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Dual Tankless Water Heater Question


Jerry Simon
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New construction with dual tankless water heaters tied/teed into one supply line. Both heaters were fully functional, and ran at the same time while I ran hot water all about the house.

Two thermostat/control panels mounted next to the heaters. Only one was on/lit-up. The other panel/thermostat appeared to be dead (but again, both heaters functioned). Might they be tied together on the one control (though it appeard the control wiring went from each control to only one heater each...can't really recall, though, if this is how they were wired).

We don't see many tankless heaters around here, so does this sound kosher? If so, why have two controls installed in the first place? Or am I missing something? I also pushed (mashed?) the various buttons on the *dead* control, and nothing.

Thanks much for any help.

Control on the left is the apparently dead one.

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It doesn't make any sense to have them tee'd together. They should each serve a zone, maybe one the kitchen, 1/2 bath and laundry and the other for the master and family bath.

Plumbing two together would only make sense of there was a single point of use that could use more than 7gpm of hot water. Was this a carwash?

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Originally posted by Chad Fabry

It doesn't make any sense to have them tee'd together. They should each serve a zone, maybe one the kitchen, 1/2 bath and laundry and the other for the master and family bath.

Plumbing two together would only make sense of there was a single point of use that could use more than 7gpm of hot water. Was this a carwash?

Hi Chad...

No, big 4,000 SF house, with huge (3-4 person) whirlpool tub...that's my guess why they're teed together.

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I'd call it classic misapplication of tankless technology. One sizes the tank to accommodate the requirements of the house; one doesn't install two small tanks because there's a big tub in the joint. That's storage tank thinking, not tankless. The whole idea is you figure your gpm, install a heater that hits the gpm, and fill the tub because it heats on demand.

Or, you put one unit @ the point of service at the 2nd fl., and one in the bsmt. to handle the first floor & bsmt. use, each one sized appropriately for the specific fixture demand.

I think tankless technology is completely misunderstood by all the Chicago plumbers. I've yet to see an installation that made any sense. The really aggravating thing is putting one big tank in the bsmt., and then having to wait 5 minutes for hot water to get up to the 2nd fl. fixtures.

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I once installed a system with three tankless heaters, three spearate mixing valves... all to serve the same system (7 scrub rooms for 14 operating rooms. Each heater could be isolated, the system could be fed from the loop, cold water feed, or the regular (remote) hot water system.

The system was over loaded with redundancy, the thinking of the "intelligent ones" was that in case of a catastrpohe, if one system was out, there would be another as back up... they took it a step further with the third system.

As far as waiting 5 minutes for hot water... that is why you put install a loop with a circulator pump, controlled by an aquastat on the end of the loop. Once you do that, you will have "instant" hot water.

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Originally posted by Bain

It's like when you see two standard water heaters plumbed together in series, rather than parallel.

It's such a stupid thing to do, but it's the norm around here.

That's not uncommon here. Makes sense to me. First unit makes cold water warm, second unit makes warm water hot. If you have two in parallel, would you run two completely independent hot lines to two different zones, along the lines of what Chad said?

I think it'd be slick to put in a couple more valves and a little extra plumbing between the two, so you could isolate one when it fails. Haven't seen it yet.

That's my pipe dream.

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There was actually a really good article in JLC about this a few years ago. The goal is to have cold water entering the inlet pipes of both heaters, so they work equally to supply hot water to the house. The outlet pipes are tied together downstream of the heaters. When plumbed in series, the first heater does the majority of the work and wears out much more quickly than the second. Also, with equal amounts of cold-water-entering and hot-water-exiting the two tanks, the burners fire at about the same time, which results in a greater volume of hot water supplied to the house.

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