Hi, we have two Tankless Hot water sysems, and are experiencing problems in that hot water, never gets so hot that you cannot stand it, and showers are cool to warm to hotter then back to cold, back to hot, it does not happen all the time but it is happenning more now. Faucets start cool to luke warm and above luke warm, does anyone have this experience and how did you correct the problem?

I installed an electric tankless a few years ago in a spec house. I had the same exact problem. I spent hours on the phone with the mfgr. They sent me all kinds of thermocouples and parts to replace. Finally after a month or so, we got it to work decent.  Gas units are a whole different animal though. There's many variables that affect the temp output. Keep us posted on what you find.

Good Luck.

Bunt.

how old are they and do you have hard water?

Hi, the units were installed into a new home, two years old, home has be up for sale no permanent owner. I do not know if water is hard, we are able to make soap suds, which I indicates to me that it is not hard water. If it was hard water how would it affect the system or vise versus ?

Can I assume you've tried programming them to a slightly higher output temp? (If they're set to 120F at the factory, some scald-valves on shower units will never quite allow enough HW through to make it nice & steamy.)

The "cold water sandwich" startup slug of cool/lukewarm water is a standard feature of gas tankless heaters. When flow turns off and the flame goes off the forced draft fan continues for many seconds to purge exhaust from the flues, and in the process it cools off the water in the heat exchanger a bit. Then, when the flow begins anew there's a delay in getting the flame going, so some cold water passes through the heat exchanger without being heated. This results in variable temps over several seconds. If you have a series of relatively short draws and the distance between the tap an the HW heater is long that can sometime make for very odd temperature profiles along the length of the pipe. But in long draws (baths, showers) it should eventually get "pretty hot" and stay that way.

One (expensive and not space-friendly) solution to these quirks would be to plumb in a small indirect-fired HW tank of the type used with a hydronic heating boiler, using the Rinnai as the boiler. It has the advantage that the Rinnai would then be running a closed loop of "dead" water, so issues like water hardness fall into the background. There would be some standby losses (but not nearly as great as a standard tank HW heater), and you may have to play around with pump sizing and programmed temperature output to keep the Rinnai from short-cycling. (It really needs to be designed, not hacked.) Probably not the right solution here...

Dana1, thank you for your information, your solution sounds like a good start. is this something you have applied to a Rinnai Gas Tankless Hot water system? (with success) I will pass this along with hopes your suggestion may receive some condideration, as a layman, you make some sense to me, as the Anti-Scald Valves have been mentioned to me as a potential problem in this type of sysem, which could cause exactly what you say. Adding a HW tank to help solve this problem also was mentioned but no discussion as to whether it would be feasible to modify the system.  I would need some type guarantees at this time to assure taking the extra steps would be benifical.

Thanks again for your input

I've yet to implement that sort of solution at my home, but may be using a tankless HW heater as my boiler by next heating season, and would use an indirect-fired HW tank for in that fashion, along the lines of this guy's system:

http://www.heatpro.us/designtree/documents/tanklesssys.htm

But others have done variations on the theme, with & without isolating heat-exchangers somewhat cheaper/smaller desings utilizing a 2-8 gallon electric HW heaters as a buffers, letting the thermostat in the electric HW heater fire up the circulation pump for the on-demand.  It's all very Rube Goldberg contraption kludgey design, but it does work:

http://www.tractorbynet.com/forums/attachments/related-topics/61100d1158263375-tankless-water-heater-code-question-cold_water_sandwich.pdf
 
But first shot, try bumping up the Rinnai's output to 130-140F and see if that at least cures the cold-shower ills.  Careful- the higher you go, the greater the scald hazard, which is why the factory presets tend to be 120F these days.  If you go with a buffer tank solution you can set it a bit on the high side and use an adjustable tempering valve on the output as scald protection and to keep the temp of the water entering the plumbing guaranteed dead-nuts constant, even with the characteristic hysteresis of the tank's thermostat and the possiblity of less-than perfect mixing of hotter water from the Rinnai entering the tank.

Hello Art

 

I think I have the solution you are looking for...

With all due respect to the others' suggestions, I would recomend exactly the opposite. Turn down the t-stat on the WH. What you are experiencing is a fairly common problem for folks with tankless water heaters that have not been 'trained' on their WH.

Likely, the WH is cycling off and on as a result of too hot of water temp. If the water is up to 140 deg, you are going to be diluting it with cold water so as to not get scalded while in the shower. As you turn down the hot water (in the shower) to have a comfortable temperature, the WH flow drops down to less that 1/2 gal per minute. Thats when it cuts out.
 
A lot of plumbers are still used to tank type WH's where sometimes you can turn up the t-stat to make lots of hot water, dilute it with the larger volume of cold water, and not be as much at risk of running out of hot H2O.

With a tankless WH, this issue never arrises. Endless hot water, as long as you have propane to fire the heater. Turn DOWN the Rinnai WH back to the factory setting, around 115 - 120 degrees, and you will have LOTs & lots of hot water, evenly heated!

When adjusted properly, you will be very satisfied whith the performance.

HTH

Mark

Hi, Thank you for your response, as this is the second time I hear what you have mentioned and not understanding the mechanics of heating systems, what you say make sense to me somewhat, as I compare other solutions, I am very hopeful that this situation will be corrected getting all interested parties involved, I will let you know what the solution is when system is corrected, as the Rinnai, came highly recommended as being a front runner in the Industry.

 

ArtN

That's a very good point- if somebody cranked it way the hell up, that'll be nearly guaranteed happen!  I encountered a Bosch several years ago that somebody had cranked to ~150F (in order to be able to run the washer & shower at the same time or some such) that would sometimes even self-extinguish with a safety-overtemp error that had to be reset whenever the flow was too low or was suddenly reduced.