cloud_swift,

There are two parts to figuring the size of a tankless heater:

One is to calculate the gpm of all of the fixtures that are reasonably likely to be operating at the same time. You do not make any allowance for mixing in cold water. So, for example, if you have a two showers at 2.5 gpm and and two bathroom sinks at 1 gpm each you have 7 gpm.

The other part is calculating your temp rise: you need to subtract your water inlet temp (at the coldest) from 105 to come up with the required temp rise. So, for example, let's say you have 45 degree water in the winter. To get that to 105 you would need to raise it 60 degrees. For the calculation, use the 105 number - not 110 or 120. This removes the "how much cold water is mixed in" out of the equation. Later, you can set your heater to 110 or 120 and it will be fine.

With the GPM number and the temp rise number you can consult the chart that the manufacturer provides that will tell you how many gpm a unit will heat at a given temp rise.
You should know that most units with the same gas BTU input will heat about the same gpm unless you get into condensing units - which will do slightly more.

A note: You may not use your old flue with your new tankless. Period. The cost of installing the new Cat III stainless flue may be a deciding factor in what unit you choose. Consider a condensing unit that vents with PVC if the vent costs are too high.

"With a model that is too weak you will experience water that changes from hot-warm-cold-warm-hot and it"

Not true unless you buy a cheapo undersized unit from HD or an electric tankless, and exceed their output capacity.

Today's quality gas on-demand tankless systems have water control valves regulating the output volume maintaining temperature at or close to desired setpoint. If your demand exceeds GPM capacity of unit capability, only the total flow or volume is affected, not temp.

Jake, Yes I understand that aspect of the calculation, but how do I know what temp to use for the winter intake water? You mention 45 degrees but I've never measured mine. We have an 18" deep fish pond and I know that in winter it goes down to below 5 degrees C (or about 40 degrees F) because that is the temp where we have to stop feeding the fish. I'm guessing that 40 is a safe assumption for my intake water since I know there is never an issue with freezing of the underground pipes (less than once a decade or so it gets cold enough here that above ground outside pipes can freeze).

Also, is it okay to size based on an assumption that we never need to run a tub faucet and a shower at the same time? The tub faucet runs about 5 gallons per minute so it is about the same as running the two showers.

Will a tankless water heater that can supply 6 or 7 GPM at a 70-75 degree temperature rise turn on with the flow from a single sink faucet? Not sure what GPM a sink faucet would be. Maybe 0.5 GPM because it may not be turned on full blast?

cloud_swift,

You mentioned that you were worried that if you went with a larger unit, you were afraid that the minimum flow rate would be too high for a bathroom faucet to activate it.

It doesn't work that way.

You should always check the min flow rate needed to activate the burner. Smaller is better. Look for around 0.25 -0.35 gpm. That number doesn't get bigger as the water heater capacity gets bigger. The two aren't related. All of these units use modulating gas valves so they adjust the amount of heat needed to heat whatever amount of water you are using - up to their stated max.

There was one unit, The Bosch Aquastar, that used water movement to power the ignition spark. It was a bad design and it required almost 1 gpm to make it work. Stay away from the Bosch units.

To ZL700's point... the new gas units also will reduce water flow to maintain proper temperature. You still don't want to undersize your unit. You should size it for reasonable combinations of faucets being on at the same time, but you don't have to design it for all of the faucets being on at the same time. I usually look at morning time with a couple of showers going and a sink or two.

To elaborate on what Jake says sizing wise-

I would make sure you can do 2+ showers in a 2.5 bath house to be safe. The overdraw of a good unit means that everyone splits the flow. So should two showers be running and someone does laundry, exceeding the capability of the unit by 2 GPM, the shower that may normally get 2.5 GPM, now may be 1.75-2 GPM till the washer is filled. An inconvenience but only temporary, remember, with all this going on you still never run out of hot water.

The common mistake so often done is to size the heater to the roman tub filler on a whirlpool bath. Perhaps the spout may be rated for 13 GPM so everyone tends to think with mixed water, the tankless must do 10 GPM. While true if you desire the fastest fill, but is it necessary to size the unit for an occasional bath? If you installed a unit that would do 7 GPM + 3 GPM of cold, for a total of 10 GPM, is it such a big deal that it took 3-5 minutes longer to fill? Many times removing this max flow rate requirement allows a single smaller unit to cover the overall normal needs.

My main uncertainty at this point is what to use as the intake water temperature. The plumber that came today said I could figure 60 degrees F for Sacramento in winter which sounds high to me. I was expecting something between 40 and 50 degrees - our pond gets to 40 degrees in the winter but perhaps the water pipes underground don't get quite as cold as that.

If winter water temp was 55 or higher, then 105 takes a 50 degree rise. The Noritz 751 would supply 6.7 GPM at that temp rise which sounds like plenty for this situation. On the other hand, if intake water temp goes down to 40, the temp rise would be 65; the Noritz would only supply 5.2 GPM which seems marginal to me (though its enough for the two showers). The alterantive would be to use the Noritz 841 but that only increases the output at 65 rise to 5.8 and it costs a lot more - list price is about $1K more but I haven't gotten price quotes yet. Also haven't seen prices on the other brands yet.

The Noritz 841 is condensing, but since we only need a straight vent run of about 10 feet, I doubt that PVC will save us much over stainless.

In this case, there are no appliances drawing from this water heater. There is another water heater that services the kitchen, utility room and other baths. The big tub just has a regular tub spout that does about 5 GPM but the tub is tiled so it has a lot of heat mass so filling it one uses mainly hot water to end up with a hot bath after the tile has taken heat out of the water. If we run the water heater at 105 or 110 degrees, the tub would probably be filled with just hot water going in.

The $1k difference is for the condensing. I would say to look for another brand that goes above 199k btu input in non-condensing if you think you need it. I think there are some 225k units.

Remember, you are going to set your water temp on the heater to whatever you need (mine stays on 120). The point is that at 120, I mix in cold water and so the relationship between the temp rise and gpm stays the same as being designed around the 105 degree output with no cold mixed in.

I would also recommend measuring the actual GPM of the shower head with a 5-gal pail for 1 minute.

You will probably be pleasantly surprised how much it actually flows and with both on at same time.

We tested the shower head and the tub filler (actually my DH had already done a rough test on it but we repeated with a bit more accurate method). All tests were done running with cold water only (because the hot water heater has been taken out) - I assume that once there is hot water, the maximum draw from the hot water would be about the same.

Flow was about 2 GPM - it varied a bit in two measurements done at different times of the day but well under 2.5 GPM both times. The tub filler runs at about 5 GPM.

Full on flow at the sink faucets is about 2 GPM too but usually I think they would be on lower than that. I checked what a 0.5 GPM flow at the sink looks like and it is fairly thin stream so I'm not worried about being able to turn the faucet on high enough to trigger hot water production by the heater.

We're thinking of getting the direct vent unit installed. We are waiting to see how much it adds to the cost. It would let us seal up vent holes in the floor and ceiling of the water heater closet which probably bring some heat into the house in summer and let some escape in winter. Also that would let us use the bottom half of the closet for storage with the tankless water heater in the top half without worrying that something might block airflow from a vent in the floor.

The flow at the heads through the heater will be less due to the pressure drop, so everything could be about 10% less.
Don't worry so much about the tub filler flow as I stated earlier, whether it is 4 or 5 GPM, I don't think you would notice the difference.

As to your inlet water temp, Sacremento doesn't get that cold. Remember that your pipes run underground and that they don't freeze. People in Wisconsin might see 40F water, but they have 18' of snow in the front yard. I live in VA and I might see 55F from my water when it's 15F outside, so 55F might be good for you.

We got a call back from our water department. They said that because they use mixed source water (ground water and river water) that the winter temperature runs around 55 to 60 degrees. My DH and I measured the temp we like our showers - 105 for him and 110 for me. With a 55 degree rise to provide my shower temp, the Noritz 751 can produce 6.1 GPM which should be plenty for two showers even with some simultaneous sink use.

The Noritz 751 installation was finished today and it seems to deliver hot water to my bathroom sink as fast as the old water heater. Any delay for it to turn on is overwhelmed by the delay getting the cold water out of the pipe. They did a nice job of the installation so the bottom half of the old water heater closet is available for storage.

The estimate if we had gone with the Noritz 841 came in about $400-500 more than the 751 so I can see how for those with a longer duct run the 841 could come out cheaper.

Good for you. I'm happy that you took the time to let us know how it went.

Don't forget to use the 30% tax credit and also see if your gas company offers any rebates.

Cool. Also, don't forget that you can just set the remote at 110F and be good to go. Our Noritz (N-069) is at 105F. You'll be less likely to have it shut off if your summertime cold water temp gets high enough. And it reduces somewhat the heat loss in the pipe between the heater and point of use (hotter water pipes lose heat faster).

We had our first showers with the new heater this morning. It's great. With it set to 110, the water temp was almost hot enough for me but right at the edge of what I like so we kicked the temp up to 115 so that I could mix in a little cold water and get it just where I like it. Then we did some experiments. We turned on the other shower with all hot water and it handles both fine. We added a sink faucet with the hot water turned on full and there still wasn't any noticeable drop in pressure. Two showers plus two sinks did cause a drop in output but it wasn't terrible (and usually the sinks don't get turned on full blast).

Our intake water temperature right now is about 60 degrees so in winter time it doesn't drop much - maybe 5 more degrees which would drop the output by about .5 GPM. That should still be enough for both showers even if someone turns on a sink.

Jake, we won't forget the federal tax credit. Our gas company rebate excludes tankless. I don't know why - perhaps they figure the federal credit is enough incentive. Their rebate for storage water heaters is only $30.

Thull, I don't understand what you are saying about summertime cold water temperatures. Is there a problem if the temperature rise is too small? It probably doesn't affect us because our summertime intake water temperature stays pretty cool. The ground (well) water temperature doesn't vary that much seasonally and, despite our hot summer daytime temps, even our surface (river) water is fairly cool year round because it is coming from higher elevations.