Tekmar 256 setup


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Old 05-23-11, 07:16 AM
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Tekmar 256 setup

Setting up a Tekmar 256 with a Burnham RSA 125 oil steel boiler in an old gravity system with cast iron radiators. Circulator runs off of a relay.
Do I need a minimum temp?
Flue gas condensation issues with a steel boiler?
What are optimal min and max temps? I have it at 120/170 now.
Is there a way to wire it to keep the circulator from running when Tekmar goes into WWSD and the thermostat has a call for heat?
Thanks for the help (and thanks for the help in prior posts on the layout of this system; it works great)!
 

Last edited by seaworthy41; 05-23-11 at 07:44 AM.
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Old 05-23-11, 03:35 PM
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Do I need a minimum temp?
Flue gas condensation issues with a steel boiler?
Yes and yes. The type of boiler has nada to do with condensation, but in fact a steel boiler might be more prone. This would be because of the temperature gradient that occurs across the steel, between the fire and water. The thinner steel would probably be more apt to be cooled more by masses of cool water than a thicker cast iron structure.

Remember that flue gas condensation (FGC) is the exact same phenom as dew on the grass in the morning, except the grass dew is less acidic (hopefully!).

If you do a little googling and understand how and why dew forms, the same principles apply to FGC, the temperature is generally higher though...

So, bottom line is that the 'dewpoint' of flue gas from an oil fired system is somewhere around 120F. I say 'somewhere around' because if you research it on the web, you will find numbers in that vicinity.

Any time that your flue gases contact a surface that is cooler than that dewpoint, the water vapor will condense to water and take with it all the associated acids that are in that gas stream.

Understand that the water RETURNING to your system is going to be at least 20F COOLER than the SUPPLY water. If you set your BOIL MIN at 120, you almost guarantee that the water coming back will be around 100 or so... maybe less. This cool water entering your boiler will cool the steel surfaces on the fire side to less than the dewpoint and condensation will occur.

If this condensation is allowed to exist for a long period of time and NOT DRY during the burner cycle, the obvious will happen... corrosion... FAIL.

The important point is that you have to make sure that the return water to the boiler gets up to temperature fairly quickly, and that the burner cycles are long enough to DRY the inside of the boiler and chimney before shutting down again.

Is there a way to wire it to keep the circulator from running when Tekmar goes into WWSD and the thermostat has a call for heat?
I'm sure there is. It might require some different controls and/or wiring.
 
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Old 05-23-11, 08:24 PM
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Thanks for the info Trooper. Does a boiler bypass change the minimum temp setting?
 
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Old 05-24-11, 02:35 PM
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Does a boiler bypass change the minimum temp setting?
Not much if at all... what a boiler bypass does is protect the boiler by increasing the AVERAGE temp in the boiler. It does this by limiting the flow through the boiler. There won't be as much of the cool water coming back to the boiler, and the supply out will get hotter, faster.

A boiler bypass isn't magic... it won't really let you run the boiler cooler, it only provides the protection from condensation.

The biggest problem for a boiler is LARGE VOLUMES (which your system seems to have, a converted gravity system with CI rads, right?) of cool water dumping into the boiler for a LONG TIME (which could be the case with your system without the bypass).

My recommendation would be to try 140 as the BOIL MIN and see how that goes.

As for shutting down the pump, please describe again how your system is wired again, maybe we can provide some suggestions.

Isn't your pump wired to the 260?
 

Last edited by NJT; 05-24-11 at 04:10 PM.
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Old 05-24-11, 08:22 PM
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The tekmar is a 256 not a 260 so it doesn't have pump control. I have the thermostat on a call for heat going to a DPDT relay, which powers up the circulator, and which on a low voltage NO side, gives a demand for heat to the Tekmar control. The triple control is now just a high limit. So on a call for heat, the relay gets activated and the circulator comes on, but even though there is a call for heat from the relay to the Tekmar control, the WWSD setting (currently at 70) keeps the boiler off. My concern is that I get circulated water, and electric usage, when the boiler is not working anyway.

What is a good max temp with CI radiators. I saw a few prior posts aiming for about 170.

If I am setting my delta T at 40 degrees for the bypass, does that mean that I need a minimum setting of 160? It would seem to make the outdoor reset a waste as I am back around the numbers for the triple control.
 
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Old 05-25-11, 04:38 AM
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Your max temp is determined by the heat loss of your home and the sq ft of edr your CI radiators have.
 
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Old 05-25-11, 02:54 PM
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I have the thermostat on a call for heat going to a DPDT relay, which powers up the circulator, and which on a low voltage NO side, gives a demand for heat to the Tekmar control.
One zone? you said 'the thermostat' which leads me to believe there is only one.

If that's the case, why can't the thermostat go directly to the 256, and the output of the 256 fire the relay, and the low voltage contacts of the relay fire the boiler?

Then, I wonder why you have or if you even need the relay.

Isn't there a C1 and C2 terminal on the boiler aquastat? Can't you just get rid of the relay, wire the circ to the aquastat, thermostat to the 256 and 256 to the TT on the boiler?
 
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Old 05-25-11, 03:01 PM
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If I am setting my delta T at 40 degrees for the bypass, does that mean that I need a minimum setting of 160?
No. Set the BOIL MIN to 140, BOIL DESIGN to say 170, and BOIL MAX to 180.

Remember that the delta will only be 40 when the boiler is at it's hottest. It will decrease at lower temps.

Boiler reset temp is always going to be limited on a standard non-condensing boiler, there's no way around that.
 
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Old 05-25-11, 08:23 PM
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The boiler has one zone.

The thermostat can go directly to the 256, but the output of the 256 is powered only when the sensor on the supply side indicates that the output of the boiler is not the required temp. It is only powered part of the time. The system needs circulation much more of the time than just when the control is calling for a higher temp.

I have the same problem if I run the circulator off of C1/C2 of the triple control. Since the TT contacts are powered by the 256 only when the boiler output temp is too low, I do not get circulation when it is needed.

I need circulation whenever there is a call for heat. The contol will only give me an output circuit when the temp is too low in the boiler.

A 260 is much more robust and has options that are not present on the 256. But the cost of the 256 is a much more reasonable proposition.

I may be stuck with occasional circulation when the control WWSD kicks in. I dont see another option.
 
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Old 05-26-11, 02:30 PM
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OK, I gottit now... sometimes I'm a bit slow on the uptake!

There's almost always a way, but the cost/complexity might not be worth it.

If the 256 had an output that indicated when the unit was in WWSD, then it would be easy... the more I think about it, the more I realize it might not be possible!

As you say, it might be easy enough to just accept the fact that there are a few times during the year when the circ will run for no reason. I guess you could always crank the t'stat down a notch or two...
 
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Old 05-27-11, 01:00 PM
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Another way to resolve the circ issue is to up the WWSD temp a couple of degrees more than the thermostat setting (go to 72 or so). Probably not a real big deal either way.
So I am going to go 140/170. I have strap on thermometers to set up the delta t, so I could set up supply at 140 and see what I have for return, and try to dial in a lower supply temp while staying over 115 or so on the return. Maybe the risk of condensation from dialing it in tight is not worth the savings on the few degrees lowered min. temp. Maybe I can't help myself but try to squeeze out the last little bit left on the job!
Thanks for the help!
 
 

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