Tankless hot water heater in addition to HPWH

[Nsherman2006]

Hello,

I'm remodeling a bathroom and am planning on installing 2 showers. I'm also installing a 50 gallon heat pump water heater (already have it, so stepping up to a larger tank isn't a feasible option). I'm concerned that a long shower with both showers running might exhaust the capacity of the tank, and was wondering if it's advisable to install an on demand tankless heater after the HPWH in order to prevent cold showers?

I'm assuming that if the water is up to temperature, the tankless heater will remain off, and only kick in when the temperature drops. If so, thoughts on sizing such a unit? Are there any 120V units that might get me by (space in the breaker box is at a premium with both solar and a generator each taking up 2 slots)

Thanks!

-Neal

 

[wwhitney]

A tankless electric unit that plugs into a 120V receptacle and draws 15A would have a power draw of 1800W. That would be 102 BTU/min, or 12 deg F/gallon/min. So in other words it would be able to heat water for a 2 gpm showerhead by about 6 degrees F.

That's not going to work as an extender that only runs when your tank runs out of hot water. You could use it as an extender that runs during the entire shower--by delivering hot water to the shower that's 6 deg F warmer, you'd mix in more cold water and draw less hot water, so your tank wouldn't be drained as quickly.

But really it's not worth it and you're best off skipping the idea. If you find that running out of hot water is a problem in the future, you can address it then. About the only thing worth doing now is making choices that would make it easier to add a second water heater in the future.

Cheers, Wayne

 

[Sponsor]

 

[WorthFlorida]

I've read a few reviews on web sites and one thing a lot get wrong with a heat pump water heater. They are hybrids and have two electric elements as any other standard WH. The program on theses allow heat pump only or both. Most set to HP only thinking that is will save on the electric bill and in no way can it meet most demands. HP get its extreme efficiency when water is not used keeping the water at or near the set temperature with the heat pump.

Two ways you can meet a higher demand. One is, if the HP WH allows it, set the temperature to 140 degrees, then add a temperature reducing valve to adjust the output of the hot water supply to 110-120 degrees. A second way is to add a point of use electric water heater. This site has dozen of choices for most applications including "booster WH". https://www.chronomite.com/home

As wwhitney suggest try it without the add ons. You could just run empty conduit or pre-wire to the location where a "point of use" WH would be installed.

 

[Nsherman2006]

Thanks!

The physics explanation ruled out 120V heaters real quick. I will set the hot water temp on the HPWH as high as it goes and use a tempering valve to drop it back to safe temps. Also, the "booster" heaters appear to be exactly the solution to my problem, and they tap into the same 30A 240V circuit as the water heater itself.

You guys rock!

 

[Fitter30]

• Since tankless water heaters pull lots of amps and if you don't have 400 amp service I'd check out propane. Heat pump wh would turn on the electric elements just after a few minutes and still wouldn't keep up.

 

[Dana]

• Since tankless water heaters pull lots of amps and if you don't have 400 amp service I'd check out propane. Heat pump wh would turn on the electric elements just after a few minutes and still wouldn't keep up.

A propane tankless is a pretty lousy "finishing heat" solution here, since the minimum firing rates and minimum flow rates are too high to be useful at the low delta-Ts when the main tank is fading. This was an oft-tried solution for solar thermal tank back up that rarely (if ever) worked well, and it NEVER works correctly if plumbed in series with the tank.

For the kind of money it takes to buy a backup water heater, a drainwater heat exchanger of any decent size would add a substantial amount of "apparent capacity" for showering mode (albeit nothing for tub fills), assuming you can find at least a 4' section of vertical drain downstream of the shower.

The best part about this solution is that it doesn't use any electricity (or propane), just takes heat from the water going down the drain an putting into the incoming water stream. That makes the"cold" side of the shower mixer much warmer, requiring less flow from the water heater, and with the warmer (tepid, not cold) water going into the water heater the recovery time is quicker.