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Hold-off control for Burnham 206 gas boiler?

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jesmed1
jesmed1 Posts: 1,605
in Gas Heating

Trying to improve cycle efficiency of sister's Burnham 206 gas boiler. The boiler is properly sized for heat loss, but the house is over-zoned, resulting in short-cycling. I've done the following to reduce short-cycling:

  1. Installed new thermostats with adjustable differential/swing, and set differentials higher.
  2. Disconnected some thermostats at the zone controller and jumpered the disconnected zone terminals to be controlled by one thermostat centrally located in those zones.
  3. Turned the aquastat high limit down to 160. We will monitor during the winter and adjust if needed.

I've checked the supply water temp during boiler runs to verify the aquastat make/break temps, and it does break at 160 and make after SWT drops to 125. So the built-in differential is 35.

I would like to improve on that by effectively increasing the differential so that SWT has to drop lower, say to 100, before the boiler can fire again. So on a call for heat, the zone controller would energize the circs, but the boiler would not fire until the SWT has dropped to 100. This reduces average water temperature and increases boiler run time, both of which should improve cycle efficiency.

What controls are available to accomplish this?

Comments

  • Steamhead
    Steamhead Posts: 18,933

    @jesmed1 you're looking at either a HydroStat or some sort of outdoor-reset control. What aquastat is on the boiler and where is it mounted? What zone controller does she have?

    Co-Founder, All Steamed Up, Inc.

    Semi-retired

    Baltimore, MD, USA

  • jesmed1
    jesmed1 Posts: 1,605

    @Steamhead The aquastat has no cover, no label, no markings. I assume it's whatever Burnham puts in their Series 2 boilers at the factory. It's located in the upper left corner on the front of the boiler, behind the cover. Zone controller is a Taco SR506.

    IMG_20261008_200618611.jpg
  • 109A_5
    109A_5 Posts: 4,549

    You don't want the boiler temperature too low it will cause condensation. You may be better off coming up with a control strategy that waits for when more than one zone needs heat before the boiler is activated, although comfort may suffer.

    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
    hot_rodjesmed1
  • Steamhead
    Steamhead Posts: 18,933
    edited 7:28PM

    @jesmed1 with that setup I'd use a reset control such as a Tekmar 256. This control adjusts the boiler water temperature according to the outdoor temperature. If the boiler has its own circulator, I'd wire the 256's boiler contacts in series with the boiler's aquastat, so the circulator would continue to run even when the 256 shuts the burner down.

    The 256 has a minimum boiler temp setting which can be set so the boiler doesn't cool down enough to cause condensation.

    You didn't mention if her 206 has a stack damper. If not, I'd add one, so the boiler would not cool down so much between burner firings.

    Here's the manual:

    https://www.watts.com/dfsmedia/0533dbba17714b1ab581ab07a4cbb521/81348-source/638820695720000000/256-d-09.pdf

    Co-Founder, All Steamed Up, Inc.

    Semi-retired

    Baltimore, MD, USA

    PC7060jesmed1
  • jesmed1
    jesmed1 Posts: 1,605
    edited 12:35PM

    @Steamhead Thanks, that sounds good. I've already installed a new vent damper to replace the one that came with the boiler but at some point had been removed.

    I understand the condensation concern and will try to mitigate that by programming the reset so the boiler always finishes above condensing temps. Because of low water volume (and mostly baseboard rads), the boiler heats up quickly, so regardless of starting water temp, it will finish at whatever the reset target temp is. So I would program reset target at 140 minimum and 160 maximum. The largest differential I could program into the 256 would be 42 F, so (for example), at the lowest point on the reset curve, the boiler would start at 98 F and finish at 140 F. At the highest point on the reset curve, the boiler would start at 118 F and finish at 160 F.

    But if that still isn't adequate protection against condensing, I could use the minimum temp setting as you say and set the minimum to 130.

  • pecmsg
    pecmsg Posts: 7,578

    Limit the number of micro zones or install a Buffer Tank or both.

  • jesmed1
    jesmed1 Posts: 1,605
    edited 9:14PM

    @pecmsg Yes, thanks, I've combined some zones and have also discussed installing a buffer tank. We decided not to go with the buffer tank for now because she will probably replace this boiler in a few years with a mod-con, and if she still needs a buffer tank with the mod-con, it can be a much smaller one.

    I picked up the Tekmar 256 this afternoon and will install it tomorrow.

  • Steamhead
    Steamhead Posts: 18,933
    https://forum.heatinghelp.com/discussion/comment/1905927#Comment_1905927

    You should also get some shielded 2-wire cable to run to the outdoor sensor, if you haven't already. This is mentioned in the manual.

    Co-Founder, All Steamed Up, Inc.

    Semi-retired

    Baltimore, MD, USA

    pecmsg
  • pecmsg
    pecmsg Posts: 7,578
    https://forum.heatinghelp.com/discussion/comment/1905927#Comment_1905927

    a buffer tank is relatively cheap.
    it can still be used with a Mon con

  • pecmsg
    pecmsg Posts: 7,578
    edited 10:29PM

    you are aware mod cons are only highly efficient when supplying below 140° water.

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