Shower runs hot then cold then hot......

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I have an old Worcester Bosch Greenstart HE plus combi boiler. 3 months ago the shower, which has been working perfectly for last 5 years, would start off at correct temperature and then after about 2 minutes would run cold. Just as it was running cold I heard the boiler fire up and after another minute the water would return to hot again. However this cycle will repeat itself until the shower is turned off.

Initially I assumed it must be either a fault with the boiler or the thermostatic cartridge in the shower. As we were having the boiler serviced anyway decided to get the boiler checked out first. Boiler engineer checked the boiler and said all was fine and that he guaranteed it would be the thermostatic cartridge. The main test he did was run permanent hot water on downstairs tap nearest the boiler for about 5 minutes and the temperature didn't alter. I had already tested other hot water taps in the house and only the upstairs shower has this fault.

The shower head is at the highest point in the house and so presumably will have the lowest pressure of all hot water outlets.

I changed the thermostatic valve yesterday and it has made no difference.

So I assume it must be either a problem with the boiler or decreased water pressure affecting the boiler. Although we have tested the shower at times of day when the water pressure is higher and result is the same.

Any help would be appreciated.

Thanks

Nigel
 
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Assume you have cleaned any shower filters?.

Can you just run the shower into a container for exactly 30 secs, measure this into a 1 or 2 litre container/milk bottle, X2 and you have the flow rate in liters per min, LPM.

Also, if the boiler has HW temperature control, set it to its lowest setting and see if boiler runs continuously, measure this flow as well. then.
 
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Remove the shower head and see if the temp still fluctuates through the hose. If it doesn't, there's your problem. Descale or replace.
 
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Assume you have cleaned any shower filters?.

Can you just run the shower into a container for exactly 30 secs, measure this into a 1 or 2 litre container/milk bottle, X2 and you have the flow rate in liters per min, LPM.

Also, if the boiler has HW temperature control, set it to its lowest setting and see if boiler runs continuously, measure this flow as well. then.
Hi,

Thanks for the speedy response. Re filters I am not sure there are any apart from in the shower head (which I have cleaned). The flow rate as it is now is 9.4 l/m.

I turned the hot water temp to minimum (40C) - it was set to just below max at 60C and pressure has slightly increased to 10.2 l/m although that difference may be down to my measuring.

The boiler now seems to run continuously without temperature fluctuations
 
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Remove the shower head and see if the temp still fluctuates through the hose. If it doesn't, there's your problem. Descale or replace.
I regularly descale the shower head so I'm fairly sure that isn't the problem. The flow through the shower head is also fairly good.
 
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Hi,

Thanks for the speedy response. Re filters I am not sure there are any apart from in the shower head (which I have cleaned). The flow rate as it is now is 9.4 l/m.

I turned the hot water temp to minimum (40C) - it was set to just below max at 60C and pressure has slightly increased to 10.2 l/m although that difference may be down to my measuring.

The boiler now seems to run continuously without temperature fluctuations
OK Thanks, assuming a shower temp of 40C and a cold water temp of 20C then by calculation the combi flowrate was 4.7LPM at 60C, boiler output 13.1kw. The boiler will have a minimum flow swith probably set to - 2.5/ 3.0 LPM so possibly a erratic/faulty flow switch or temperature sensor. The boiler output is 14.23kw at 10.2LPM & 40C.
Can you confirm the minimum output of the boiler is < than 13/14kw? Also confirm the
Shower temp is 40C. I will get the mains temp here which should be similar to yours.

Mains temp here is 20.5C so negligible difference in calcs.
 
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OK Thanks, assuming a shower temp of 40C and a cold water temp of 20C then by calculation the combi flowrate was 4.7LPM at 60C, boiler output 13.1kw. The boiler will have a minimum flow swith probably set to - 2.5/ 3.0 LPM so possibly a erratic/faulty flow switch or temperature sensor. The boiler output is 14.23kw at 10.2LPM & 40C.
Can you confirm the minimum output of the boiler is < than 13/14kw? Also confirm the
Shower temp is 40C. I will get the mains temp here which should be similar to yours.
Hi - I have checked the shower temp with thermostat on shower set to max and it registers 45C (using an accurate cooking probe). So not sure why the 5C discrepancy.

For the minimum heat output my manual says:
Min. rated heat output net 40/30°C 12.9kw
Min. rated heat output net 50/30°C 12.8kw
Min. rated heat output net 80/60°C 11.4kw

Thanks Nigel
 
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Is/was the shower measured temp 45C with boiler HW set to 60C? The shower heat demand may be borderline below the boiler min output leading to shower temp increasing to its SP+5C which will cause it to cycle on/off.
 
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Is/was the shower measured temp 45C with boiler HW set to 60C? The shower heat demand may be borderline below the boiler min output leading to shower temp increasing to its SP+5C which will cause it to cycle on/off.
Hi - No it was with the boiler still set to minimum (which is marked as 40C)
 
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That results in a boiler demand of 17.4kw so well away from boiler min output, can you repeat the problem test with boiler at 60C and shower at 40C?, measure flowrate and shower temp.
 
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That results in a boiler demand of 17.4kw so well away from boiler min output, can you repeat the problem test with boiler at 60C and shower at 40C?, measure flowrate and shower temp.
Hi I moved the boiler hot water temp to max (60) and then adjusted the thermostatic valve on shower to 40 The flow rate was 8.5 l/m. during this 30 second test (once water was at correct temp) the boiler didn't cut out at all. When I did the original flow rate tests the temp was set lower at about 35C
 
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If the actual shower temp was 45C (mains temp 20.5C) then the boiler flowrate is 5.09LPM @ 60C and its output is 14.53kw which is > the boiler minimum output, so problem at any lower showering temp is probably a combination of low mains pressure + high mains temperature, there may be a filter on mains water supply at the boiler?.
I couldn't shower at a temperature of 45C, 40c max and 38C preferably, for me.
 
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If the actual shower temp was 45C (mains temp 20.5C) then the boiler flowrate is 5.09LPM @ 60C and its output is 14.53kw which is > the boiler minimum output, so problem at any lower showering temp is probably a combination of low mains pressure + high mains temperature, there may be a filter on mains water supply at the boiler?.
I couldn't shower at a temperature of 45C, 40c max and 38C preferably, for me.
Hi, and thank you for your continued help. Yes we both normally shower (especially in these temperatures) at lower than 38 - probably around 32-35C. As you say it may be that in this unusual weather the temperature differential between I/C water temp and required temp is too low. I'm not sure if there is a water filter in the water supply to boiler. Are you talking about an integral filter within the boiler or a fitted filter added before the boiler housing? As far as I can see there is no filter before the boiler - its a straight 22 mm copper pipe run from main straight into boiler.

So do you think there is a fault now with the boiler (flow switch / temp sensor) or is it just the strange circumstances of the unusually hot weather and that by turning water temp down to 40 for remainder of summer is the solution. Although on very hot days in previous summers we haven't had this issue.

Thanks Nigel
 
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I'd say Nigel probably just the unusual combination of the poor turndown to 12.8kw plus the high mains temperature plus the poorish shower flowrate. There doesn't seem to be any filter on the boiler mains but I thought there may be hot and cold ones somewhere on the shower inlets.
 
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I'd say Nigel probably just the unusual combination of the poor turndown to 12.8kw plus the high mains temperature plus the poorish shower flowrate. There doesn't seem to be any filter on the boiler mains but I thought there may be hot and cold ones somewhere on the shower inlets.
Thanks very much for your help. I will leave hot water set at 40C and see if there is anything I can do to improve the flow rate. There is a lever valve that's not full bore on the input but not sure if I changed that it would make any difference? Maybe I'll check with neighbors to see what flow rate they get.
 

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