Is this short cycling?

ARainyDay

Hi all, I have a combi boiler (28kWh) at 65 degrees flow temp. It services 4 radiators (all finned, 1 x double 3 x single) and a towel rail. TRVs are around 2.5 with unused room at 1. Thermostat in good working order. I’ve noticed that when the boiler reaches the programmed temperature, it will cycle. It will fire up for 5 minutes, then turn off for 5, so essentially 10 minutes for a complete cycle (so boiler firing 6 times in total per hour). This will continue until the next programmed temperature change. Is this what is known as short cycling? Or is it the boiler just trying to maintain the set temperature?

Ballymoney

That’s how our Honeywell thermostat works, although I have it programmed to 20 minute cycles. Ours will fire up the boiler for about 7-8 mins every 20 mins to maintain the chosen temperature.

ARainyDay

Ballymoney said:

That’s how our Honeywell thermostat works, although I have it programmed to 20 minute cycles. Ours will fire up the boiler for about 7-8 mins every 20 mins to maintain the chosen temperature.

Oh, I wonder if I can do that, what model is your Honeywell? I have a CM921, I must check a manual

Ballymoney

ARainyDay said:

Oh, I wonder if I can do that, what model is your Honeywell? I have a CM921, I must check a manual

I also have the CM921. If you fiddle with the settings you can alter the cycle length. I can’t remember exactly how but it’s all in the manual. 

ARainyDay

Thank you- I’ve just taken a look at the online manual  thanks for your help!

[Deleted User]

From the Honeywell website:

It is generally considered that boilers with an output greater than 30KW will not be in a domestic installation. Also older boilers of a higher output often have a large cast iron heat exchanger.

If a boiler is less than 30KW then the recommended settings are as per the specification/data sheet for the product, cycle rate 6 and minimum on time 1 minute.

If the boiler is greater than 30KW but has a modern, low water content heat exchanger then these recommendations would be as per a bouiler under 30KW. If, however, it is an older boiler, with a large cast iron heat exchanger the recommended settings would be cycle rate of 3 and minimum on time of 4 minutes.

Edit: Just to add a note if caution. In these expensive times, there is a tendency to turn down TRV settings to the absolute minimum. This effectively reduces the size of the heating system which, in turn, means that your boiler is over-sized for the system that it is trying to heat. 

If heat isn’t dissipated through the pipes and radiators then the boiler needs to cycle to keep hot water flowing through the heat exchanger. This avoids heat exchanger damage.

Ballymoney

On the back of your original post I’ve kept an eye on our system today. Turns out in this cold weather our thermostat will kick the boiler in to action for 12 mins out of every 20 minute cycle. This is to maintain a temp of 19c. So the boiler / pump is only idle for 7-8 mins of every 20 once the house is up to temp.

This feels like quite a lot tbh and probably explains the 145kwh of gas we’re using a day 😄

Astria

Ballymoney said:

On the back of your original post I’ve kept an eye on our system today. Turns out in this cold weather our thermostat will kick the boiler in to action for 12 mins out of every 20 minute cycle. This is to maintain a temp of 19c. So the boiler / pump is only idle for 7-8 mins of every 20 once the house is up to temp.

This feels like quite a lot tbh and probably explains the 145kwh of gas we’re using a day 😄

According to my logs, our boiler was on for 11 hours yesterday but used "only" 76 kWh.

It's on 24/7 though, as 2 things: breathing in cold air makes me sneeze constantly, and hate getting out of bed to visit the bathroom when it's freezing.

FreeBear

Have a look at the manual for the boiler - It should tell you if the output can be "modulated" down to a lower output. With a smarter thermostat that can instruct the boiler to modulate down, the cycling should be reduced. But with only 4 rads, the boiler may be overpowered for the amount of heat that the radiators can dissipate.

markin

If the stat is set far higher than most rads it will keep calling for heat.


Different brands can be a little different.

0 = Off · * = 7°C · 1 = 10°C · 2 = 15°C · 3 = 20°C · 4 = 25°C · 5 = 30°C.

ChaunceyGardiner

I have 3 radiators, 1 large, 2 small, and a boiler than can't seem to modulate for toffee. When the radiators are hot, it can fire for as little as 30 seconds at a time. Now, that's short-cycling!