Varying shower temperature

[Deleted member 99988]

Background:
I live on the first floor in a block of 24 flats with a water pressure booster set (this has been serviced and is in full working order).
I have a Weissmann vitodens 100-w boiler.
One shower is a Bristan mixer shower, the other is an Aqualisa thermostatic quartz A2 mixer shower.

Both showers in my flat vary in temperature (and pressure) when in use. I have noticed that when the shower is in use the boiler kicks when required but gets to 88 degrees approx and turns itself off. When the temperature cools down again it kicks back in. So you get hot-tepid-hot-cold-tepid showers. This happens at any time of day.

The same happens with my hot taps (ie boiler going on and off).

To complicate things - the water flow to my flat (and others in the block) varies between 8-15L/minute. I have confirmed this with a weir gauge in my own flat and other flats. This variation happens several times over a minute. See attached video in dropbox link.

I have no plumbing experience but have tried to read up online about this. I have spoken to various plumbers and still no answer. My theories are as follows:

The boiler works hard to heat the high flow rate of water. When the flow drops it seems to overheat so shuts off (the flame signal on the lcd screen disappears).
Could the PRV in my boiler have failed?
Could a temperature sensor in the boiler be faulty?
Could the secondary heat exchanger could be a problem or blocked? This sounds an expensive thing to fix first.

Solutions I have thought of:
Get a PRV on my mains supply so my pressure is more constant - perhaps giving a flow of 8-10L/minute? Would this help the boiler not to fire up SO much and then overheat?

Further questions:
Is it reasonable to expect that my water flow is fairly constant? 8 - 15L/min seems a huge variation to expect the boiler to deal with.

Attached items:
Dropbox - video.mov

 

[dancinplumba]

you need a GSR to sort boiler out, there is an issue with it.
pressure should be constant with booster set unless its not big enough

 

[Deleted member 99988]

you need a GSR to sort boiler out, there is an issue with it.
pressure should be constant with booster set unless its not big enough

I had a British gas annual boiler service approx 18 months ago. This was after I had the above problem and they confirmed that the boiler was working fine. I did mention this problem at the time but they pointed it away from the boiler.

Could it be they don't (or didn't) check for other problems?

 

[dancinplumba]

I had a British gas annual boiler service approx 18 months ago. This was after I had the above problem and they confirmed that the boiler was working fine. I did mention this problem at the time but they pointed it away from the boiler.

Could it be they don't (or didn't) check for other problems?

yes , probably

 

[JCplumb]

I have a similar issue with my thermostatic mixer shower on a combi, need to turn the desired water temp on the boiler down to about 55 to stop it happening.
I'm no gas man but I figured the shower slows the hot down more the hotter it gets so at some points is throttling the hot water enough to make the boiler think it's satisfying temp demand so the water temp then cools until the boiler sees the need to to fire up again.
Try turning your water dial to 3 on the boiler and see if it still happens.

 

[Chalked]

I had a British gas annual boiler service approx 18 months ago. This was after I had the above problem and they confirmed that the boiler was working fine. I did mention this problem at the time but they pointed it away from the boiler.

Could it be they don't (or didn't) check for other problems?

Annual service 18months ago!

 

[Deleted member 99988]

Should the boiler be getting up to 88 degrees?
I tried turning down the water temp but it still goes pretty much as high.

 

[JCplumb]

Not a gas engy, but when I turn mine down to 55 it doesn't go above 60.

 

[johnduffell]

Could be your plate heat exchanger is scaled up so the heat is going into the primary water but not getting to the dhw quick enough.
Try turning the target dhw temp as low as possible, we use 35c target and turn the hot tap on only. Saves gas anyway that way. But it could be time for a new plate hex anyway.

 

[Deleted member 99988]

What do you think about the fluctuations in water flow?

 

[johnduffell]

What do you think about the fluctuations in water flow?

I thought you said that was an issue with the incoming water supply? You'd have to get onto the free holder about that or the water company.
If forced to guess I'd say there's a restriction somewhere in the pipework upstream of the flats, and there's a header tank that's keeping itself topped up.

 

[Deleted member 99988]

I thought you said that was an issue with the incoming water supply? You'd have to get onto the free holder about that or the water company.
If forced to guess I'd say there's a restriction somewhere in the pipework upstream of the flats, and there's a header tank that's keeping itself topped up.

There is an issue with the incoming water supply. It varies between 8-15L/min on all cold taps. Same issue in other flats on same floor, floor above and ground floor.

Water company have checked the supply to the booster set. The company that service the booster set have replaced it and recently serviced it. No change. There is not a header tank.

Where else could the fault be?

 

[Harvest Fields]

There is an issue with the incoming water supply. It varies between 8-15L/min on all cold taps. Same issue in other flats on same floor, floor above and ground floor.

Water company have checked the supply to the booster set. The company that service the booster set have replaced it and recently serviced it. No change. There is not a header tank.

Where else could the fault be?

You cannot gauge flow from your taps. It needs to be tested from the point it enters the property. All taps let out different flow rates.

 

[Deleted member 99988]

You cannot gauge flow from your taps. It needs to be tested from the point it enters the property. All taps let out different flow rates.

I see that, but it's not the absolute flow I am worried about. It is the pressure/flow fluctuations I am experiencing.

 

[Harvest Fields]

Are you saying that for example. Sometimes your bath tap is 8 litres and sometimes it is 15?

 

[Deleted member 99988]

Are you saying that for example. Sometimes your bath tap is 8 litres and sometimes it is 15?

Exactly this. The same happens on my bath tap, my basin tap and my kitchen tap (difficult to measure qualitatively here due to aerator). It fluctuates 1-2 times over the course of a minute. Did you see the video attached to my original post?

 

[Harvest Fields]

Just watched it. Looks like the the booster might be cutting in and out. Does it happen every time you switch a tap on or just mornings/evenings. Have to tried it with the taps filter removed from the nozzle

 

[Deleted member 99988]

It happens at any time of day/night.

They have changed a setting on the pumps and the flow is now pretty constant (around 10-11L/minute).

I still have the problem with the hot water.

If I use:
Hot water --> boiler overheats (90+ degrees), flame cuts off and water never even gets hot.
Heating --> works fine (temp gets to 65ish degrees C and stabilises)
HW + heating --> overheats.

 

[johnduffell]

Did you read my earlier post about your plate heat exchanger?

 

[SavageW]

Sounds like you have two issues mate, if the flow rate has now been sorted an as you said now a constant between 10-11lpm, as others have said I’d be saying the plate heat exchanger is blocked/restricted. Ring British Gas as you have a contract with them. Report a problem with your hot water, I know you said you reported it to the BG engineer when you had your annual service but some blokes only do the servicing work so they ain’t gunna get to involved in finding the fault.