Strange hot & cold water temp in shower, from combi following service

[d000hg]

We have a fairly new Baxi combi boiler with full service history. It generally works pretty well - heating runs at a high temp, water can be scalding - but there are a couple of odd things we've noticed.

1. My wife reports the shower in our ensuite keeps going cold for a bit, then back up to heat. This is not due to other people running the hot tap, or water at all (often I'll be in bed with nobody else here).
2. I like a hot bath. Recently, even running only the hot tap it is only what I'd call warm, not hot.

2. happened last winter so I wonder if either colder water supply or the fact the heating is on means it just cannot heat the water enough. Does heating and hot water use the same bit of the boiler or are they separate? I do find if you run the tap less, the water can be hotter.

1. is new. We just had the boiler serviced by BG a few weeks back and my wife is sure that's when it changed.

What should we do, and can anyone help me understand likely causes? Thanks

 

[flameport]

Does heating and hot water use the same bit of the boiler or are they separate?

Same, there is a diverter valve in the boiler so it can either heat the hot water, or the radiators.
Not both at the same time, unless the valve is faulty, in which case you will get radiators heating up when a hot tap is turned on and hot water output will be poor.
There may be other problems.

I do find if you run the tap less, the water can be hotter.

Normal. Lower flow = water is in the boiler for longer and gets hotter.

 

[d000hg]

Ah so the boiler is smart enough to prioritise hot water over heating? I was considering turning the heating off to run a bath!

It seems to only be this one shower with the issue, not other taps etc.

So the water temp at on the boiler is best case, it doesn't reduce flow to attain that if it's struggling?
Maybe it is as simple as the cold weather. Heating water from 15-50 on demand is pretty impressive when you work out the math, if it drops to 5C that's a huge difference in energy.

 

[ianmcd]

exactly your boiler is set to raise the water temp at a given flow rate so say it is 35 deg at 11L/min fore example if the incoming water is summer is say 10 deg then the final water temp will be 45 deg in winter if water incoming is 5 deg final water temp will only be 40 deg, hence if you slow the water flow the water is hotter as it picks up more heat going through the boiler, these are just made up figures as an example and most likely the figures for your boiler will be different , was just to explain to you how your boiler works

 

[d000hg]

All makes sense. Just leaves the mystery of the shower temperature not being stable. But I haven't experienced it first-hand so I've asked her to try and record more specifically what is happening, try the other shower to see if it makes a difference.

I believe many boilers keep a tiny amount of pre-heated water - often I find turning the hot tap on I get a brief rush of warm/hot water after the pipe runs through, then it cools down before getting hot again. I don't know how much water the pipes hold exactly but I guess a few litres which makes running the hot tap just to wash your hands seem a huge waste of water and energy... you wait for the water to get hot then just as it does, you turn it off leaving the pipes full of hot water that is wasted.

 

[ianmcd]

you will always get a slug of cold water, the water in the boiler will be hot but by the time that a fresh demand is senced and the boiler reacts there will always be a bit of cold, once the demand is activated it should be constant though

 

[Hot&Cold]

We just had the boiler serviced by BG a few weeks back and my wife is sure that's when it changed.

Could just be a coincidence, If you have a suitable thermometer you can measure the hot water temperature from a fully open bath tap.The temperature should average out similar to the setting of the boiler hot water thermostat. While reading the thermometer note the lowest stable and highest stable temperatures.

The system could be working correctly a lot depends on the boiler model and the propertys plumbing layout

 

[Madrab]

Turn on the shower at the usual temp, does the boiler cycle on and off?

 

[d000hg]

Turn on the shower at the usual temp, does the boiler cycle on and off?

Sorry for maybe a daft question but how do I tell?

 

[Madrab]

Turn on the shower and go to the boiler, do you hear it running then stopping then running then stopping. What Baxi is it?