Honeywell DT90e with an Oil Fired Boiler and Underfloor Heat

Hi

I have an underfloor heating system where the hot water is supplied by an oil fired boiler.

The underfloor heating manifolds are controlled via an older design of the DT200 Honeywell room stats. The system works fine.

I would like to change these to DT90E Honeywell room stats.

I note that the DT90E is a TPI design where as the DT200 is a simple on / off.

I have read that the DT90E should be set up with a Min On/Off time of 4 minutes and a cycle rate of 3 cycles per hour when used with an oil fired boiler.

I understand that the DT90E is a direct replacement for the DT200.

I have it in my head that the boiler will be switched on and off for 4 minutes 3 times per hour.

I know that this is wrong, but will the underfloor heating system, which operates through thermal actuators, still work the same way as it does with the DT200s ?

Many thanks.

Hi

I was wondering if anyone could help ?

self builder 123 said:

Hi

I have an underfloor heating system where the hot water is supplied by an oil fired boiler.

The underfloor heating manifolds are controlled via an older design of the DT200 Honeywell room stats. The system works fine.

I would like to change these to DT90E Honeywell room stats.

I note that the DT90E is a TPI design where as the DT200 is a simple on / off.

I have read that the DT90E should be set up with a Min On/Off time of 4 minutes and a cycle rate of 3 cycles per hour when used with an oil fired boiler.

Click to expand...

yes if the DT90E is controlling the boiler
set it to on/off 1 minute and 12cycles per hour if its controlling the acuator

self builder 123 said:

I understand that the DT90E is a direct replacement for the DT200.

Click to expand...

correct

self builder 123 said:

I have it in my head that the boiler will be switched on and off for 4 minutes 3 times per hour.

Click to expand...

no!!
these are minimum limit times
for eg if it is set to control an oil boiler it will be set to 4
this means if the stat is satisfied it will switch off then stay off for at least 4 minutes before switching on again regardless of temperature set! so if the temp dropped within a minute say then the boiler will be prevented from firing till the 4 minutes are up

self builder 123 said:

I know that this is wrong, but will the underfloor heating system, which operates through thermal actuators, still work the same way as it does with the DT200s ?

Many thanks.

Click to expand...

yes but see answers above

Matt

Cheers matt1e

You are right - each of the DT200s are connected to a thermal actuator which in turn controls the oil fired boiler. The DT90E will work in the same way.

What does the cycle rate mean for a thermal actuator which is connected to an oil fired boiler ? 12 cycles per hour - does this mean that the actuator will be opened and closed every 5 minutes i.e. 12 times per hour thus firing the boiler 12 times per hour if there is a demand for heat from the DT90E ?

hi
not quite, the cycle rate is the amount of times per hour that the unit will switch on and off during the on time and within the proportional band
the default proportional band for the DM90 is 1.5 degrees but to make it easier to understand we will persume its been set to 2 degrees
so say for example the heating is on from say 10 till 12 and the temp is set for 20 degrees this means that if the room temp is
above 21 degrees the output will be fully off
below 19 degrees the output will be fully on
inbetween 19 and 21 (the proportional band width) then the output will cycle on and off for what ever times per hour are set in the cycle rate
but it will only do this between 10 and 12 in the example above (the set on time)
this gives better temp control

Matt

self builder 123 said:

I have read that the DT90E should be set up with a Min On/Off time of 4 minutes and a cycle rate of 3 cycles per hour when used with an oil fired boiler.

Click to expand...

As you say the DT90E is a TPI controller. It controls how long the boiler is on depending on the difference between actual and required room temperatures. When the difference is high the boiler will be running continuously when it gets within about 2C of the required temperature it will start cycling.

If the cycle rate is three minutes per hour, then the cycle length is 20 minutes. How much of that 20 minutes the boiler is on will depend on the difference between actual and required temperatures. The smaller the difference, the less of the 20 minutes will be on.

The minimum ON time (not OFF time) is just what it says. The boiler will stay on for this time, even if the TPI controller asks for less. I don't know why this is necessary - probably due to the way the boiler works and the time it needs to get up to temperature.

As you are controlling the boiler through thermal actuators, it might be a good idea to ask Honeywell what settings to use.

D_Hailsham said:

As you are controlling the boiler through thermal actuators, it might be a good idea to ask Honeywell what settings to use.

Click to expand...

seconded
you may req the oil boiler settings if the actuators are switching the boiler

Matt

Hi

Many thanks to you both - I will contact Honeywell as you suggested.