Hi,

The thing to know is that gas tankless water heaters can work very well at a reasonable cost if the entire thing is thought out well. If not, they can be a lot of trouble.

Retrofitting from tank to tankless is more expensive than installing tankless in the first place, AND can sometimes be less than optimal unless it's possible re-think the plumbing system ( a major remodel, for example). Issues such as gas supply to the home, gas pipe size to the heater, venting, distance from the heater to the point of use, pipe insulation, circulation return line, water hardness, etc. all enter into the mix.

There are some good reasons to go tankless, the most compelling is the continuous hot water. Space saving can be another. Solving venting issues (eliminating or abandoning a masonry chimney, for example) is another. Although gas tankless is very efficient and can be a good choice from a "low energy use" standpoint, the cost savings - particularly for a retrofit - will never come close to any level of "payback" vs a standard gas tank.

Gas tankless heaters take longer to supply hot water. It takes several seconds for them to fire up, then heat the water, and for that hot water to get in the pipes. A standard tank water heater has hot water ready to go, and the water is often warm in the first 3 feet of pipe. Gas tankless can benefit from insulated hot water lines and recirculation loops on longer runs. A typical situation where a gas tankless heater is a bad retrofit is where the heater is in the garage at one end of the home, and the master bath is at the other end and the hot water pipes are uninsulated and without recirculation. Granted this is a bad setup for a tank heater too - but you notice it more with a tankless.

I don't know the details of your situation - if there is a recirculation loop. On a house that size, it will make a difference.

The water heater you mentioned (200k btu/hr) will heat 9.5 gallons of water from 70 degrees to 105 degrees per minute. We assume a temperature of 105 for showers. If the inlet water temperature is lower than 70 degrees in Texas, your water heater will reduce the water flow to maintain the temperature. We assume 2.5 gallons a minute for a shower. So, assuming 70 degree inlet temp, 2.5 gal (standard shower head) = 3 showers at the same time with 2 gpm left for other uses. It can produce this amount of hot water indefinitely.

Just so you are aware: a 200k BTU / hour gas tankless is as large as they make for residential use and you would want nothing smaller. All brands of gas tankless heaters that are condensing units will produce the same amount of hot water - there's no changing physics.

There is an option between gas tank and tankless that can be a good solution in your case:

There are water heaters that have a large tank + a high input condensing heater. These provide hot water faster than tankless and can provide some level of continuous hot water (4.5 gpm in your situation) and recover very quickly. I'm not endorsing (or not endorsing) the particular brand in the link... just is an example. It has a 50 gal tank and a 100k btu / hr input and vents with plastic pipe vent. One of the advantages is that the existing gas line will usually supply 100k btu / hr where it will usually not supply 200k btu / hr for a tankless.

Thank you so much for your thoughtful and detailed response. I really appreciate it!

I don't know the details of your situation - if there is a recirculation loop. On a house that size, it will make a difference.

Our situation is that 1) I've been freaked out for years about having these tanks in the attic over our heads while we are sleeping (why oh why do they do that in TX?), so I probably will sleep better without that; 2) Currently there are 2 tanks, so the plumbers have said they would utilize that existing plumbing and the recirculating plumbing lines would serve the further side of the house; 3) We have a 4000 sq ft house which is quite wide, so the recirculating pump will stretch over to the other side of the house and hook into the existing plumbing lines that come from the second tank; 4) Our house is often very full with lots of family and friends staying over (about maybe 6 times a year) and when that happens, all the bathrooms are being used, kitchen, etc.

AND can sometimes be less than optimal unless it's possible re-think the plumbing system ( a major remodel, for example).

The plumbers I've had here so far are from larger companies that employ all licensed plumbers and they have said that no re-work of plumbing is needed. I don't have any way to verify that, but based on what I see in the attic and where the current tanks are located it seems to make sense.

Issues such as gas supply to the home, gas pipe size to the heater, venting, distance from the heater to the point of use, pipe insulation, circulation return line, water hardness, etc. all enter into the mix.

In the proposals they did put in there that they had to increase the size of the gas pipe from the heater to the outside of the house and they included that in the bid. They also put in the bid that the venting would be replaced and vented through the roof and a roofer would do the work like flashing. They did bid a recirculating line. I think it's about 20 feet one-way and looping back. We have a whole house water softener.

Solving venting issues (eliminating or abandoning a masonry chimney, for example) is another.

The venting would not go through a masonry chimney. It would go from the unit through the roof.

Although gas tankless is very efficient and can be a good choice from a "low energy use" standpoint, the cost savings - particularly for a retrofit - will never come close to any level of "payback" vs a standard gas tank.

We are not so worried about pay-back because this is our forever home. We don't have jobs that would require us to relocate and we have tons of family within walking distance of us, so we are not moving til someone rolls us out in a wheelchair.

Gas tankless heaters take longer to supply hot water. It takes several seconds for them to fire up, then heat the water, and for that hot water to get in the pipes.

Is that also true if you have a recirculation line?

A typical situation where a gas tankless heater is a bad retrofit is where the heater is in the garage at one end of the home, and the master bath is at the other end and the hot water pipes are uninsulated and without recirculation. Granted this is a bad setup for a tank heater too - but you notice it more with a tankless.

I think in our case the heater will be at the top of the attic stairs, where tank #1 is, and then the recirculation lines will loop over to where the 2nd tank is currently.

The water heater you mentioned (200k btu/hr) will heat 9.5 gallons of water from 70 degrees to 105 degrees per minute. We assume a temperature of 105 for showers. If the inlet water temperature is lower than 70 degrees in Texas, your water heater will reduce the water flow to maintain the temperature. We assume 2.5 gallons a minute for a shower. So, assuming 70 degree inlet temp, 2.5 gal (standard shower head) = 3 showers at the same time with 2 gpm left for other uses. It can produce this amount of hot water indefinitely.

The winters here are short, so the ground water isn't usually very cold. It's Houston area.

Just so you are aware: a 200k BTU / hour gas tankless is as large as they make for residential use and you would want nothing smaller. All brands of gas tankless heaters that are condensing units will produce the same amount of hot water - there's no changing physics.

Thank you for that information.

There is an option between gas tank and tankless that can be a good solution in your case:

There are water heaters that have a large tank + a high input condensing heater.

It took me a long time to get comfortable with tankless; I don't think I have brain band-width to start considering other hybrid technologies!!

Thank you so much for this information.