Can one tankless replace two tanks?

[Jacob]

I am remodeling a one story 4500 sqft house and plan to convert to a tankless water heater. The house was designed with two water heater when it was built in the 60’s. One on each side of the house. One serviced two full bathrooms and I plan to add a laundry closet to this end of the house. The other services two full bathrooms, the kitchen and laundry room with a sink. they are setup as two separate systems.
Here are my ideas.
1. One tankless heater with recirculating pump to service the whole house.
2. Two tankless installed in series with recirculating pump servicing the whole house.
3. Two separate systems each with is own tankless water heater.

I am installing all new PEX water line through the entire house. Making this more like new construction. Using the existing copper isn’t practical with several water fixtures being relocated and all waterline being under the slab. The PEX will all run through the attic, which will be a conditioned space as I am spraying the roof deck with foam.

 

[Dana]

What's the biggest tub you need to fill?

What is total flow if both bathrooms are running showers? (Test them with a bucket and a stopwatch.)

Installing two tankless in series is a lousy solution- take that off the list. At high flow the pressure drop across even one tankless can be enough to cause issues with shower mixers, etc.

At the high summertime humidity in Euless TX a single heat pump water heater and a recirculation loop probably makes the most sense. About 2/3 of the heat going into the water is taken from the room air, lowering the overall cooling load, but more importantly, in your area that would be mostly latent- load, taking the heat of vaporization of the moisture it's removing from the air and stuffing it in the tank. (That technology has gotten a lot better in the past dozen years.)

 

[Sponsor]

 

[Jacob]

What's the biggest tub you need to fill?

What is total flow if both bathrooms are running showers? (Test them with a bucket and a stopwatch.)

Installing two tankless in series is a lousy solution- take that off the list. At high flow the pressure drop across even one tankless can be enough to cause issues with shower mixers, etc.

At the high summertime humidity in Euless TX a single heat pump water heater and a recirculation loop probably makes the most sense. About 2/3 of the heat going into the water is taken from the room air, lowering the overall cooling load, but more importantly, in your area that would be mostly latent- load, taking the heat of vaporization of the moisture it's removing from the air and stuffing it in the tank. (That technology has gotten a lot better in the past dozen years.)

Thanks for the input. The largest tub is 68 gallon. I’m not sure on the shower flow rate yet and can’t test with bucket, because I am still remodeling and haven’t lock in on the fixture or built the showers yet. It seem all the new shower fixture have lower flow rates than older models. I was hoping one unit will keep up with demand. The cold water isn’t very cold in the south, so a lower temperature rise is required than say a northern climate.

 

[Sylvan]

If you know the available pressure and the size of the piping your using the GPM and velocity can be figured out mathematically Close enough without having to figure friction loss

 

[Dana]

Thanks for the input. The largest tub is 68 gallon. I’m not sure on the shower flow rate yet and can’t test with bucket, because I am still remodeling and haven’t lock in on the fixture or built the showers yet. It seem all the new shower fixture have lower flow rates than older models. I was hoping one unit will keep up with demand. The cold water isn’t very cold in the south, so a lower temperature rise is required than say a northern climate.

Thanks for the input. The largest tub is 68 gallon. I’m not sure on the shower flow rate yet and can’t test with bucket, because I am still remodeling and haven’t lock in on the fixture or built the showers yet. It seem all the new shower fixture have lower flow rates than older models. I was hoping one unit will keep up with demand. The cold water isn’t very cold in the south, so a lower temperature rise is required than say a northern climate.

The deep well temperatures in your area are in the mid to high 60s, so a mid-winter incoming water temp on a city main could be in the 55F range.

With 55F incoming water a 65 gallon heat pump water heater set to 135-140F could deliver 68 gallons of 110F water into that tub (with margin), but a 50 gallon water heater (any type) would be pushing your luck, unless that tub doesn't need the full 68 gallons.

A tankless wouldn't run out of hot water, but the fill time could be tedious- the max flow rate at the tub is limited by both the burner's BTU rate capacity and the pressure drop across the heat exchanger in the tankless at higher flow (and only to a lesser degree the pressure drop from the distribution plumbing.) Even at 4 gpm the pressure drop across a typical 199K tankless is on the order of 10 psi, at 6 gpm it's twice that. This is what it looks like with a Takagi H3, but most others are pretty similar:

With tank water heaters the only major pressure drop is the plumbing, making them much better at filling big tubs in a reasonable amount of time.