Remeha boilers and Honeywell CM9XX thermostats

[mysteryman]

Perhaps they can't be bothered in the face of opposition from the very people who should be installing it!

The real issue here is one of complete understanding. A boiler with external on-off controls can only modulate to a fixed flow temperature, which has to be kept high in case of need in severe weather. The control mentioned by the OP is an on-off control, so the boiler is short-cycling which is not the best form of control. If you add a downstream flow temperature sensor you can on-off control it at variable temperature, but then the burner is not modulating. Still not the best form of control. If you add a device to make it modulate at variable temperature so as to match the building's heat demand, you have the best form of control. This has the additional advantage that the variable flow temperature will be below condensing temperature most of the time [or all the time with underfloor heating]. This device is readily available. It is weather compensation. In order to modulate at variable temperature, it needs to be 'inside' the boiler, or as an integral part of the boiler. It can't be properly effective as an external add-on.

The successful manufacturers have thought all this out for you, and they do indeed have all the answers, even to questions you may not have asked.

 

[DeltaT2]

I thought with Remeha you could use any OT Protocal controls??

 

[D_Hailsham]

They don't know what the boiler software is set to do.

If they don't know, they should find out!

There is much controversy, not confined to DIY forums, over the compatibility of modulating boilers and thermostats with TPI, such as the Honeywell CM900 series, DT90E etc. Honeywell and other manufacturers of this type of control all say that they are suitable for any type of boiler. But have they tested them on all types of boiler or are they just relying on the few boilers they have in their laboratories?

the broag starts low an ramps up over a fixed time. When the return starts to rise, it stops ramping up

Are you sure about that? According to my copy of the Installation manual the boiler starts off at a medium output (Parameter P20 - 2500rpm). it then modulates up or down (Range 1300 to 3500rpm) depending on the boiler temperature.

My point is that the cm9xx series is a variation of the OT version, just with the OT removed and changed to on-off switching.

What is your evidence for this?

 

[mysteryman]

Probably, but OT controls get 'inside' the boiler as I described.

External on-off controls are not the best way, although they will work.

 

[D_Hailsham]

A boiler with external on-off controls can only modulate to a fixed flow temperature, which has to be kept high in case of need in severe weather. The control mentioned by the OP is an on-off control, so the boiler is short-cycling which is not the best form of control.

But the CM9XX type control is not a simple on/off control. When the room temperature is below the start of the TPI range (1.5C below target) the boiler will run flat out, so the water temperature will rise to the value set by the boiler thermostat. But when the room temperature enters the TPI range and the on/off ratio changes, the water may not reach the temperature set by the boiler stat. I found this out with my original Apollo Fanfare boiler. Sometimes the water temperature was only 45-50C, even though the stat was set to high (82C). The house was still at 21C and the boiler was coming on for a minute every 10 minutes and sometimes every 20 minutes or longer, depending on how quickly the house cooled down.

I wouldn't agree that the control is causing the boiler to short-cycle, though that depends on how you define short-cycle.

 

[D_Hailsham]

Probably, but OT controls get 'inside' the boiler as I described. External on-off controls are not the best way, although they will work.

Couldn't agree more. I would love to be able to install an iSense, but it would cause an upheaval running new cables from the motorized valve and cylinder stat in the upstairs airing cupboard to the downstairs boiler. So I am left trying to do the best with what I've got.

 

[D_Hailsham]

Had a reply from Remeha (actually baxi parts dept)

Here is my original email to them (blue) and their reply (green):

For the past 5 years my boiler, a Myson Apollo Fanfare, has been controlled by a CM927. The boiler gave up two weeks ago and has been replaced by a Remeha Avanta 18V boiler.

I have read on various DIY forums that the Remeha is not compatible with the CM927 due to the way the Remeha's modulation system clashes with the CM927's TPI logic.

Could you advise if this is correct and, if so, what I can do about it?

Thank you for your recent contact in regards to the programmer on your Apollo boiler.

As the model number for your boiler has not been provided, I have looked at the models 30i, 40i and 50i.
Our part number for the programmer is B4161 and can be purchased through any of our stockists.

Unfortunately, we are unable to comment on the compatibility of the programmer that you have fitted.

I have replied, suggesting that they send my query to the technical dept Remeha in Holland.

 

[Boilerman2]

Why not go into Engineer mode on the CM control and adjust the boiler firing rate, if you slow this down, the boiler will fire for longer periods of time

However using the Celcia 30 Opentherm control would be a lot better

 

[mysteryman]

How does that alter the boiler firing rate? As I understand it, the CM is just varying the frequency of starts, and the burner will modulate to the set point on its own thermostat.

You can't get variable temperature modulation with external controls.

 

[mysteryman]

How do you delete an accidentally duplicated post?

 

[D_Hailsham]

How does that alter the boiler firing rate? As I understand it, the CM is just varying the frequency of starts, and the burner will modulate to the set point on its own thermostat.

You can't get variable temperature modulation with external controls.

But you do get it.

While the room temperature is outside the Proportional Band (more than 1.5°C below the Target temperature) the boiler will run continuously, controlled by the boiler thermostat, so the water temperature will reach that set by the boiler stat.

When the room temperature gets within the Proportional Band the boiler is effectively controlled by the CM9XX. The proportion of the Cycle Time (normally 10 minutes long) that the boiler is on depends on how close the actual room temperature is to the Target temperature. The closer it is, the less time the boiler stays on. So when it is very close the boiler may only be on for one minute and off for nine minutes.

During the nine minutes off the water in the system will be cooling down, but the one minute the boiler is on may not be sufficient to bring the water back up to temperature, so the mean temperature of the water drops.

If the house is well insulated, so the room temperature drops slowly, the boiler may go more than ten minutes without needing to fire and the water temperature drops further. When the boiler fires it may only be for a minute or two, just sufficient to raise the room temperature a fraction of a degree, so again the water temperature will not reach the temperature set by the boiler stat.

As for changing the parameters, the only ones available are:

Cycle rate (3, 6, 9 or 12 per hour) - normally 6, i.e 10 minutes per cycle, for gas boiler;
Minimum ON time (1 to 5 minutes) - normally 1 for a gas boiler.

 

[mysteryman]

So you are running the burner flat out in short bursts - you are not modulating at variable temperature.

Good modern premix burners are at their most efficient at around 30% of full output. Weather comp will keep them in that area much of the time. External on-off control cannot do it.

 

[DeltaT2]

Had a reply from Remeha (actually baxi parts dept)

Here is my original email to them (blue) and their reply (green):

For the past 5 years my boiler, a Myson Apollo Fanfare, has been controlled by a CM927. The boiler gave up two weeks ago and has been replaced by a Remeha Avanta 18V boiler.

I have read on various DIY forums that the Remeha is not compatible with the CM927 due to the way the Remeha's modulation system clashes with the CM927's TPI logic.

Could you advise if this is correct and, if so, what I can do about it?

Thank you for your recent contact in regards to the programmer on your Apollo boiler.

As the model number for your boiler has not been provided, I have looked at the models 30i, 40i and 50i.
Our part number for the programmer is B4161 and can be purchased through any of our stockists.

Unfortunately, we are unable to comment on the compatibility of the programmer that you have fitted.

I have replied, suggesting that they send my query to the technical dept Remeha in Holland.

Aye, I think you got one of the 'Heat Team' big hitters there DH???!!! My understanding is that since Remeha became part of Baxi, their best 552 BBU engineers is on the case...........

 

[DeltaT2]

So you are running the burner flat out in short bursts - you are not modulating at variable temperature.

Good modern premix burners are at their most efficient at around 30% of full output. Weather comp will keep them in that area much of the time. External on-off control cannot do it.

Thanks MM, that's interesting. But can we establish if it's only OT that can modulate the burner, where On/Off controls will have a limited 'band width' & switching??

 

[D_Hailsham]

So you are running the burner flat out in short bursts - you are not modulating at variable temperature.

I didn't say it was modulating at variable temperature, as happens with weather comp systems.

As for running the boiler flat out, I was standing next to the boiler just now when it lit up. So I checked the boiler output (fan speed); it was 1300rpm, which is the minimum output. So my assertion that the boiler always starts at medium output seems to be incorrect. It ran for about a minute at 1300 rpm and then stopped.

I now want to know what the fan "starting speed" is all about.