Weil McLain WTGO-4 Series 2 Tankless Coil Fluctuating Domestic Hot Water

mattmay310

I have a Weil McLain WTGO-4 Series 2 Oil Boiler with a Beckett Burner and a Tankless Coil and the Domestic Hot Water fluctuates when there is a big draw from it when I use the bath tub. It isn’t as much of a problem with the shower and the faucets but really fluctuates when using the bathtub. I have cleaned the coil and changed the mixing valve. The aquastat seems to be working properly. I have never had this issue in the past. Any idea what the problem could be? Faulty aquastat probe/sensor? Buildup on the outside of the tankless coil? Thank you in advance.

EdTheHeaterMan

What process did you use to clean the coil?

Since this is a series 2 boiler you have had this for some time and you have not experienced this before. Something has changed from before. Have you replaced any faucets to the tub? The coil will not run of hot water as long as the boiler temperature is above 150° and the water going thru the coil is at or less than 3 gallons per minute (GPM)

What are the temperature settings inside the aquastat? I would check to see that they are ast lease 160° LOW and 180° High with a 20° DIFF

mattmay310

I cleaned the coil with vinegar. I changed the cartridge to the delta faucet that goes to my tub because it was dripping. I have changed that twice to make sure that wasn’t the issue. The aquastat is set to 170 low and 190 high with 20 Diff.

HVACNUT

What process did you use to clean the coil with vinegar? Is it set up to back flush?

How many heating zones?

Did anyone change the nozzle size?

Its known that even with a mixing valve, drawing a tub through a 5 GPM coil must be done slowly. Can you give us actual high and low fluctuating temperatures at the tub?

EdTheHeaterMan

Changing the cartridge on the faucet would not be my first attempt to fix this problem. I would measure the water flow out of the problem faucet. You need a watch, clock or phone app that can measure 30 seconds. You also need a 5 gallon pail, a 1/2 gallon bottle and a Sharpie marker. Put half a gallon of water in the pail and mark water level in the pail with the sharpie. Add another half gallon and mark the surface on the pail with the Sharpie. Continue this until you have marked every half gallon on the side of the pail until you get the 5 gallons. That should be 10 marks.

Now with your new measuring device, place the pail under the faucet to collect ALL the water that will exit that faucet. Turn on the hot water only and start the timer for 30 seconds simultaneously. Turn off the hot water faucet when you get to 30 seconds. Now let the water surface in the pail settle and read the number of half gallons you have in the pail. That is the number of gallons per minute your faucet is moving through the tankless coil. (if it is less than 5 GPM then you can redo the test for a full minute for a more accurate measurement)

Compare that GPM you find with that test, with the GPM rating of the tankless coil. A 3 GPM coil will not work well with a 5 GPM flow rate into the tub. 

If you read the instructions for connecting the tankless coil to your potable water system, It should include a flow rate restrictor and a mixing valve.

I wonder if the flow rate restrictor has corroded away causing the orifice to increase in size allowing more water through it. Or did the original installer never put the restrictor in the cold water piping and just used a partially open valve to create the restriction needed to make the flow rate match the coil. When you had the vinegar flush service, you probably opened the valve 100% and therefore the GPM restriction is no longer there.

Just some random thoughts 

EBEBRATT-Ed

And the GPM the coil can provide is based on 200 deg. boiler water.