Any advice is appreciated.
I built a green duplex a couple of years ago in west L.A.; it's working pretty good overall except; it's a 3 story building; I have showers on the 2nd and 3rd floors and each unit has a tankless w/h mounted on the exterior wall at grade level.
It takes a couple of minutes for the water in the shower to heat up; a waste of gas and water.
I tried installing a "Chilipepper" recirc. pump. (no dedicated return line; just a typical plumbing system; totally seperate ea. side) in the rental unit side; it does deliver hot water in about 10-15 seconds or so (good); but the tenant reports the water goes back cold during a shower; that won't work!
Probably the recirc. pump is pulling the hot water faster but the lines themselves (copper) are still cold (?). The Chilipepper unit shuts itself off as the water gets warm.
I don't think any other sort of recirc. pump will work any better; maybe a pump that stays on for some time would be better but I'm skeptical.
Now I'm thinking about an electric instantaneous w/h for the shower; you turn on the shower to hot; the instant w/h provides hot water; then I'd need the instant w/h to shut down as the gas tankless w/h has heated up the lines.
I've also thought about what the call "electric showers" or "electric showerheads". There seem to be quite a few of these; almost all seem to be made in the UK; I guess they're popular over there.
The problem with that is I have a typical US prefabricated shower unit w/ typical shower valve and shower head locations; I guess I'd have to re-route some of the plumbing to make the "electric showerhead" work and obviously add power to that location; no really big deal but the showers won't look too good then. The "electric showers" seem to all be a small plastic box that mounts on the inside shower wall with an adjustabe showerhead routed from the box; not really made for a US style retrofit as far as I can see.
Again; it would be nice if whichever instant type w/h I tried to use would shut itself off and just let the water flow as the tankless unit gets going.
Or...would I be better off just going totally instant heating on the showers; and forget the gas tankless w/h for showering?
It seems someone needs to invent an instant electric replacement shower mixing valve that fits in the typical size of a typical US shower mixing valve; ...that also shuts itself off as the water from the main w/h takes over.
Electricity tends to be pricier than gas in L.A.; I would like to use the gas more and electricity less but the time delays for the showers to get hot really is an energy and water waster.
Any thoughts gentlemen?
Any assistance is appreciated.

are the lines between the water heater & the shower insulated? I would guess that the root of the problem is that the heater is mounted so far from the shower.

No; not insulated. Yes; I screwed that up. But being in west L.A.; a very mild climate; I figured that insulating hot water lines was only for a colder climate. I think insulating the hot water lines would only help for after you've already heated the lines up once.
For example; when I get up in the morning; the lines would be cold whether they're insulated or not.
They're not really that cold; a typical morning temperature this time of year is about 50; maybe in the 40's if it's really cold.
Most of the run of the lines is within the overall building insulation; but part of it is on the ground floor where the garages are and that has no insualtion.
The more I think about it I think just a very small instant electric w/h put into the hot water line before the shower mixing valve will do the trick.
That way the shower mixing valve and showerhead do their thing like they always do; the instant w/h should make the shower hot right from the get go.
Again; any thoughts are appreciated.

What is the water temperature during a shower at the water heater? At the showerhead? But based on the water initially being hot and then cooling, I'll guess that you have a lack of capacity - could be that the heater needs cleaning.

Thanks for all replies.
Maybe I should have been clearer; the water in the showers stays perfectly hot w/o using any recirculating pump; it just takes about a minute and a half or 2 minutes to get hot; then it'll stay hot.
The tenant says for some reason when the Chilipepper recirc. pump is used the water gets cold again(?). Maybe she is turning the shower handle back to cooler because it's too hot?
The tankless w/h's are Paloma Gas units; I bought the larger capacity Paloma units; 19k btu I think? They're mounted outside; they seem to be working correctly.
The Paloma units have an interior digital control; we've got ours set at the max of 140 deg. When the showers get hot you do have to turn the water to the middle or it'll stay too hot.
I have noticed the water temp. in the showers when using the tankless units does sort of move around a bit; it's not perfectly stable like a w/h with a tank.
I guess that's just how the shower mixing valves are working with the tankless units and the distance from the ground floor tankless units to the 3rd floor showers. It's not really that far though; the vertical difference is maybe 18', and the horizontal distance is maybe only 8'; seriously; they're fairly close.
The time to heat up has been the same since new; the whole building was new construction from the ground up finished Jan. 2008; the tankless units have been used now about 35 months.
I'm still leaning towards a very small instant electric w/h just before the shower mixing valve. If I go that route I'll need to figure out whether to use the instant heater ONLY for the shower heating or whether to run the instant heater in line w/ the larger gas tankless w/h downstairs; probably the former.
Thanks for any insights.

Craigj;

I also have a tankless propane mounted outside and it takes a while to get hot water beacause our was a retrofit and located about 35 ft from the original tank heater, so it take just that much longer to get to where it needs to be.

The reason the water gets cold is 140 is too high and is in conflict with the anti-scald valve in the shower, the valve will actually stop the hotwater flow and then there is a void in the pipe because there is no call at the tankless unit.
The solution for ours was to set at 115 therefore letting a contant flow at the showervalve without the antiscald trying to shut it off.

OK; thank you very much Mr. Kavala; that is helpful to know; I should have thought of that.

Yeah; I could just leave it as it is and live w/ the 1 1/2 to 2 minute wait; no big deal, but it is still wasting water and gas; I'd like to improve that if I can.

I should have mounted those tankless units closer to the shower and I should have insulated the pipes I guess but man; they're really not that far apart.

A regular w/h would probably take a bit to warm up too I guess w/ the same distance and no insulation on the pipes.

I'm still thinking about an instant unit; but I'm afraid that will cause some other unknown problem w/ the larger gas tankless unit if I mount them inline, or if I disconnect the tankless unit from the shower and use only an instant unit for the shower I'm afraid the instant unit won't provide enough temp. rise and flow. I'd probably have to go w/ a smaller 100v instant unit because I don't have any 220v up there.

Thanks again gentlemen and of course any more comments are welcome.

craigj;

I insulated my pipes where possible, ........... I think that the instant units are OK for hand washing, but showering may be too much

If temperature variations are the issue, you can add a thermostatic mixing valve near the shower. If you look up the temperature rise and gpm of the small instant units, it's pretty small.

Craig,
If the showers are equipped with water saver shower heads (<2gal/min), a small thermostatically controlled POU instant water heater located near the showerhead would provide an immediate supply of hot water. When the tankless finally delivers hot water to the showerhead, the POU heater automatically shuts down. These units aren't cheap (~$250), but they work great. I have a 40A Eemax unit installed at my kitchen sink, but could have gone smaller. The downside is these water heaters are powered off 240VAC, which can complicate and drive up the cost of installation.

Craig,

If the water goes cold after the Chilipepper, it may be a function of the Paloma.  I don't know the details of the Paloma, but the Bosch 635ES that I had in my previous house would go through a post-purge cycle where the exhaust gas is vented out after shutoff of the tankless, and cools the heat exchanger down as well.  So, when the Chilipepper runs, it heats the water, then it turns off, the tankless goes through post-purge, and you fire up the shower, and it heats up again, but that section of water in the heat exchanger during the post-purge is cooled, and thus a "cold water sandwich" in the pipe where it goes hot-cold-hot.

Some of the newer tankless designs eliminate that, so it may be a selection of equipment.  The Monitor tankless that I tried since at a showroom, I was able to turn on and off and on and off at will the hot water and I never got the cold water sandwich effect, whereas the Bosch I installed, we just learned to live with it, such as when washing dishes we would leave the hot water running above the minimum flow rate so that the tankless wouldn't shut off, but I'm sure it caused some issues with the clothes and dish washer where the water coming in wasn't as hot because it was mixed with the cold water sandwich.

Victor
www.ecobuilthome.ca
A 4350sqft Net Zero Energy initiative

The "cold water sandwich" effect is why I've stayed away from tankless water heaters. Can you recall the brand and model of one that is designed to eliminate that nasty characteristic?

Using a small (3-5 gallon) electric tank heater as a buffer tank near the load, disabling it's heating element and letting the tank thermostat control the recirculation pump to heat the tank would be a very decent fix to a number of tankless quirks and long-call path issues. Set the tank to 120F or something, and the tankless to 140F, and intstall a mixing valve on the output of the tank set to 110-115F. That way there's always a couple of minutes-worth of shower-flow stored locally for zero-delay, and there's never a cold-water sandwich issue.

[edited to add] Clark:  Models that have a mini-buffer inside the unit will work.  IIRC all of the Naviens with an "A" after the model number have a small pump & tank inside, that is time-of-day programmable to avoid overnight standby losses when no hot water use is anticipated. But there are others- read the specs.

Clark,

This was the one I played with: http://www.monitorproducts.com/products/waterheaters

I kept turning it on and off several times, and no temperature fluctuations at all.

I'm also liking the Navien units very much; Dana1 is right, the A models have a recirculating pump that basically takes the minimum flow rate down to a trickle from the typcal 0.5-0.7GPM, and the Navien units are up to 98% efficient.  We actually have a Navien unit installed in our house as a backup heat source to the Daikin Altherma ASHP as opposed to using it for domestic hot water, and the specs are very impressive indeed, along with the price being very competitive against the rest of the tankless water heaters.

Victor
www.ecobuilthome.ca
A 4350 sqft Net Zero Energy initiative

Victor, thanks for the heads up on the Monitor tankless and Navien water heaters. And the best of luck on your home building project. --Clark

Thanks Clark.  We're almost complete, about 3 weeks from an occupancy permit, and the solar panels should be on in the next month or so as well.

I'd throw a caution on either of the two tankless brands I've mentioned.  Not from a functional perspective, but from a serviceability perspective, particularly if there isn't a local rep in your area.  Unlike a Bosh/Paloma/Takagi/Other popular brands where there's a presence in most markets, these ones are less well known, and if there's an issue and if there isn't a strong rep in the area, troubleshooting and/or parts may be an issue.  Navien is making significant inroads from what I hear, and with their specs (particularly with the low minimum flow rate) I think this one is a winner.

Victor
www.ecobuilthome.ca
A 4350sqft Net Zero Energy initiative

I'm sure it varies a lot by region and locality, but I've read some pretty wretched installation & support issues with Navien in the past year or so (installations by incompetent unqualified non-factory-certified installers, with no backup from local distribution, etc.) I'm sure it's not a situation unique to them, but be sure this isn't the first installation by whomever sets it up, and that there is a reasonably local & responsive distribution chain. That said, I personally know a few satisified Navien owners (one of whom uses it as backup for a solar space-heating/DHW combi.)

There are many Navien xxx-A installations where neither the installer nor the owner seem able to RTFM well enough get the time-of-day on the buffer programmed correctly, making the apparent delay even worse than non-buffered versions. It's not hard, just follow the instructions, but clearly not a product for those who still have 12:00 flashing on their microwave oven 3 years after purchase. :-)

Noritz & Rinnai seem to have pretty good support in most of the US, with Takagi trailing a bit (although their USA telephone support gets good reviews, talking even less-technical homeowners through system debug.) Navien seems to be competing more on price/performance than on service.

We're planning a house and these situations have convinced me a boiler is the way to go. Feed the radiant and domestic hot water in one efficient no-nonsense boiler. At least that's my understanding. Do you even need a water tank with a boiler? Or will it heat up the water like a tankless (hopefully faster though).

That's what I did. I coupled a 40 gallon indirect water heater with a boiler that gives priority to domestic hot water. You must have a tank that separates the higher pressure potable water from the low pressure water in the boiler loop. Hot water recovery is very fast and standby losses are small. This setup is probably the most efficient way to provide domestic hot water.

Unless your boiler has a real monster of a burner (indicating you live in a drafty McMansion in cold country if it's sized properly to the heating load) it's unbuffered output won't be anywhere near that of a gas fired tankless. But buffered with an indirect tank you can get higher instantaneous flow rates (gallons per minute) than with a tankless, as well as a respectable gallons per hour hot water rates. The size of the indirect you'd need depends on both the BTU output of the boiler and how much hot water you'd need over a reasonably long time frame (gallons per hour).

With an indirect you can run gpm rates much higher than the burner alone would support (even higher than with a tankless) but only until the indirect's temp dropped below an acceptable level. Bigger indirect==more time at high flow rates. It's not bad way to go- it's far more efficient than standalone tanks if you do it right, and doesn't short-cycle the burner into lower-than-advertised efficiency the way a tankless on-demand does on small draws. It's not as efficient in HW heating mode than a condensing tankless, but is comparable to that of "standard efficiency" tankless than scores an 0.82 on an EF test. But a condensing boiler more than makes up for that in enhanced space-heating efficiency.

Then there's idiots like me, using a tankless Takagi as a boiler, plumbed to a "reverse indirect" that is both the heating-system's buffer, and heats the potable hot water with an internal heat exchanger. The tankless never runs less than 10-12 minutes at a time so it suffers minimal cycling losses, and rarely modulates to more than 1/3 of full output, even with all zones calling for heat while somebody is in the shower. (A drainwater heat exchanger on the main drain was critical in getting full-on shower performance during zone calls for heat without having to crank up the output temp on the tankless or the heating system's water temp. Tub filling can be an issue mid-winter with the zones running.) There are many ways to skin the combined-load cat...