Tankless water heaters

[GaryS]

My single-story, 2600sqft home in North Texas currently has two, 50 gal. electric water heaters. They are situated about 65' apart, with one servicing the kitchen and the guest bath, while the other serves the master suite and utility room. Neither are high efficiency models.

The master suite heater is pre-heated by a geothermal heat exchanger, which keeps the water temp almost warm enough to shower and wash clothes.

Since both tanks are nearing the end of their estimated age, I would like to replace both with electric tankless units.

There are now only 2 adults living in the house and it seems a waste to keep 100 gal of water heated all the time, so I'm thinking we could save quite a bit of engergy by replacing the old tanks with tankless units.

Does this sound like a sensible, cost effective application? Should I use only one heater and a recirculatiing pump with a storage tank near the geothermal unit?...or should I just replace both with tankless heaters and no storage?

[LazyPup]

Many people are being lulled into the false notion that tankless water heaters are more efficient than tank type heaters. While it is true that the tank type water heaters of the past had tremendious radiational losses that is simply not true of the new state of the art tank type water heaters.

Recent changes in the plumbing codes now demand that water heater vessels have a minimum of R-12 insulation. In response to that many manufacturers have switched from the old fiberglass tank liners to a more modern and efficient thicker foam insulation, some of which are approaching R-50 and higher. In turn, the new water heaters are equiped with high efficiency burners and redesigned flues and heat exchangers that are approaching 85% or higher fuel efficiency for gas heaters.

Because there are absolutely NO FLUE LOSSES in an electric water heater the energy efficiency of tank type electric heaters is nearly 100%.

By contrast, while there is no radiatonal losses from a tankless heater, they are notoriously ineficient when one considers the amount of fuel consumed per pound of water heated.

While tankless water heaters do offer instant on hot water at remote small demand such as a kitchen sink or half bath lavatory location, the convenience comes at a great expense of energy, and only the larger tankless heaters have the makeup capacity to meet the needs of a truely hot shower or a large soaker tub.

In addition, an HVAC heat recovery system such GaryS has on his geothermal heat pump will only work if there is a tank type storage vessel on the system.

In most instances if a home is equiped with a state of the art tank type water heaters and both the hot and cold lines are properly insulated the overall energy cost of producing hot water in the home will range in the order of 50% to 65% of the cost of producing the same amount of water with the typical tankless type units.

In GaryS's situation it would be my recommendation to install a high efficiency, high recovery 50 gallon tanktype water heater at the master bath and maintain the geothermal heat recovery unit.

For peak efficiency I would then consider two tankless heaters on the other end of the house, one small 2.5 or 3 gal/min in the kitchen area. (2.5gal/min is code minimu for a kitchen sink and 2.75gal/min is code minimum for a dishwasher).

Given that there are only two people residing in the home and the second bath is a guest bath, the overall demand in that area would be minimal when considered on and annual average, therefore I would consider a second 3 to 5gal/min tankless water heater to meet the demands of that location. (Code minimum for a shower is 3gal/min.) You would then only be expelling energy in this bathroom in those instances when you actually have guests.

You could then eliminate the present hot water line to those areas and thereby eliminate the radiational losses from that line.

[sparks98]

I live in Philadelphia, Pa, so my water is usually much colder than in Texas and I believe that electricity is more expensive. I liked the idea of 'Endless Hot Water', so I first tried an electric tankless type hot water heater. I didn't like it at all. Water temp was too inconsistant per flow rate and it cost about $75 in electricity to take a shower. I then went with the gas tankless model by AquaStar (Bosch). I absolutely love it and will never again use a tank. It does require much more fuel, but ONLY WHEN HOT WATER IS NEEDED. Bottom line...the wife and I can shower as long as we like AND the gas bill IS CHEAPER! Also, the water temp stays the same no matter how little or heavy the flow at the faucet. They make two models, I have to smaller one and you can run a shower and washing machine at the same time. They last WAY LONGER than tank models and NO FLOODED basement every 10 years.

Good Luck

[GaryS]

Thanks for the responses. I was unaware that tankless, electric heaters were that inefficient!

I love my geothermal heating and cooling, so it only makes sense to use the pre-heated water from it and to keep a storage tank heater in that location. Too bad it's the one in the attic, since that's the one I'd like to replace.

Am I correct to assume that there are no plumbing or electrical changes to make if I simply replace the one tank with a tankless unit? The reason I ask is the kitchen heater is now located only 5' from the sink and dishwasher and I couldn't put a tankless heater any closer than it already is. Since the guest bath is almost never used, there shouldn't be much radiant loss without a demand from that source...correct?

Unfortunately, using a gas unit is not an option for me. Despite natural gas wells all around us, it's not available in our old development. The few neighbors who do use gas, are stuck with propane tanks in their yard.

[LazyPup]

There are many factors that must be considered when debating whether to install a tanktype heater versus a tankless.

First is the physical location of the installation. Any electric water heater, whether tank type or tankless may be installed at any convenient location, basement, utility room, attic space,gargage, crawlspace or in a utility closet in the bathroom or laundry area and in the case of the small point of use types such as used in a kitchen or bathroom, they may even be installed in a cabinet space.

Gas water heaters may be installed in a basement, utility room, crawlspace, attic or garage but THEY MAY NOT BE INSTALLED in any bathroom, bedroom, or closet or utility space that opens into a bathroom or bedroom. (International Residential Code IRC2005.2 or Uniform Plumbing Code UPC 605.2).

When installed in a garage, basement or utility space where there is the potential for storing flamable liquids (lawn & garden equipment, paints, oils, gasoline containers etc.) the tank must be elevated a minimum of 18" above the finished floor(IRC1307.1 & UPC501.0), and when installed in a garage the tank must be elevated or protected by Bollards to prevent any possiblity of an automobile striking the tank(IRC 1307.3.1 & UPC 510.3)

While the proponents of tankless heaters make the claim that it is an endless source of hot water, in fact the actual "Makeup" rate of tankless water heaters is far below the rate of modern high efficiency tank type heaters.

It can be argued that a tankless heater has no stored hot water therefore it has zero "standby losses", but with the advent of high efficiency foam insultion the standby losses is in many cases less than 1% for a tank type heater if properly installed. The 1 or 2% standby losses is then offset by the higher efficiency of the tank type burners and in the end, the advantage goes to the tank type unit.

The radiational losses of the associated piping system will remain equal regardless of which type of heater is used, therefore the key is to locate the heater as close to the point of demand as possible and insulate all water lines.

Many people erroneosly believe that it requires more energy to heat the entire storage vessel than it does to heat the actual amount of water per demand in a tankless. Once a tank type water heater is initially heated to the demand temp, the only additional heat required is "Makeup energy" to heat the actual amount of water consumed plus the minimal amount of energy lost to radiation from the tank which was previously discussed therefore the actual BTU requirement per demand remains nearly constant for both types.

1BTU will raise 1 pound of water 1 degree of farenheit.

The cold water supply typically enters the structure at about 50degF. (may be colder in northern climates in winter months.)

The code maximum for a residential water heater is 120deg.F unless all tubs and showers are equipped with anti-scald valves.

Therefore the differential of water temps is 120-50= 70degF.

Allowing 1 degF/lb x the differential of 70degF =70BTU/lb of water.

At maximum density of 40degF water weighs 8.43lbs. (at 50degF water weighs approximately 8.2lbs)

70BTU/lb x 8.2lbs = 574BTU/gal at 100% combustion efficiency. This factor will remain constant whether a tank type heater, tankless heater of a pot hung on a campfire.

The fuel consumption variable is now determined by the efficiency of the burner, which is typically 50% to 60% for a gas tankless and 75% to 85%+ for a gas tank type burner.

The additon of a heat recovery unit on the HVAC system will then reduce the temperature differential of a tank type unit from 70degF to 10 to 20degF. while also reducing the operating cost of the AC by approximately 20% to 30%. This option is not available on tankless heater installations. Distinct advantage, tank type heater.

The claim is made that tankless heaters have an overall reduced maintenance cost because they do not have water vessels that corrode out.

While the typical contractor grade glass lined water heater tanks
do have a life expectancy of 6 to 10yrs that number can be nearly doubled by opting for the high end stainless steel vessels. In most cases premature failure of water heater vessels is a result of improper installation or maintenance. The prime cause of corrosion failure is the ommission of the code required dielectric couplings between copper piping and steel vesssels. The second leading cause is the failure to replace the sacrificial anodes in a timely manner, and properly blowing down the sediments in the tank as prescribed by the manufacturers.

Many tankless heater manufacturers offer a lifetime parts warranty, BUT, i have been installing tankless units at the customers request since the mid 80's and I have yet to see a tankless wate heater in constant use survive 6 yrs without requiring maintenance. (The labor is not warranted by the manufacturer.)

Therefore I will stand on my previous recommendation. A tankless heater offers an advantage for a point of mimimal demand, such as a kitehen, half bath lavatory or a second bathroom that has minimal use but would not be a wise choice as a whole house heater or maximum demand heater.

[jackal]

quote:Originally posted by sparks98

I live in Philadelphia, Pa, so my water is usually much colder than in Texas and I believe that electricity is more expensive. I liked the idea of 'Endless Hot Water', so I first tried an electric tankless type hot water heater. I didn't like it at all. Water temp was too inconsistant per flow rate and it cost about $75 in electricity to take a shower. I then went with the gas tankless model by AquaStar (Bosch). I absolutely love it and will never again use a tank. It does require much more fuel, but ONLY WHEN HOT WATER IS NEEDED. Bottom line...the wife and I can shower as long as we like AND the gas bill IS CHEAPER! Also, the water temp stays the same no matter how little or heavy the flow at the faucet. They make two models, I have to smaller one and you can run a shower and washing machine at the same time. They last WAY LONGER than tank models and NO FLOODED basement every 10 years.

Good Luck

This is directed towards sparks, where did you buy your aquastar? I purchased mine through Lowe's. And where did you buy the venting supplies you used?
Thanks,
jackal

[Rewired]

I bought one of those tankless gas heaters about 4 years ago to replace the power vented pile of scrap that was installed a few years earlier ( vent motor failed twice, Recall notice about venting problems and carbon monoxide poisoning) Not to mention it was the biggest one we could fit in that space and it could not keep up with 3 people having showers all in succession.
So far it has been one of the best purchases yet, as I have not had to suffer a cold shower ever again, BUT there are a few quarks about it that took some time getting used to such as when the temperature dial is turned all the way up, it limits the flow through the unit so the water picks up more heat. That only makes it a bit of a pain when you are trying to fill a bathtub, it just takes a bit longer.
Also if you change the flow going through the heater quick, the unit doesn't really react fast enough, the water may get a bit cooler or a bit warmer briefly, no big deal though.

A.D