Series or parallel water heater

[Cam Wood]

I just finished my house in April. I have a 38 gallon electric water heater in the crawl space. I knew the WH wouldn't be able to keep up with the jacuzzi tub but couldn't afford a second WH until now. The question is do I install them in series or in parallel to maximize available hot water and life of the heaters?

 

[Jadnashua]

It's really a little messy trying to make them work in parallel. I'd put them in series. You should also get a little more hot out of the pair that way.

You can make the existing WH function like a larger one if you run the temperature up and put a tempering valve on the outlet. That will premix some cold giving you some more volume at the set temperature (nominally, about 120-degrees, where the tank could be 140 or higher).

 

[Sponsor]

 

[Fitter30]

Series the first heater collects the majority amount of minerals. Parallel piped in reverse return takes a little more piping but it's the easiest way to have even flow even if the heaters aren't a match.

 

[Bannerman]

When plumbed in Series, cold water will always enter the 1st tank so its electric heating elements will be utilized 100% of the time. As hot water will exit the 1st tank and flow to the 2nd tank, with identical temperature settings for both tanks. the 2nd tank's electric heating elements will be only utilized when the 1st tank's capacity has been depleted which will result in cooler water entering the 2nd tank. The 2nd tank will then mostly provide additional hot water storage, but minimal heating capacity to reduce recovery time.

When plumbed in Parallel as shown in Fitter30's drawing, each heater will always supply 50% of hot water consumed. Since cold water will equally enter each tank, the heating element run time will be equalized between both tanks and the recovery rate will be doubled.

If the water contains sufficient minerals for scale accumulation to be a concern, suggest then installing a water softener system to remove calcium and Magnesium minerals which are the most common cause of scale formation.

 

[MACPLUMB]

In Parallel Per MFG'S SPECS, as the best way to get the most hot water

 

[Cam Wood]

When plumbed in Series, cold water will always enter the 1st tank so its electric heating elements will be utilized 100% of the time. As hot water will exit the 1st tank and flow to the 2nd tank, with identical temperature settings for both tanks. the 2nd tank's electric heating elements will be only utilized when the 1st tank's capacity has been depleted which will result in cooler water entering the 2nd tank. The 2nd tank will then mostly provide additional hot water storage, but minimal heating capacity to reduce recovery time.

When plumbed in Parallel as shown in Fitter30's drawing, each heater will always supply 50% of hot water consumed. Since cold water will equally enter each tank, the heating element run time will be equalized between both tanks and the recovery rate will be doubled.

If the water contains sufficient minerals for scale accumulation to be a concern, suggest then installing a water softener system to remove calcium and Magnesium minerals which are the most common cause of scale formation.

If I run in series, will both WHs initially be fully heated, giving me 76 gallons of hot water?

 

[Bannerman]

will both WHs initially be fully heated, giving me 76 gallons of hot water?

As they will both fill with cold water, then the thermostat in each tank will activate that tank's heating elements to heat the water until the temperature setting in each tank is achieved. Once up to temperature, with any further hot water use, only the 1st tank's elements will be activated unless so much hot water is consumed to where cooler water begins to enter the 2nd tank.

Your original question:

The question is do I install them in series or in parallel to maximize available hot water and life of the heaters?

The correct answer is then Parallel to equalize both WH's operation and increase the hot water recovery rate (heating capacity).

 

[BenG123456]

I have two 50-gallon heat pump water heaters next to each other in series with valves in place to bypass one if needed. They are in series because when I ran the plumbing originally the first slot was going to be for a desuperheater on a geothermal system but that project fell through (bad land for drilling). So instead I put a 2nd heat pump water heater in its place.

These water heaters are pretty cool because they track the electrical usage. I have both set to heat pump only mode (so they can be on the same 30 Amp circuit). I have the first WH set to 120F and the 2nd set to 125F. Right now the first WH is using about 80 kWh per month and the 2nd is using about 20 kWh per month. So 80% of the work is being done by the first WH.

I'm going to add plumbing to switch to parallel flow by closing and opening certain valves. Maybe put globe valves if needed to equalize the flow when running in parallel. Parallel is what I would have done if having two water heaters was the original plan. After a couple showers the first WH will run for a few hours while the 2nd one will only run for 30 minutes or so. Recovery time should be better if I get both of the compressors running for the same amount of time. Right now there's a lot of heating capacity left on the table while the 2nd WH is sitting idle and the first is doing all the work!