HONEYWELL DT90E ROOM THERMOSTAT

[slj33]

hi,
i have a 15 year old ideal classic gas 244 boiler, and have just had a new DT90E room thermostat fitted. the problem is the default settings for gas boilers is cycle rate 6 / min on off time 1 min, and the
Proportional band width 1.5.
with these settings the boiler cycled on and off constantly. the temp of 18 was reached and maintained well, but the constant cycling seemed unnecessary and annoying!
the heating engineers who were not that familiar with the thermostat recommended changing the settings to the oil boiler settings of : cycle rate 3 / on off 4 mins and leave the pb at 1.5

i have done this but wondered if this was in fact the best thing to do and also should i up the pb from 1.5 (it can go up to 3)?

as i said the stat does control the temp well - but the cycling bothers us as we feel we may use more gas ? rather confused by the settings as we did not have a room thermostat before!

also the boiler has a thermostat on it - dial, from 1-6 what is the best/most efficient setting for this.

many thanks for any help offered.

 

[Dan Robinson]

Boiler on 4 or 5.

Leave stat as was from the factory.

The boiler cycled because it is the design of the stat.

have you measured the gas usage?

 

[slj33]

hi thanks for the reply

not measured it as such - its been colder anyway so prob used more - but it was cycling literally every few mins - on for 1min approx - worried about the wear and tear etc. the engineers were not sure about it, they thought it was more suited to a modern condensing boiler. its just disconcerting to hear the boiler going on and off when temp is reached ! should the Proportional band width be increased ? would that reduce cycling? really do not understand that one!
the reason for installing it was to save gas so ........

thanks

 

[Dan Robinson]

its just disconcerting to hear the boiler going on and off when temp is reached

What do you expect it to do then?

Cycling 6 times an hour shouldn't do too much harm to the boiler.

Cycling less may mean the heating doesn't work as efficiently.

Efficiency gains from this system are not an exact science though.

 

[MJN]

Repeated firings are a common behaviour of systems controlled by Honeywell TPI controllers as a search of this forum would reveal.

Whilst it does produce extremely stable temperature control many are of the opinion that it is a touch on the aggresive side and, most importantly, that the configuration options available (cycle rate and min on/off time) don't extend quite as far as some might like. It could result in premature boiler failure as stresses will inevitbly be higher during such operation, however the seemingly fragile nature of most modern boilers is such that removing this risk may not reduce your chances of failure all that much.

Some have modified their setups with time-delay relays to remove the short(er) firing cycles however you might consider this a customisation too far and prefer just to live with it and drop the cycle rate to 3 times/hr. That, and keep reminding yourself of the added comfort such stable temperture control brings!

Mathew

 

[snb]

Repeated firings are a common behaviour of systems controlled by Honeywell TPI controllers as a search of this forum would reveal.

Whilst it does produce extremely stable temperature control many are of the opinion that it is a touch on the aggresive side and, most importantly, that the configuration options available (cycle rate and min on/off time) don't extend quite as far as some might like. It could result in premature boiler failure as stresses will inevitbly be higher during such operation, however the seemingly fragile nature of most modern boilers is such that removing this risk may not reduce your chances of failure all that much.

Some have modified their setups with time-delay relays to remove the short(er) firing cycles however you might consider this a customisation too far and prefer just to live with it and drop the cycle rate to 3 times/hr. That, and keep reminding yourself of the added comfort such stable temperture control brings!

Mathew

The use of these tpi controls do exactly the opposite of what you suggest.

They actually reduce cycling of the boiler.

But what it appears is the OP did not have a room stat (interlock) before and was probably experiencing temp over shoots.

The only way to get around boiler short cycling is to adjust the boiler output (if this is possible)

This is a common problem with oversized boilers being installed in the past as the boilers were fixed output. Over the last 10 years and since the introduction of condensing boilers most boilers will modulate to reduce short cycling.

This is where there is still a need to have the system designed correctly and the boiler sized correctly. The old way of you can't have to big can cause loads of problems with heating

 

[MJN]

The use of these tpi controls do exactly the opposite of what you suggest.

On the contrary. In the absence of a proportional demand input the only way for the programmer to modulate heat output is through adjusting the boiler firing duty cycle.

Over the last 10 years and since the introduction of condensing boilers most boilers will modulate to reduce short cycling.

The problem is that few modulate down low enough to avoid having to cycle when the stat is still calling for heat.

Mathew

 

[slj33]

thanks for the replies.

having played around with the settings, and having read posts on this subject, it seems that the OIL boiler settings do seem to suite our old GAS boiler the best, as suggested in a post on this forum.

i am starting to understand the priciples of the tpi now! but am still unsure of the proportonal band width setting - should i change this from 1.5 as i am using the cycle 3 / 4 min on settings?

sorry for the ignorance!

 

[MJN]

There's no need for apologies - this is a common subject of confusion and debate, not helped for the fact that whilst the principle of TPI controls are well known and documented the same cannot be said for manufacturer's actual implementations. We are often left with little choice but to work out how these stats *actually* function from empirical experimentation and observation. Fortunately, the Honeywells are so common that they are fairly well understood.

The proportional bandwidth setting dictates the size of the 'window' within which the proportional behaviour (i.e. firing periods being adjusted within a duty cycle) occurs. Outside of this window it functions as always on or always off just like any other stat.

To illustrate, if your target temperature is 21C and the proportional bandwidth setting set to 1.5C then the proportional window is therefore 20.25C to 21.75C (or is it 19.5C to 22.5C? D_Hailsham needs to mark this homework - he seems to know these stats inside out!).

Let's say that the house is at 16C then, being below the proportional window, the stat would request the boiler to permantly fire (the boiler might itself cycle, but it's not the stat doing that) i.e. the stat wouldn't do anything fancy and would just hold its relay contacts permanently closed.

Only when the house temperature reached the lower end of the proportional window (20.25C, or 19.5C if my window assumption above isn't correct) does the stat then go into proportional mode. If it determines that we only need 50% of the boiler's heat output to reach/maintain the set temperature then it'll fire the boiler for 50% of the duty cycle (e.g. 10 mins out of every 20 if set to 3 cycles/hour). Ideally it would just tell the boiler 'I need 50% output please' but most boilers don't allow such control so it has to achieve a similar result by cycling on/off/on/off/... and adjusting the relative on/off period lengths instead.

As the temperature increases even more towards our target it'll adjust (reduce) the duty cycle further e.g. for a desired 20% output it'll fire for 4 minutes out of every 20.

When the target temperature is reached then, asssuming there is still loss from the house, the stat will still be required to call for heat occasionally. To put it another way, there is constant heat loss and so there needs to be constant heat input too. These are the repeated firings you are seeing whilst apparently 'up to temperature'. As discussed, it might not be all that mechanically sympathetic but it is highly effective at maintaining a stable temperature and avoiding under/overshoot.

Incidentally, there is a minimum on/off time setting in the stat - this will dictate what the lowest duty cycle rating will ever be. For example, if it calculates that a 10% boiler output is required yet the minimum on/off time is set to 3 minutes then it won't make any call for heat at all because 10% of our 20 minute cycle is 2 minutes (i.e. below the acceptable minimum). Some have increased there minimum on/off time further through the use of external time-delay relays as mentioned.

If the stat does happen to overshoot the target temperature depsite its best endeavours, and the overshoot exceeds the upper limit of the proportional window, then it won't call for heat at all until the room temperature falls back to within (or below) the window again.

If you want to reduce the apparent number of firings, and are willing to accept a slightly coarser (but still very capable) temperature control ability then reduce the cycle rate to the minimum (3 times/hr) and increase the min on/off to max (5 mins?). I don't see benefit in changing the proportional bandwidth window size in this scenario because the more you reduce it the more you reduce the opportunity to benefit from TPI control.

Mathew

 

[vicvapour]

On category 2 of the system setting, would it help if the minimum time on ~(1:Ot) was set to 5 minutes, and Proportional bandwith was changed from the default 1.5 to 3.0 degress celcius, this may even out the peaks and troughs of your boiler cycling.

 

[D_Hailsham]

The problem is that few modulate down low enough to avoid having to cycle when the stat is still calling for heat.

Some manufacturers, e.g Baxi, are at last getting the message and producing boilers which are more suitable to modern, highly insulated, houses and modulate down to 2 or 3 kW.

Geminox have had boilers which modulate down to 1 or 2kW for many years. But they are beyond most customer's budgets.

 

[MJN]

[Ref Vicvapour's post] We may have posted at the same time (and I did add a bit to mine after posting) but, yes, I think it would help increasing the minimum on/off time as you say. I don't believe it's worth reducing the window size however if considering the stability trade-off that then has to be made.

Mathew

 

[MJN]

D_Hailsham, it's about time too. As you say, it seems to have been a long time coming that this capability be provided. I can only assume there are difficulties with supporting such a large output range e.g. flame stability at the lower end vs sufficient heat transfer at the upper?

Mathew

 

[mysteryman]

All of the above makes an eloquent case for weather compensation, which feeds in just the right amount of heat to match losses. The best boilers have an anti-cycle function as well.

 

[D_Hailsham]

To illustrate, if your target temperature is 21C and the proportional bandwidth setting set to 1.5C then the proportional window is therefore 20.25C to 21.75C (or is it 19.5C to 22.5C? D_Hailsham needs to mark this homework - he seems to know these stats inside out!).

According to Honeywell's explanation of TPI, if the PB is 1.5C and the required temperature is 21C, then the band is from 19.5C to 21C. There's no point having the band going above the set point!