indirect DWH coil cleaning procedure to restore water heating effeciency

Old 01-24-11, 11:22 AM
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indirect DWH coil cleaning procedure to restore water heating effeciency

The following procedure was e-mailed to me from Heat Transfer Products the manufacturers of SuperStor indirect water heaters.

Indirect coil transfer test instructions

If you have had a decrease in hot water delivery over a period of time and the indirect will not supply the amount of hot water you had in the past and or the hot water comes out hot at first if the indirect was sitting for hours with no use and then goes cold after a few minutes and will not get warm unless you shut the hot water off for a while, this could be caused by liming of the coil. Liming is when the water has left mineral deposits over a long period of time on the coil’s outer wall building up thickness. This will prevent the heated boiler water that is entering the coil from transferring through the coil into the stored water for domestic use.

To verify that this is the problem, please follow the directions below:
# -1 Shut off the boiler.
# -2 Drain the entire contents of the indirect.
# -3 Refill the indirect with new cold water.
# -4 Turn on the boiler.
# -5 The temperature entering the coil from the boiler (normally 180*) should be at least +30* higher than the water coming out of the coil and back to the boiler. This is called the delta. Example: 180* going in 150* coming out. If you have a wide spread delta then the indirect is transferring the hot boiler water as it should and the problem lays elsewhere. See mechanical reaction time below. If you are not transferring efficiently the delta or temperature difference will be close, example: 180* going in 180*, 170* coming out, then the coil is not transferring and will need cleaning.

Coil cleaning Procedure’s

When attempting to clean the coil there are two options to choose from. The first one is the easiest has no cost, and can be done at the time of the above test.

Option # -1 Thermal shock, Drain all the water from the indirect and let the boiler water go through the coil so it heats up the dry coil for 10 minutes. Turn on the cold water for 3 to 5 minute so as to cover the coil. When the cold water slams against the hot coil the coil will contract and some of the deposits may flake off. Flush out the water and repeat this procedure. This procedure may not work if the coil has had deposits on it for a long period of time. Shut off the boiler, refill tank, and repeat the procedure above. Operate shower to see if there is an increase in recovery. If this did not improve its recovery try option two.

Option # -2 Purchase a chemical called CLR or Lime away (biodegradable & non toxic) from your local hardware store. Or go on line to Schifffahrt, Yachten, Häfen und Booten and order this chemical from the manufacturer. This is the strongest of the chemicals and is highly recommended and is biodegradable and non toxic. Drain the water from the indirect. For a 20 to a 60 gallon tank you will need approximately 5 gallons of this chemical if you are to do it effectively. For an 80 or 119 gallon tank you will need 10 gallons. Turn off all power to the boiler. Drain the indirect. Remove the aquastat from the indirect and unscrew the well that the control was on by turning ccw. Note: ( this is a ¾” or ½” NPT thread) You will now see a threaded connection on the surface of the tank. You will need a centrifugal pump for pumping water. Hose number one: Select a hose that is approximately 10’ long, will fit through the threaded button is flexible but rigid enough so it will not buckle, and cut of the male end if needed so it fits in to the tank through the NPT threaded button, and place the opposite female end on to the pump outlet / discharge. Hose number two: Place a washing machine hose on to the inlet of the pump and the opposite end into a 5 gallon bucket. Fill the bucket with water and prime pump so it starts to inject the water in to the indirect add the chemical and more water to the bucket of water as it is pumping so that at least 8 gallons of water and the chemical are injected in to the indirect. Turn off the pump while pinching the hose going from the pump in the bucket and attach it to the drain valve on the bottom of the indirect. Open the drain valve while turning on the pump and let it run for 10 minutes so as to scrub the coil. Close the valve and turn off the pump and let the chemical sit in the indirect for approximately three to six hours. Turn on pump while opening valve and let it run for another 10 minutes. Remove hose, replace the aquastat well, drain tank and flush well. Repeat the indirect coil recovery test procedure above to check for recovery and then operate shower to see if there is an increase in recovery and or shower time. If there is no increase in recovery consult the technical support department at the factory for direction.

Mechanical Reaction Time
Recovery can be compromised by controls and or valves. If you have an aquastat it will have a fixed or adjustable differential and will open on temperature rise. The differential is used to bring the boiler on or open a zone valve and or turn on a circulator that delivers the hot boiler water to the indirect’s coil. The differential can be fixed or adjustable. Note: (if adjustable it normally works a range of 5* to 30*).

The aquastat is set to a limit temperature and is the temperature selected to be delivered to the fixtures. When hot water is drawn from a fixture cold water enters the indirect thus cooling down the water in the indirect. The differential senses the drop in temperature and closes the aquastat and start’s the recovery process. If the differential is set to wide then the reaction time between release of hot water and energization of the boiler may cause a lack of hot water.

Check to be sure the differential is as tight as possible if using an adjustable type and if using a fixed type determine the fixed number. If it is to wide then this may be a contributor to the lack of recovery. Consider changing it to an adjustable type.

If using zone valves you may find that the elapsed time between the valve being energized and it opening and turning on the boiler/ circulator is 90 seconds or more partially on the older style valves. This delay and a wide differential may contribute to lack of recovery. To help in recovery you could install a mixing / tempering valve so that the water in the tank can be stored hotter (150*) than the desired delivery temperature normally 120*, and set the mixing valve to the desired delivery temperature to gain recovery.

Last edited by NJT; 01-24-11 at 03:41 PM. Reason: removed personal information
Old 01-24-11, 02:24 PM
Join Date: Jul 2009
Location: Toronto
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Glad I install triangletube indirects. They self clean
Good post none the less

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