Repair Watts 70 mixer?-shut off water

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TomR

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I have oil fired tankless hot water. The boiler is about 165 F and pressure is 25-30 psi. Our tap hot water is 145-155 F. No matter what I do to the mixer valve (Watts 70A), it doesn't change so I guess I need to repair valve. Kit looks like about $35. There is a valve in cold water line that splits cold water between the mixer valve and supply to tankless hot water. Neither the hot water into the mix valve or the tempered water out has a valve.

So, I have questions:
1) Is the boiler temp and pressure acceptable, and if not, how can I adjust?
2) Should I repair or replace the Watts 70A? If replace, are there better alternatives?
3) If I turn the furnace off, close the cold water valve, and loosen the bonnet on the mixing valve, I guess the tankless and the domestic hot water lines will drain. Am I correct?
4) After I repair/replace, will everything be "normal" when I turn on the cold water and oil burner?

Thanks in advance for any help or advice!
 

Dana

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If you're comfortable with doing sweat soldered connections, replacing the whole thing is about the same money as the repair kit. But it's probably quicker to install the repair kit. Every thing should return to normal

For orders of magnitude more money you can stop using the tankless coil and install an indirect fire water heater tank, which will deliver better hot water service and improve the as-used AFUE of the (very likely) WAY-oversized boiler, by lowering the boiler's average standby temperature, and allowing more maximal use of the boiler's thermal mass with larger min-to-max temperature swings. With the tankless coil the minimum standby temperature needs to be pretty high, contributing to much bigger standby losses, and reducing the temperature swing between standby and the high-limit, which adds a lot of annual ignition cycles. With an indirect water heater and a retrofit heat purging boiler controller you should be able to reduce annual oil use by at least 10%, usually more, depending on just how oversized the boiler is for the space heating load, and how oversized it is for the system's zone radiation. (If it's been short cycling on zone calls due to insufficient zone radiation it could save 20% or more.)

If you want to better estimate on how much that would save, first, run these numbers on your fuel use, using +5F as the outside design temperature (that's Worcester's 99th percentile temperature bin, Framingham's is +6F, either one is good enough for these purposes.) With the 99% heat load number you'll then be able to estimate the oversize factor. For a sense of how the oversize factor and tankless coil affects the as-used annual efficiency relative to the steady-state efficiency, see Table 3 of this document. With a retrofit heat purge controller and lower standby temperatures the oversizing efficiency hit will be closer to that of system #3. With your current setup you're looking (best case) at system #1, assuming the radiation is sufficient for emitting 100% of the boiler output with no cycling on zone calls.

There is currently a $500 MassSave rebate subsidy for installing indirect water heater and abandoning the tankless coil. (Though there is none for installing the heat purging boiler control, do it anyway. The Intellicon HW+ is cheap and effective enough, but there are others.)
 
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