Water too hot when heating is on

(15 Posts)
ginyogarepeat Sat 09-Nov-19 13:27:03

First year in a new home (house is 20 years old). Over the summer when heating wasn't on, turned on hot water as and when we needed it and all fine. Now that winter's here and we're using the heating more, the hot water is running very, very hot - despite thermostat set at below 60. What could be the cause of this?

I also noticed earlier that en-suite sink tap upstairs - cold tap initially ran a bit warm and then got cold after a few seconds. Doesn't seem be an issue with downstairs taps and I'm not sure if it's related to hot water issue?

DH and I fairly clueless about boiler issues - would I be better getting a plumber or a boiler engineer to check it out professionally?

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PigletJohn Sat 09-Nov-19 14:02:37

Have you got a hot-water cylinder? What colour?

Photos of the pipework and controls would help.

ginyogarepeat Sat 09-Nov-19 14:12:27

I've taken a few @PigletJohn. Green cylinder tank. Two switches beside it in the press - first is marked 'hot water' and is on. Second is off and unlabelled?

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PigletJohn Sat 09-Nov-19 14:54:22


the hot water cylinder should be heated by your gas boiler. It also has an electric immersion heater that can be used when the boiler is broken. Do not use the electric immersion heater at other times. Turn off the electric switch marked "water heater" and leave it off until your boiler breaks. Energy from electricity costs about four times as much as energy from gas.

At the bottom right hand side of your cylinder, held in place by a stretchy white cord, is the cylinder thermostat. This controls the temperature of the cylinder when the boiler is running. It will look rather like one of
these. You can adjust it to between 50 and 60. Plumbers will tell you that 60C sterilises the water more quickly, which is true, but the way cylinders work, the top will always be hotter than the bottom. Depending on cylinder size, you may want to adjust the setting a bit so it is hot enough to run a full bath, but not scalding. 60C is pretty hot (try a thermometer).

When the boiler is running, and your timer/programmer is set to HW on, it will heat the cylinder each time the cylinder stat, which is near the bottom, senses that the water is below target temperature. The bottom of the cylinder is where cold water enters to keep it full, so this may happen each time you fill a sink. This may result in the water at the top of the cylinder being uncomfortably hot. You can control this by setting the programmer to HW On about twice a day, for about an hour, starting half an hour before you expect tp be having baths and showers, and turning HW Off half an hour after your last bath or shower time. Depending on the size of your cylinder, and how much water you use, this may be about right. Many modern cylinders (of a different design) are very large and can supply a day's hot water after being heated once. This is economical on gas because the boiler and pipework are only heated once. You can reduce heat losses even more by lagging the hot pipes, especially around the boiler and cylinger. They will be 22mm diameter (some 28mm or 15mm). Measure before buying so you get the right size.

I can see your green cylinder, and probably the other controls, are around 20 years old, which fits with the age of your house. There may be wear in the motorised valve, which can cause the cylinder to get too hot, but try my other suggestions first. if the motorised valve (the blue box) is at fault, the large copper pipe running through it will get very hot when the radiators are on, even if HW is Off.

When the boiler is off, try using the immersion heater for a day. If the water gets scalding hot, the immersion heater thermometer is faulty, which can be very dangerous with an older design as the water may boil. The thermostat can be easily changed by a plumber or heating engineer or electrician. It is in a sealed tube so no water comes out when you change the thermostat. This thermostat can also be adjusted, but only after turning off the power.

ginyogarepeat Sat 09-Nov-19 15:04:53

Thanks so much for this @PigletJohn. We use oil as energy, not gas - does this make any difference? We have an immersion switch in the kitchen which is always off, am confused by a second one in the press! If this second one has been on all summer why didn't we have the same issue with the hot water? It's only now that the heating's on that we're having this problem. Yes, thermostat box was first thing we checked when this started, was moved down to min temp - about 50 degrees, which hasn't made a difference.

Our control panel downstairs is what we use to control heating and water. Can boost either for an hour or use timer.

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ginyogarepeat Sat 09-Nov-19 15:06:40

Sorry, meant to add that is what we currently do re hot water - programme for hour in morning and same again in evening for showers/baths.

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PigletJohn Sat 09-Nov-19 15:13:38

ok, feel the large pipe that goes through the blue box. That may be the problem. If it is getting very hot whenever the radiators are on, that's due to wear. 20 years is a fair life. If it turns out to be faulty it needs to be changed by a heating engineer or experienced plumber. It uses water and electricity, and some draining will be needed.

Your boiler service person could doubtless do it, but check it first.

ginyogarepeat Sat 09-Nov-19 16:34:37

Ok - heating's on downstairs so I've gone up and felt the large pipe coming out of blue box - it's very hot so that's likely to the problem then?

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PigletJohn Sat 09-Nov-19 16:35:31

Is the HW set to "off?"

PigletJohn Sat 09-Nov-19 16:36:44

on the boiler programmer, I mean.

post a photo of it if not clear please.

ginyogarepeat Sat 09-Nov-19 16:37:16

The hot water switch beside the cylinder was on - I've switched it off now.

Can't figure out what that switch is for - it's not immersion as that's downstairs.

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ginyogarepeat Sat 09-Nov-19 16:38:11

On control programmer downstairs (I assume you mean controller that we can use to time water and heating) yes - hot water is off.

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ginyogarepeat Sat 09-Nov-19 16:39:54

Sorry I was a bit confused there - switch in press marked 'water heater' was on (it's been on since we moved in - immersion switch is downstairs and never used). Programmer used to control water and heating - heating on but water off.

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PigletJohn Sat 09-Nov-19 16:50:40

"Programmer used to control water and heating - heating on but water off."

Ok, in that case the pipe to the right of the blue valve should not be very hot. It may be somewhat warm due to heat conduction.

If it gets very hot, when HW is Off, whenever the radiators are being heated, then it is worn out and not sealing (or there might be a wiring fault, which is less likely). The valve is supposed to close to prevent heat being provided to the cylinder when not demanded.

ginyogarepeat Sat 09-Nov-19 16:55:29

Thank you!

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