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Letters: September/October 2010

September/October 2010
This article originally appeared in the September/October 2010 issue of Home Energy Magazine.
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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. 

Carol Bomben
Eden Prairie, Minnesota

Refrigerator expert and former Home Energy Associate Editor Jim Cavallo replies:

The EIA (Energy Information Agency) gives a figure of 1,359 kWh per year as the average refrigerator consumption per household.  The Census of Housing gives about 1.5 refrigerators per household as an average. And with an average price of electricity of 11.5 cents per kWh (EIA), that gives $104.19 as about the average cost of a typical refrigerator in a year.

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?

Bill Rupp
Asheville, North Carolina

Home Energy Water Heater Expert Larry Weingarten replies:

To try and surround the first question about adequate hot-water delivery, I looked up specs on your unit and it says it can produce just over a 50ºF rise in temperature at six gpm. So what is the temperature of your incoming cold water? To get 105ºF out, you would need 55ºF cold water. At 3 gpm, you should be able to melt ice to get a comfortable bath! So this begs the question of gas supply. Do you really have adequate supply without too much pressure drop? There will be a tapping on the heater to check gas pressure. Is scale buildup causing reduced heat exchange? Your heater has a warning light that goes on when it is scaled up. Instructions do say to service the unit yearly, and there is a specific warranty exclusion for damage caused by scale.

The sandwich effect can be treated with a small electric tank heater. This is also useful for low-flow draws where the tankless heater won’t continue to fire. I’ll warn you not to hook up a standard recirculating pump to the tank, or you’ll be spending a lot more on electricity to keep the plumbing toasty. I would not count on the electric heater to do much in the way of recovery, especially for a bath or shower. For example, the AO Smith EJC-6 is a 6-gallon unit available in 120 or 240 volts. It is either 1,650 or 3,000 watts. Both inputs would be considered slow recovery. By way of comparison, a whole-house electric tankless heater can draw 28,000 watts.

One other thing to consider is the plumbing. Open a number of hot taps around the house to put a large demand on the heater. Does flow trickle down as it does in the tub, or do you get better flow? If it’s better, look into the tub/shower valve itself. The new thermostatic or pressure-balanced valves have created some head-scratching situations.

So do look into a small tank-type heater (and check its anode periodically). Insulate the hot lines as much as possible to reduce heat loss between heater and tub. If your heater is a year old, and particularly if your tankless heats hard water, have it serviced and descaled. At the same time, have the gas pressure checked under operating conditions to see if the supply is adequate. Check settings on the tub/shower valve to see if trouble is lurking there.

One last question: Did the unit ever heat water for the tub better than it does now? If so, gas line sizing is less of a concern. Hope the opinions help!

Bill Rupp:

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.

Larry Weingarten:

Thanks for the answers. It seems to me the unit is not heating nearly as well as it can do and clues point toward the gas supply. Something seems to be interfering with gas flow. There is a pressure regulator that may need adjusting after eight years. Usually in water heaters, there is a screen or filter in the gas supply/control valve to prevent any debris from damaging or getting stuck in the automatic valve. If your heater has such a screen, that’s one more place to look for a flow problem. One other thing to look at is the burner assembly, and clean any gunk or ash from it to allow good gas flow to the flame. You’ve eliminated my other concerns and have done a nice job with the plumbing to boot! I’ll be interested to learn what you discover.

Bill Rupp:

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?

Larry Weingarten:

You have good gas supply and relatively warm water, and the unit is not bringing water temp up at flow. It seems clear the heater ... isn’t. I’d follow the gas and see where the obstruction is. Is there a bug in the main orifice? Is there ash in the burner? You’ve narrowed it down a lot. If you can find a phone number for them, perhaps a call to Rinnai customer support with the info you’ve gathered will shine a light on the problem. In a prior generation of their heaters, there were small parts that couldn’t take getting wet and would quit working as they should. Perhaps there is a sensor that is off. I’m sure they have experience with how your heater misbehaves. 

Bill Rupp:

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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