Shower hot then cold then hot again

Anne8584

Given how the system has been plumbed in in the past the water goes up to the bathroom level before coming down to the kitchen tap, so I guess if someone recently used the kitchen tap, which they did this evening, then that water would then come through the shower as mentioned. 

Anne8584

Otherwise central heating comes on quick and radiators haven't needed bleeding, I check these yearly when it gets colder. So this goes against potential sludge build up. Anyway, will book in another service. 

neilmcl

Anne8584 said:

Given how the system has been plumbed in in the past the water goes up to the bathroom level before coming down to the kitchen tap, so I guess if someone recently used the kitchen tap, which they did this evening, then that water would then come through the shower as mentioned. 

As I mentioned I don't see this as anything wrong, just the way it works sometimes. I'd definitely recommend running the tap first before the shower to get the temp up to full.

coffeehound

There can also be a temperature wobble as the cold feed to the shower starts warm and then cools down; especially with the strange warm-cold weather we've had lately.  Perhaps also the shower mixer might be getting unresponsive rather than a boiler problem; maybe due a descaling.

Anne8584

I fitted a new shower valve about a month ago as the shower head had started dripping, the issue I describe started shortly before that. So I know its not the shower itself. 

Jeepers_Creepers

Anne8584 said:

I ran the shower for a while, part filled the bath, then ran the hot tap for a bit, while observing boiler readings. The light was on throughout. When the shower went on the temp rose to just above 60 degrees then after a minute or so dropped quickly to 54.5 and stayed there. I then turned of the shower and turned the tap on. When the hot tap was on the temp went up to 65 degrees then down to 62.5 up to 63 then down to 62.5 and stayed there. 

Could well be nothing more than what Neil and Coffee suggest.

If it were a blocked P2P, then it would usually carry on doing this sort of hot-cold cycling.

What's the difference between a hot tap and the hot of the shower? The latter goes through a mixer, most likely thermostatically controlled. That has the effect of adjusting the hot flow to suit whatever temp the shower is set at, and some boilers aren't as quick to react to this, and can modulate down too much if the hot demand is reduced suddenly. It's interesting that the boiler only supplies the hot at 55oC during the shower - this is usually too low for many shower thermostats to operate smoothly.

You mentioned you replaced the shower's flow cartridge recently? What about the temp control side - the thermostat?

I think it would be worth removing this and sitting it in a cup of descaling liquid overnight before rinsing and replacing.

Certainly, if this cold fluctuation only happens once per shower, and at near the beginning - and the boiler otherwise seems fine - I wouldn't bother getting anyone out to look at it. Ask your GasSafe about this behaviour at the next service. Otherwise, wait until it gets worse - if it is sludge (but no reason to suspect it is), it will get worse...

Meanwhile, happy brrrr-aaaah! showers.

neilmcl

Anne8584 said:

I ran the shower for a while, part filled the bath, then ran the hot tap for a bit, while observing boiler readings. The light was on throughout. When the shower went on the temp rose to just above 60 degrees then after a minute or so dropped quickly to 54.5 and stayed there. I then turned of the shower and turned the tap on. When the hot tap was on the temp went up to 65 degrees then down to 62.5 up to 63 then down to 62.5 and stayed there. 

One thing to bear in mind is that your shower will have a maximum, safe, hot water cut off at around 44 degrees so it would never drawer a full 60 degrees from the boiler so it doesn't surprise me that the temp on the boiler will be lower than that from a a hot tap.

southcoastrgi

Exactly what neil said, sounds perfectly normal to me, your boiler has 5 yrs warentee 

Jeepers_Creepers

neilmcl said:

One thing to bear in mind is that your shower will have a maximum, safe, hot water cut off at around 44 degrees so it would never drawer a full 60 degrees from the boiler so it doesn't surprise me that the temp on the boiler will be lower than that from a a hot tap.

From what I understand about modern modulating combis is that the DHW should be delivered at pretty much the temp asked for on the dial - even if the scaling is no more sophisticated than 0 to 'hot' like mine. The position of this DHW control dial should correspond to a hot water delivery temp that is maintained fairly constantly. Ie, if you turn your hot tap on half, then you should get a half-flow at whatever temp is set, say 60oC. Turn it down to a trickle, and it should still be 60oC. Turn it up full, and the boiler will do its damnedest to maintain that 60oC. Ie, the flow rate changes, but the boiler's power output modulates to serve that flow and maintain that temp as much as possible.

So, a thermostatic shower which is set to a typical 39oC will still be drawing hot from the combi at the set temp - say 60oC. It then blends this with cold to arrive at the shower head delivery temp.

Why would the boiler turn its delivery temp down for the shower to below what it supplies to a tap? Unless the DHW demand was sooo limited that the boiler could barely modulate down to that rate. Why would the boiler drop its temp to below 63oC when supplying the shower when it doesn't for a tap?

Something is weird.

Also, this hasn't been the normal behaviour of the shower and boiler; Anne mentioned it started just over a month ago or summat?

I wonder if it's something to do with the thermo cartridge? Sticky in operation, perhaps, slow to respond, restricts hot flow until the boiler turns right down - possibly shutting the burner off for a bit.

A mystery.