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Gravity conversion

We have a customer that wants a new boiler.  Zone the exisiting CI rads and add infloor in the basement.  Indirect water heater.    So far so good.

The problem is it needs to be running by the middle of March for some grants and the temp today was about -25f.  

The existing boiler is a Viking with returns on both sides and a header off the top that is a bullhead manifold.   There is a 1" tapping on one of the returns, and a 1" on the header where the relief valve is installed.    Expansion tank in the ceiling and no pumps.

What I'm thinking of doing is tying the modcon into the two 1" tappings.  The boiler should then become a buffer tank.  

We don't need to actually fire the modcon until the inspection in March and by then we shouldn't need as much heat.  Then once it is nice out we can remove the old boiler and do the rest of the work.

Does anybody see any major problems with this?  Anybody try it before?


  • Ironman
    Ironman Member Posts: 7,350
    Path of Least Resistance

    Since water takes the easiest path, what will stop it from flowing backwards through the old boiler? You'll need an isolation valve in one of the lines to the old boiler. Gravity boilers usually didn't have these when they were installed. You'd actually need two since every gravity boiler I've seen has twin supply and returns.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • rich pickering
    rich pickering Member Posts: 277
    pump at the modcon

    We would pump from the modcon to the boiler return, through the boiler, and back to modcon from the header.  Hopefully we will still get gravity flow to the rads. 
  • Ironman
    Ironman Member Posts: 7,350
    edited February 2011
    Parallel or Series

    Let me see if I'm following you correctly. Are you placing the mod/con in series or parallel with the old boiler? If in series, yes it should work fine. If in parallel, you'll just be flowing through the old boiler, not the system.

    Also, watch out for the usual things when converting from gravity to forced flow. I'd get rid of the overhead expansion tank and install a new fill/back flow with the mod/con. I don't know what brand you're go to use, but some have an internal pressure switch that requires 12# min. to operate.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • rich pickering
    rich pickering Member Posts: 277
    vito 100

    planning on a vito100 once I confirm the rads and the heat loss. 

    The flow is through the boiler but not the system if that makes sense.  Until demo, think of the vito as a conversion burner.   The boiler will be removed in May/June depending on weather. 

    And I'm not going to remove the burner from the old boiler until we are ready for demo, just in case.    After all, what could possibly go wrong? 
  • Mike Kusiak_2
    Mike Kusiak_2 Member Posts: 604
    It might work

    What if you piped the modcon supply into the existing header at the top and the modcon return into the existing return at the bottom of the old boiler. I think this would establish gravity flow since the hottest water would be at the top. If for some reason the gravity flow did not start, then the modcon would  be pumping the hot water into the top of the old boiler where it would collect and stratify the water in the old boiler. Eventually the old boiler would contain hot water at the top which should definitely start the gravity circulation.

    Just a thought, but can't think of a reason why it wouldn't work?
  • rlaggren
    rlaggren Member Posts: 160
    It sounds like you won't have the system installed

    At least not the complete new system, at the time of the inspection. I understand what you're trying to do v.v. some kind of $$ grant date rule, deadline, whatever; basically it sounds like you're just hanging a new M/C with some kind of connection to the piping

    somewhere to meet the letter of a paper requirement and coming back to do the other half later.  We've done stuff like that, but usually the inspector is at least partially in the loop because the "installation" looks pretty weird at the normal inspection date. But maybe you got that covered already.

    Just a thought.

    disclaimer - I'm a plumber, not a heating pro.
This discussion has been closed.