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Need help with an improperly balanced DHW system

josepha Member Posts: 20
I have a multi family with 10 risers (5 kitchens and 5 baths) and the hot water stubbornly doesn't want to reach the furthest riser (bath).

Supply is provided by by 1" coming out of a Holby mixing valve at the steam boiler with a new adequately sized domestic hot water coil. We placed a recirculating pump with aquastat on the 3/4 return line just before the boiler. Though this has modestly improved hot water to some fixtures in the building, it hasn't made much of a dent in the problematic riser (both the hot water supply and return) remain luke warm on that riser. We've even tried to set the aquastat on the pump so high that it continually runs, but not much effect either.

The only time it gets hot on those fixtures is when water has be running for like 10-15+ min, and even then it's not consistent.

On one particular occasion we lost hot water for an hour on one whole side of the building (opposite the boiler) and I happened to notice that the unit directly above the boiler was taking a long shower and had sufficient hot water. It could have been an coincidence, but shortly after the shower was finished, hot water came back to the whole building. This has not been noticed to have happened since.

All of this leads me to believe that the water is not properly balanced across all risers (maybe at some point it was, but no longer).

I was going to place another recirculating pump at the bottom of the return of the problematic riser (horizontally right before it meets the common 3/4 return to the boiler where the previously installed pump sits some 30-40 feet away.

Any thoughts if this would help?
Should I instead look into trying to restrict/better balance all of the risers across the building?



  • Larry Weingarten
    Larry Weingarten Member Posts: 3,288
    Hello, Have you done any testing for cross connection?
    Yours, Larry
    delta T
  • Jamie Hall
    Jamie Hall Member Posts: 23,275

    Hello, Have you done any testing for cross connection?

    Yours, Larry

    Excellent point. Let's think about that one problematic riser for a moment. Make sure nothing else in the building is calling for hot water. Now when you turn a hot water faucet on in that riser, you should get hot water out of the faucet in no more than a few minutes, and once hot it should stay hot. If that isn't happening, cold water is getting into the hot water feed from somewhere. You first want to check that the problem isn't on that riser -- shut all the other risers off (you do have valves?) both hot and cold. If the problem persists (leave both hot and cold open on that riser) the problem is on that riser. No Joy? Open both hot and cold on another riser. Rinse and repeat until you hit it. Then look for a cross connection of leaking single handle faucet... worth a shot, anyway.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Alan (California Radiant) Forbes
  • There's a cross-connection problem somewhere. It could even be at the replaced recirculating pump. Is there a properly functioning check valve after the pump?

    If that checks out, I would turn on the hot water at the faucet that doesn't get hot and start turning off other fixtures at their supply valves (angle stops). You only need to shut the cold supply. When the water at the running fixture gets hot, you've found the cross-connection.

    You can't do this at every fixture since you typically don't have shutoffs at showers or bathtubs.

    You can also find the problem by sound as you may be able to hear water flowing through the errant fixture.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,288
    Hello, Do you have Moen or Mixit faucets/showers?

    Yours, Larry
  • josepha
    josepha Member Posts: 20
    edited June 2019
    Thanks all!

    @Larry Weingarten @Jamie Hall
    I don't know how long the issue has been going on since no one complained , All said that it at least predates their tenancy (ie at least 4 years), so it's difficult to track to any previously bath repairs on that line that could have caused some cross contamination, let alone some failure.

    Though I think the shutoff to that riser in buried in the ceiling somewhere (sigh, gotta fit that also), all other riser cutoffs are available and have been able to isolate to only that riser and same problem persists.

    Also, I think I found one instance of cross contamination in the shower body of one of the units (floor 2 of 4) and replaced that shower body guts. It’s better in at least that unit, but same in others.

    @Alan (California Radiant) Forbes
    The recirculating pump was newly added. There is a check valve after the pump and appears to be working (as when pump is running though before the valve gets hot, after stays cold).

    I’ve tried some form of isolating fixtures, though problem has been by the time I get around to doing so, the fixture is already hot sometimes 5min, sometimes 10-15+ so I can’t tell for sure. Also varies, time of day and season (boiler running or not).

    Overall, I thought it was either an imbalance/distance thing or cross contamination issue and am faced with 1) running around to try and isolate any cross contamination issues in remaining fixtures and or start replacing shower bodies. Or 2) maybe adding a pump to the return at the bottom of that problematic riser. I figured if it works then problem solved (for now) and if not, almost certainly a cross contamination issue, but willing to take that risk. Question really that I have... is it a futile effort to do so?

    If I go with #2, then was looking at the Taco 005-SF2 or, if that does not have enough head, then the 008-SF6?

    Thanks again!
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,288
    Hi, If you have access to all units, I’d shut off cold supply to the water heater and open a hot tap towards the end of the main line. Assuming it runs, then look at fixtures with an IR camera. That might help you to pinpoint the cross connections.

    Yours, Larry
  • Zman
    Zman Member Posts: 7,569
    The most common cross connects are janitors sinks with soap or Y adapters and shower heads with on off valves on the heads. People like to leave the shower mixer on in a certain position then turn it off at the head. The valve cross connects the whole stack.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • jumper
    jumper Member Posts: 2,245
    Some buildings have dedicated pipes from each fixture to manifold in utility room. More pipes but skinnier and far less joining.
  • 136lin
    136lin Member Posts: 30
    Ideas in no particular order- riser is partially obstructed with rust or other "junk", what material is the piping, galvanized steel, copper, pex?, have you eyeballed the apartments affected?, does anyone have a portable dishwasher?,, check laundry room for "y" fitting. As was mentioned, are there any Moen faucets? All of these we had found to cause cross connections'

    Also remember KIS (keep it simple) and Occams razor-"The simplest explanation for some phenomenon is more likely to be accurate than more complicated explanations."