow and return temperatures going back into the boiler and the Delta T is acceptable the most recent plumber told me.

The most recent plumber said that he thinks the mixing valve in the UFH manifold is faulty and he would replace that in terms of stopping the whistling. Other plumbers have suggested that 3/4" pipe to the upstairs rads should have been used instead of 1/2", however radiators upstairs are piping hot when heating runs to them, so suggests circulation not an issue. Original installer says he uses 1/2" on all jobs of this size (3 bed semi) and never has a problem and he is adamant it is a boiler issue.

I have attached some pictures of the boiler and pipework, as well as the UFH pump and mixing valve.

I would love to hear peoples opinions here, seems like it's a real head scratcher to some of the plumbers who have looked at it, so I really need an expert opinion from someone who fixes/repairs these type of issues. Could it be a boiler problem i.e. faulty thermostart, dodgy PCB?

Thanks in advanceCapture.PNG
 
N

Northern Joe

Hi Simon,

Agree an auto bypass between flow and return was suggested as a solution by some plumbers, others thought it wouldn't help. Is the 2 port motorised valve not the blue instrument above the logic strip?
 
Messages
1,630
Hi,

First time poster. I've been banging my head against a wall this past month on google searches and soliciting advice of a couple of plumbers as well as the boiler supplier.

I got a De Dietrich 28-33kw Combi boiler installed just over a year ago, with UFH downstairs (3 zones) and radiators upstairs (4 upstairs in total). I started to notice the following problems at the start of last month.

Boiler cycling in and out - Initially I thought this was more so on the UFH circuit, the boiler would fire up and would soon shoot up away over the set temperature on the boiler and cut out, before cooling down, restarting and repeating the process. I've since seen that this is happening on the upstairs radiators also, and the cycling is more frequent.

Whistling Noise coming from UFH TRV - I established that I had a whistling noise coming from the TRV on the manifold, and that if i turned the TRV up/down in temperature it would temporarily stop the whistling before it would start again once it reached the set point temperature. This was also causing more frequent cycling. To stop the whistling, I removed the TRV completely and reduced the output temperature on the boiler to 55c, so the temperature going through the manifold now is between 45c -50c.

From studying the behavior of the boiler closely, with the TRV off, the boiler will cycle in and out for the first few minutes, but as it reaches it's set temperature the burner light stays on the set temperature remains consistent.

Also, with more load and demand being called for i.e. a couple of UFH zones on at once or a UFH zone and rads on, the burner light will stay on and temperature remain consistent.

The provider of the boiler says it is not a boiler issue and boiler doing it's job (although they didn't do that thorough of a check), other plumbers that I have had out seem to think that installing a bypass might solve it, however other plumbers don't. The flow and return temperatures going back into the boiler and the Delta T is acceptable the most recent plumber told me.

The most recent plumber said that he thinks the mixing valve in the UFH manifold is faulty and he would replace that in terms of stopping the whistling. Other plumbers have suggested that 3/4" pipe to the upstairs rads should have been used instead of 1/2", however radiators upstairs are piping hot when heating runs to them, so suggests circulation not an issue. Original installer says he uses 1/2" on all jobs of this size (3 bed semi) and never has a problem and he is adamant it is a boiler issue.

I have attached some pictures of the boiler and pipework, as well as the UFH pump and mixing valve.

I would love to hear peoples opinions here, seems like it's a real head scratcher to some of the plumbers who have looked at it, so I really need an expert opinion from someone who fixes/repairs these type of issues. Could it be a boiler problem i.e. faulty thermostart, dodgy PCB?

Thanks in advanceView attachment 47091

What is the minimum boiler output on CH?

When you removed the TMV completely was the boiler just circulating water into the top of the manifold, through the UFH and then returned to the boiler?.

Is there one TMV for all three UFZ zones?

What is the approx flowrate through each UFH zone, by adding the flow tube rates?

Did your plumber give you any idea of his acceptable deltaT?
 

SimonG

Plumber
Messages
20,002
Hi Simon,

Agree an auto bypass between flow and return was suggested as a solution by some plumbers, others thought it wouldn't help. Is the 2 port motorised valve not the blue instrument above the logic strip?

Yep, didn't notice that, looks like that's the one for radiators, but wheres the one for the underfloor heating?
 

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