Hot water too hot.

[dannynolan]

We have a Worcester Bosch Greenstar CDI boiler and a hot water tank with thermostat (see picture).

With the cold weather our radiators were not getting hot enough so we have turned up the boiler. The radiators are now hot enough but the hot water from our taps is far too hot.

Does this mean that the cylinder thermostat is faulty or might there be some other reason?

in the airing cubpoard

 

[BlueLoo]

Why do you have a combi boiler and a hot water tank together?

Is the tank defunct/decommissioned?

 

[dannynolan]

It's a condensing boiler.

 

[BlueLoo]

It's a condensing boiler.

Greenstar CDi Compact overview
Our Greenstar CDi Compact combi boiler is suitable for small, medium and large-sized properties with one or two bathrooms.


Are you sure?

 

[dannynolan]

Sorry my fault, I should have given the complete name - Greenstar CDI Conventional

 

[Munroast]

the top of the tank will pretty much go to the temp of the output temp of the boiler, I suspect the water at the same level as the thermostat will be the temperature on the thermostst

 

[BlueLoo]

Sorry my fault, I should have given the complete name - Greenstar CDI Conventional

Ok, because it would explain why the HW was getting hotter .

Obvious answer would be someone has turned the immersion on.

that not being the case, then you look to see how the boiler is heating the hot water. its usually by indirect internal coil, controlled by a valve or somesort.
Perhaps that zone valve has failed and is not closing when it should, or, that stat is knackered and not telling the valve to close.

Some easy checks: look at the zone valve to the tank from the boiler. in a cold/off condition, is the valve indicator showing the valve shut?.
adjust the thermostat up and down. it should "Click" as the contacts open and close. If not, then a continuity test on it may be useful.

What system do you have? can you take a picture of the zone valve?

 

[BlueLoo]

the top of the tank will pretty much go to the temp of the output temp of the boiler, I suspect the water at the same level as the thermostat will be the temperature on the thermostst

Only if it isn't zoned. (unusual),

 

[Harry Bloomfield]

Does this mean that the cylinder thermostat is faulty or might there be some other reason?

There could be other reasons and the stat is not set particularly high, but try turning it down some more. It should click at around the temperature it is 'seeing'.

 

[Munroast]

Only if it isn't zoned. (unusual),

could you explain this further - may be my tank /system has always been faulty. Because the top of my tank always sort of matches output temperature no matter what the thermostat says. I have ours set to just below 40 and the top of the tank just after heating will be over 60

 

[Dereekoo]

Do you have you a 3 way valve on your system that diverts flow to heating and/or to hot water cylinder

 

[CBW]

The cylinder thermostat isn’t strapped to the cylinder, check it’s making contact 1st.

 

[BlueLoo]

could you explain this further - may be my tank /system has always been faulty. Because the top of my tank always sort of matches output temperature no matter what the thermostat says. I have ours set to just below 40 and the top of the tank just after heating will be over 60

Tank temps need tyo be ~60 for health reasons (legionnaires, etc) so setting at 40 is a no-no. The thermostat should be 1/3 from the bottom as hot water rises and you don't want cold water stagnating.

You have to know if your tank is direct or indirect heated. If its indirect, there is a coil which goes through it, fed from the boiler. This feed is usually controlled by a 2/3 port zone valve.
As the water temp drops, the stat closes, sending power to the xone valve. the zone valve motor opens the valve, and in turn, closes a microswitch when fully actuated which then sends a live feed to the boiler to tell it to open up.

If this valve fails closed, you won't get boiler water flow through the tank to heat it up. If it fails open, the boiler will always heat it up whenever its switched on.

The valve wear over time and can begin to pass, shut or not.

 

[BlueLoo]

The cylinder thermostat isn’t strapped to the cylinder, check it’s making contact 1st.

Sometimes the reasons are blaringly obvious lol.

 

[Munroast]

Do you have you a 3 way valve on your system that diverts flow to heating and/or to hot water cylinder

think so, and the valves all work just fine