System Boiler and hot water production

27 Jun 2018
Reaction score
United Kingdom

I just moved in a new house where there is a system boiler (should be roughly 10 years old), everything seems to work fine by I have a question in regards the hot water production.

Basically, one it's time to produce hot water the burner kicks in and usually in 20-30 min the target temp of 60 degrees is reached so the burner goes back to rest. What it is strange is that at this point, while there is no hot water demand, the burner starts again pretty much at interval of 20min for 5 min, is this normal?

I would have expected that the hot water cylinder would be able to preserve the heat for much longer. SHould I consider that there is a fault either in the hotwater cylinder thermostat or in a water leak from the hot water pipes?

If anyone could explain me this or confirm that this is just normal, I would appreciate it.

Sponsored Links
Are you or anyone else at the property complaining of the water being excessively hot, if no then unlikely to be a faulty cylinder stat. Is the domestic water tank constantly trying to refill when no demand?
If we open just the hot tap, yes it's boiling hot. I didn't stand in front of the hot water cylinder to check if there is demand to refill the water while none use it.
Sponsored Links
Look at what number the cylinder stat clicks off at when the water is up to temperature, this should give you a good idea of whether the stat is faulty or not.
Ok, I did some checks:

1) The temperature at the tap is pretty much wahat I would expect 59-60 degrees

2) with no demand I can’t observe any water refill in the cylinder.

This is my first time I deal with a system boiler and I wonder if this is just a normal behaviour.

Anyone can comment on it?

Make and model of "system boiler" please. Pics of hot water tank and surrounding pipework/valves would help to ID your problem.
Also check that cylinder stat is securely attached and properly grounded to the bare metal of the cylinder. And are you in a hard water area?
The stat is attached securly.
Here the pics of required. The water is medium hard, in my location.


  • C6219F15-E364-41FA-A254-922DF29F20CE.jpeg
    104.9 KB · Views: 257
  • D328AA90-9D77-4365-8B12-A241AC911BC7.jpeg
    208.9 KB · Views: 263
  • 585F5B1D-EF63-49F4-BB6C-E5F5A72D29E1.jpeg
    112.5 KB · Views: 242
OK. So it's an unvented cylinder - I should have asked that! So scrap what I said above as its not relevant although limescale might still be.
Maybe it's the hysteresis of the thermostat that has changed over years of use ?
water leak from the hot water pipes?
its possible, also check no hot water is dripping through the tundish. have a look through the tundish (the black plastic item with existing scale deposits,hopefully its historical). see,feel if hot water is dripping through.

process of elimination before calling in a tech.

The cylinder should have a service record :)

Most people would be happy the icos is actually working :cautious:
What temperature is your boiler stat on? It needs to be higher (IME with my Vaillant, by 5-10 degrees) than the cylinder thermostat. You can do a cursory check on the cylinder stat to see if it's working by listening for an audible click when adjusting it - try turning it down slowly once the boiler switches off after heating the cylinder up, you'll probably find it's slightly too high to prevent the boiler cycling. If that's the case, either turn the cylinder stat down a bit or turn the boiler stat up a bit.

A more thorough check would involve checking the outputs of the cylinder stat and zone valves, but you may be better getting a professional in if that is necessary, as unvented cylinders have a number of safety elements which require a G3 certificate to have work carried out on them.
One thing to try is to turn the hot water thermostat down by 5C, then observe the boiler operation. In my experience the output temperature from the icos boiler is only about 65C when the knob is in the position you have, so it's possible the cylinder thermostat doesn't get 'satisfied'. If this is the case the boiler will display 'c' and cycle as you have described in your original post until the hot water timer switches off. When the cylinder thermostat becomes satisfied and all else is well, the boiler will display '0' after about 2 minutes.

So if that works, and you like your water hot, then turn the cylinder stat back to the setting shown in your photos and turn UP the boiler temperature by about half a division. Be wary of going much higher than 60C, as rapid scale formation will occur. Likewise beware going less than 55C to maintain protection against legionella growth in the hot water system (unless you can maintain a weekly regime of heating to 60C).
PS, that icos is a 'regular' boiler, not a system boiler, their system boilers have the pipework exiting the boiler at the bottom, and they only made the system boiler in 15kW and 24kW sizes.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links