Hot water for new shower

[Richdomb]

I'm adding new addition including a walk in shower with a Delta multi head shower. Looking to ensure enough hot water for an decent shower. I have a 40 gal propane power vent water heater that replaced a 40 gallon
standard vent that was removed due to chimney going away. The shower is 30 to 35' from current heater I had an idea of putting old water heater right under new shower as non working storage tank and have a recirculating to old heater. Does this seem at all practical. and if so do you have any suggestions on the details. Thanks for any replies.

 

[Jadnashua]

What is the gpm rating of the combined shower heads you intend to use? Typically, a 40-gallon tank would be nearly useless for anything other than a few minute shower with a high flow rate. The spec sheet on the WH should give you a first hour rate, but keep in mind, that's literally one hour's use. That might be say 120g/60min=2gpm, not even enough potentially to feed one standard head as the incoming water dilutes the temperature of the tank. FWIW, you can typically get around 75-80% of the capacity of the tank at the nominal temperature before it starts to cool off when trying to empty it fast since the burner won't have much time to reheat much of it.

Then, you need to consider how large the supply line is and what it is made of. A 3/4" line in copper shouldn't be asked to flow more than about 8gpm on the hot side or friction losses to the pressure available start to add up along with internal erosion of the pipe and noises.

 

[Sponsor]

 

[Richdomb]

What is the gpm rating of the combined shower heads you intend to use? Typically, a 40-gallon tank would be nearly useless for anything other than a few minute shower with a high flow rate. The spec sheet on the WH should give you a first hour rate, but keep in mind, that's literally one hour's use. That might be say 120g/60min=2gpm, not even enough potentially to feed one standard head as the incoming water dilutes the temperature of the tank. FWIW, you can typically get around 75-80% of the capacity of the tank at the nominal temperature before it starts to cool off when trying to empty it fast since the burner won't have much time to reheat much of it.

Then, you need to consider how large the supply line is and what it is made of. A 3/4" line in copper shouldn't be asked to flow more than about 8gpm on the hot side or friction losses to the pressure available start to add up along with internal erosion of the pipe and noises.

 

[Dana]

An atmospheric drafted propane water heater has huge standby losses compared to an electric tank, due to convection through the center-flue heat exchager, the insulation clearances around the burner, and the (usually) much thinner insulation. The power used by the recirculation pump will add up too. A cheap 40-50 gallon electric (wired-up and functional) that takes it's input from the new propane water heater is a better bet.

The power used to cover the standby losses of the electric water heater could even be lower than that of the recirculation pump scheme, but the heavy lifting of heating the water will be done at the higher output rate of the propane burner, not relying (much) on the electric elements for anything but covering the standby losses.

With gusher showers and high priced energy such as propane or electricity it's usually well worth including a drainwater heater exchanger to feed both the cold input to the primary water heater and the cold side of the shower with partially heated water. For high flow showers it's important to use a heat exchanger with the lowest possible pressure drop. The EcoDrain V1000 series is the new industry leader on that, but Renewability's multi-coiled PowerPipe series isn't bad even in a 4" x 60". (Others will sometimes have a problem delivering the 6, 8, 10 gpm through the heat exchanger in a high flow app.) If PowerPipe you'll probably want the C4 series for the lower pressure drop if the anticipated peak flow is much more than 6 gpm.

The fattest and tallest that fits will "pay back" quicker than smaller units on fuel savings, despite the higher marginal cost of the unit. Expect to spend about a grand, but for multi-spray units the cost per shower can be pretty high, and a decent heat recovery unit will return more than half the energy that's going down the drain back to the water heater & shower, cutting those costs in half.

By returning a good fraction of the heat, the "apparent capacity" of the water heater is extended, since the now warmer water going into the cold side of the mixer reduces the amount of hot water needed in the mix, and the warmer water entering the water heater requires less heat to come fully up to temp.

Natural Resources Canada developed a standardized test protocol for making apples-to-apples efficiency comparisons between different vendors and models. They have a downloadable spreadsheet of all third-party tested drainwater heat exchangers in their database here.