Danfoss TP700 or Honeywell CM907?

[jpm777]

Hi,

I recently purchased a Honeywell DT90e and it has been driving me crazy as it seems too sensitive, turning the heating on and off constantly.
I agree the temperature remains quite even, but often the boiler fires and the rads barely even get luke warm. Surely this is a waste of gas, particularly in light of turning the heating off results in the house maintaining temperature to within 1 degree for about an hour!
Anyway, I was thinking a 7 day timer with its 6 different time periods may give me more control, as I could automatically set the next period to turn the heat down after a period of heating time (when it will have reached an acceptable temp).
The question is, would a honeywell CM907 be better, or would the danfoss TP7000 which I note has the option to turn off TPI be better suited to my circumstances?
Your opinions please....

Thanks,

Jpm777

 

[mikely]

If it is the Time Proportional control that you don’t like then the option to turn it off may be attractive. An argument put forward for TPI is that, because it operates the boiler in regular short bursts, the water temperature remains relatively low (as you note), and therefore the efficiency of the boiler is improved. I have a CM907 which I think is excellent, but working in a system with a thermal store, so the boiler operation is decoupled from the house thermostat.

There is interesting background here, although it does not prove the case:

http://www.energysavingtrust.org.uk...densing-boilers-TPI-control-project-extension

 

[D_Hailsham]

I recently purchased a Honeywell DT90e and it has been driving me crazy as it seems too sensitive, turning the heating on and off constantly.
I agree the temperature remains quite even, but often the boiler fires and the rads barely even get luke warm.

The rads only need to give off enough heat to make up for that lost since the boiler last went off. They do not need to be kept at max temperature all the time.

Heating up the house from cold does take a lot of heat, but maintaining the required temperature takes much less. It's like driving a car! While getting up to speed from standstill, the accelerator will be nearer the floor so the engine produces more power. When up to speed the accelerator will be open just enough to maintain speed.

If you have a well insulated house, the rads will sometimes be only lukewarm. You have to get used to it as many people (particularly of the female gender) expect to find that the rads are boiling hot all the time and think there is something wrong if they are not.

 

[jpm777]

Heating up the house from cold does take a lot of heat, but maintaining the required temperature takes much less. It's like driving a car! While getting up to speed from standstill, the accelerator will be nearer the floor so the engine produces more power. When up to speed the accelerator will be open just enough to maintain speed.

If you have a well insulated house, the rads will sometimes be only lukewarm. You have to get used to it as many people (particularly of the female gender) expect to find that the rads are boiling hot all the time and think there is something wrong if they are not.[/quote]

Thanks, that does indeed make logical sense. Is it the tpi part of the stat that makes it come on so regularly, or is it the inbuilt sensitivity to maintain temp? This would help me choose between the two stats, unless you can particularly recommend one over the other?

Thanks.

 

[D_Hailsham]

Thanks, that does indeed make logical sense. Is it the tpi part of the stat that makes it come on so regularly, or is it the inbuilt sensitivity to maintain temp? This would help me choose between the two stats, unless you can particularly recommend one over the other?

It's a mixture of both. (helpful, eh!)

TPI works mainly during the heating up from cold phase. The boiler will run flat out until the temperature is about 1.5C (this can be changed) below the set point. The boiler will then start cycling. Initially 9 minutes on and one minute off with reducing on times and longer off times (minimum 1 minute on, 9 minutes off) as the actual temperature gets nearer to the set temperature. The purpose of this is to prevent the overshoot you get if the boiler is kept running flat out until the set temperature is reached. It's just like lifting the accelerator as you near the required speed.

The sensitivity of most digital stats is approx 0.5C, i.e. it will maintain the temperature within ±0.5C. Some can even maintain it within ±0.25C, but that's not really necessary.

What all this avoids is the stat clicking off at 20C but the room continues to warm up to 21C or 22C, due to the heat remaining in the rads. So someone complains they are too hot and turns the stat down to 18C. But the house then cools down to 17C, so they then complain they are too cold and turn it back up to 20C.

 

[mikely]

Is it the tpi part of the stat that makes it come on so regularly, or is it the inbuilt sensitivity to maintain temp? This would help me choose between the two stats, unless you can particularly recommend one over the other?

Yes it is the time proportional output control that makes the boiler fire so regularly. By default the DT90 is set to 6 cycles per hour so you should find that it fires no more than this and sometimes less. The thermostat is not switching so regularly because it is sensitive, but because it is using a time averaged output over 6 cycle periods per hour to achieve a constant heat output and thereby maintain the set temperature.

To continue the car analogy, you don’t lift your foot completely off the accelerator when you achieve your required speed, you slowly adjust the throttle setting to achieve precisely the speed you want. This is what the TPI controller is doing.

 

[UpgradeME]

The question is, would a honeywell CM907 be better, or would the danfoss TP7000 which I note has the option to turn off TPI be better suited to my circumstances?
Your opinions please....

I used to have a TP7000 (I purchased and fitted it) and although it did it's job, the user interface to change / input the heating times is so unintuitative, really poor, making it a real pain to use. For this reason, I would not recommend a TP7000 to anyone.

 

[JohnD]

I have a Danfoss one (I thought I heard the Honeywell was not so reliable?) and it is not intuitive to use, but it is set up now so I just leave it. Occasionally I will step the "now" temperature up or down for 1/2/3 hours, and I use the holiday setting when I go away, but the daily settings I don't change. I think I turned off its "predictive" function because I didn't understand it.

I have TRVs and an Alpha pump which slackens off as the house comes up to temp, as well as the boiler modulating down so I didn't see the point, and I didn't want the controls taking decisions behind my back. The room stat is in a large open-plan area with two large rads with TRVs, and one fairly large without, so it cuts down the heat input as it nears the set temp.

 

[DP]

Danfoss for me any day

 

[UpgradeME]

Danfoss for me any day

Whilst you may have fitted many, I'm guessing your've never 'lived' with a TP7000 DP?

Reliability is great for an installer, usability is as important for the user!!

 

[jpm777]

Is it the tpi part of the stat that makes it come on so regularly, or is it the inbuilt sensitivity to maintain temp? This would help me choose between the two stats, unless you can particularly recommend one over the other?

Yes it is the time proportional output control that makes the boiler fire so regularly. By default the DT90 is set to 6 cycles per hour so you should find that it fires no more than this and sometimes less. The thermostat is not switching so regularly because it is sensitive, but because it is using a time averaged output over 6 cycle periods per hour to achieve a constant heat output and thereby maintain the set temperature.

To continue the car analogy, you don’t lift your foot completely off the accelerator when you achieve your required speed, you slowly adjust the throttle setting to achieve precisely the speed you want. This is what the TPI controller is doing.

What I am desperately trying to avoid is buying another stat that cycles the boiler all the time and unnecessarily. To keep the car analogy going, as my house is well insulated and therefore holds heat, it is like a car that can be put into neutral, engine turned off and then coast maintaining the same speed for an hour or so. Starting the engine to let it tick over during this time would seem like a waste of fuel.
Maybe a different 7 day stat altogether then? I'm worried the Honeywell one would be too sensitive and that the danfoss by way of turning off the tpi may be better. It seems it doesn't have a good user interface.
Any other ideas please?

 

[Dan Robinson]

Jpm, I think you're missing the point of the tpi.

 

[jpm777]

Jpm, I think you're missing the point of the tpi.

Well ok Dan thanks for that. I thought by mentioning the danfoss one and that it could be turned off meant I was on to something. Would you care to explain your wisdom further then?!

 

[Dan Robinson]

The controller is cycling the boiler on purpose.

Tpi can also be turned off on the Honeywell.

It has all been explained already. I will few read the thread, but it seems you are worrying over nothing.

*edit

Read the thread and still not sure what you're worrying about.

 

[jpm777]

The controller is cycling the boiler on purpose.

Tpi can also be turned off on the Honeywell.

It has all been explained already. I will few read the thread, but it seems you are worrying over nothing.

*edit

Read the thread and still not sure what you're worrying about.

Ok Dan thank you for the swift reply. Are you certain that the tpi control can be turned off on the Honeywell cm907. It's not mentioned anywhere in their product literature.
Sorry for it being long winded, what I'm worried about is wasting money with the boiler firing more regularly than my property requires, ie I need a bigger differential between on and off.