tankless water heater

[shovelshort]

I am getting ready to replace my old leaking tank water heater with a tankless. I am running natural gas. It is a Takagi TK-2. There is not a recommended plumber in my area for this brand. The plumber I have been talking to has only installed a tankless water heater one other time.

My questions are; can I use the existing 5" flew that was being used and shared previously by my regular water heater and furnace (two story home, water heater and furnace will be in the basement)? Will any regular pressure relief valve work with this unit? Should I assume that the gas and water pressure that worked fine for my old water heater will be enough for the tankless?

Is there anything else I should be looking into for this conversion?

[LazyPup]

When tankless water heaters first became available I fell for the hype, jumped on the supposed high tech band wagon and installed dozens upon dozens of them, but based upon actual service records for the last ten years I will no longer even consider installing one, I simply don't need the headaches.

One of the first problems you encounter is that, although they do provide an endless supply of hot water the question becomes,what is "Hot" and is that supply sufficient to meet the demand? While a tank type heater heats the water to a preset temperature tankless can only increase the temperature of the incoming cold water by a specified differential which is typically 70degF maximum. In southern climates where the incoming water is arriving at approximately 55degF a tankless can heat the water to 55+70= 125degF however it must be understood that the greater the differential between cold supply water and the output demand temperature the slower the water must move through a tankless heater. While many tankless are rated to produce 4 or 5gal/min at maximum differential that is cut down to .6 or .8gal/min. This is a major concern in northern climates where the incoming water temperatures can drop to 35degF in mid winter. This means that the absolute best the tankless can do is produce 35degF + 70degF = 105degF output and that is running at the full fire rate and the lowest output GPM.

The proponents of tankless would have you believe their advantage is that there is no standby losses when there is no demand, however they often site standby losses for tank type heaters based upon the types of heaters built in the late 70's. Formerly tank type heaters only had a thin fiberglass insulation that provided about R-4insulation but the codes have been modified and now require all tank type heaters to have a minimum of R-12 insulation. In order to meet the new standard most water heater manufacturers have switched to a cast in foam insulation that typically exceeds the R-12 minimum and some are now approaching R-50 insulation values. This means that the standby losses from the storage vessels is now nearly a non-issue.

We must then consider that if a whole house type tankless is installed at the same location as a tank type was previously installed the radiation losses from the hot water distribution piping remains identically the same.

Now let us consider the energy requirements. Typically a 50gal tank type gas water heater requires a 1/2" gas line supplying 36,000BTU's of energy. By contrast the tankless requires a minimum of a 3/4" gas line with a BTU range of from19,500BTU to 140,000BTU on high fire. This means that in most instances when retrofitting an existing structure we often need to run a completely new gas line from the street and have a different meter installed.

Whole house electric tankless water heaters typically require 220v @ 120amp and here again, very few residential electrical service connections are capable of taking on an additional 120amp load without some very expensive upgrades to the Service Entrance Equipement and main service panel.

Due to the high temperatures encountered in tankless heaters most require double walled stainless steel flue venting and many local codes will not permit connecting them to a chimney shared by an HVAC unit.

Another advertising point that is commonly used by Tankless manufacturers is long life service. Most of them have stainless steel heat exchangers and advertise a 12 to 15yr service life, on the other hand, while the tank type manufactures offer 8 to 10 year warranties the average lifespan of a tank type water heater is 12yrs. (I recently changed the Rheem AUI 40 glass lined gas water heater at my mothers house that i helped my dad install in 1965).

While the tankless do have a long life stainless steel heat exchanger they also have some rather complex control components and based upon the service records of those I personally installed, I have yet to see one go 5 yrs without requiring maintenance. The down side here is that when they do fail you often have to order parts from distant suppliers in California or New York and it can take days or weeks to get the parts.

Now consider the initial cost. Typically a wholehouse tankless will cost $900 to $1200 while its tank type counterpart is available in a $250 to $350 range.

I still remain a strong proponent of smaller point of use tankless units at remote locations such as a 1/2bath or a kitchen which is a fair distance from the primary water heater location, but i would never recommend a whole house tankless as a retrofit to an existing structure.

If you choice to consider a tankless is based upon fuel economy there are a number of options that will meet your needs for far less money.