Tankless water heaters information home page:
This article series explains the operating characteristics of tankless water heaters, also called instant water heaters or "on demand" water heaters. Tankless water heaters may be installed to serve a building, building area, or as point of use electric water heaters.
The tankless point of use heaters we discuss include Bosch, Chromolite, Bradford White, Eemax, Instant-Flow, Noritz, Rheem, Rinnai, Stiebel, Takagi, Titan & other brands. We give contact information for each tankless water heater manufacturer. We also describe electric shower heaters: Dur-o-Matic, Marey, & other brands.
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A tankless water heater saves energy by turning off completely when no hot water is being drawn - there are no standby losses - no tank of hot water needs to be kept hot when hot water is not being used.
Sketch of a point of use tankless water heater (at left) courtesy of Carson Dunlop Associates.
Although some instantaneous water heaters can provide up to 9 GPM (and use 200,000 BTUH), smaller units cannot provide very large hot water flow rates in gallons per minute.
If the hot water usage rate is expected to be high, a very high BTUH unit (efficient but not necessarily economical) or multiple smaller point of use tankless water heaters may be needed to satisfy all building occupants and uses.
Consumer Reports points out that a tankless water heater may be efficient but not necessarily economical to operate. What they mean is that it is certainly efficient to avoid heating water when no one is using it. But if water usage is high, a high-BTU tankless water heater may consume more total energy than a large but more conventional oil or gas fired hot water tank and burner.
As the name suggests, and as you can see in our photo at left of an electric Instant-Flow water heater, tankless water heaters have no tank, and therefore no storage capacity at all. When the faucets and fixtures in the home are idle, the water heater is dormant.
No stored hot water means no standby losses, and, if you match the heating capacity of your tankless heater to your hot water flow rate requirements, there is no delay in hot water supply and no running out of hot water.
The Instant-Flow® water heater shown is a Model S-48L/240 240-volt unit rated at 4800 watts and is installed on a 20-A electrical circuit.
Tankless heaters are supplied in all-electric models (shown here) and gas (or possibly oil) fired units that include a burner, heat exchanger, venting system, and the necessary controls.
When a hot water faucet is opened or the dishwasher or clothes washing machine calls for hot water, the tankless heater detects the water flow and ignites the burners.
These powerful burners quickly heat the water inside the small diameter heat exchanger. As hot water is drawn out, fresh cold water is drawn in and the cycle continues. A significant advantage of this system is that you can't empty all of the hot water out of the tank because there is no tank - just continuous hot water.
The other major advantage over conventional water heaters is energy savings. Tankless water heaters have no large reservoir of water that has to be kept hot around the clock in case hot water is needed. One side effect of the lack of storage is that all of the water in the system is cold when the system has not been in operation recently. When a hot water faucet is operated, it may take more time for hot water to be delivered than in a conventional system.
As you can see by our photo (left) of an Ariston tankless water heater, tankless or demand or "instant" water heaters are much smaller than conventional waters with storage tanks, and are usually wall-mounted.
However for non-electric tankless heaters, the burner on a tankless or demand water heater must be much larger than on a conventional heater since it has to instantly heat incoming cold water to its target hot temperature.
Most tankless water heaters are either electric, or they are fueled by natural gas or propane and are vented through a side wall of the house.
As Rheem points out, "The venting must be Category III, Stainless steel, and it applies to the adapter, the vent pipe, all elbows, and terminal vent. This is important because the flue gases and by-products of combustion can cause condensation. Stainless steel will not deteriorate like type B venting under these conditions."
Tankless water heaters are often much more efficient than conventional water heaters, using modulating burners, direct venting and/or condensing combustion systems.
Most systems include a mixing (tempering) valve and a means of setting a maximum water temperature to avoid scalding. This tempering valve mixes some cold water with the hot water leaving the unit to reduce the temperature.
In one sense, there should be no need for a tempering valve, since we do not need to keep the reservoir water at 140° to prevent Legionnaires Disease.
On the other hand, since we don't know the flow rate of the water through the heater, the tempering valve may be needed since the burner cannot modulate to deliver exactly the temperature we want due to the infinitely variable water flow.
If the flow rate is low, the water moves more slowly through the heater, and picks up more heat from the burner.
Some tankless systems include a remote control, which can be used to monitor the performance of the system, display error codes or change the desired water temperature.
Also see RADIANT HEAT FLOOR MISTAKES where we describe use of small boilers or tankless coil type water heaters for radiant floor heating systems.
Readers should also see ALTERNATIVE HOT WATER SOURCES to distinguish among these types of hot water producing systems
and see ELECTRIC SHOWER HEATERS for a discussion of point of use heaters at shower heads.
The original text of this article was provided courtesy of Carson Dunlop - that text has been edited and may not entirely reflect CD's views. Page top sketch of a point of use tankless water heater courtesy of Carson Dunlop Associates. Our OPINION is that readers looking for plenty of hot water heated efficiently should also consider the heater we describe at Indirect-fired Water Heaters.
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I recently installed an electric (120 amp) tankless water heater in a cabin - it actually has 6 ganged heating coils that turn on incrementally as the demand dictates. I also installed a scald less shower faucet - which (I think) decreases the hot water draw when water pressure decreases.
My dilemma is: two or three times during a typical shower, the water temperature will increase rapidly (too hot), and then decrease rapidly (too cold) - and then settle at the "agreed upon" temperature for a minute or two. The water system is on a residential well that uses a 40-60 lb pressure switch.
I theorize that when the pressure approaches the low (cut-in) point, the faucet demands more hot water (which causes the tankless heater to crank up more heating coils), but about the time that new hotter water reaches the shower (about 13 seconds from the water heater to the shower head) the pressure increases toward the 60-lb (cut-out) range - causing the faucet to demand less hot water - causing the water heater to shut down some of the heating coils.
So, If I set a closer differential between the pressure switch cut-in/cut-out pressures (like maybe raising the cut-in to maybe 50 lbs), could I eliminate that yoyo effect? Or could there be a different cause? - Cabin Jack 1/19/2012
Jack, Why not install an automatic mixing valve to regulate the temperature regardless of water pressure?
Also see TANKLESS WATER HEATER REPAIR GUIDE where we discuss other reasons that hot water temperature may vary when using an instant or tankless water heater.
I am thinking of replacing our hot water tank: which is best gas or electric? would prefer electric to avoid venting - Sid Archer 10/7/12
Sid there is not a single right answer to which type of water heater is best, as you want to match the heater to the intended use. Depending on where you live, electrical rates may make gas fuel look less costly to operate, possibly more costly to buy and install, including cost of a chimney or vent system.
(May 11, 2015) Anonymous said:
Dose a gas tankless water heater require a pop off valve?
Good question Anon.
Some tankless water heaters such as the POWERSTAR AE7.2 Point-of-Use Electric Tankless Water Heater and also the Rheem Tankless Electric RTE 13 are advertised as not requiring a pressure/temperature relief valve, but even there the manufacturer notes that the TP valve may be required by local codes.
But certainly there are tankless heaters that are installed with the valve and that include it OEM. And there are TP valves designed for these heaters such as the Rheem AP12993C Pressure Relief Valve and the Watts Model 3L TP valve. So the answer, as Mark Cramer says, is ... It depends...
Though it's always safe and not harmless to add a properly-located TPR valve.
Give us the brand and model number for more details.
(July 14, 2015) Anonymous said:
I just bought a Rheem RTE 13 with supply and outlet pipes on the side of the box.
For winterizing a cabin against freezing, how do I drain the water left inside the tank.
The Rheem Tankless Electric RTE 13 is a tankless water heater. There is no "tank" to drain. However there will be water in the coiled copper tubing inside the water heater that must and can be drained via service valve found at the bottom of the coil inside the unit.
Continuing: if you don't have a service and installation manual for your water heater or if you need more assistance
Manufacturer’s National Service
Department at the following address:
1241 Carwood Court
Montgomery, Alabama 36117
When contacting the manufacturer, the following information will
a. Model and serial number of the water heater as shown on the rating
plate attached to the jacket of the heater.
b. Address where the water heater is located and physical location.
c. Name and address of installer and any service agency who
performed service on the water heater.
d. Date of original installation and dates any service work was
e. Details of the problems as you can best describe them.
f. List of people, with dates, who have been contacted regarding your
Watch out: DO NOT TURN ON THE WATER HEATER AGAIN when de-winterizing it and restoring it to service until you have made certain that the heater has been completely re-filled with water and air has been purged from the system.
Finally, you can contact Rheem directly\
Rheem Water Heating and Heating & Cooling Products
101 Bell Road • Montgomery, AL 36117-4305
1-866-720-2076 • www.rheem.com
Rheem Canada Ltd./Ltée • Water Heater Division
125 Edgeware Road, Unit 1, Brampton, Ontario L6Y 0P5
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