A tankless heater will add a few seconds to the amount of time it takes to get hot water - possibly 5-10 seconds. The reason is that the unit must fire up, and because a tank unit will have hot water in the first 6 feet or so of pipe.

That said, I think there's more going on here than an additional 5-10 second wait.

If you have "low water pressure" in your kitchen for both hot and cold I would guess you really have low volume rather than low pressure. I would look to see if the aerator is clogged. Something is restricting the flow of water to the kitchen, and it's not the water heater (since you have better volume in the bathroom). Low flow would greatly increase the amount of time to get hot water.

Are the hot water lines insulated? My guess is that unless it was done later, a house built in 1940 doesn't have insulated hot water pipes. You need to do that. Also, are the pipes in a slab? I've seen situations where uninsulated pipes in a slab, in a cold area allow virtually all of the heat from the hot water to be lost before it got to a shower. The slab had to heat up before the water would get warm.

Last thing I would do, since this home is new to you, is to have the tankless heater descaled and inlet screen cleaned. You don't know when the last time it was done (unless it's marked on the heater) - and so it would be reasonable to do it so you have a record of it.

So:

1. fix the water flow issue to the kitch sink
2. make sure hot water pipes are insulated
3. descale the tankless and inlet screen

BTW: in what city are you located?

***BTW: in what city are you located?

This is an important question... your heater is a 140,000 btu heater (small) that will only produce 2.9 gpm of 120F water if inlet temps are 40 F.

If you are in a cold climate that is currently experiencing record low temps, you may be bumping into the limits of this size of water heater.

Thanks for your responses! I live in Sacramento. We usually get a few weeks of at or below freezing temperatures, but nothing too bad really.

Im not sure if the house is on a slab. There is a crawl space under the house, so maybe that means there isn't a slab?

Given what I have described, is this something a decent plumber can handle? At first I didn't even know if this was a plumbing issue. Sounds like insulating pipes would be really expensive.

This is something a plumber could handle.

I'm going to assume you are not on a slab (because you have a crawlspace)

I'm going to assume inlet water temps aren't the issue

So... I would find out why you have low flow to the kitchen and shower (clogged aerator, shower head, etc).

Insulating the hot water pipes isn't bad if they are accessible. Pipe insulation comes in 6' sections that is pre-split and pre-glued. You snap it on and remove the adhesive strip to seal it. Hardest part is cutting it at elbows (not at all hard, I use a miter box)

and have the plumber descale the water heater and clean the inlet screen. He will run vinegar through the heater to descale it.

Here is a link that might be useful: Foam pipe insulation

Thank you very much!

You should be able to read the flow on the digital controller on your water heater. Check in operating instructions and find the sequence. I do not have a Nortiz, but can read the actual flow through the unit to within .1 gpm. That info can determine what is going on with your whole system. You could also have too small a gas line. Tankless water heaters are designed to guarantee temperature at the expense of pressure, meaning if you turn every faucet in the house on and you exceed the capacity of the unit it will slow down the flow to ensure the temperature set point is met. Check out the ground water temp with a thermometer. Check the unit specs for output at a specific temp rise. I suspect your ground water temp is close to 60F, so set the unit at 120* see what Noritz says the output will be at 60* temp rise. If the unit reaches that flow rate then your gas line is likely ok. FYI, the number one mistake in tankless installs is improper gas line sizing.

Since there is reasonable water flow in the half bath, I think it's wrong to focus too much on the water heater. Also, the low-flow is a problem for the cold as well as the hot water.

A low volume of water will result in long lead times for hot water - esp if the pipes aren't insulated.

The water flow rate must be fixed first.

That heater can output 3.8 gpm at a 60 degree temp rise. So with 60 degree cold water and 120 degree hot water, it will provide enough hot water for a 1 shower (2.5 gpm) and one bathroom sink faucet ( 1.0 gpm) at the same time.

Good point about the gas line -- but at this time there's no reason to suspect a problem with the heater at all. I recommended descaling it because the homeowner has no way of knowing when it was last done.

So with 60 degree cold water and 120 degree hot water, it will provide enough hot water for a 1 shower (2.5 gpm) and one bathroom sink faucet ( 1.0 gpm) at the same time. Yes, but does the OP really stand in a shower under 120ðF water? Any cold water added to the hot flow at the tap effectively increases the capacity by lowering the hot flow rate.

Dadoes,

Of course you are correct... my bad. Got caught up in what Jackfre was saying.

Even though the outlet temp of the heater is usually set at 120, that's not what the shower is set to. For sizing purposes, 105 is the temp used.

So, should have been 105 for the outlet temp - so a 45 degree temp rise and 5.3 gpm - roughly enough for 2 showers.

So that other people understand, tankless heater manufacturers give a temp rise chart to show how much hot water the tankless will produce at a given temp rise. BTW: it's the same chart for any heater of the same input BTU's and the same efficiency.

Here is a link that might be useful: Temp rise flow chart.

One thing I just did was take out the little screen in the shower head, which really helped the pressure, and in turn, I think reduced some time to receive hot water. It was gross and full of some nasty material. Cant explain why the kitchen faucet takes so long though.

Temp rise flow chart is great. Now at least I understand how it works. I know this sounds stupid but I actually thought raising the temperature on the unit might provide hot water faster...I know now the probably the opposite is true. I guess we will have hot water faster in the spring and summer.

Im going to have a plumber out to descale and tuneup the water heater.

Remove the screen on the kitchen and you will find similar stuff.

Flush the faucet without the aerator and then clean the aerator out in vinegar.

Increasing the flow through the faucet will shorten wait times.

does vinegar do the same thing as CLR?

Does the same thing,,,, but with vinegar, it's food grade so you don't have to worry about anything contaminating your water.