Thermostat faulty or normal operation?

[Outback]

Hi,

This is my first post so please be gentle.

Setup:
Worcester Bosch Greenstar 24i System boiler
Danfoss RET B-RF wireless thermostat (RX-1 receiver with Danfoss RET-B wireless thermostat)
Danfoss FP715 Si Time clock

All new and installed earlier this year by a reputable company that holds all the required certifications etc.

Something a little odd has been happening and I am not entirely sure if this is normal or the sign of a problem. The system appears to be operating 100% correctly most of the time and I am getting hot radiators and water with no issues.

My concern is that I am getting what I think it odd behaviour once the house is up to temperature. Sometimes, I am finding that the boiler will go off and stay off for say half an hour during which time the house temperature obviously drops. The thermostat seems to then call for heat when the temperature drops to the setpoint but, the thermostat only seems to call for heat for a minute or two and then stops. This repeats over and over so that I am getting several very short runs from the boiler. It's as if the themostat is reading a temperature drop and then almost immediately seeing the temperature going back above the setpoint. This sometimes happens three or four times in the space of 20 minutes and I am guessing that all the firing up for short times isn't going to do the boiler any favours.

I thought it might be due to something simple like a draft so, I moved the thermostat to another room and get the same results. Any opinions as to whether this is a problem or normal operation? Seems a little odd to me and I would have thought that the thermostat would have some internal deadbanding or debouncing built into it?

Thanks in advance!

 

[mikely]

I think this is normal operation. The RET B incorporates chrono-proportional regulation which means that, once up to operating temperature, the stat cycles the heating on and off such that the time averaged (chrono-proportional) output of the system balances the heat loss from the house. During relatively mild weather, you will find that the heating will only come on for short bursts because not much heat is needed to maintain the set temperature.

Switch settings on the unit allow you to select different cycle rates for the chrono-proportional control (3 or 6 cycles per hour). You can also disable the chrono-proportional control and switch to conventional on/off control. So you could see which works best for you. You should find that chrono-proportional gives a more accurate control of temperature.

 

[David937]

Someone was recently telling me that these type of controls wear out plant much quicker, how true is this?

This was his explanation:

Add up the cycles per hour, mult by the hours on per day then factor
that into the spec'd cycle life of zone valves and boiler ignition
circuits and you'll find that the life is shorter than you think. 3
years I think in my case when calculated.

 

[D_Hailsham]

Someone was recently telling me that these type of controls wear out plant much quicker, how true is this?

This was his explanation:

Add up the cycles per hour, mult by the hours on per day then factor
that into the spec'd cycle life of zone valves and boiler ignition
circuits and you'll find that the life is shorter than you think. 3
years I think in my case when calculated.

So Honeywell etc would never think of checking the effect PI control has on the life of their motorized valves??

I have yet to see any published information on the life of zone valves etc, so I don't see how the comparison can be made.

The only problem I know of is that, when the "on" portion of the cycle is very short, say less than a minute, a motorized valve may not be able to respond quickly enough. Danfoss recommend turning PI control off if you have motorized valves, though I don't know if that only applies to their valves.

I have been using a Honeywell CM927 for over three years and have never noticed any problems.

 

[Outback]

I think this is normal operation. The RET B incorporates chrono-proportional regulation which means that, once up to operating temperature, the stat cycles the heating on and off such that the time averaged (chrono-proportional) output of the system balances the heat loss from the house.

Thanks for that mikely. I wasn't sure if it was normal behaviour for the system or a sign of a problem. I had a look at the back of the RET B and it is indeed in chrono-proportional mode. I was wondering what that actually meant since the description in the user manual is about as useful and informative as stale cheese.