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Boiler cycling on and off with a low-flow shower head

Bert Matter | Posted in GBA Pro Help on

Just wondering if anyone has come across this before. I have a Baxi Luna 310 fi modulating boiler for both in-floor heating & domestic hot water (35,000-105,000 Btu). With a low-flow shower head (approx. 6 litres / minute), the shower water goes from hot to cold constantly. I noticed the boiler cycles on and off the same way. The boiler requires a minimum flow rate of 11 litres / minute to prevent it from cycling on and off.

I know I could install a wasteful minimum 11 litres / minute shower head or maybe a small buffering tank 5 or 10 gallons ? so the water would maintain a constant temperature. Here in Ontario the building code requires hot water storage tanks to maintain 140 degrees F to prevent Legionnaires disease.

Any advice would be appreciated!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Bert,
    Yes, many people have encountered this problem. It's one reason that I usually advise anyone who asks about tankless water heaters that they will probably be happier with an old-fashioned tank-type heater.

    Tankless water heater manufacturers have been trying to address this problem by introducing new models that stay lit at lower flow rates. Evidently the Baxi Luna is a unit to avoid.

    You might call the manufacturer's technical help line to see if they have any suggestions (or at least to register your complaint).

    Possible solutions include swapping the unit out for a different model that handles low flows better, or installing a 5-gallon electric-resistance buffer tank.

  2. Keith Gustafson | | #2

    I wonder if anyone has tried using a thermostatically balanced shower valve in this situation. It would seem like it would help. If you turned the shower [with your current valve] on full hot and it maintained temp I think it would work.

  3. Aj Builder, Upstate NY Zone 6a | | #3

    The obvious solution.... Remove restrictor on the shower head. There is no shortage of water where I live.

  4. GBA Editor
    Martin Holladay | | #4

    AJ,
    Your solution will work -- assuming that the showerhead has a removable flow restrictor. But the net result will be higher energy bills than would be the case if the water heater operated properly and the low-flow showerhead could be used as designed.

  5. Bert Matter | | #5

    correction on minimum flow rate for the boiler to remain on is 2.5 litres / minute, not 11.
    While the shower is on, it uses 6 L / minute with the shower tap set midway between
    hot & cold. So I did an experiment. With the shower set full to hot, the boiler remained on the entire time, but the shower temperature was too hot. The solution : the boiler has a domestic hot water temperature control dial. I had it set to max , so when I turned it down , the effect on the shower
    was that I had to turn the shower dial almost all the way to the hot side. This in turn meant I had met the minimum 2.5 litre/ minute for the boiler to remain on.

  6. Expert Member
    Dana Dorsett | | #6

    The Baxi Luna is something of a pig in this regard. There are MANY tankless water heaters that modulate down to under half the 35.4 KBTU/hr min-mod fire of the Luna 310.

    I've seen similar issues with low-end Bosch tankless on-demand hot water units of 15-18 years ago- the min-fire output is simply too high for low flow rates at comfortable showering delta-Ts. There is no solution other than higher flow or buffering tanks- the excess heat has to go somewhere. Most Asian tankless units will modulate down to under 20KBTU/hr some under 10KBTU/hr.

    The standby loss of a buffer tank would be pretty low compared to the efficiency hit of doubling the hot water flow in the shower, but from a sheer economics point of view it's probably a payoff == never sort of deal.

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