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Aqua booster / tankless coil storage tank setup question

Bobb Member Posts: 1
Live here in Conn. and have a FHW utica boiler with a tankless coil for DHW. The system is about 9 years and has 40 gal Johnson wood 40gal storage tank (7 years old). The storage tank has sprung a leak and will be replaced. It currently has the typical set up of cold water entering the coil then HW going to the storage tank. Water is recirculated to coil to maintain temperature when needed. This has worked well except the cold water entering the coil causes the boiler to short cycle often i.e., when washing dishes.
So, my question is, now that I'm replacing the tank I would like to change the configuration and have the cold water enter the storage tank boiler input line and then only have the coil heat the stored water on the recirculation loop when needed as the stored water cools. I hope to gain having only warm water enter the tankless coil and thus reduce the short cycling. I don't see any problems with this but want to get other opinions of what I may be missing.
FYI, know many feel storage tank aqua boosters are antiquated. But since the all parts other then tank are in place its hard to justify any other upgrade. Heat loss will be no worse then an indirect since, it basically the same tank. The boiler is rated at 86% efficient. We're in a single family ranch and never have a problem with hot water in current situation. Overall, I am satisfied and only want to improve on current design if possible. Otherwise will just install in current set up replacing only the leaking tank. Thoughts?


  • STEVEusaPA
    STEVEusaPA Member Posts: 6,506
    If I understand what you’re describing, I’d switch your boiler to cold start and put an aquastat sensor in the storage tank to call the boiler to fire only when you need to recharge the storage tank.
    That would prevent your boiler from firing for simple hand washing.
    But if you’re doing all of that, maybe simply an indirect on its own zone would be best
    HVACNUT Member Posts: 5,829
    Heat loss is not the issue. As @STEVEusaPA wrote, having the boiler maintain a minimum temperature vs. cold start is what will save fuel by converting to an indirect.
    Any time you're drawing water through the coil and therefore dropping the boiler temp, the boiler must cycle.
    I think an upgrade would be to raise the temperature on the new tank to 140° and install a thermostatic mixing valve to temper back down to 120° for DHW.