Choice of thermostat advice please

[diselsamm]

Hi

I'm having issues with my wireless Honeywell thermostat. It's sat in the living room, set at 18C currently. It makes my Worcester Greenstar 18Ri to short cycle. So, the moment it gets cold outside my boiler works for about a minute, then stops, and starts again in about 5 minutes, then repeat. Obviously the temp in my living room does not fluctuate so much. I've got a total of 8 radiators at home up and downstairs, and happy to agree the boiler is oversized for my property. But I can clearly see the thermostat is the one calling for heat (know this from the led light on the receiver).

I've been thinking of having the thermostat and receiver replaced with maybe a Drayton Wiser or Greenstar Comfort I. Any suggestions on the choice of thermostat?

I've a Honeywell Y6630D1007 wireless thermostat and a Honeywell ST699 programmer and want to rip them off.

 

[ShaunCorbs]

Tbh you will get the same issue as

 

[diselsamm]

Tbh you will get the same issue as

Why is that Shaun?

As I understand it the boiler turns on when:

• the thermostat calls for heat
• smart TRVs call for heat (I've got the traditional/dumb ones)
• the boiler itself when it cool down, even when rooms are not cold

So if a replacement thermostat has a wider/adjustable hysteresis and "better" ant-cycling software, why it would replicate the current problem? Actually, what is causing the current problem then?

 

[Chuck]

Actually, what is causing the current problem then?

All boilers have a minimum output power, typically about 6kW. If the radiators are not dissipating least this amount, e.g. because TRVs are shut due to warm weather, the boiler will switch on and off with a duty cycle that matches the average output to the average demand.

It's always worth checking that the system is correctly balanced and setup in these cases. For example, if the TRVs are all set at 17°C and the system thermostat setpoint is at 18°C then the boiler is going to have to do a lot of cycling.

Some thermostats allow you to increase the minimum cycle period for their demand to twenty minutes (usually used for oil boilers). This will, however, mean you need to increase the setpoint temperature to stay comfortable during the 'off' periods. Another approach is to increase the thermal mass of the water in the system by installing a 'volumiser' or 'buffer' tank and/or replace the boiler with one selected specifically for its low minimum power. For many people, these cures are worse than the problem.

 

[ShaunCorbs]

You could just wiring in an anti run / cycle timer

There all digital stats 2-5dc differential it all depends on the room eg sun influence

Then room load are the rest shut off and just one trv open etc

 

[diselsamm]

You could just wiring in an anti run / cycle timer

There all digital stats 2-5dc differential it all depends on the room eg sun influence

Then room load are the rest shut off and just one trv open etc

That is interesting. What specific product (anti run/cycle timer) would you suggest for a Worcester Greenstat 18ri? How is that affecting the running time?

Let me elaborate on my problem: My problem is that without a significant change in the temperature in my living room my wireless thermostat (sat on a shelf, no direct sun exposure) is calling for heat. To be specific, it is not the boiler itself per se deciding to start running but the wireless thermostat. And I know this because when I check the led light on the transmitter is green when this happens. Unless what I am being told is that:

• my wireless thermostat is iffy
• my boiler is cycling because its own onboard thermostat detects a drop in temperature
• because one(s) of my regular TRVs is constantly asking for heat

If read between the lines, are you suggesting that in order to have a change in the behavior of my boiler, I need to install something like a Wiser and smart TRVs, or just a anti-cycle timer?

 

[diselsamm]

All boilers have a minimum output power, typically about 6kW. If the radiators are not dissipating least this amount, e.g. because TRVs are shut due to warm weather, the boiler will switch on and off with a duty cycle that matches the average output to the average demand.

It's always worth checking that the system is correctly balanced and setup in these cases. For example, if the TRVs are all set at 17°C and the system thermostat setpoint is at 18°C then the boiler is going to have to do a lot of cycling.

Some thermostats allow you to increase the minimum cycle period for their demand to twenty minutes (usually used for oil boilers). This will, however, mean you need to increase the setpoint temperature to stay comfortable during the 'off' periods. Another approach is to increase the thermal mass of the water in the system by installing a 'volumiser' or 'buffer' tank and/or replace the boiler with one selected specifically for its low minimum power. For many people, these cures are worse than the problem.

If I assume on average that my radiators have an output of 1 kW, and having 7 of those, and 1 towel rail, my 18Ri should fit in ok? I do not usually experience this cycling issue during the day, when it is warmer. It happens only when it starts to get colder below about 12C outside. And I explained to Shaun above, it looks to me the wireless thermostat is causing the cycling, and not the boiler itself.

The system has been properly balanced with a temp prob, maybe the radiator in the kitchen can be touched a bit. Now, in all fairness I've turned down the TRV in the sleeping room upstairs as if left fully open it is extremely noisy with water running noises. All the rest of the TRVs are left fully open. The only exception is the towel rack which does not have a TRV, but we did have a bypass valve installed.

Please could you tell me a bit more about increasing the thermal mass of the water using a volumiser or buffer thank? I used to have a gravity system which was pressurised now, eliminating the small tank with the floater. I still have the big water tank that is feeding the hot water cylinder.

Doing a research on possible thermostats I came across Drayton Wiser or Greenstar Comfort I as plausible candidates. Would you think of something else instead these? Thanks!

 

[gmartine]

The ideal solution is to down rate your boiler's output to 15,12 or even 9kw which can be done very cheaply on WB ri's with a code plug key available from fleabay or a merch, best ask your service engineer.

Best you or eng calculate the heat loss of whole house, I've converted a few of these to 9 or 12kw where the min load is now about 3kw ish which should improve things.

 

[diselsamm]

gmartline, I did not even expect that would be possible without changing the burner size etc. Please could you link me such a product? I found this as an example, is this what you're referring to WORCESTER GREENSTAR 12RI CODE PLUG KEY NG4 12KW 87144210340 | eBay - https://tinyurl.com/5burhppm
Thank you!

 

[gmartine]

Correct.

Also check pump speed, try a lower setting.