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Heat exchanger Questions

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by Kemer, Feb 25, 2013.

  1. Kemer

    Kemer Member

    Joined:
    Feb 26, 2008
    Messages:
    213
    Loc:
    Northeast Pa
    Can anyone tell me if there are tell tale signs that a heat exchanger is blocked.I am not getting good heat transfer and wondering if it's my pump or blocked exchanger I have a taco 008 vdtf6-1 and the pump seems to be running full out all the time (the percentage light is on steady)

    Thanks Harry

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  2. Karl_northwind

    Karl_northwind Member

    Joined:
    Feb 13, 2012
    Messages:
    182
    Loc:
    Central Wi.
    if it's a flatplate HX, you can turn on both circulators (zones, whatever will get water circulating on both sides of the HX)
    once things should be flowing (you will need to have heat available on the supply side of the HX) feel the back plate.
    (opposite side from the connections) you should feel a gradient temp from hot to not-so-hot on the supply ports. you should feel an opposite gradient on the load side of the HX.

    if you have hot at the hot in, and the rest of the HX is about the temp of the load side, you have low flow thru the supply side. that can be plugged somewhere in the system, plugged strainer, bad circ control or setting on the VDT circ, or mostly closed valve. no heat at all, means a closed valve, dead circ, totally plugged, no flow.

    now leave the hot in/out loop running, and close a valve on the load side loop (or turn off the pump),
    if the whole HX heats up even temp, or got there from the beginning, but no heat comes out the load side, you have a flow problem on that side.

    if the plate hx heats up and you confirm flow on the load side, think about scaling problems if it's exposed to DHW.

    is this between the GARN and a pressurized system with the VDT circ on the garn side? I'd check for a plugged strainer.


    karl
  3. logger6644

    logger6644 New Member

    Joined:
    Jan 30, 2013
    Messages:
    55
    Loc:
    Upstate New York
    The key issue is flow and as stated in another post there can be other reasons for flow loss besides the heat exchanger. I have temperature-pressure gauges all the lines on my plate exchanger. I can monitor both temperature and pressure drop. The pressure gauges are not sensitive enough to be of practical use but the temperature gauges will indicate problems. In my case when running normally the temperature drop is similar on both sides, indicating that my flow rates are similar on both sides when the system is actively drawing heat. If the proportion is different in your normal state that's just your normal, indicating a higher or lower rate on one side. The heat in just has to equal the heat out and so under normal (unblocked) conditions these temperature drops should remain proportional. If you see this change then it indicates a blockage in the side that the temperature differential rises. Now you still have to track down the reason flow has changed. If we did have accurate enough pressure gauges it will be where you see the pressure drop, but since we usually don't you have to do a process of elimination.

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