Remeha boilers and Honeywell CM9XX thermostats

[mysteryman]

Yes, you need either OT control or weather comp to modulate to variable temperature, and OEM kit has to be simpler and better.

I appreciate that on a very light load at constant temperature, the burner may stay on low fire, but under those conditions it ought to be modulating at a much lower flow temp anyway.

 

[ALEC1]

remehas start at min, and after a set period ramp up if the return has not risen...in other words if the first ramp is longer than one minute it won't get out of lower power

what were the people at honeywell thinking.. they have a perfectly good controller that works stonkingly well with these boilers...its an OT one, widely available in the rest of europe...but no the UK!

 

[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.

Does this mean that any condensing boiler with premix burner is better to be oversized for the heat load, to enable it to work at it's most efficient MM??

 

[DeltaT2]

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.

How are you checking the fan RPM DH, by Amp draw??

 

[D_Hailsham]

How are you checking the fan RPM DH, by Amp draw??

The display on the front of the boiler allow the user to view: Flow temperature, Return temperature, Hot water temperature (combi), Outside temperature (with external sensor), Ionization Current and Fan RPM.

 

[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.

Does this mean that any condensing boiler with premix burner is better to be oversized for the heat load, to enable it to work at it's most efficient MM??

No, weather compensation will do that. We were talking about conditions of light load, which is 95% of the heating season. Boilers and heat emitters should be sized to suit the load.

 

[DeltaT2]

How are you checking the fan RPM DH, by Amp draw??

The display on the front of the boiler allow the user to view: Flow temperature, Return temperature, Hot water temperature (combi), Outside temperature (with external sensor), Ionization Current and Fan RPM.

Thanks DH, I didn't know that.

 

[D_Hailsham]

what were the people at honeywell thinking.. they have a perfectly good controller that works stonkingly well with these boilers...its an OT one, widely available in the rest of europe...but no the UK!

I asked earlier for your evidence that the CM9XX series is a cut down version of an OT thermostat. Could you provide it please?

 

[ALEC1]

Only observation by comparing the OT version with the on-off version . Compensation controllers have the same TPI logic as the 9xx versions but send different signals...so there is a difference in the signal, one asks for temperature the other sends an on-off signal..

Perhaps I was over enthusiastic in the use of the word "same"

 

[D_Hailsham]

Compensation controllers have the same TPI logic as the 9xx versions but send different signals...so there is a difference in the signal, one asks for temperature the other sends an on-off signal.

I can see that both OT and 9XX controllers could use the same TPI logic to determine what the boiler needs to do. In the 9XX controller this can only be interpreted as an on-off signal of varying proportions. But with an OT controller it will be a modulation rate (0 - 100%). What determines the rate is either the error in room temperature or the water temperature determined by the control curve for a WC system.

CM9XX systems are one-way communication only; OT systems are two-way.

 

[ALEC1]

I am not sure that the OT controller calls for a% of modulation...it seems to call for temperature, according to the deviation from set point on the thermostat...the broag pcb controls the modulation..

I have an I sense and had a honeywell OT controller on the broag before...

 

[Dan Robinson]

I am not sure that the OT controller calls for a% of modulation...

It doesn't. OT is just the protocol for transmitting data backwards and forwards. Unless you have the modulating thermostat and/or weather sensor, all an OT programmer will do is allow the interrogation of the boiler controller for current status. That data will depend on the sensors present too.


A Honeywell CM90x or CM92x controller has a slightly different PCB to the OT variant. The main difference I presume being no relay. Just whatever is uses to send and receive the data.

Remember there is at least two versions of OT.

 

[snb]

i have fitted over 200 remehas with cm907/927 and have only had a handfull of problems.

These were all on large houses with heating demands over 20kw.

If you read the technical spec it does state for the honeywell cm series to adjust the firing rate and minimum on period.

I always alter these to 5min minimum on period and leave the the number of cycles to 6.

The remehas intial fire rate is 2000rpm to start with (about 5 secs) before dropping to the 1300 rpm from where it start to ramp up as required to maintain a 10 degree differential on the flow and return. (Which has been changed to 15 degree differential on boiler from july2010)


The change made above was due to problems with very big systems and large cylinders with 20plus kw coils which would cause the system to take a long time to heat up as the differential would remain often with 40 flow and 30 ish return temps.

On systems like these the software can be altered but they won't give the code out to many people. Or you can get around it by changing the minimum fan speed to 2000rpm

 

[Dan Robinson]

What? It can't be told easily that is has to heat a cylinder? Secret codes to change the ramp?

Glad I don't fit them

 

[ALEC1]

with the isense the delta T goes as low as 5c..I suspect it has to behave differently for on-off controls...

the board takes a cylinder sensor dan...