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Adjusting the Flow

Kungur Member Posts: 49
I started up my new radiant heat retro yesterday. I am using a coal fired stoker boiler. So far everything is running great and comfortable. But I have a question. I know I will want to messing around playiny with the flow meters and here is my question.

When the flow is decreased does the amount of heat decrease? Also and likewise if it is increase does the amount of heat increase? The resaon I ask what,to you guys may be a simple question, is I thought Iread somewhere that the heat increases with a decrease in in flow.

Hey, I am just a carpenter/woodworker. If it was made out of wood I would not be asking !!


  • Mark Eatherton
    Mark Eatherton Member Posts: 5,839
    Heat and flow....

    As with any hydronic question, the only correct answer is, "It Depends"!

    Delta T has been tossed about here recently as it pertains to keeping a condensing boiler in the condensing mode all of the time. An admirable idea,but its effect on emitters is less than desirable heat output, for meager gain in thermal efficiency.

    Flow is essentially mass. As you move more mass, you have more POTENTIAL heat delivery. The potential is based on EXTRACTION or EMITTER capacity, and the real time heat load which generates the DT.

    If you have an old boiler, I am going to assume you have old cast iron radiator/emitters. So, you have a significant amount of high mass extraction capacity. A high flow rate across these emitters may result (eventually) in a relatively low DT, but it ends up creating a higher AVERAGE emitter surface temperature across all of the emitters, which equates to more heat delivery. If you slow the flow down too much, the majority of the heat is given up at the front of the circuit, leaving very little heat and usually uncomfortably cool conditions down stream. But moving higher amounts of mass (GPM) requires the consumption of more kilowatts of electricity, and there is a finite point of return on the investment in that area. A reasonable expectation is in the 20 to 40 degree F range. When started cold, the DT may be greater than that, but once the mass is charged and rolling, the DT will drop. This is the perfect setting for continuous circ with outdoor reset, and non electric TRV's. Once you get this train moving at a steady pace, it will be hard to stop, and maintenance BUT's will be minimal. Real time load.

    In Europe, they design some of their series circuit systems around a 30 degree C differential, but that flow will reverse directions every once in a while (7 to 10 minutes) to spread the heat around and avoid heat starved units on the ends of a circuit. If the circuits are in parallel, and TRVs installed, they will self balance flow and heat output wise, so are not usually reversed.

    Then there is laminar flow considerations. If the flow is too slow, the water has a hard time giving up its heat to the surround materials (cast iron, copper, plastic pipes etc) due to lack of turbulence and fluid surface tensions and should be avoided.

    The opposite end of this is flowing water too fast, creating crevices and canyons in the piping that will eventually fail. Even plastic tubing is subject to hydraulic erosion corrosion. It just takes longer for it to work its way to the surface.

    Believe it or not, your coal fired boiler is subject to condensate corrosion, and it is more toxic to the combustion chamber metal than most appliances due to the sulphur content in the flue gasses.

    The bottom line is this, so long as heat is being delivered equally to all emitters, don't choke the flow anymore than necessary. Let the pump do its job, and everything down stream will do its job just fine, and you won't be thinking about it, because you will be comfortable. Sometimes, we have a tendency to over think these things....

    The rule of thumb here in America is to design around a 20 degree DT, and let the system do what it may. In the typical baseboard system, designed around a 20 DT, it is not uncommon to find a 7 to 10 degree DT, even at design condition. This is not an indication that flow is too high, as much as it is an indication that the load we thought would be there is NOT there.

    Now, you probably know more than you wanted to , but such is the nature of the beast, called comfort :-)

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
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