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Low mass boiler and cool return temps

Dave M
Dave M Member Posts: 36
Small Peerless Pro water boiler in an older house with a fairly large amount of water in the radiation system. It's a mix of fin tube and cast iron radiators, three zones. The boiler is low mass, less than 4 gallons of water. If the boiler is in standby and near full temp and the downstairs zone calls for heat the boiler temp will drop for 5 or 10 minutes until it starts to rise again. The cold water returning pushes it down for several minutes. It can take 10 or 15 minutes for the water temp to get back above 160. High limit is 180. I watch the digital temp readout on the L7224U aquastat. 

There is a bypass pump (Taco 007) direct from the boiler supply to the return. A sensor in the supply turns this bypass pump on when the return temp is below 130. It doesn't turn off the pumps to the zones though. Even with this bypass pump it takes quite a while for the boiler temp to recover.  The bypass pump piping has a ball valve that is part way off. Is it OK to run a 007 pump full speed from supply to return with only a 5 or 6 feet of pipe? If so I would open that valve fully.

I'm just concerned about flue gas condensation when the return temps cool that long. This boiler was sold as a cold start. It does have the L7224U with a no-call standby low limit set for  140. The boiler does heat the house OK. I am wondering if fully opening that valve in the bypass line would help (and is OK) and if shutting of the zone circulators when the output temp is low is necessary and would help. Thoughts?



  • furnacefigher15
    furnacefigher15 Member Posts: 514
    Cold start boiler

    If the boiler is a cold starter, why is it running running in standby?

    Don't fiddle with the valve, it was set that way to throttle back the pump. If wide open, it may prevent heat from getting to the zones properly.
  • icesailor
    icesailor Member Posts: 7,265
    Low mass boiler, high mass systems:

    A better choice for you would have been a primary/secondary with a Taco 4-way "I" series valve. It gives you outdoor reset in the system so you can make it run at whatever it needs to run at and the system temperature will settle out and run continuously at some Delta T. The boiler will not be subject to all that crazy flow because the 4-way is always modulating the boiler and system temperature. It offers boiler protection by sensing boiler return water temperature and gives you two dip switch temperatures.
  • Paul48
    Paul48 Member Posts: 4,469

    That aquastat has the ability to turn off the low limit, so it can be used as a cold start boiler. If it is maintaining, obviously the low limit is not turned off. It functions the same as any triple aquastat....high limit,low limit and differential. The low limit and differential should limit the time the boiler spends condensing if set properly.It shuts the circulator off if the boiler temp falls below the low limit and diff. setting.Between that and your bypass you should be able to do a pretty good job.
  • Paul48
    Paul48 Member Posts: 4,469

    Starting a bypass circulator based on supply temps is like closing the barn door after the horses get out.
  • icesailor
    icesailor Member Posts: 7,265
    By passages:

    You take a low mass boiler. Put a boiler pump and pump through the boiler. Put two properly closely spaced tees in the primary loop connected to the 4-way. Connect the secondary supply and return in the other side of the 4'way. Put a system pump wherever you prefer. Connect the outdoor reset on the 4-way, the system temperature probe on the supply out to the system, and the other on the water returning back to the boiler on the boiler side of the system, set the system reset to what you want, and if you have boiler reset, set that higher, and you are good to go.

    Stop condensation in boilers
  • Steve Whitbeck
    Steve Whitbeck Member Posts: 669
    edited December 2011

    Install a Taco injection control and use the bypass pump as the injection pump. Repipe the boiler as the instructions show. depending on the size of the boiler reduce the injection piping from the boiler to reduce water flow. If the boiler is 175,000 BTU or less I would use 1 inch pipe from the boiler to the system. I also would use a weighted check valve in the supply loop from the boiler to stop gravity rise.
  • Dave M
    Dave M Member Posts: 36
    Taco 4-way seems best

    Thank you all for the input. Seems I'm hearing the consensus is that the Taco 4-way modulation is the most effective solution.  Keeps the boiler temp up while allowing hot water to the zones gradually as it's available.

    BTW the sensor for the current bypass pump is on the boiler return (not supply). Although with a very low mass boiler there isn't that much delta between supply and return when you're heating a fairly high mass load.

    The aquastat was set to maintain and not cold start partly because it's in a not too dry basement and it helps prevent rusting in summer.

    I will look into the Taco 4-way further. Thanks again.

  • Paul48
    Paul48 Member Posts: 4,469
This discussion has been closed.