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cast iron & mono flow

Ron Beck
Ron Beck Member Posts: 6
If the boiler is cast iron or steel, does the boiler have a boiler bypass? The boiler bypass takes cool return water and pipes it into the hot supply, reducing the flow rate through the boiler. This gets hotter water out to the supply. It will blend with the cooler return water and varies the temperatures to the system to whatever is required for heating at any outside temperature. As the outside temperature changes so does the system water temperature. Use a full size pipe (same as manifold) for the bypass, and a valve to control the flow through the boiler. If the pump is on the return side of the boiler and you are not moving it, you will need to put a tee between the pump and the boiler. Pipe from that tee to a tee in the supply. Between the tee in the return and the boiler, place a valve to restrict water flow into the boiler. If you install a valve in the bypass it must be full open. This works very well in systems which are larger than the boiler DOE output, monoflo systems and cheap ways to increase the comfort level in older homes with excess radiation. Oh, also gives boiler the all important protection and saves fuel.


  • Howard_5
    Howard_5 Member Posts: 5
    cast iron & mono flow

    I recently inspected a newly replaced boiler. The old boiler was 125,000 BTU gas and the new boiler is gas 100,000. There was no change in any piping except near the boiler. Circulator remained-on the return. The house is a 1200 square foot ranch. The heat is cast iron board with mono-flow tees. The problem is the new boiler never gets up to high limit, so it stays on forever to raise the temperature on the main floor even a few degrees. When I shut the supply and return valves the boiler heats up to 180 within a few minutes. However with the valves open the boiler temperature never passes 165 degrees. The board is not air bound, it gets very hot to the touch. The new boiler was sized according to the baseboard and I re-calculated the heat load and the boiler should be large enough. Any ideas to help solve the mystery?
  • why?

    Why the pump installed on the return side ? Did the bolier burner shut off at 165? Your limit control may be set at that number... I sure hope u have a indoor/outdoor reset control...
  • ralman
    ralman Member Posts: 231
    I am just a home owner with the same problem.

    I have a 1950 1.25" monoflo piping system, cast iron baseray baseboard, oil fired peerless 120,000 BTU output, 1500 square foot ranch. My boiler never shuts down for high limit. I feel it is due to the large water content in the system that returns to the boiler. My aquastat controller activates the circulator on a heat call. Thermostat calls for heat never last more than 5 minutes on very cold days and that short cycling problem is worse above 35 degrees outside temp. I have 67 gallons of water in the system, 17 gallons of water in the boiler. I have to guess at my circulator GPM which I'll use 10 GPM. 10 GPM x 4 minute burner cycle = 40 gallons total moved through the circulator. 84 total gallons - 40 gallons moved through the circulator = 44 gallons of water that did not travel through the boiler. The problem I feel is that the thermostat is satisfied so quickly that high temperature water in the boiler never completely travels from the boiler through the entire system and back to the boiler. This results in colder temps in baseboards at the end of the loop. I have 187 feet of baseboard and 360 feet of iron pipe. I feel the amount of metal itself robs a lot of BTU's from the high temperature boiler water. This results in very low return water temperatures which are keeping the boiler from ever getting close to high limit. If I overide my thermostat by holding my aquastat relay on, the boiler and circulator will run continuously. I did this for 15 minutes and my boiler did not reach the 180 degree high limit setting but the supply and return temps were higher. I figure that if I would have continued to overide, eventually I would have reached high limit.
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