hydronics mike - tekmar 256 diff issue -

soot_seeker_3

hi mike,

i have a tekmar 256 (manuf 10/04 - firmware 2.1?). most settings are on default w/ terminal set to '4' conv tube - min set 145f. the automatic differential setting seemed to cause short cycling w/ my old cast iron bolier so i decided to try a manual differential setting. the data sheet (and the control) says you can select up to a 42f diff. i set it to 24f (and later 26f) for a 12f swing above & below target - yet the control will only go +/- 10f no matter what (it acts like i've set it 20f). is there some other unmentioned over-riding setting that is stopping it?

thanx,

ss

hydronicsmike

what is going on with...

...the heat/Boiler Demand at the time? Is it permanent?

Please call me to sort out 250-545-7749, ext. 214

Looking forward to speaking with you.

Mike

soot_seeker_3

more bizzare....

hi mike,

i'm runnin'around here due to deadline so i haven't had a chance to call - but yes there is 1 zone that tends to stay on much of the time when the outside temp falls below 37f. i just went down & checked the 256 an hour ago & the situation is stranger now. i have the diff set for 24f (also tested at 22f) & i watched it for about 2 cycles. the first time around the 256 forced a 32f diff (16f up & down). the 2nd time it did a 26f diff. (13f up / down). last night when the target temp was 160f it forced the 20f diff & today when i checked it the target was around 145-146f. i've noticed over the past couple months that when the diff was set to AD (auto) that the 256 seemed to stay at a ~12f no matter what the target - this was causing a slight short cycling since the tt was closing on 6f up / down. so now i figured i'd try the manual setting but it's almost like the manual setting is allowing the AD setting to work & the AD setting just keeps it fixed at 12f.

????

ss

hydronicsmike

Interesting, but doesn't make much sense

I am a little at a loss here and would like to look further into it. The only time I cn see the differential overriden, is when the Demand goes away, or if the Boilers minimum On-time is not met with the differential specified. How long are the cycles? Could you pelase provide concrete feedback on the on-time? The next time the boiler fires, please record that a Heat Demand is present and the burner symbol is displayed in the screen of the Control(must be, or Boiler wouldn't fire and if it does, it wouldn't be because of the Control - IE: Boiler T-T is energized by something else), the Target temperature and the Temperature the Boiler Relay (Symbol in screen) comes on and when it turns off,a s well as running time in seconds.

I have tried to simulate what you are experiencing on a work bench and was unable to duplicate what you are seeing. On the bench, the Control maintains the differential as specified, if the Controls minimum on-time is satisfied.

A call will go a long way. At your convenience, any time is good for me.

Regards,

Mike

soot_seeker_3

will attempt to document -

hi mike,

i will try to document it best way possible. one thing seems for sure - it is not (immediately) paying attention to my diff setting - maybe it takes some time to kick in? does your control immediately respond to a new diff - even if in the middle of a cycle? i put a datalogger on there yesterday & will see what i get over the next 24 hrs - sampling at every 10secs. last night i set the t256 for a 30f diff & 150f boiler min. exited the set-up & shut-off the system for 10 secs. powered up, went in & checked to see that the settings were retained (they were). t256 calculated a 153f target & brought the supply temp up to 164f & stopped (burner icon off) (almost like it was now seeing the 22f diff that i had programmed 3 settings ago). so i got 11f up instead of 15f. didn't have time to watch a downward run but will attempt to observe again today.

ss

hydronicsmike

Soot

hydronicsmike

Soot

Got more info now? Please call me 250-545-7749, Ext. 214

Mike

soot_seeker_2

yup, more info -

hi mike,

again, i was not able to call today due to scheduling & snow storm on long island but i wanted to give you this info for the moment.

i'm still working on wiring the demand signal into one of the datalogger channels so that i can can give you a more concrete picture of what's happening here. but i did spend some time observing the behavior of the t256 over the past couple days and i think i'm starting to see part of what the control is doing - whether it's correct or not i don't know.

it would 'appear' that the manual fixed differential setting is over-ridden by a fast falling supply temp. in other words, with the diff set to 30 deg F, you would expect a 15 deg F up & down about the target temp. i *have* seen it do this at least once in the past few days but more often, when a demand comes on and includes a major zone that has been idle for a while and as such the supply temp starts falling *fast*, the t256 will 'put the brakes on early' and rather than wait for the full 15F drop, it may come on at say 8F down depending on how fast the supply temp is falling - i have witnessed this a few times now. sometimes it picks 8F, sometimes 11F, etc - just depends on how fast the supply temp is falling. in the case where a minor or light zone is running and the supply temp is falling slowly, the t256 seems to attempt to wait for the full manual differential. i don't see this mentioned in the manual but that is what it seems to be doing. maybe you can confirm this with the programmers.

that behavior on the down side portion of the cycle doesn't bother me since i gather it's essentially trying to predict & save the boiler from thermal shock (although the aquastat's circ pump low temp safety would normally kick-out anyway) but the problematic other side of the coin here is that when the supply temperature starts on the upward portion of cycle, the t256 seems to limit the upside differential as well - meaning that instead of going up to the target plus 15F, it appears to cut-off a bit short of the 'fixed' 15f degrees (say 10 or 11F). now, i don't know whether the programmer has the unit watching the speed of the supply temp going up and is trying to avoid overshoot or if the unit makes note of the fact that the supply temp still rises a few degrees after it shuts off due to the 2min post purge - but it seems that if the 'fixed' differential was altered during the down-cycle, then it also gets altered for the up-cycle. obviously i would prefer that it would just go the whole differential up since that extends the total boiler cycle time & it really has nothing to lose by doing so (remember i'm trying to use the fixed diff to extend the cycles since the auto setting seemed to allow some short cycling).

to make things stranger (& blow part of my theory away), i glanced at it once this afternoon during an up-cycle & even though the target was set at 153F (and the diff was still set at 30F), the control ran it up to 170-171F before shutting off (that's 18-19f up & the target was still the same at the end of the cycle). that's the first time i saw it do that so i'm still at a bit of a loss here.

by the way, i got a chance to look at the date code for the unit - it says oct 2004 and then there is a 34 0589 - i'm not sure if that is a revision indication or a serial number but maybe that will help you identity something.

until i get further here, maybe someone there can tell you if the t256 uses some sort of 'anticipation' algorithm that over-rides a manual fixed diff. i've tried cycling power and a few other other things including switching between adv & installer mode - nothing seems to make a difference. i also noticed that the manual indicates that the target temp display is available in the installer mode but on my unit i can only see it when in advanced mode so i've left the switch there (is this just a typo?).

thanx,

ss

hydronicsmike

Soot...

...TRG is only available in Advanced Access Level and not Intsaller. This is a typo as you suggested.

The numbers you recorded are Date Code and Serial Number.

Anyways, we could go on and on and contemplate as to what may be going on. Fact is, you need to call me if you want a solid answer. There are just too many variables and questions I would like to ask. You have already done an incredible job logging the data and posted it here. This will alos be very helpful in finding out what is going on. But you can most likely appreciate that as one question is answered, another may come up.

Lets figure this out. You can call me whenever is convenient for you. 250-545-7749, Ext. 214. Looking forward to talking with you.

Regards,

Mike

Mike

soot_seeker_2

tried to call - ext busy - will try again -

tried to call - ext busy - will try again -

soot_seeker_2

thanx for the phone call!

mike,

thanx for the discussion this evening. i'm duly impressed with your integrity & the tenacity that you showed while trying to get to the bottom of all this - no matter how 'abrasive' some might think an approach may be.

i think for the time being, we can sum all this up in the wise words of the old carly simon tune:

Anticipation, Anticipation
Is making me late
Is keeping me waiting...........

remember, it's a feature going down, but it's bug going up.

later gator,

soot_seeker