Honywell DT90E thermostat-Increased gas consumpttion

[Roger Parkes]

Hi all,

I installed the subject thermostat to replace a previous Honywell on/off type exactly 12 month ago. I understand the PI function and how the boiler is cycled as the set point is approached. All the settings are at default and the system is operating perfectly providing an accurate 21.5deg C room temp with no overshoot. However my gas consumption has increased from 5200kWhr (average previous 3 years) to 8350kWhrs this year.
The boiler is a combination condensing type, 4 years old and seviced annually by british gas. The gas rate has been checked and is ok. Is it possible the increased cycling of the boiler would cause this, or ny other cause I am missing?

 

[Howsie]

Cycling does use more gas than modulating (IMO) but I would be surprised if it were that much more.

Have you changed your brand of underwear or left a window open?

 

[Roger Parkes]

This year I guess has been colder but not enough to account for the big increase. Previous stat was not modulating, normal on/off with heat anticipator. Have not changed underwear this year :blush5:

 

[doitmyself]

the system is operating perfectly providing an accurate 21.5deg C room temp with no overshoot.

Does the house feel any warmer than it did with the old thermostat?

If so, have you tried lowering the temperature?

Are you calculating your consumption using actual meter readings or estimated ones?

 

[Roger Parkes]

Does the house feel any warmer than it did with the old thermostat?

If so, have you tried lowering the temperature?

Are you calculating your consumption using actual meter readings or estimated ones?

Temperature level feels the same but is constant and any lower would not be comfortable. With old stat comfort only achieved by fiddling with the set point to counter overshoot and undershoot.
Meter readings are actual, here is consumption based quarterly Moving Annual Total, which makes the trend clear.

Gas consumption MAT
Quarter	Gas kWhr	MAT kWhr
1/09	652
2/09	1,000
3/09	201
4/09	1,960	3,813
1/10	2,160	5,321
2/10	423	4,744
3/10	256	4,799
4/10	2,684	5,523
1/11	1,552	4,915
2/11	413	4,905
3/11	323	4,972
4/11	1,802	4,090
1/12	3,107	5,645
2/12	1,915	7,147
3/12	323	7,147
4/12	3014	8,359

 

[Roger Welton]

You can try fine tuning the tpi settings to increase the minimum run time / max cycle rate

DT90E Wired Digital Thermostat - Honeywell UK Heating Controls

the installers guide shows you how.

 

[dancinplumba]

what make of boiler is it, the trouble with 'tpi' is that it causes boiler cycling when near to set temp, boiler fires on full rate, then throttles down, fires on full rate then throttles down. imo

 

[Roger Parkes]

Thanks Roger Welton and Howsie Your comment re the boiler throttling down when the temperature is close to the set point may be very relevant. The boiler is a Main Combi 24HE. The tpi is set to a one minute minimum off period and the cycle rate set to six times per hour and the proportional band to 1.5deg.C. I think this explains why the boiler cycles so rapidly and achieves such close control. The way forward may be to relax all thee settings and accept a slightly wider control. Does this type of boiler lose efficiency when operating throttled back at low capacity and does a boiler use more gas when stopping and starting in the same way that a car uses more fuel with frequent start stops? Is anyone aware of any reports/tests which have been carried out on this thermostat? If all else fails I will reinstall the original thermostat which was only changed in attempt to save energy as advertised. :thinking: y

 

[dancinplumba]

yes they do use more gas when stopping and starting, to save energy, you need modulating and weather compensating controls which directly control boiler burner modulation. BUT, your boiler does not accept these controls, it will only accept
on/off controls

 

[Roger Parkes]

I agree modulating control will be more efficient than on/off but in view of Honeywell claims I was expecting a gain of TPI versus old on/off. Your comment re loss of boiler efficiency has clarified what is happening, so will relax tpi settings and see what happens. Will monitor and report back in due course.

 

[doitmyself]

I agree modulating control will be more efficient than on/off

That's only true if the boiler has been correctly sized and can modulate low enough. Most boilers are not only oversized but also do not modulate low enough, so they run in on/off mode when the outside temperature exceeds 3-4C.

 

[Roger Parkes]

Point taken.

 

[bilco_21]

I am intrigued to see if TPI controls are indeed any more efficient than the standard on/off controls. I recently changed to the DT90E stat as opposed to the white honeywell T6360B. With our TPI controls the boiler never really gets to modulate as the stat turns it off before it gets to that temperature range. I've wondered about relaxing the TPI control/range but never really knew how to do it correctly, so i would be interested in your results if it stops the on/off of the boiler every 10 mins.

 

[Roger Welton]

I noticed a benefit, but I have a boiler with only on/off controls (oil)

 

[doitmyself]

With our TPI controls the boiler never really gets to modulate as the stat turns it off before it gets to that temperature range.

Is the house getting up to temperature and is the temperature being maintained? If so there's nothing to worry about; the stat is doing what it says on the tin.

Is you stat actually running for a short time every 10 minutes?

 

[bilco_21]

The house does reach the required temp of 20.5 quite quickly. The boiler definitely cuts in and out every 10 mins sometimes for 4-5 mins sometimes for 1 minute. My only question is when the stat shows the temp has been reached and has been stable like that for 1-2 hours does it still need to be cutting in and out for just 1 minute at a time?? Surely this must waste a certain amount of gas because the rads don't get enough heat in them in 1-2 minutes of boiler operation to give off any heat?!?!???

 

[doitmyself]

My only question is when the stat shows the temp has been reached and has been stable like that for 1-2 hours

Q. What causes the temperature to stay stable?

A. The thermostat, by ensuring the the boiler provides just enough heat to make up for the loss over the 10 minute cycle period.

Systems are sized so the house can be heated from cold (-1C) to warm (20C). Lets say this is a 21kW boiler and rads. When the house has reached the required temperature you need enough heat to make up for the temperature drop when the stat switches the boiler off. If the stat has a 1C differential, so the temperature drops from 20C to 19C before the boiler comes on, you will need 1kW of heat to bring the temperature back up to 20C.

The output of a radiator is temperature dependent, so the lower output can achieved by running the system with a lower flow temperature. If you check the flow temperature when the house is running at a stable temperature you will probably find that it may only be 40C to 50C.

 

[handsomejackuk]

i have this same thermostat and dont think it runs for long enough between cycles is it better to cut down the default number of on off cycles from 6 to 4 to ensure a longer burn time ?

upstairs bedrooms are run in micro bore, and i have throttled back donwstairs radiators but upstairs and the furthest away radiators dont seem to have enough time to heat up...

 

[doitmyself]

i have this same thermostat and don't think it runs for long enough between cycles is it better to cut down the default number of on off cycles from 6 to 4 to ensure a longer burn time ?

Have you monitored the boiler, when the house is up to temperature, to see what the actual cycle rate is and how long it runs for?

I have throttled back downstairs radiators but upstairs and the furthest away radiators don't seem to have enough time to heat up...

System needs balancing correctly. How far have you throttled back the downstairs rads?