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Community and Q&A

Point of use tank vs on demand recirc

Brandon Avery | Posted in Energy Efficiency and Durability on

I am stuck between installing two point of use electric 1 gallon water heaters (one in master bath and one in kitchen) with the incoming water being preheated by central tank water heater OR simply installing an on demand recirculating pump. The main purpose being to eliminate the wasted water and wait time for hot water.  Both the kitchen and master bath are between 20 and 30 feet from the main water heater. I plan on insulating the hot water lines regardless.

I like the simplicity of using a small point of use tank but am concerned about standby losses.  I know they make tankless electric point of use but I don’t like the idea of such high demand loads.

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Replies

  1. Mark Walker | | #1

    I've been trying to make this decision too. What I saw is that the really small point of use tanks (or tankless) have a rather short lifespan.
    I'm leaning towards a crossover valve and a recirc pump (returning in the cold line).
    To preheat the water to less than 120/125°F runs the distinct risk of Legionnaires Disease.
    A crossover and pump can be set up to stop pumping when the hot arrives. Just activate it with a doorbell type button in the master bath.

  2. Expert Member
    Zephyr7 | | #2

    The standby losses for the small point of use water heaters will be minimal. A recirculating pump will have losses too: both energy to run the pump, and the thermal losses of the pipe loop which acts like a radiator connected to your water heater. You may be able to eliminate the energy use of the pump by using a thermosiphon system, which I highly recommend instead of the pump if your system can be configured that way. My guess is that the standby losses for the small heaters will be less than the recirculating system, although I can’t be certain, but the reliability is certainly going to be less with the two small heaters (three tanks to fail instead of one). Replacement of even one small water heater is going to cancel out, from a green perspective, a small energy loss premium for a recirculating system.

    Note that there are also on-demand recirculating pumps so that might be another option for you.

    Bill

  3. Mark Walker | | #3

    Here are FHB's details: https://www.finehomebuilding.com/2013/03/07/hot-water-circulation-loops

    In my own house, I have a full loop and demand pump. The timer on the Grundfos is worse than bad, so I leave it on and plugged into a smart outlet (phone/wifi control). It's scheduled for wake up and kid-bedtime. All other times are on demand via the phone app. The pump turns on for up to 5 minutes, but the pump turns itself off when it senses hot water. All my pex is insulated, so the water is still comfy to wash hands an hour later.

    I wish I had simple buttons in the bathrooms to activate the pump.
    I avoid putting a LOT of water into my septic tank.
    It's great to have hot water first thing in the morning.
    A thermosyphon will always be losing energy.

  4. Ryan Lewis - Zone 4A | | #4

    They have small point of use _tankless_out there. HTP just showcased one that can be used inline with an existing hot water heater. No idea about how often they would fail..

  5. Charlie Sullivan | | #5

    I think the energy equation depends greatly on what your main water heater's technology is. If it's a heat pump, the recirculation is probably a better energy choice; if it's straight electric, the point of use heaters are a good idea. If it burns natural gas, you'd have to decide whether you want to compare on cost or climate impact and have the relevant data for your energy suppliers.

  6. Expert Member
    Akos | | #6

    Brandon,

    Provided your pipes are inside the conditioned envelope, losses from a recirc setup only matter during the cooling season. The rest of the time, the heat from the pipes just heats the building so it is not lost.

    Bypassed based setups tend to fill the cold water pipe with lukewarm water, this might not be the best say for a kitchen where you want cold water from the tap. It works great for showers and tubs though.

    Any recirc setup, either pump or thermosyphon needs some controls. Running it all the time does waste a lot of heat in the summer time (speaking from experience here). For a thermosyphon setup you can add a motorized ball valve on the return line for control.

    A nice side benefit of pumped recirc with dedicated return is that if you have access to the underside of the floors in the kitchen or bathroom area, it is not too much work to add in floor heat. Run the return through the floor joist with some heat plates and add in insulation underneath.

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