Recommended Boiler and cylinder temp settings

Barrovian

Well-known Member
I'm hoping someone give me some advice. I have a 10 year old British Gas 330 condensing boiler and a hot water cylinder in the airing cupboard which has a thermostat setting on the side. I'm always struggling trying to get a balance between hot water that is more than lukewarm and radiators that don't take your skin off it you touch them. I've searched high and low and all the info I find seems to suggest the boiler temp should be set at around 75c for the radiators and 60c-65c on the cylinder thermostat. Does that sound right? Trouble is with those settings for me is, the hot water isn't quite hot enough and the radiators get too hot. It's almost as if I need the boiler temp (for the radiators) to be set lower than the temperature of the hot water cylinder, but how would that work? I don't think there is anything 'wrong' with either the boiler or cylinder, my CH system is serviced once a year. It's more that I'm having trouble deciding on the optimum temp settings for my home.
 

Rodders53

Distinguished Member
HW cylinder temp: 60C to kill Legionella - otherwise whatever's comfortable water wise.
Don't trust the thermostat number on the cylinder. They can be inaccurate. (Mine's way off). Use a known-to-be accurate thermometer in the water from a nearby tap to check it.
Or clip on pipe stats or similar means to measure the actual temperatures rather than the dial markings on the controls?

Showers and Baths should have thermostatic mixers fitted: 38 C is nominal and max circa 45 C as there's such a scalding risk that Govt. Elfin Safety aka Building Regs demand them now.

Boiler temp must be equal to or slightly above the HW demand temp or the HW can never reach it.

But I suspect the cylinder stat is stopping flow to the coil long before the water is up to the temperature it thinks it is set to?

My (oil) boiler is set to around 60C and my cylinder is set on something like 75 (almost max? iirc?) .. but the water exiting the tank reads a bit over 55C on the exit pipe 'stat and around the same at the taps when running hot. THAT is very hot water.

When we first moved here we found the HW wasn't very hot, which I tracked down to the cylinder stat being inaccurate.
 

Xenomorph

Active Member
I think the recommendation for cylinder setting is 60C. Could be wrong here, but I believe that if it's too hot, and you live in a hard water area, it can cause limescale build up on the pipes. Someone correct me if I'm wrong.
Put it this way, I had my cylinder thermostat set to 60C at my previous house, and the water was always hot enough.
How old is your cylinder? Could it be a faulty thermostat, if the water is not getting hot enough?
 

Barrovian

Well-known Member
@Rodders53,@Xenomorph
Thanks both for your detailed replies and advice. I had not thought about the fact that the cylinder thermostat may be off. I’ve noticed that if I set the boiler temp to 65 the radiators are fine but if I set the stat on the cylinder the same, the water doesn’t seem that warm. It’s not ‘cold’ by a long shot just not ‘hot’ ( You can tell I know what I’m talking about can’t you😂) I’ll get the thermometer out and test the water temp that’s coming out of the tap. From what you say, I should be aiming for it to be between 38c & 45c. If it’s way below that then maybe the stat is faulty.
 

Rodders53

Distinguished Member
No.
Water from a hot only tap should reach the water in tank temperature of 55-60 degrees in a fairly short time.
So whatever the thermostat peaks at after drawing off hot at a sink over a period will be what the tank is at.

From a thermostatic mixer tap it will be limited by that mixer - so don't measure there!

In UK domestically if you use a lot of hot water it may not be necessary to Legionella heat tank to 60C... and run HW cooler. (But that's another discussion).

Ideal HW temp in a tank for one house will differ from another. A small tank used for regular deep baths will need to be hotter than a big tank only used for showers and an occasional washing up of greasy pots.
 

Barrovian

Well-known Member
No.
Water from a hot only tap should reach the water in tank temperature of 55-60 degrees in a fairly short time.
So whatever the thermostat peaks at after drawing off hot at a sink over a period will be what the tank is at.

From a thermostatic mixer tap it will be limited by that mixer - so don't measure there!

In UK domestically if you use a lot of hot water it may not be necessary to Legionella heat tank to 60C... and run HW cooler. (But that's another discussion).

Ideal HW temp in a tank for one house will differ from another. A small tank used for regular deep baths will need to be hotter than a big tank only used for showers and an occasional washing up of greasy pots.
So bear with me, what I need to do is test the temp of the water coming out of a hot only tap, see what the temp is there and that’s the temp of the water in the tank, regardless of what the stat on the cylinder says? And I should be aiming for between 38 & 45 from the tap?
 

Rodders53

Distinguished Member
So bear with me, what I need to do is test the temp of the water coming out of a hot only tap, see what the temp is there and that’s the temp of the water in the tank, regardless of what the stat on the cylinder says?
Yes... once it's running fully hot (choose a hot tap close to the tank to waste least water).
That should tell you if the thermostat on the HW tank is correct (accurate) - or not.
And I should be aiming for between 38 & 45 from the tap?
No.
Whatever temperature you want it to be / to store the water in the HW tank at. If worried about Legionella it should stored above 60C so the tap will be close to that figure (but maybe a few degrees lower).

If you have very young children, elderly or vulnerable people using the hot taps (perhaps sometimes unsupervised) then a lower hot water temperature is safer for them. But that can be achieved via thermostatic mixers at the sinks if required?

Storing the hot water at too low a temperature may mean a bath mixed to 38C (say) uses all the hot water stored before the bath is full... that depends on the tank size and bath size (as well as the cold water temperature mixed with the hot).
 

Barrovian

Well-known Member
@Rodders53 thanks again for your detailed reply. You've given me some pointers for things to check so I'm going to adjust settings over the next couple of weeks and monitor tap water temp and see what happens.
 

Barrovian

Well-known Member
@Rodders53. Hope you don't mind me tagging you but I just wanted to run my findings past you. If I set the boiler to 65c and the cylinder thermostat to 60c, the hot water from the hot tap reaches approx 40c max. If I leave the boiler at 65c but bump the cylinder to 70c the hot water remains at 40c and the pump runs constantly. If I increase the boiler temp to 75c, then I get hot water at about 50c and the pump turns off. So to me, it seems that the cylinder thermostat is thinking the hot water in the tank is some 15 - 20 degrees hotter than it actually is. Is that a reasonable assumption? I'm hoping to sound slightly knowledgable for when I call a plumber. Incidentally, if if flick the electric immersion on I very quickly get to water at just over 60c. I'm tempted to just use that to heat the water until I can get a professional out.
 

Fergal82

Distinguished Member
@Rodders53. Hope you don't mind me tagging you but I just wanted to run my findings past you. If I set the boiler to 65c and the cylinder thermostat to 60c, the hot water from the hot tap reaches approx 40c max. If I leave the boiler at 65c but bump the cylinder to 70c the hot water remains at 40c and the pump runs constantly. If I increase the boiler temp to 75c, then I get hot water at about 50c and the pump turns off. So to me, it seems that the cylinder thermostat is thinking the hot water in the tank is some 15 - 20 degrees hotter than it actually is. Is that a reasonable assumption? I'm hoping to sound slightly knowledgable for when I call a plumber. Incidentally, if if flick the electric immersion on I very quickly get to water at just over 60c. I'm tempted to just use that to heat the water until I can get a professional out.
Are you running the hw on timed or constant?
If your giving the tank adequate time to heat up (let’s say an hour for arguments sake) then it sounds like your cylinder stat isn’t doing it’s job properly,
 

Barrovian

Well-known Member
Are you running the hw on timed or constant?
If your giving the tank adequate time to heat up (let’s say an hour for arguments sake) then it sounds like your cylinder stat isn’t doing it’s job properly,
Hi @Fergal82. Thanks for taking the time to reply. HW is timed to come on at 0600 - 0900 and 1600 - 1930. I've also tried running it all day, but it never gets over 40c from the tank with the settings as I described above.
 

Fergal82

Distinguished Member
Hi @Fergal82. Thanks for taking the time to reply. HW is timed to come on at 0600 - 0900 and 1600 - 1930. I've also tried running it all day, but it never gets over 40c from the tank with the settings as I described above.
Assuming that it’s a standard sized tank (36x18) then that’s plenty of time to heat it to whatever setting you have over 40c. (50-55 should be fine, anything higher will feel like it’s going to take your hand off)

The temp at the boiler is your primary setting (as you’ve established, will effect the temp of the radiators to touch) I would say anything between 60 to 70 should be sufficient. The higher it’s set the quicker the hw cylinder should get to temp. It shouldn’t effect the hw temp though as that is regulated by the cylinder stat.

There are other things that can effect hw cylinders (scale,wear&tear) but the first place to start with your problem would be to change the cylinder stat. Nice, easy and cheap!
 

Barrovian

Well-known Member
Assuming that it’s a standard sized tank (36x18) then that’s plenty of time to heat it to whatever setting you have over 40c. (50-55 should be fine, anything higher will feel like it’s going to take your hand off)

The temp at the boiler is your primary setting (as you’ve established, will effect the temp of the radiators to touch) I would say anything between 60 to 70 should be sufficient. The higher it’s set the quicker the hw cylinder should get to temp. It shouldn’t effect the hw temp though as that is regulated by the cylinder stat.

There are other things that can effect hw cylinders (scale,wear&tear) but the first place to start with your problem would be to change the cylinder stat. Nice, easy and cheap!
Thanks @Fergal82. Fingers crossed that it’s the cylinder stat then. I’ll get on the phone tomorrow. Obliged to you.
 

The latest video from AVForums

65-inch LG C1 Review coming soon to AVForums
Subscribe to our YouTube channel

Full fat HDMI teeshirts

Support AVForums with Patreon

Top Bottom