Letters: September/October 2010
Refrigerator Energy Bite
What is the average cost of energy to run an average size refrigerator? I’m asking the question for my 90-year-old dad, Chuck Johnson, in Livingston, Texas.
Eden Prairie, Minnesota
Hot Water Detective
I use a Rinnai tankless water heater (Model V2532FFU), which works well most of the time, but I have a couple of problems. We keep the temperature set at 116–120ºF, which provides the hot water we need except when filling a bathtub. The tub faucet will accommodate 6 gallons per minute (gpm) flow but works fine with 3 gpm flow. When running hot water from the Rinnai water heater, the flow drops to a trickle. Our plumber measured the flow at about 1.3 gpm. By reducing the water temperature, the flow increases as it should according to Rinnai specifications and the tub fills fine, except the temperature is too cool.
The second problem is the so-called sandwich effect, characteristic of tankless heaters, wherein hot water is followed by cold if the hot water is turned off for a minute or so. Rinnai says the best way to eliminate the sandwich effect is to put a small water heater (electric) in series with the tankless heater that will serve as a buffer. My plumber says that won’t work but hasn’t given me a reason.
I wonder if the small water heater might not solve both problems. If the Rinnai water temperature is set between 96ºF and 100ºF the flow is fine. If the small heater is set for 118ºF to 120ºF we would get the temperature desired. This should also eliminate the sandwich effect. It seems that if the small electric heater is being fed water at 100ºF, it shouldn’t have to work too hard to boost it to 120ºF. If use exceeded the storage of the small tank, the hot water should still be at 100ºF.
Is there something I am missing?
Asheville, North Carolina
Thank you so much for your observations and suggestions with regard to my hot-water problem. Let me answer your last question first. Yes, the water heater worked fine for about six years. It was installed in 2002, and we first noticed flow problems about two years ago. It has gotten worse since.
The tankless heater is fueled with propane. We have not checked the gas pressure but will do so. The water heater needs 10 inches of water column (WC) (0.37 psi gauge) to 14 inches WC (0.52 psig), according to the manual. I have no reason to suspect low gas pressure, since all other gas appliances function properly.
The cold water for the heater comes from a deep well and is stored in a large (500-gallon) tank in our basement. It is filtered through a whole-house filter that is supposed to remove iron and any particles above 2 microns. I don’t know the temperature of the incoming water yet but guess that it is about 60ºF or higher.
On house plumbing, the water flow does drop to a trickle if many hot-water faucets are opened at once. This isn’t as evident as in the bathtub, because most faucets are flow limited anyway.
The heater has been serviced regularly, and most recently the flow sensor was replaced (about one month ago); and more recently (two weeks ago) the heat exchanger was replaced. We are pretty sure that scale is not the problem.
We do use a recirculating pump to get hot water flowing to the bathroom. It is a Metlund unit that runs for about one to two minutes until the water is hot, then shuts off.
If we go to the small electric water heater for storage and mixing, we will probably use a 6- or 10-gallon unit set for about 115ºF. We will then set the Rinnai tankless heater to about 100ºF. The water from the Rinnai flows fine at 100ºF and is warm enough to complete a shower if we exhaust the storage of the small electric heater.
My propane supplier’s representative has just measured the gas pressure into the Rinnai water heater. The specification is 10 inches WC minimum to 14 inches WC maximum. The measurement at the test point was 12.4 inches WC with the unit off and 11.2 inches WC while operating. This would seem to be normal operation, unless there could be a flow restriction inside the heater. Also I have measured the incoming (cold) water temperature and it is about 65ºF. A 50ºF rise should get it to 115ºF, which is about what we normally use. Does this point to anything else?
Your are correct on both observations, but these weren't the problem. Even a visit from the Rinnai service representative left him scratching his head. He replaced several parts, including the igniter and the vent device. That solved some problems but not the low flow. Finally, with the help of a Rinnai technician on the phone, my plumber determined that the culprit was a printed circuit board in the unit that controls the flow sensor and had failed to provide the needed signals. A new printed circuit board is on the way! I really appreciate your help.
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