I have purchased a Honeywell Evohome and am running with radiator zoning valves. I am having issues with the way this system is regulating the temperature.
My experience very much echoes that in this thread
however I thought I wouldn't hijack it as it is long enough
I have come from a Honeywell CM907 digital stat, which was very good at maintaining the temperature, to within +/- 0.2 degree. It used to do pulsing of the boiler when just under the set point, and stop when it reached it. I was hoping get that sort of control in all my rooms using the zone valves (where suitably thermally disconnected from each other).
I have found that the ability of the Evohome to hold a set temperature is very poor. It consistently over-shoots by 1.5 degrees before the heat demand stops.
System:
Honeywell Evohome with BDR-91 relay and HR92 zone valves. Combi boiler.
Firmware: v25, dated 16 Jan 2014
I will describe the results of some observations.
----
I used Evohome internal sensor and in single zone/thermostat mode. I set the temperature to 18.0. The boiler was fired continuously until 17.5 was reached. Then it went into the proportional band, and commenced 1 minute pulses every 10 minutes. This continued until the room temperature - as measured by Evohome itself - was 19.5 degrees.
I then installed the zone valves, in the hope this behaviour would go away.
I did a long constant temperature test on the lounge zone. From a current room temperature of 16.0, I set it to a constant 18.0 overnight. Initially, it kept pulsing and got to 19.5 degrees before closing the valve and stopping calling for heat. Overnight it gradually fell, and by 3 in the morning it was approaching 18.0. On arrival at 18.0 it did the same thing and got back to 19.5 deg C.
Conclusion: this 1.5 degree overshoot cannot be resolved by adjusting my temperatures down by 1.5 degrees, since it does eventually approach the right temperature for a while, then when triggered overshoots again. This is a serious bug in the way the control system has been implemented.
Also this overshoot behaviour is not related to a high radiator temperature, as I repeated this with cool radiators (boiler down low) and a slower slow temperature rise still resulted in a deliberate overshoot to 1.5 degrees more than requested.
---
This next observation was independent heating of other zones, which also doesn't work unless the other zones are set down cold.
The lounge was set constant at 18.5 degrees, and lounge room temp was indeed stable at 18.5 degrees (presumably having peaked earlier in the day and come down).
Bedroom 1 and bedroom 2 were set very low, and were both at 16.0 degrees.
Bedroom 1 and 2 zones were then set to 18.5 degrees to trigger the heating.
The heating came on, and the bedrooms started heating up. But the lounge radiator was also allowed to run, even though no heat was needed.
The bedrooms only started to reduce the calls for heat at 19.0, and (no surprise) stopped calling for heat at 19.5 (both of them).
The lounge also kept heating up, and the lounge rad valve gradually started to close at lounge temp of 19.5, and closed down fully at 20.0 (set point of lounge + 1.5 degrees).
Conclusion: Any zone calling for heat, will keep going until it gets to 1.5 degrees over the set point. Regardless of which zone is calling for heat, all zones will run until they are 1.5 degrees more than the set point, until their zone valves are instructed to close.
-----------
This is offering very poor control - since all zones will go up by 1.5 degrees even if only one calls for heat. Indeed it defeats the object of zoning, might as well just use normal TRVs!!
and yes, I have also tried removing the battery and doing a cold boot and rebinding all my valves 'slowly'. No change.
-----------
Some interesting quotes from other thread...
Quotes:
"When using Evohome in thermostat-modus (only Evohome+BDR91, no radiator valves), it will continue slowly heating up the room untill the temperature reaches 1.5 degree above the requested temperature. After that, it will finally stop requesting heat. This process of slowly heating up above requested temperature can take 2 hours easily. It's not a short accidental overshoot. It's a continuous, well orchestrated slow build up of overshoot on overshoot on overshoot. It's a huge bug in my opinion.
Some people have suggested to put my requested temperature a degree lower in Evohome, to compensate for this Evohome behaviour. This is not a solution. Yes, it will prevent Evohome from heating up the room too much, but it won't prevent Evohome from letting it cool down too far afterwards."
"I don't think TPI is the problem. The old Honeywell Round does TPI as well, I think, but works fine. The problem is a bug in the implementation in Evohome is my guess. It never skips a TPI cycle. But it should. But it doesn't. If there's a bug that big in the system, they will HAVE to make new firmware available. The product is too expensivee not to. This would mean they would need to give everyone their money back for a non-functioning system."
------------
I am attempting to get an answer from Honeywell engineers via their support email. What I hope would happen, is that they admit this is not intended behaviour; that they find the bug; that they fix their firmware; and that they release the new firmware with PC bootloading software and instructions for users to update it. That would cost them, and what I actually think will happen is probably the opposite of all that, and I will return the product to the supplier as not fit for purpose.
1 degree is pretty much spans too cold - ok - and too hot. That was about the accuracy of the old analogue clicky thermostats. So if Honeywell consider that 1.5 degree overshoot as a design feature, it is definitely not fit for purpose.
My experience very much echoes that in this thread
however I thought I wouldn't hijack it as it is long enough
I have come from a Honeywell CM907 digital stat, which was very good at maintaining the temperature, to within +/- 0.2 degree. It used to do pulsing of the boiler when just under the set point, and stop when it reached it. I was hoping get that sort of control in all my rooms using the zone valves (where suitably thermally disconnected from each other).
I have found that the ability of the Evohome to hold a set temperature is very poor. It consistently over-shoots by 1.5 degrees before the heat demand stops.
System:
Honeywell Evohome with BDR-91 relay and HR92 zone valves. Combi boiler.
Firmware: v25, dated 16 Jan 2014
I will describe the results of some observations.
----
I used Evohome internal sensor and in single zone/thermostat mode. I set the temperature to 18.0. The boiler was fired continuously until 17.5 was reached. Then it went into the proportional band, and commenced 1 minute pulses every 10 minutes. This continued until the room temperature - as measured by Evohome itself - was 19.5 degrees.
I then installed the zone valves, in the hope this behaviour would go away.
I did a long constant temperature test on the lounge zone. From a current room temperature of 16.0, I set it to a constant 18.0 overnight. Initially, it kept pulsing and got to 19.5 degrees before closing the valve and stopping calling for heat. Overnight it gradually fell, and by 3 in the morning it was approaching 18.0. On arrival at 18.0 it did the same thing and got back to 19.5 deg C.
Conclusion: this 1.5 degree overshoot cannot be resolved by adjusting my temperatures down by 1.5 degrees, since it does eventually approach the right temperature for a while, then when triggered overshoots again. This is a serious bug in the way the control system has been implemented.
Also this overshoot behaviour is not related to a high radiator temperature, as I repeated this with cool radiators (boiler down low) and a slower slow temperature rise still resulted in a deliberate overshoot to 1.5 degrees more than requested.
---
This next observation was independent heating of other zones, which also doesn't work unless the other zones are set down cold.
The lounge was set constant at 18.5 degrees, and lounge room temp was indeed stable at 18.5 degrees (presumably having peaked earlier in the day and come down).
Bedroom 1 and bedroom 2 were set very low, and were both at 16.0 degrees.
Bedroom 1 and 2 zones were then set to 18.5 degrees to trigger the heating.
The heating came on, and the bedrooms started heating up. But the lounge radiator was also allowed to run, even though no heat was needed.
The bedrooms only started to reduce the calls for heat at 19.0, and (no surprise) stopped calling for heat at 19.5 (both of them).
The lounge also kept heating up, and the lounge rad valve gradually started to close at lounge temp of 19.5, and closed down fully at 20.0 (set point of lounge + 1.5 degrees).
Conclusion: Any zone calling for heat, will keep going until it gets to 1.5 degrees over the set point. Regardless of which zone is calling for heat, all zones will run until they are 1.5 degrees more than the set point, until their zone valves are instructed to close.
-----------
This is offering very poor control - since all zones will go up by 1.5 degrees even if only one calls for heat. Indeed it defeats the object of zoning, might as well just use normal TRVs!!
and yes, I have also tried removing the battery and doing a cold boot and rebinding all my valves 'slowly'. No change.
-----------
Some interesting quotes from other thread...
Quotes:
"When using Evohome in thermostat-modus (only Evohome+BDR91, no radiator valves), it will continue slowly heating up the room untill the temperature reaches 1.5 degree above the requested temperature. After that, it will finally stop requesting heat. This process of slowly heating up above requested temperature can take 2 hours easily. It's not a short accidental overshoot. It's a continuous, well orchestrated slow build up of overshoot on overshoot on overshoot. It's a huge bug in my opinion.
Some people have suggested to put my requested temperature a degree lower in Evohome, to compensate for this Evohome behaviour. This is not a solution. Yes, it will prevent Evohome from heating up the room too much, but it won't prevent Evohome from letting it cool down too far afterwards."
"I don't think TPI is the problem. The old Honeywell Round does TPI as well, I think, but works fine. The problem is a bug in the implementation in Evohome is my guess. It never skips a TPI cycle. But it should. But it doesn't. If there's a bug that big in the system, they will HAVE to make new firmware available. The product is too expensivee not to. This would mean they would need to give everyone their money back for a non-functioning system."
------------
I am attempting to get an answer from Honeywell engineers via their support email. What I hope would happen, is that they admit this is not intended behaviour; that they find the bug; that they fix their firmware; and that they release the new firmware with PC bootloading software and instructions for users to update it. That would cost them, and what I actually think will happen is probably the opposite of all that, and I will return the product to the supplier as not fit for purpose.
1 degree is pretty much spans too cold - ok - and too hot. That was about the accuracy of the old analogue clicky thermostats. So if Honeywell consider that 1.5 degree overshoot as a design feature, it is definitely not fit for purpose.
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