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.

 

[diselsamm]

Thank you gmartline! I will discuss this with my heating engineer and see with him into heat loss calcs and choosing one of these keys.

 

[Chuck]

Looking at the Honeywell Y6630D1007 datasheet, it has a minimum on time of one minute and a cycle rate of six cycles per hour. These are factory set and can't be changed. It doesn't have a setpoint hysteresis band because the time delays have a similar effect.

In addition, the boiler has its own anti-short-cycling behaviour. This is proprietary but is something like "If the boiler has cycled more than twice in the last ten minutes wait ten minutes before firing again." (This is from memory, I don't have my notebook to hand.) I think your boiler has an 'Info' button you can push to make it display its status so you can keep an eye on what's happening.

 

[gmartine]

No such thing on that regular boiler Chuck, only two control knobs and a LED.

 

[Chuck]

No such thing on that regular boiler Chuck, only two control knobs and a LED.

Good catch, thanks. I missed the 'R' on the model number. OP should forget what I said about the boiler behaviour, I don't think it's applicable to that model.

 

[diselsamm]

Gmartline, my current circulation pump is a Grundfos UPS3 15-50/65, which replaced the standard red-coloured Grundfos pump.
The current one is set at constant speed 1. The pump got other speeds as well. What speed mode and speed would you use?
I do not have a UFH and previously the system was with a F&E tank, now pressurised with indirect vented solar cylinder and expansion vessel.

​

 

[gmartine]

Lowest possible speed as long as radiators get up to temp and you have enough flow rate so setting 1 should normally be fine. You do have to set the pump curve on those, proportional or constant pressure depending on your system.

One other thing relating to your original enquiry, the stat really shouldn't be short cycling as much as it is. Just make sure it's correctly postioned out of draughts and high enough and the connecting corridor is warm enough or keep the door closed or it will be susceptible to the behaviour you describe.

 

[diselsamm]

Lowest possible speed as long as radiators get up to temp and you have enough flow rate so setting 1 should normally be fine. You do have to set the pump curve on those, proportional or constant pressure depending on your system.

One other thing relating to your original enquiry, the stat really shouldn't be short cycling as much as it is. Just make sure it's correctly postioned out of draughts and high enough and the connecting corridor is warm enough or keep the door closed or it will be susceptible to the behaviour you describe.

Thank you Gmartline. The pump is not that model where you can set the curve per se, but can define the mode and speed, if I understand correctly.

As to the thermostat I do suspect it is malfunctioning as the place it is sat on is the living room, on a shelve, about 1.2m above the floor. No draughts, and temp is stable between 18 and 19C with this weather and heating setting. Funny though my thermostat is sat at 17C but the room temp is 1-2C higher.

How these code keys operate? Is it similar to the way the remapping on a car operates? I assume once my heating engineer puts on a smaller output one, he will also have to make adjustments to the burning mxing rates, etc? I will have a conversation with him about this next week.

 

[gmartine]

By curve I meant mode.

No other adjustments required except to swap out the key code, just make sure to label the boiIer with it's new rating.

The different keycode vary in resistance only so I guess that pulls down a particular voltage which triggers the respective "map".

The whole ri range is so simple it's the same boiler from 9 to 24kw cept the key code.

 

[diselsamm]

Update:
So I had the plug replaced and the boiler is "detuned" to 12kW. My impressions: very positive. The boiler is much quieter. And the cycling issue is in my opinion gone. Thank you gmartline!

I would like to replace my current thermostat and need your recommendation please.

I currently have

• Honeywell Y6630D1007 Wireless Analogue Room Thermostat
• Honeywell ST699 2-channel programmer.

and I think I would like to keep the programmer and replace the thermostat and its receiver. I am thinking to replace them with Y87RF2024. What are your thought? Thanks!

 

[diselsamm]

Update:
So I had the plug replaced and the boiler is "detuned" to 12kW. My impressions: very positive. The boiler is much quieter. And the cycling issue is in my opinion gone. Thank you gmartline!

I would like to replace my current thermostat and need your recommendation please.

I currently have

• Honeywell Y6630D1007 Wireless Analogue Room Thermostat
• Honeywell ST699 2-channel programmer.

and I think I would like to keep the programmer and replace the thermostat and its receiver. I am thinking to replace them with Y87RF2024. What are your thought? Thanks!

spoke too early, the cycling issue is not resolved. So focusing now on the the replacement of the thermostat

 

[diselsamm]

Update2: After a further monitoring, it seems the cycling was actually resolved. The boiler turns on about every 10 minutes, which is as expected. At this point I am not sure anymore whether it is worth it to replace the receiver and thermostat.

 

[John.g]

I cant find any info on that stat as to its hysteresis or if it is a TPI controller, I have a basic digital roomstat set to its minimum differential setting of 0.3C, (it has no TPI) it switches in when the room temp is 0.15C below its SP and switches out at at SP with ~ 0.2C overshoot after room heats up after initial heating say in the morning, its surprising how long it takes to increase the temperature by that 0.35C, over 10 minutes normally with ~ 25 minutes off time (oil fired boiler).

 

[diselsamm]

John, I am not sure whether your screenshot is for your thermostat, but it is exactly the same for mine. 6 cycle ph, 1 minute min ON time.
The way I think about this is that a good thermostat should be able to monitor the outside temp and learn/so anticipate the heat loss of the house and work around it. In the real world I do not know how closely my existing thermostat is able to do this and whether there is a sizable margin for improvement or it is good enough. In the end my boiler is an on/off thing which by default means its control is not precise.

 

[ShaunCorbs]

John, I am not sure whether your screenshot is for your thermostat, but it is exactly the same for mine. 6 cycle ph, 1 minute min ON time.
The way I think about this is that a good thermostat should be able to monitor the outside temp and learn/so anticipate the heat loss of the house and work around it. In the real world I do not know how closely my existing thermostat is able to do this and whether there is a sizable margin for improvement or it is good enough. In the end my boiler is an on/off thing which by default means its control is not precise.

You can get that but you will have to change your boiler as it’s a dumb control wise

 

[John.g]

John, I am not sure whether your screenshot is for your thermostat, but it is exactly the same for mine. 6 cycle ph, 1 minute min ON time.
The way I think about this is that a good thermostat should be able to monitor the outside temp and learn/so anticipate the heat loss of the house and work around it. In the real world I do not know how closely my existing thermostat is able to do this and whether there is a sizable margin for improvement or it is good enough. In the end my boiler is an on/off thing which by default means its control is not precise.

That info is from your stat, cant find any info from Honeywell. All these "clever" control systems like evohome will constantly cycle the boiler on and off even when the heat demand is > than the boilers minimum output to give pinpoint temperature control. Weather Compensation is best utilized IMO by controlling the boiler target temperature but the boiler has to be able to use this.

 

[diselsamm]

To be fair I am not sure what better a boiler is there on the market compared to mine. Mine is a detuned 12 kW greenstar RI. 45dB noise, modulating in the 4-12kW range. Simple and simple to fix. I have a one zone Y system.

 

[John.g]

Your heat demand will rarely be much lower than 4kw but if your existing stat has TPI control then it will cycle it on/off, you should be able to buy a relatively fairly basic digital stat, if you do purchase one ensure that its one with a low differential of 0.2C/0.3C and where the TPI control can be disabled or one without TPI, like mine.

 

[diselsamm]

I am thinking about Honeywell Y87RF2024, which will be a direct replacement for my current setup, keeping the old programmer. Unfortunately I can't find much info on how that thermostat operates. This and I am not sure of the benefits of this replacement thermostat over the analogue one I currently have and which seems to be working fine.

 

[gmartine]

Smarter stat systems uses geolocation to get local temperature if weather comp not used, my very old mother (92) has your boiler (9kw) bolted on to tado in a 3 bed semi. Seems to work ok, target temp is 22.2 deg C.

 

[diselsamm]

I am not sure I am comfortable with my skills to replace my Honeywell controls with Tado. I think I also have to take down the programmer which will cause cosmetic issues (I know!). I am comfortable to replace myself my receiver and thermostat with the Y87RF2024, and keep my programmer, which works fine. The question is, is it worth it?

 

[gmartine]

I was answering your query about a stat being able to monitor the outside temperature #23. It needn't be Tado but most smart systems connect to an outside weather service via the internet and adapt your heating in an anticpatory manner, that amongst some other features will save you something.

 

[diselsamm]

thank you gmartline. By the way I am very happy how well the key plug worked out. I can't recognise my own boiler now.

 

[gmartine]

Good to hear, when I get a chance I'll take a pic of my mother's boiler as I've noted the temperatures on the flow temp dial. From memory one oclock is about 60 deg C which hers is set to. Worth playing around with that of you haven't already, obviously it'll take longer for the room to get up to temp so allow for that if you experiment.

 

[diselsamm]

Now that you reminded me, I will dial it down as mine is set near to max atm. It's a long story but previously the boiler was not able to go over 68C and we had to keep the dial at near max. Basically the boiler was overheating and cutting out. This happened due to an extremely problematic conversation of the system to sealed one, resulting in circulation problems both in the pipework and in the main heat exchanger.

 

[gmartine]

Running the system at 80 or so degrees is far from ideal and will exacerbate the symptoms of your original enquiry. Never mind the return flow temp being excessive and above dew point or out of condensing mode it sounds like you have more fundamental issues than simply replacing the room stat will resolve.