Cycle Rate for Old Boiler

[michaeld121]

I've got a very old Potterton boiler. Recently the conventional old thermostat was on its way out, so I replaced it with a Honeywell CM67. My main priority is to keep the boiler going until I plan to replace it in 12 to 18 months. I'm looking for advice on what the most sensible settings for the Cycle Rate (currently left at the default of 6 per hour) and Minimum on Time (currently 3 mins) would be.

I've found that if it's cycling within the proportional bandwidth (set at 1.5 deg C) it tends to come on, start to warm up the first couple of radiators and then switch off after the minimum on time, before it really does any good. Is it likely to do any harm to the boiler if I increase the minimum on time to 5 mins?

Or would the safest thing be to just use the 'Manual' thermostat setting & run it like an old fashioned thermostat until I replace the boiler?

Thanks,

Michael

In case it's any help: the boiler is a Potterton Kingfisher CF55 with a Grundfos Super Selectric Pump & a Honeywell Motorised Mid-Position Valve.

[Geronimo W. Christ Esq]

Are you saying the radiators are too cold, and the stat is not keeping the system on for long enough ?

Sounds like the stat is too low, or incorrectly positioned. What temp is the stat set to ? Is it close to a heat source ?

[John Stumbles]

Do Honeywell not give any guidance in their instructions? I think it would be a good idea to prevent short cycling if you have an old cast-iron non-fanned-flue boiler, for efficiency's sake.

[michaeld121]

When it's within the proportional bandwidth the heating tends to come on every 10 mins (as per the cycle rate) for the 3 minutes (ie minimum on time). This will warm all of the radiators eventually - but takes a very long time.

I'm wondering if it might be worth upping the minimum on time to 5 minutes and increasing the cycle rate to 9 per hour. That way it will work in 5 minute bursts every 7 or so mins & should heat things up much more quickly. I've been through Honeywell's instructions and they aren't particularly comprehensive.

Based on what I've heard & read the stat is in a sensible place & it's not close to a heat source - it seems to be more to do with the way the thermostat controls the heating.

Michael