58 degrees, 18" below grade is close here on the truly left coast.
Size the appliance for the number of fixtures operating simultaneously, so when 15 of you and your wife's relatives decide to use your "dingy digs" as a staging area and crash site for their visit to Disney Land----------

I generally am with your contractor: there's not enough difference in price to not use the largest one.

I didn't do that on my own install 5 years ago, and it's something I would have done differently. It's only an issue for me in the coldest part of winter (below 0) when I'm limited to 4 GPM.

Even though you don't foresee using 7 GPM, the next family might... or if you have guests.

I appreciate the comments and the confirmation on the water temp. As to where I land, I know the $200 is not a huge differential, but heck it's still 200 bucks. (And it's $400 if I go with the smallest unit.)

Thinking of flow: there's only one shower in the house so it will be impossible for two showers going at once. The tub is rarely used, but it would be easy enough to avoid filling it while someone is in the shower. Dishwasher has a timer to run at night. That leaves the washer (not sure how much water it uses) and running the hot water at various sinks while showering. With just two adults in the house, that washer doesn't run all that often. On the rare times a relative stays with us, it's only one person (and we still just have one shower to share.)

Am I wrong in thinking that if a 2-person household in a smallish house in So Cal can't get by with a smaller tankless then no one can?

Last question: is the gas used in a tankless a function of how much hot water is being run through it? Or would a smaller tankless be more efficient in general?

The amount of gas that's used is a function of how much water is going through it and the amount of temperature rise. I didn't look at all of the units, but the largest one goes from 19k BTU/H to 199k BTU/H.

Smaller isn't going to be any more efficient.

Clearly, you can do what you want. My general advice is to go with the larger one, but if you don't ever see yourself needing it, then don't.

The fact that that a smaller unit is not going to be more efficient is a helpful data point. For the record, $200 only equals about 0.1% of the total cost of this project. OTOH, I'm always looking for a few small victories regarding costs on this remodel.

And yes, the unit that the plumber recommends is 199K BTU. (The others are 180K and 150K--if I downsize I'd pick the middle one.)

Thanks for indulging me as I figure out what to do.

I understand. $200 is still plenty of money.

You might look at the Rheem units - it's what I have and have been very happy with it. The 199k unit is $1320 including vent kit and service valves.

Hold the contractor's feet to the fire on installation costs. There's really no reason for high installed costs. On new install or significant remodel, a tankless is about as hard to install as a standard tank.

"there's only one shower in the house so it will be impossible for two showers going at once. The tub is rarely used, but it would be easy enough to avoid filling it while someone is in the shower"

I am confused: you are doing a total gut remodel to a 2 bathroom home, but only one bathroom will have a shower. The other will have a tub only, with no shower. Am I understanding this correctly?

The value of your house will be affected negatively, much more than the additional cost to add an overhead shower during the extensive remodel. You and your partner may not see the need for 2 showers in a 2 bathroom house that's only 1,500 sq ft. But to my family of 5, the 1,500 sq ft house would be just right, if only it had another full bathroom. Think about the family of 5 that may be looking to buy your house in the future. It makes sense to think about the future owners of a house when you're doing a complete remodel. Don't just think about your immediate requirements.

And I'd go with the largest unit. 199K BTU is pretty standard for a tankless. It's not overkill. I would be a little concerned that the middle sized one is going to be discontinued and parts may not be available ten years down the road. I doubt that many 180K BTU units are sold, when the company also offers the 199K BTU model.

Interesting point about the lack of a second shower. I guess it didn't occur to us since our current home (we're living in our current home while we remodel the one we bought) doesn't have a second shower. We're putting a clawfoot tub in the house we're re-modeling, which also has the potential to be re-sell unfriendly. But my wife has always wanted one, so I told her we should go for it.

Perhaps it makes sense to plumb for a shower even though we wouldn't use it and while it's possible to shower in a clawfoot tub, it's not very comfortable to stand in.

mtimmer,

Even in smaller homes the issue is the ability of two adults to get ready for work at the same time. Having two showers is pretty important for resale if you only have 2 bathrooms.

A claw-foot tub is nice if you have the room in addition to the shower - but I would want 2 showers.

I would go witht he 150 or 180. I'm in N CA and have a 180 Rinnai RL-75. Understand that a shower head will flow from 1.5-2.5 gpm. On my 2.5 gal heads I run 2.1 gal of hot mixing with .4 rpm of cold to arrive at the 2.5gal flow rate. I can run both showers all day long and one other faucet. Understand that tankless will guarantee temp at the expense of flow, so it you are trying to put more water thru the unit that it can handle it will slow down the water flow to deliver the set point temp. You will see a reduction in water pressure in that circumstance. My claw foots vol is 36 gal. Running the water heater it takes about 8 min. to fill the tub.

I would not buy the unit with the built in pump. I would locate the water heater so you do not require any recirc. I have seen plenty of systems where the energy loss in recirc cost more than the cost of heating the water. On my re-model the longest run I have to any faucet from my tankless is 16'. You can and should try to do the same. I chose the .82 Energy factor (EF) over the .95 because we simply don't use enough hot water to make it pay, and the condensing .95EF units are a bit more complicated. Also, pay attention to the actual flow rate of the fixtures. You do not want to large pipe supplying the fixtures. A faucet or shower running 1.5-2gpm does not need 3/4" supply. Tankless do not make hot water until the faucet is open. If you have to large a pipe you then end up with a longer wait

I agree with the comments on the second shower especially from a re-sale point. On my claw foot we have a separate shower in that bathroom so if you want you can enjoy the tub without curtains/doors etc. For resale and when people visit the second shower is nice. Then again, I don't have any friends so it is a moot point for me.