honeywell DT90E room stat

[Supernatural323]

Hi All.
I have an old Ideal ELAN 2 propane fanned flue gas boiler
80F P

I decided to go digital with my room thermostat, so I purchased the honeywell DT90E.

Not sure what I think of it, Keeps a very good steady temperature but the boiler is on off all the time even when the temperature displayed on the room stat is the same as temperature set.

Is this energy efficient to be on/ off all the time, wouldn't it be better that it turns on when it goes 1C below desired temperature and turns off when it gets to it, I know people might say it will overshoot by 0.5C or so, but you can balance this out.

My question is
1) Is a TPI Thermostat more energy efficient
2) Will it **** my old boiler up?.

Has anyone any good suggestions in regards to the setup, I don't understand any of it?

Many thanks for reading.

 

[D_Hailsham]

Keeps a very good steady temperature but the boiler is on off all the time even when the temperature displayed on the room stat is the same as temperature set.

The display only moves on steps of 0.5C, so 20C could be anything between 19¾C and 20¼C.

Have you timed how long the boiler is on when the house is up to temperature? The minimum should be one minute every 10 minutes.

wouldn't it be better that it turns on when it goes 1C below desired temperature and turns off when it gets to it, I know people might say it will overshoot by 0.5C or so, but you can balance this out.

But you can't balance it out. If you drop the setting by 0.5C to account for overshoot, you have immediately dropped the temperature at which the boiler comes on by 0.5C. This might be too cold for some, so they will turn the stat up again. You have to maintain the temperature within close limits to prevent this. Try turning the stat down by half a degree and see if anyone notices.

1) Is a TPI Thermostat more energy efficient

The manufacturers will say: "yes"; but an independent trial said: "no". The jury is still out as the trial was not statistically significant.

There is no doubt that TPI provides a better control over temperature and consequently a more comfortable environment.

2) Will it **** my old boiler up?

No.

Has anyone any good suggestions in regards to the setup, I don't understand any of it?

Which settings are you thinking of?

TPI controls are only new to the domestic market. They have been the norm in commercial installations for years.

 

[Supernatural323]

Have you timed how long the boiler is on when the house os up to temperature? The minimum should be one minute every 10 minutes.

What does "os" stand for?


Thanks for getting back to me.

 

[Dan Robinson]

os is a type for "is"

 

[Supernatural323]

Hi,
Ok it is on for about 1.20 Min and off for about 15 Min.

What I don't get is, the thermostat is set to 19C but the temperature is 19.5C and the boiler continues to turn off on, why does it come on when it is 0.5c above set point?


Regards

 

[D_Hailsham]

os is a type for "is"

I've corrected it.

Type is a typo for typo.

 

[Dan Robinson]

You saw my deliberate mistake

 

[Supernatural323]

Is there a way of turning the tpi off then?

 

[mikely]

Is there a way of turning the tpi off then?

No you can’t turn off the TPI function because that is what makes it work as a temperature controller. You can alter some of the settings that affect its operation, but it depends what you are trying to achieve.

If you want the boiler to fire less often, you can reduce the cycle rate to 3 per hour (default is 6), but temperature control may not be as good. See the Installer mode settings: http://www.honeywelluk.com/Documents/Full-Specification/pdf/1078.pdf

 

[D_Hailsham]

What I don't get is, the thermostat is set to 19C but the temperature is 19.5C and the boiler continues to turn off on, why does it come on when it is 0.5c above set point?

Don't forget that 19.5C is not an exact temperature; the readings only change in 0.5C steps.

Do you have a well insulated house? If so there could be a slight amount of overshoot. Increasing the proportional band may help to reduce this.

 

[Supernatural323]

Don't forget that 19.5C is not an exact temperature; the readings only change in 0.5C steps.

Hi D_Hailsham,

I get about it only operates in 0.5C, But if the set point is 19 C and the thermostat reading is 19.5C, then this temperature might not be 19.5C but is closer to 19.5C than 19C. But why would the boiler still turn off/on to try and maintain a temperature which it has allready past.

With this system, when it reaches temperature, the top half of the radiators are always warm, not as hot as it can get just a little warm.

When my house got up to temp last night I turned the boiler off, it took 1 hour for my house to loose 1C.

What I want to know is what uses less gas, on/off for an hour, Or 1 good blast when the temperature reduces by 0.5C - 1C or so.


Sorry if you think my questions are silly, but times are hard and I'm trying to improve the system even though it feels as if I'm not.

 

[D_Hailsham]

I get about it only operates in 0.5C, But if the set point is 19 C and the thermostat reading is 19.5C, then this temperature might not be 19.5C but is closer to 19.5C than 19C. But why would the boiler still turn off/on to try and maintain a temperature which it has already past.

That's a difficult one. I have had a CM927, which is the programmable version, for over four years and never seen it do what you say.

When my house got up to temp last night I turned the boiler off, it took 1 hour for my house to loose 1C.

The spec says:

Temperature control accuracy: ± 0.5°C (or better) at 20°C, 50% load and 3°C /hour temperature ramp

Your temperature ramp is much lower, so the current Proportional Band might be causing some overshoot. Try increasing the Proportional Band - parameter Pb.

With this system, when it reaches temperature, the top half of the radiators are always warm, not as hot as it can get just a little warm.

That shows the system is working properly. In the current mild weather the rads do not need to give out their maximum output. The thermostat is effectively adjusting the water temperature so the rads are giving off the correct amount of heat.

What I want to know is what uses less gas, on/off for an hour, Or 1 good blast when the temperature reduces by 0.5C - 1C or so.

My experience is that TPI maintains the room temperature very accurately and saves money.

 

[mikely]

But why would the boiler still turn off/on to try and maintain a temperature which it has allready past.

Once it is up to temperature the thermostat is trying to achieve a constant average output of the heating system in order to maintain the target temperature. It does this by cycling the system on and off as you have observed so that the average (Time Proportional) output is constant. Over time it should adjust this average output so that it settles at the target temperature. As D_H suggests, the system may be overshooting a little so it may be worth increasing the proportional band width. Does the temperature settle after 2 or 3 hours of operation?

What I want to know is what uses less gas, on/off for an hour, Or 1 good blast when the temperature reduces by 0.5C - 1C or so.

I think the supposed benefit of the regular ‘short burst’ operation is that it lowers the average heating return temperature, which increases the efficiency of the boiler. However this will have more effect on a modern condensing boiler and even then is probably only a few percent.