Fast hot water - tankless heater as backup or recirculator

Question

My master bathroom is the farthest from my electric water heater (traditional with a tank) on the other side of the house. All other faucets are much closer and receive quick hot water. During the bathroom remodel I've been spoiled using the guest bath that is directly adjacent to the hot water heater with instant hot water. I've looked into recirculating pumps but they seem to waste quite a bit of electricity having hot water in the pipes all the time. My pipes are mostly in the unconditioned crawlspace, however, they are insulated with fiberglass wrap or foam.

I was wondering if instead of a recirculating pump could I get a smaller model tankless water heater like the Rheem RTE 13 or the Ecosmart 13 and connect the inlet to my hot water line at the master bath. This would in theory have the tankless heater heat the water when it receives the cool water from the pipes and then stop supplying heat when it receives heated water from the tank. It seems like this is a similar arrangement to solar hot water backup, see Where can I find good resources on solar thermal backup options?.

One review of the Ecosmart says thermistor issues result in inconsistent heating when the inlet temperature is too high (> 120F). http://www.amazon.com/review/R2AH7NY585PJN3

Based on the calculators/ground water temps in Eastern Washington to supply my entire hot water demand for the bathroom would require a larger 18kW heater and it would be great to be able to get a away with a smaller model in the proposed back-up arrangement.

Does anyone have experience using a tankless heater in this way? Can it be done or should I simply go the recirculating route, although most evidence suggests this is a waste of electricity. Also, I would prefer not to loop my hot water down my cold water line so I'll probably run a separate PEX hot water line back to my hot water heater if necessary.

Answer 1

We have a circulation pump that was in the house when we bought it. The instant hot water is nice, but it is a huge energy waster, as you suggested. Our solution was to put switches on the circulation pump. It's plugged into a receptacle near the water heater, so it was as easy as making that a switched receptacle. We have switches (timer type that turn themselves off soon after you turn them on) next to the shower and by the kitchen sink. It only takes about 30 seconds for the circulation pump to get the hot water to you, so we just turn it on while getting ready for the shower (or while stacking the dishes). So convenient, but without the wasted energy or water.

Answer 2

If you want fast, don't use a tankless water heater!

We have a Rheem RTG series heater. It has excellent capacity and performance, and has reduced energy costs significantly.

However, part of that efficiency is greatly lowered availability of hot water. If no hot water has been run for awhile (20 minutes?), the standing water in the pipes have cooled to "not hot". Even though I have rerouted pipes through the crawlspace for directness, and insulated heavily, turning on any faucet requires a long wait for hot water to arrive.

Before, with an electric tank, hot water would migrate into the hot feed for some distance, easily checked by touching the pipes when no water was running. Upon turning on the nearest faucet, it would be a second or two for hot water to arrive. The farthest faucets (about 35 feet away) would take 15 to 20 seconds to receive hot water.

With the tankless heater, hot water is not sitting anywhere. It has to be made once the water starts moving. First, there is a half second time for the flow detector (in the tankless heater) to trigger the igniter logic. Then it takes one to two seconds for the natural gas burner to light. Then it takes at least a few seconds for the water in the heat exchanger to get warm. There must be at least 15 feet of pipe inside the exchanger, so there is a surprisingly long delay before hot water reaches the pipe outside the heater. Then there is the full length of cold pipe from the heater to the faucet.

The net result is that with an inefficient old hot water tank, hot water reaches a faucet quite soon and with a rapid increase in temperature. With the tankless, it takes a minimum of 25 seconds before it begins to feel slightly warmed, and at least another 8 seconds of ramping up temperature before it feels hot. Even for faucets five feet of pipe away from the heater!

I just measured kitchen sink performance. The heater has been off for at least 8 hours. With the faucet wide open (for hot only), it took 58 seconds for the water to first feel "not cold" and a total of 72 seconds for it to be full hot (140 °F/60 °C). The kitchen sink is 21 feet floor distance away from the heater, a total of 33–34 feet of pipe.

Answer 3

The problem with that is, half measures don't work.

The idea of putting an electric heater at the point of use is a fine one. The problem is the heater needs to be big enough to actually do the job. Corner-cutting on this will always produce an unsatisfactory result, and that is the #1 complaint about electric tankless, tepid water (because the buyer cheaped out and bought an inadequate unit).

If the heater is actually big enough to do the job, then you don't need the main heater at all. If the heater is not big enough, you will be unhappy with the tepid water coming out until the hot water arrives.

Even if it works "well enough" (105F, say), it's a safety problem because you get your shower adjusted to satisfaction and BLAM, 140F water arrives from the big heater and scalds you instantly, before you can reach for the knobs. Tanked heaters must run that hot to avoid legionella. The cure is a blending shower faucet, make sure to get one.

Low flow showerheads help electric heaters work much better, as they reduce flow. Flow is the electric's nemesis, since it is limited by available electric service.

The cure to "waiting for hot water" is put the shower next to the water heater, duh. When people put their water heater 75' from their shower, a long wait is what they sign up for. That is why (properly sized) electric tankless are so wonderful: with the exhaust back at the power plant, you can put them anywhere.

Also it helps to reduce hot water pipe size: each reduction from 3/4 -- 1/2 -- 3/8 will cut in half the time it takes for water to warm up, because it reduces the inventory of cold water in the pipe. A guy who fits 3/4 hot water pipe "because it's better" also signs up for a long wait.

Answer 4

You could do that, and i continue to think about, but I will probably wait until we do a complete bathroom remodel. In the meanwhile I use this:

http://thegreenlandlord.wordpress.com/2010/07/26/how-much-does-the-showerstart-water-saver-valve-save/

Turn the shower on full hot and walk away for a couple minutes. When you come back the water in the bathroom will be hot, and you won't have wasted much water.