Keston Celcius 25 & UFH

31 Oct 2006
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United Kingdom
I have a Keston C25 supplying a cylinder & underfloor heating.

Both cylinder and UFH are wired together to SL on the boiler.

My cylinder thermo will call boiler and it comes and stays on until temp reached when thermo cuts live via Drayton ZA5

However my UFH will supply live to the SL on the boiler but the boiler cycles on and off for short periods such as 10 secs. Is this normal behavior and if so what is controlling this cycling ?

Thank you
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A crap boiler with twitchy electronics.

Where is the thermostat set? Try halving it.

Iirc they go full tilt straight away them throttle back. For ufh on its own the boiler will be overwhelmed and throw a paddy.
Yep I've read all the bad about the k25 but you get what you given when you buy a new place!

Thermostat is set to 3/4 on the boiler. Doesnt make much diff where I have it but what the hell makes it a different animal when the UFH is calling it compared to the HW ?

I replaced both actuators (heads only , valves fine).

Prior to this the HW live (with UFH live disconnected) when switched off via the actuator would fire the boiler for 3 seconds every 15 mins. Valve was closed but I didnt test actuator with a meter (and its thrown) so cant be sure it was faulty actuator or boiler itself. But when I stripped actuator, micro switch (which is a sealed unit) looked fine e.g. clicked when pushed

But with UFH only connected and switched off via its own actuator it never fired boiler.

New actuators later and with each of HW and CH connected separately and with no live I have no boiler firings which is correct

but I have this dam annoying boiler cycling for the UFH..!
You may be missing the point of my post.

When the boiler fires it is chucking 25kW or so of energy down the flow. But the UFH is not going to absorb a fraction of that so the temperature gradient across the boiler's flow and return sensors jumps rapidly and upsets the boiler.

Try firing it with the thermostat on 50%. Just before you expect it to shut off, manually open the hot water zone valve.

Does the boiler miraculously behave?

I am of course assuming that during a ufh only demand cycle there is a constant 240v at the boiler's s/l terminal?
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With 50% on boiler stat, I still have the same short runs except less intensity.

Opening the HV valve seems to extend the run cycle by a small amount but not to the point where it stays on until UFH stats call it off.

So how can I correct this (apart from new boiler lol) ?

Is it caused by the valve below not being set correct. Not sure 100% what it does myself but looks like it adjusts flow / return flow


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