Remeha boilers and Honeywell CM9XX thermostats

[gaswizzard]

You may have checked it many times, but there is still the 0.5C steps on the display to take into consideration.

The .5c steps were taken into consideration as per the above.

So it's not wired up to a boiler? If that's the case, how do you know when the stat has turned the boiler on or off?

The relay in the cmxxx is either open/closed (on/off) , this is varified with the flame signal on front display , these units are not as complex as some may think.

 

[D_Hailsham]

The .5c steps were taken into consideration as per the above.

You would need a separate thermometer, with an accuracy of ±0.05C to do that.

The relay in the cmxxx is either open/closed (on/off) , this is verified with the flame signal on front display.

There is some debate about what the flame means. The flame can be displayed without the boiler actually being on.

these units are not as complex as some may think.

That depends on what you mean by complex. There is some fairly clever circuitry to calculate the required on time per cycle. This depends on the difference between actual and target temperature and the rate at which the temperature changes. That's why it is called TPI (Time Proportional Integral).

I have a Powerpoint Presentation, downloaded from Honeywell USA, about the CM9XX stats. It says that the cycle rate setting is just a maximum and the length of a cycle can be varied by the TPI logic. This would account for your 8 mins ON, 30 seconds OFF - its about 7 cycles per hour.

 

[gaswizzard]

There is some debate about what the flame means. The flame can be displayed without the boiler actually being on.

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The flame symbol represents integral relay on , with flame signal off there is no power via B on the CMxxx , with target temp set .5c below room temp the CMxxx will switch the relay for a period of 5 min on/5 min off , it's that simple.

 

[gaswizzard]

The flame can be displayed without the boiler actually being on.

That maybe the case but the green light on receiver box would be on obviously due to power via terminal B , ie: MV or boiler powered.

In laymens terms green light on reciever will always be on when flame signal is showing on CM controller , or in the case of the 907 MV/boiler will be powered.

You have one so why not carry out some R&D.

 

[mikely]

The flame symbol represents integral relay on , with flame signal off there is no power via B on the CMxxx , with target temp set .5c below room temp the CMxxx will switch the relay for a period of 5 min on/5 min off , it's that simple.

The flame symbol on the CM90x does represent relay on, however on the CM92x, it doesn't; green light on the receiver does. Quoting from the Honeywell installation guide:- ‘The receiver box receives the heat demand (0-100%) signal from the room thermostat. The room thermostat will display the flame symbol on the LCD display whenever more heat is required. Depending on the demand the receiver box will switch the heating device on to match the current requirements of the system.’

When the room temp is at the target, the heating demand is determined (by the proportional + integral controller) as a time proportional output that balances the heat loss from the building. If the demand level is 50% then 5mins on/ 5 mins off will work, but if it is 20% then the cycle time has to be longer – 5mins on/ 20mins off.

 

[gaswizzard]

The flame symbol on the CM90x does represent relay on, however on the CM92x, it doesn't; green light on the receiver does. .

As long as flame symbol is on the green light on relay box will be on , if not then it is an RF issue (slow to respond) , with the two units close to each other the green light on relay will illuminate within a few seconds of flame symbol , this in turn will power boiler/MV just the same as any other conventional type thermostat.

 

[MJN]

The flame symbol on the CM90x does represent relay on, however on the CM92x, it doesn't; green light on the receiver does. .

As long as flame symbol is on the green light on relay box will be on , if not then it is an RF issue (slow to respond)

That is not true. Re-read what Mikely says. The flame signal on the wireless versions merely represents a demand which, as stated, could be anywhere between 0 and 100%. The receiver box will control the boiler (and green light) to suit. To illustrate with an example, if the stat calls for a 10% output then it'll illuminate the flame symbol continuously for the cycle, however the green light on the receiver will only be on for 10% of the time.

The flame symbol on the wired version however does indeed operate in direct correlation with the relay.

Mathew

 

[MJN]

<duplicate post>

 

[mikely]

As long as flame symbol is on the green light on relay box will be on , if not then it is an RF issue (slow to respond) , with the two units close to each other the green light on relay will illuminate within a few seconds of flame symbol , this in turn will power boiler/MV just the same as any other conventional type thermostat.

I don’t think so. As D_H states, ‘The flame can be displayed without the boiler actually being on.’ This also concurs with the Honeywell description. The thermostat sends a 0-100% demand to the receiver, not an on/off demand.

 

[gaswizzard]

Why would the wireless be any different to the wired? , whenever i've encountered the green light illuminated on reciever the boiler/MV are powered.

 

[gaswizzard]

If honeywell actually explained how these units function instead of just giving the installer a parameter table to work with then things may be a bit clearer.

Interesting all the same.

 

[mikely]

It does seem odd that in the wireless version they split the control function between thermostat and receiver. They must have had some good reason.

 

[ALEC1]

I suspect there are two receivers one OT and one for on-off switching..

may be there are several versions of OT too...

 

[mikely]

Yes, that's a good point. The same thermostat unit can then be used for either.

 

[gaswizzard]

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.

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These type controllers should not be used with conventional boilers , short cycling like you have described can cause condensation issues due to return water temps constantly below flue gas dew point temp , was the boiler Hex corroded? , seen this a few times with regard to these controllers.

It's a pity the manufactures don't mention this.