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Return Temperature To Low on radiant heating using McLain Weil boiler

I have a McLain Weil Boiler GV-6 series 175000BTU.
That boiler heats the house that is divided in 5 zones, and a 45 gal indirect transfer water heater.
Usually it works fine, but on the past days, with outside temp in the 10-20F, the house is quite cold, about 60F.
All thermostats are set to 70F and are pulling the 24V lines (requesting heat all time). The control line from the control panels to the boiler is pulled to 0V (request heat).
The boiler cycles with about 150 seconds ON and 120 seconds OFF, a cycle of about 4.5 minutes.
The baseboard radiators have a temperature about 100F, rather than the expected 150F.
I tried bleeding the air from from the loops, but that did not help.

I measured the return temperature (after the combined 5 loops) and that is about 105F. The output of the boiler is 180-200F (200F is reached when the boiler turns ON).
Just after the connection between the boiler loop and the radiator loops, the water is about 120F.
See picture below with the pipes and temperatures on them.

I thought that the circulator inside the boiler is not working, but I hear its sound.
I have 5 zones, one water heater, all using Taco Model 007-F5 circulator pumps.
The water heater uses same Taco circulator.

I would appreciate any suggestions about what may cause this situation, and how to verify and repair the system?



  • neilc
    neilc Member Posts: 1,804
    what about that circulator outside the boiler?
    is it working also?
    post a better picture there, floor to ceiling, showing that circ.
    known to beat dead horses
  • lchmb
    lchmb Member Posts: 2,997
    if that's the series 1 or 2 I would suspect the mixing valve..
  • Gilmorrie
    Gilmorrie Member Posts: 160
    I think you are over-pumping - with five zones, each with a Taco 007. When they all call for heat - wow. At least, you could benefit from a boiler bypass - or fewer zones or with five zones, but with zone valves.
  • Gordy
    Gordy Member Posts: 9,514
    Looks to me like a flow issue on the primary (boiler) loop. Circulator, or mixing valve is suspect.
  • hot_rod
    hot_rod Member Posts: 16,285
    I believe it is a Hoffman brand mix valve on that series, not easily disassembled, but I would check there is all the pumps are working and the system worked in the past

    Remember the heat emitters dictat the boiler operating condition. How do the heat emitters match up to the boiler output .
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • dcalbaza
    dcalbaza Member Posts: 3
    Thank you all for the replies. It was indeed a mixing valve in the boiler that got stuck open. WM has a technical buletin SB0401R1 about how to fix this.