To hot - Boiler Control and Thermostat CG8

AlwaysLearning2024 · October 31

Hi everyone,

Trying to wrap my head around a “too-hot” condition in my 4-family apartment building.

Current Installation:

WM CH8 series 11 (202,000 output)
2-Pipe reverse return hydronic system with fin-tubed recessed heat emitters.

The previous boiler control (which I do not remember, yet used small mechanical inserts that set times, and a knob to set the temp — very easy), was replaced with a Tekmar 256 with outdoor reset and pipe-probe.

I’ve had too-hot issues since the 256 was installed.

256 parameters are:

Outdoor design is set at 10 degrees (Cincinnati, OH)
Min 140 / max 180.
Design 68 degrees

With these settings, I’m still nearly 78 degrees in room.
There is no external room temp sensor.

Curious if anyone has some tips or a better solution.

Thank you!

Jamie Hall · October 31

Constant circulation in the radiation and primary/secondary piping?

If so, start playing with the outdoor reset curve on the Tekmar. Your problem is running the ater through the radiation at too high a temperature.

AlwaysLearning2024 · October 31

thank you @Jamie Hall . Yes, constant circulation. 1 zone.

I’ve been using the “automatic” boiler differential on the 256.

How would I start to adjust the outdoor reset curve on the 256v

EdTheHeaterMan · October 31

Here is the manual Tekmar 256. at the bottom of page 2 it reads "To make an adjustment to a setting in the control, press and hold simultaneously for 1 second, all 3 buttons. The display will then show the word ADJUST in the top right corner. Then select the desired item using the Item button. Finally, use the and / or button to make the adjustment."

EdTheHeaterMan · October 31

Check to see if the 256 is set for the type of Terminal Unit your system uses (Terminal Unit is their way of saying the tyoe of radiator you have).

Screenshot 2025-10-31 at 10.56.36 AM.png

You describe "recessed fin tube" as the radiators you have. In this case you set the Tekmar 256 to BOIL DSGN to 180° and the BOIL MIN to 140°. This appears to be where you are already set. Try to adjust those numbers to BOIL DSGN 172° and BOIL MIN to 140°. See if that reduces the room temperature to 70°. This will move the curve for the coldest day to be a little cooler. it may not adjust the top of the curve (the warmest days) enough to notice.

Next try moving the BOIL MIN to 135° and see if that helps. Always wait 24 hours between adjustments to see how each change makes the room temperatures to change.

Remember to record all your settings and then record the results after at least 24 hours of operation. It sometimes takes time for all the parameters to get your temperature just right. Eventually you will get a feel for that control and how it works in your building.

AlwaysLearning2024 · October 31

Thank you, @EdTheHeaterMan .

Yes, chose 4 as the heat emitter type.

I'll make the initial change of 180 to 172 and report back.

More soon!

EdTheHeaterMan · October 31

to be clear, the Tekmar 256 does not have a setting number four (4) for a heat emitter type. The chart gives you the starting point for the two settings. BOIL DSGN and BOIL MIN based on that type of emitter. Your tekmar 256 is already set that way. I am suggesting that you adjust BOIL DSGN from 180° to 172° for the first step in adjusting the reset curve.

109A_5 · November 1

The demand input may be there for a reason.

AlwaysLearning2024 · November 1

Thank you @EdTheHeaterMan

Seems there is a heat emitter type that corresponds with a number in the menu system on the 256. The choice of number "4" chooses the fin-tube variety.

Thank for @109A_5 — Seems that is connected properly and the boiler is firing.

Curious if using a 260 with a indoor room air sensor input could also help even out the heat? Or is this overkill?

109A_5 · November 1

Seems to me you no longer have the temperature set knob.

I would not consider my self a 'Outdoor Reset' expert or even a Tekmar 256 Expert.

However, I find it hard to believe there is a outdoor reset curve that would be an exact match for the buildings heat loss in every weather condition. The Tekmar 256 is only looking at outdoor temperature, not other conditions like Sun, wind speed and direction. I see the building's heat loss as variable to other conditions beyond just the outdoor temperature, outdoor temperature is the predominant weather condition.

The Tekmar 256 provides a demand input that you could connect a thermostat to. So in my opinion this input is to limit the heat demand, when needed. In my mind a thermostat would replace the knob on the old unit.

Although you did not say what temp knob on the old unit controlled, water temperature or room temperature.

mattmia2 · November 1

with a cast iron boiler you probably can not make the swt low enough to match the load of the building on mild or moderate days without the boiler condensing (unless you have a separate mixing system to make the system water cooler than the boiler).

you probably need a t-stat input to cycle it on warmer days.

EdTheHeaterMan · November 1

I am just wondering ig the old control was made by AMERICAN STABILIS? They has a device called the ENERTROL that I installed on several of my customers' boilers in the 1980s. There was a red box that contained all the ODR parts. You connect a boiler temperature sensor on the supply pipe and an outdoor sensor on the north side of the building. Two other wires were connected in series with the high limit to shut the burner down when the boiler temperature was hot enough to heat the building.

AlwaysLearning2024 · November 1

Thanks @EdTheHeaterMan and @mattmia2 .

Yes, cast iron boiler.

The old boiler control I remember has a small knob for temp, and some sort of pin timer circuit. Maybe white or beige. All analog it seemed.

EdTheHeaterMan · November 1

That sounds exactly like the ENERTROL. The pin timer was for a boiler with a tankless coil. You would set the SUM TEMP dial on the red box for the outdoor reset curve you wanted for you radiators. The pin timer was to disable the ODR when you wanted the boiler to be hot enough for taking a shower. This ment that if you took a shower when the timer was set for ODR to kick in, your may get a lukewarm shower.

Do you have a separate water heater now, or do you still have a tankless coil for DHW?

EdTheHeaterMan · 12:09AM

When using the Enertrol with or without the Pin Timer, you still used the existing room thermostat to control the room temperature in the homeowners living room. That thermostat was not removed when the Enertrol was added.

You have a one zone building with 4 units that overheat. The ODR from American Stablis or Tekmar did (does) not have any provision for room sensing. That is done with a standard room thermostat placed in one of the dwelling units to sample a representative room for the entire building. That thermostat is (was) subject to tenant tampering so you may want to use a remote sensor thermostat an locate the actual control in a locked room away from tenants.

That thermostat will connect to the system the same way it would if there was no ODR at all. The ODR just connects in series with the burner limit. The thermostat connects to the control the brings on the burner and the pump to heat the rooms. If that does not make sense… ask me to explain it in more detail.

To hot - Boiler Control and Thermostat CG8

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