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# poor heat exchange

Member Posts: 5
I have a new Weil-McClain WM97-110 boiler
*home being heated by boiler
*also used to make domestic hot water - John Wood Make hot water heater - 2004 - JW10-75IT - (serial #) W0404504319

Question: On a call for domestic hot water, which is a priority loop, boiler target is 190 degrees - supply reaches 185 degrees and return is 168 degrees. The temperature difference on the hot water heater is 12 degrees. it takes 30 minutes for aquastat to satisfy at 125 degrees. Was wondering what would be a good temperature difference on the hot water heater primary loop? Believe heat exchanger may have sediment causing poor heat exchange, very slow recovery.
Any suggestions?

• Member Posts: 17,044
Not sediment -- scale. How hard is your water? Is it treated? Scale can form particularly on the domestic water side of the heat exchanger and really cut down on its performance.

You can figure the BTUs going into your heat exchanger from the "hydronic formula" (BTUh = 500 times gpm times delta T).
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 5
Thanks Jamie. I am not sure about the GPM, but we have a Grundfos pump model #UPS 15-58FC with a 1 inch outlet. We have pump set on high speed. I was thinking somewhere about 7 gallons per minute.
That would equal 7 x 500=3500x12=42,000 BTUH. We have roughly 95,000 BTU available. Water softener was recently added with home renovation, also tank sat empty and idle with home renovation. Sounds like it could be scale making hot water heater inefficient. Any other suggestions?
• Member Posts: 10,253
Try slowing the pump down just for the fun of it. Different schools of thought sometimes over pumping will not give you good heat transfer. Sometimes the math doesn't work out although @Jamie Hall formula is correct. Can't hurt to try
• Member Posts: 17,044
If I make the assumption that your incoming water is at say 55, that BTU input will bring 40 gallons of water up to 125 in half an hour or so. For what that's worth, if anything...
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 5
Ed,
We did just that and no difference on medium and low. Our delta-T did not really change at all. Which leads me to believe it's the heat exchanger scaled up.