Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

2 boiler system EKG, be nice!

baj702 Member Posts: 44

I've never seen (irl) another boiler before. No one really talks about what they look like when they are operating, so I have no idea where any of this stands: is it normal?. I took readings every 60 seconds for two different cycles. I would really like to get some data somewhere around when it's 10F-20F outside and the heating cycle is longer.

This is a two boiler system, one is 300k BTU and the other is 275k BTU. I use the Tekmar 279 with outdoor reset. One graph corresponds to when it was 37F degrees and the other corresponds to 44F degrees.

The temp probe for the condensate temp is at the end of the main, after the F&T trap, and just before it dumps into the condensate tank.

There's a 0-3psi gauge on Boiler 1 and a 0-3psi gauge on the equalizer. They match pretty closly, but the pressure gauge on Boiler 1 is a little more jittery.

About 75% of the main is insulated. The other 25% is really hard to get to.

From the graphs:
1. It takes about 8 min for the condensate temp to start rising. It takes about another 8 min for the condensate temp to reach 175F degrees at which time the heating cycle begins. Pressure just barely starts to register.

2. Once the pressure hits 1.5lbs, boiler 2 will shut off. Boiler 1 doesn't have enough juice to continue raising the pressure, but it does slow down the descent.

3. With Boiler 2 off, the pressure begins to fall. When it hits .75lbs boiler 2 will turn back on and the pressure will rise.

The graph when the outside temp is 44 deg. It's a little warmer outside, so:
1. Steam is established a little faster
2. Pressure builds faster
3. The one boiler is more effective at slowing down the pressure drop when the other boiler burner is shut off. But it still can't build pressure by itself.


  • baj702
    baj702 Member Posts: 44
    I should add that what really makes the pressure drop when one boiler is shut off is the condensate being pumped into the boilers. The condensate is dumped into a pretty big un-insulated tank, and adding that colder water causes the pressure to drop. I think on longer cycles, where that water has all been used and so it is much hotter, that Boiler 1 is capable of maintaining the pressure and even continue to raise it so that both boilers will eventually shut off at around 1.75 psi.
    JUGHNE Member Posts: 11,021
    Does each boiler have it's own feeder pump control?
    And is there a solenoid valve on each boiler that opens when it's control calls for water?

    If you can throttle down the pump delivery it may not kill the steam pressure so drastically.
  • baj702
    baj702 Member Posts: 44
    @JUGHNE There is one control for the whole system as the boilers are interconnected, i.e. same water line for both boilers. But, that's a good idea. The pump is pretty quick at pumping - about a second per pump burst, and about 2 or 3 burst to get the water level back up - and there is a gate valve to throttle it. But if I close the gate valve some, would that put more strain on the pump e.g. it not as easy to pump?
  • Jamie Hall
    Jamie Hall Member Posts: 22,908
    Oh dear A gate valve. Well, it would be ideal if you could throttle that feed rate down with a valve -- it won't hurt the pump a bit to have more head loss to deal with -- but a gate valve is NOT the way to do it. They are intended to be either full on -- or full off.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • baj702
    baj702 Member Posts: 44
    @Jamie Hall What type of valve should I get?
  • SuperTech
    SuperTech Member Posts: 2,120
    A globe valve would be much better.