EPH Combipack 4 and Worcester Greenstar 24Ri boiler

[Gwyndy]

I have an EPH CP4 wireless thermostat controlling a Greenstar 24Ri gas condensing boiler. The CP4 was a replacement for my old Honeywell CM927.

I use the CP4 in TPI mode. (If I run it in ‘normal’ mode I get a huge overshoot on set point temperature.) The default settings are 6 cycles per hour and a proportional band of 2.0C. As the room temperature approaches the set point the boiler firing times progressively reduce, as you would expect. This is OK until the room temperature starts to fall. Just a reduction of 0.1C will cause the CP4 to start the heating cycle but with a very short time. Although the ‘on time’ is undoubtedly correct it is totally pointless. It takes the boiler a couple of minutes to sort itself out and determine the heat input required, by which time the CP4 has switched off again. There is no meaningful heat input to the system and is just wasting gas. The result is that after the initial heating period the room temperature will never stabilise at the set point.

After much experimentation I have changed the CP4 to 2 cycles per hour and prop band to 1.5C. In theory small temperature drops will result in a longer ‘on’ time and a more stable temperature but is this correct?

Incidentally, I have had long debates about this with EPH but they cannot explain what is happening.

Questions

1. Does anyone have a similar experience when using a TPI controlled thermostat?

2. Is there a better thermostat than the CP4?

 

[JonathanM]

Just how short were the burns at 6CPH?

It takes the boiler a couple of minutes to sort itself out and determine the heat input required, by which time the CP4 has switched off again. There is no meaningful heat input to the system and is just wasting gas

I don't understand this bit. Does the boiler not fire up straight away? When I briefly used TPI, the burn was for about one minute, but the boiler came on and did a proper burn. Even with that short burn you could notice a 0.1C rise.

The result is that after the initial heating period the room temperature will never stabilise at the set point.

What happened to the temperature. Did it continue to fall?

After much experimentation I have changed the CP4 to 2 cycles per hour and prop band to 1.5C. In theory small temperature drops will result in a longer ‘on’ time and a more stable temperature but is this correct?

I agree that logically this should work. My guess is that at 2CPH it will burn three times longer. Have you thought of leaving the proportional band at 2.0C in order to test this, so that you only change one thing at once.

Is there a better thermostat than the CP4?

On some thermostats you can specify a minimum burn time.

According to my plumbers merchant, your model is extremely popular. Maybe it is faulty in some way?

If I run it in ‘normal’ mode I get a huge overshoot on set point temperature.

The manual at P03 seems to be saying that for normal mode you can adjust the differential from the 0.4C default. Can you take it right down to zero?

EDIT: Do you know what year your boiler is?

 

[JonathanM]

Also, how does the behaviour compare with your old thermostat? It looks like that one was also TPI, albeit with several options you could change.

 

[Gwyndy]

Just how short were the burns at 6CPH?

All my initial observations were done with 3CPH and prop band at 1.5C. On time with a temperature drop of 0.1C is approx 1 minute, consistent with the calculated time.

I don't understand this bit. Does the boiler not fire up straight away? When I briefly used TPI, the burn was for about one minute, but the boiler came on and did a proper burn. Even with that short burn you could notice a 0.1C rise.

Yes, the boiler fires immediately. The boiler is a variable flame unit and never starts on full flame. This is normal.

What happened to the temperature. Did it continue to fall?

Yes, the room temperature continues to fall and settles around 0.4C below set point. I think that this is consistent with progressively longer burn times.

I agree that logically this should work. My guess is that at 2CPH it will burn three times longer. Have you thought of leaving the proportional band at 2.0C in order to test this, so that you only change one thing at once.

I have been investigating this issue for over 12 months. I have never made multiple changes to the CP4 settings. Prop band was set at 1.5C, the minimum, and all other changes have just been on cycles per hour. I am now trying 2CPH on the basis that this should extend the minimum on time.

On some thermostats you can specify a minimum burn time.

According to my plumbers merchant, your model is extremely popular. Maybe it is faulty in some way?

No minimum on time on the CP4. I have suggested to EPH that this is a serious omission!
The CP4 is very popular because it is not expensive. The original unit was faulty but was replaced by EPH. They have concluded that there is nothing wrong with the device and that the issue relates to its interaction with my boiler. Hmmmm!

The manual at P03 seems to be saying that for normal mode you can adjust the differential from the 0.4C default. Can you take it right down to zero?

EDIT: Do you know what year your boiler is?

The differentials, HOn and HOff which control the hysteresis, have no effect in TPI mode, only in 'normal' mode.

The boiler is one year old.

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What annoys me about the CP4 is the very short cycles that achieve nothing in terms of heat input to the system.

The heating installer tells me that the Hive thermostat is very good but, given that it was they who recommended the EPH, I am not sure!

 

[Gwyndy]

Also, how does the behaviour compare with your old thermostat? It looks like that one was also TPI, albeit with several options you could change.

The old boiler was a traditional on / off, full flame or nothing! The Honeywell CM927 did short cycle as it approached set point and as the room temperature started to fall but at least the heat input was significant. From memory there was a minimum on time setting. Control was spot on!

 

[JonathanM]

The differentials, HOn and HOff which control the hysteresis, have no effect in TPI mode, only in 'normal' mode.

I know that. I was just wondering if you ran it in normal mode, with hysteresis set to zero, it would give you what you want.

The old boiler was a traditional on / off, full flame or nothing!

That's what I have now! And that's why I turned off TPI. Burns just too short, I thought, for an old lady.

The boiler is one year old.

It sounds like you never used the old thermostat with this boiler, then?

Yes, the room temperature continues to fall and settles around 0.4C below set point. I think that this is consistent with progressively longer burn times

So, when it gets to 0.4C below set point, do the thermostat and boiler then settle down and behave satisfactorily, in keeping things steady at 0.4C below set point?

 

[Gwyndy]

I know that. I was just wondering if you ran it in normal mode, with hysteresis set to zero, it would give you what you want.

No, tried that. Because boiler runs flat out there is a big overshoot on set point and consequently requires a big fall in room temperature before system starts again. Uncomfortable!

That's what I have now! And that's why I turned off TPI. Burns just too short, I thought, for an old lady.

Few moving parts on an old boiler. Might be worth trying again.

It sounds like you never used the old thermostat with this boiler, then?

No, I didn't. Display kept failing on the CM927 and I got fed up with constantly repairing it.

So, when it gets to 0.4C below set point, do the thermostat and boiler then settle down and behave satisfactorily, in keeping things steady at 0.4C below set point?

Yes, it does. The actual room temperature is not really the issue as I can easily live with 0.4C below set point. The annoyance is the constant short cycle switching which is just adding to wear on boiler, motorised valves, and pump, and of course the waste of gas!!

 

[JonathanM]

Yes, it does. The actual room temperature is not really the issue as I can easily live with 0.4C below set point. The annoyance is the constant short cycle switching which is just adding to wear on boiler, motorised valves, and pump, and of course the waste of gas!!

It's a bit like my situation. I wanted a thermostat that gave longer burns than TPI, but didn't overshoot as much as using hysteresis. But I just accepted mine in the end.

It's quite a conundrum. I read a snippet about one Worcester Bosch boiler having a "102 second soft start" after ignition. Maybe that's what's happening, and during those 102 seconds the TPI is coming on and going off.

Are you seriously looking for another thermostat? I've checked out the specs on loads recently. It seems in recent years, they have been dumbed down, giving fewer options to configure. (Although it may be that those options are just being hidden from the manual, to avoid confusing users). And I've mainly been looking at non-programmable. These sometimes seem to show more options than programmable. But I can have a look to see if I can remember a programmable one that lets you specify minimum burn.

I have found an extremely configurable Italian thermostat, where you can change lots of settings, even specify your own TPI algorithm. But I don't know if it comes in a programmable version.

 

[JonathanM]

I remembered. You may know this already! But the direct replacement for your Honeywell seems to be the T4/T4R.

https://www.screwfix.com/p/honeywell-home-t4-1-channel-wired-programmable-thermostat/8672v

https://www.screwfix.com/p/honeywell-home-t4r-1-channel-wireless-programmable-thermostat/9543v

https://digitalassets.resideo.com/damroot/Original/10018/t4-pd-en0h8552ge23.pdf

Installer menu allows extra functions to be set at the discretion of the installer to match the homeowner’s applications and needs – Language selection – Cycle rate – Upper/Lower set point limit adjustment – Minimum ON time – Daily, weekly or 5+2 schedule option – 4 or 6 periods a day – Wired remote sensor – Fail safe mode – Clock format – Daylight saving time change (DST) – Temperature display offset – Factory reset – Binding (T4R only)

There is a cheaper T3/T3R, but it's not obvious that is has that Minimum ON time option. Hopefully someone else will know. A shame if not, because I've been told that Wolseley sell their own brand version of T3R for just £60.

EDIT: It looks like T3/T3R does have the same options, if you click on Technical Properties tab

https://homecomfort.resideo.com/sit...-TOEM&pid=T3 Programmable Thermostat(HW-TOEM)

 

[Gwyndy]

I read a snippet about one Worcester Bosch boiler having a "102 second soft start" after ignition. Maybe that's what's happening, and during those 102 seconds the TPI is coming on and going off.

I think that is exactly what is happening! It seems to stabilise at 0.4C below set point when the burn time will be longer of course.

Are you seriously looking for another thermostat?

Maybe! I am currently experimenting with the CP4 set at 2 cycles per hour and, at the moment, it seems to be behaving. I am going to ask the heating installer if they have a Hive stat that I can set up (without wiring in the receiver) and observe what it does.
Please don't worry about looking for other stats at the moment. I'll persevere a little longer with the CP4.

... the direct replacement for your Honeywell seems to be the T4/T4R.

I hadn't seen that one - seems a good spec. I did like the old CM927 despite its quirks. I got fed up with repairing the display when bits of the display went 'missing'. (It was a known problem which was never acknowledged by Honeywell.)

Many thanks for all your help and suggestions.

 

[Gwyndy]

A long time since I started this thread but I have finally bought and installed a Honeywell T4R wireless thermostat to replace the EPH CP4.

Early days, but so far so good. I am back to 6 cycles / hour recommended for gas boilers and I have set the minimum run time to 2 minutes. The system reaches set point and the temperature control is excellent. Fingers crossed!