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Water distribution problem

ARMANDO COBO | Posted in General Questions on

A new house I designed has a water distribution problem that has everyone puzzled, and I would like to find out if anyone has any ideas about it and how to fix it. Thanks.
The house is 5000sf, 3 1/2 baths, ZEH using two tankless heaters with a recirculating pump.
From the Homeowner: “Except on the kitchen island where the pipes run through the concrete slab foundation and hot water takes a little while to arrive (even after the plumbers made some tweaks), the bathrooms fed by water lines within the conditioned envelop of the house come on and are hot within about 10 seconds. After about 30 seconds of hot, the water cools significantly 10-15 degrees, such that it is very noticeable. The cool phase runs for about 30-60 seconds and the water starts to warm again. In taking a shower it stays hot until we finish, maybe another minute or two. We don’t take long showers. So I’m not sure if a cool cycle would reoccur. This happens in the master and both upstairs and downstairs guest baths.

What is maybe odd is that this hot cold hot cycle occurs if one of us takes a shower on a shower side (we have dual showers) that the other used within a few min. You would think it would remain hot. Also, even though the main lines coming into the dual shower are from a single trunk and then split to each shower controller. All of the mixer valves throughout the house are Delta Monitor 17 series that maintain constant pressure, but we did not get the TempAssur versions that also maintain temperature. Maybe we should have splurged.”

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  1. GBA Editor
    Martin Holladay | | #1

    This is called the "cold slug" problem. If you Google "cold slug" plus "tankless water heater" you'll get a lot of hits.

    There was a reference to the cold slug problem in one GBA article (an article on space heating, not domestic hot water). Here is the passage I'm thinking of:

    “Combi-systems with tankless water heaters report more problems with ‘cold slugs’ than systems that only provide domestic hot water. ‘The water heater is trying to give you 140 degree water,’ Magande told me. ‘If something happens — conditions change — the valve will try to make corrections to give you 140 degree water again, but the adjustment takes a few seconds. So you can get a cold slug of water.’ To reduce irritating cold slugs during showers, most experts advise that it’s useful to install a small buffer tank — for example, a 5-gallon or 12-gallon electric water heater — in any combi-system that uses a tankless water heater.”

    -- Martin Holladay

  2. Expert Member
    ARMANDO COBO | | #2

    Thanks Martin. I'll follow up on it. I also should have mentioned in my post that all waterlines are inside the conditioned space and none inside exterior walls; but I guess it makes no difference in this case.
    Interesting to know why it happened on this house and not on the other dozens of houses before.

  3. GBA Editor
    Martin Holladay | | #3

    I assume it has to do with the pump and the sensor that controls the pump. The water heater turns on at a certain flow rate; when the pump is energized, the flow rate changes; the change in flow rate causes the tankless water heater to adjust the flame quickly, and this need for a quick adjustment in flame height takes a while to stabilize.

    So you get a cold slug.

    Regular tank-type water heaters don't have the cold slug problem. You could add a 5-gallon water heater in line with the tankless heater to address the problem.

    -- Martin Holladay

  4. Expert Member
    ARMANDO COBO | | #4

    Just in case someone else needs more info... also attaching a good article from Michael Chandler.

  5. Anon3 | | #5

    Tankless sucks, they have a minimum flow rate otherwise the hot water shuts down. Go with the traditional tanks.

  6. Expert Member
    ARMANDO COBO | | #6

    Thanks Dana. I know from now on I'll be more forceful with clients about using regular tank WHs, which I actually prefer. I guess after installing dual tankless for many years, this was the first time I had any complaints. Go figure... live and learn! Thank y'all.

  7. Expert Member
    Dana Dorsett | | #7

    Recirculation systems can also dramatically cut into as-used efficiency of the system as a whole. EF only measures the efficiency of the tank, but always-hot distribution plumbing has an order of magnitude higher standby loss than those that are allowed to cool between draws.

    R3 code minimum pipe insulation makes sense for the latter, but it's not clear how far it needs to be upgraded for recirc systems to be reduced to comparable losses, code prescriptives notwithstanding. Recirculation systems that are only active when flow is detected or actively drawn by a push button aren't too lossy, but still more lossy than water distribution systems without recirculation.

    Something like a Phoenix Light Duty isn't exactly a "regular tank", in that it's a condensing sealed-combustion direct-vented modulating burner on an all-stainless tank, but is a far better solution than a tankless to most residential hot water. It has enough burner to support one "endless shower" at 2gpm, but can deliver far higher peak flow than any tankless, and the tank can be sized to the biggest tub that needs filling (they come in 50, 60, and 80 gallon sizes.) If you have enough water pressure and/or fat enough plumbing, like any tank water heater there's no problem at all filling a 50 gallon tub in 3 minutes or less, way beyond the realm of residential tankless units. And the short bursty sips drawn by high efficiency clothes washers and dishwashers don't short-cycle the burner into low efficiency and short lifespan the way it does with a tankless. It's not cheap- USD$2 - 2.5K for just the water heater (depending on size), but it's a simpler install than a big-burner tankless, and in most houses would be fine with 3/4" gas line plumbing. It's enough burner that it can be readily used for space heating + hot water in most houses. They also make similar products in the series with much bigger burners & tanks, but most homes would never need that much capacity. The Light Duty will out last most "regular tanks" by 2x the lifespan due to the stainless construction- it's only "light duty" when compared to commercial water heaters. (I don't work for HTP, but I like SOME of their products, and this is one of them.)

  8. Expert Member
    Dana Dorsett | | #8

    Michael Chandler's "solution" is actually a "solution-problem". He specifies a seriously oversized pump that pegs the flow sensors, causing early flow sensor failure. It pegs the maximum firing rate on the burners too, and is likely to reduce the product lifespan. A smaller pump and a bigger buffer make more sense, but it's basically re-inventing the tank water heater with a modulating burner. There are well designed well insulated condensing tanks with modulating burners available that are a far better solution (eg HTP's Phoenix series) .

    Navien's tiny internal tanklet solution is well engineered (unlike Chandler's total hack) , but cuts pretty seriously into the as-used EF.

  9. Expert Member
    ARMANDO COBO | | #9

    Some years a go at Summer Camp, Gary Klein with The Alliance for Water Efficiency in CA, gave us a presentation about hot water usage and distribution systems. I've been following his guides on the recirculating loop with on-demand for larger homes, and for small homes, following the 30'-35' rule from tank to terminal. The goal is to have hot water delivered within 10-15 seconds to any terminal. So far its been great till now with this issue of "cold slug". I sent him and email, and I would love if he could chime in here.
    Anyone wanting to look on hot water distribution systems at his work can go to The Alliance for Water Efficiency at:

  10. user-2890856 | | #10

    What Dana said .

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