tekmar 256 reset control question??

soot_seeker_3

i was hoping that 'hydronics mike' was on the wall today as i had a T-256 question. i called tekmar & they are off today (monday) for a holiday so i figured someone here might know the answer.

i'm about to install a tekmar 256 on an older system with separate line voltage (t-stat)controlled circ pumps (3) - i.e. no zone valves used. this control has a boiler demand loop that wants to see a dry contact closure. according to the application notes, in a newer system, they would want a 'parallel ladder' loop wired off all the zone valves dry contact terminals to allow any zone to indicate a 'boiler demand' to the t-256 control. i obviously don't have this (no zone valves) & would have to wire up 3 X 120v aux relays to provide this. that's the way app note note #2 is drawn. HOWEVER, in app note #1, they simply 'jumper-out' the boiler demand loop at the t-256 control terminal (heat demand always on), and don't bother with any zone valve feedback, and just run the control based on the indoor & outdoor sensor. so the the big question is : what's the real point of the boiler demand signal input?? do i need to use it in my setup? is there an advantage / disadvantage running this way? i've read all the pdfs available & there is no discussion as to why you would wire this up or not - other than to see the 'boiler demand' message 'light-up' on the LCD screen...

anybody know the answer before i call them tomorrow?

ss

[Deleted User]

That is to keep

from maintaining temp in the boiler if none of the zones are calling for heat. If you want to use the feature, in your particular case, install a flow switch to detect flow to the boiler. Wire the switch to that circuit. Should save a few fuel dollars.

If you don't, whenever the O/D temp is below the WWSD, the boiler will maintain the temp required by the reset curve.

Wayco Wayne_2

Hey!

I don't know but would like to know. I've installed 2 of them in the last week and installed them so the Tek mar relay energizes with the boiler relay, just in case it's important. I'll be watching the answers. WW

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Einsiedler_2

demand

if you want the boiler to idle & maintain setpoint at all times, then jump it & leave t256 powered up at all times.

If you want the boiler off when no call for heat (cold starts), then use 120V SPDT relay for each zone. use leg 1 to power pump and leg#2 as dry contact for boiler demand.

you must determine which strategy works best for your system. weigh out cold starts vs. boiler always on (or about to come on)
consider expected calls for heat; lots & lots of little zones (big chance for lots of cycles on demand system) or just 1 or 2 large zones (might be redundant to keep boiler up). then again terminal mass s/b considered (c.i. rads vs lower mass)

is this a single temp system? or is there also mixing involved?

lots to consider.

EIN

soot_seeker_2

interesting answers....

ok, interesting answers - but they generate a few more questions.

if you want 'cold start' (which i don't want on a 1980 JOT -w/ an afg - no mixing) why not just set the 'boiler min' value to off? (this setting replaces the value at the low end of the terminal curve). then you wouldn't have to do any wiring to that demand loop - just jumper it out. basically the way i see it is that when the demand loop is open, the control will never turn on the burner. i can see that this would be useful in a priority setup where the boiler t-256 was installed first & then months later you had to tie in, say, an independent oil fired HW tank and you were worried about the 2 burners on the same oil line or in the case where you were firing 2 boilers in parallel or series and you wanted to stage them properly (the boiler demand terminal would suddenly become important since you gave up the use of the TT terminals on the burner for the t-256's control.) other than that, the wiring shown in the application notes #2 thru #4 basically only allows the t-256 to fire the burner when there is a demand for heat and then have no control otherwise. like i said, this seems to be the same thing as jumpering-out the loop & setting the 'boiler minimum' to off - maybe there's another reason - i just couldn't think of it. as far as the WWSD operation on this control, the only thing i can gather from literature is that as long as the demand loop is jumpered, the t-256 will keep the boiler at minimum temp (say 130-140deg) (thru-out the late spring/summer/early fall, etc). that's basically the setting that i need to avoid flue condensation in a basement - old chimney situation anyway - so i would not want this loop open or else the control would allow it to go down to 'cold' room temp. as far as connecting the demand loop to the 'boiler relay' (as mentioned above), i'm not seeing that either. if by 'boiler relay' you mean off the aquastat, then basically if the boiler temp is below the hi-limit and (minus) the differential (as it should be if the t256 is doing it's job) then the aquastat demand/call would be 'on' most of the time - this however is basically ignored since the control is firing the burner according to the indoor & outdoor sensors & radiator (terminal) curve. on a cold day, and at the point where the boiler temp does happen to come up to satisfy the aquastat, then the boiler relay will shut off / break the demand loop & possiby prevent the control from reaching the boiler max temp which is 10deg higher than the top radiator (terminal) curve temp. there's no point in doing that anyway since the hi-limit (if set at say 180F) would open anyway preventing the control from reaching max temp. .. sorta redundant unless i'm missing something.

ss

hydronicsmike

Sorry, was out of town...

Gosh, the one time I am not around, somone is actually looking for me

Anyways, you've got some good answers. As I see it, you have a couple of line voltage thermostats cycling the zone pump on and off, which I am assuming are also the only pumps responsible for flow through the boiler. In other words, you dont have a seperate boiler pump. In this case, I like what Ron recommended. Use a flow switch to enable the demand.

A jumper could be applied across the heat demand terminals (7 and 9) and the control will always maintain a reset water temperature unless WWSD. HOWEVER, this is only recommeneded if there was continuous flow past the boiler sensor at all times, OR ELSE the boiler may SHORT CYCLE.

I am very sorry to have missed you yesterday, but am available from now on every day (no vacation coming up soon )
Call me at your convenience at 250-545-7749, Ext. 214.

Happy New Year to you.

Mike

soot_seeker_2

i called tekmar this morning..... another idea ..

i called tekmar this morning. i couldn't reach mike as he was busy on the line but i spoke to 'paul' (excellent rep). after explaining my situation (above) and expressing my concerns about cold starting a 25 yr old (soot prone) JOT, we agreed it might be best (and easiest) to just jumper the demand loop terminal and let the t256 continuously govern the reset temp all the time (not just during demand). [so you guys had the right ideas (above) as to how the demand loop worked - thank you.] the rep explained that tekmar is very 'cold-start' oriented & that is why you'll basically see many of their application drawings designed according to that point of view. the rep indicated that tekmar's research has shown that cold-start is really the way to go (with few if any historical problems) and that those less inclined to go that route may not be giving the boilers enough credit to hold up under that operation. certainly, the fuel savings are far greater - so it's always something to consider.

with regard to the demand wiring, the only big difference between the two setups is that the t256 control is cut out during periods of non-demand (allowing possible cold-start) as opposed to maintaining a minimum reset temp. naturally, i like the idea of the greater fuel savings associated with cold-starting and it would probably be fine for the cold months where the boiler temp would rarely get a chance to really drop that low as to become a sooting problem - but my real concern was for the fall and late spring periods where the cycles would be less & the boiler temp might actually go cold for a period of time between demands ( trust me, my boiler doesn't like that & eventually throws a soot fit). i thought that this might be solved by jumpering the demand loop and lowering the boiler temp minimum (allowing a lower idle temp) - but that wouldn't really work since it would also alter the low end of the terminal curve and throw-off the target point for the radiators during real cold snaps.

so here's the next question. if you have a situation (like above) where you would want the idle (no demand) boiler temp to be lower than the terminal curve minimum but you DON'T want it to go completely cold, why not just jumper-out the demand loop (t256 'always on') *AND* wire the heat demand signal from the zone valves/circ pumps/flow valves etc.(inverted thru a relay) to the 'unocc' loop instead and adjust the unocc value such that it shifts the curve during 'no demand' (closed unocc loop) to give you a lower idle temp - but will never let the boiler go too low since the t256 is always 'on' and then flip back (open loop @ unocc) to the OCC (value) setting which would put you back on the regular terminal curve during a demand?

any problems with this idea that you can see?

ss