Tankless Coil in oil fired Boiler doesnt produce hot water/ boiler doesn't turn on

[topaz75]

I recently bought a house with a somewhat quirky heating/ hot water setup that the last owner put in and configured himself. It's and oil fired slant fin intrepid boiler with a tankless domestic coil. Also connected is a wood fired boiler/ stove that we are not using at the moment, we are relying on the oil fired part only.

The part that's making problems now is the hot water system. It worked fine for the first week or so that we were in the house, now it ceased to produce hot water on demand. To get hot water, I have to turn up the thermostat to create space heat demand, then I will also get hot water. Just turning on the hot water faucet does not make the boiler turn on.

I checked the Honeywell aquastat, and as far as i can tell the settings are ok, HI is set to 190, LO to 160 and DIFF to 25. The LO setting definitely has no effect, if I turn down the thermostat in the living room and there consequently is no heat demand for space heating the boiler cools off to way below the LO setting, I had it cool down as low as 90F. Turning on the hot water faucet doesn't turn on the Boiler either. Only turning up the living room thermostat will.

Anything else I can check myself before calling an HVAC guy?

[jadnashua]

Sounds like the aquastat is bad. Basically, it's a temperature controlled set of switches. It should be easy to check with a multimeter. If the switches are opening and closing when they should, then the controller is suspect. Don't know how sophisticated the control on that is, but many of the older ones are pretty simplistic.

[Dana]

Since the wood-stove boiler/slantfin hybrid was a DIY hack, the hacker may have disabled the lo-limit control to be able to use the wood stove as the heat source. But if the hot water had been working fine even during the warmer spring-like days this winter and is now behaving differently, that's probably not the case.

Either way, SFAIK there is no code-compliant way to make the woodstove and boiler work together, and it's probably worth recommissioning into a boiler-only system before firing up the woodstove. (One steam explosion or fire later you may be looking at an insurance adjuster protecting the insurer's interests, leaving you to pick up the pieces on your own dime.) With any luck it won't be too difficult or expensive to back out the hack.

[Tom Sawyer]

Yes Dana, yes and in fact with a combo unit bypassing the low limit is essential if only one set of controls is used which is the poor mans way of hacking the job in. There are better ways to control both units but of course, it costs a lot more money for the controls. dump the tankless coil and install an indirect (super-stor ss40) and now you can keep the existing controls and let the thermostat in the indirect call the burner and circulator. You know, thirty years back during the 1st energy crunch we would run into every hacked, mickey mouse set up that you can imagine.....and then they all went away. Now.......they're baaaaack.

[Dana]

...Chucky is an amateur HVAC engineer, don'cha know? How do you think he ended up looking like that?