[LazyPup]

It’s easy to be impressed with the claim of an unlimited supply of hot water but the real question then becomes, what is “Hot”? Understanding that a tankless has no storage capacity it must by necessity then be rated by the amount of temperature increase it can produce as the water passes through the unit. Thus we often see whole house tankless water heaters rated at 5.6 to 7gal/min @ 24 to 45 degree differential. Now let us examine this claim in the real world.

The plumbing code requires that we bury the incoming water line a minimum of 12” below grade or in regions subject to freezing a minimum of 6” below average frost depth.

The Earths natural surface soil temperature is approximately 50 to 55degF. In summer this surface soil is warmed by exposure to direct sun while in winter the heat energy of the soil is radiated into the colder air above and the surface soil temps drop proportionally. As the heat energy is radiated out of the soil the soil temp drops proportionally. The deepest depth, which drops to 32degF(freezing) in any given region is called the frost depth however the temperatures below the frost depth are still within a few degrees of freezing and only increase gradually until they reach a point approximate 1.5 times the frost depth where the soil will remain a stable 50- 55degF.

In the northern climates our incoming cold-water temperature in summer will be 50 to 55degF but in mid winter when the ground is frozen it is common for the incoming cold water temps to drop as low as 34degF.

When designing a hot water system we must design it to meet the need in the worst-case scenario.

Now assuming for the moment that a typical hot shower is 105degF we would need to design a water heater that could supply 2.5Gal/min (the flow rate of the shower) with a differential temperature of 105 – 36 = 69degF.

With a tankless water heater as the temperature differential increases the unit must then throttle down the flow rate in order to heat the water to the desired output temperature,. As a result of this reduced flow rate we cannot size a tankless by its maximum output rate but rather we must examine the units differential flow rate chart to determine if the unit is capable of supplying our minimum demand at the maximum temperature differential we are likely to encounter. Thus while the advertisers might use the best case scenario of an installation in the sun belt and say their unit is capable of supplying 7.5gpm thereby simultaneously providing water to three showers we find that in a cold climate that same unit is barely able to supply the necessary 2.5gpm for one shower.

Making the proper choice can be difficult enough for new construction but when we consider retrofitting an older home from a tank type to a tankless we are confronted with yet another problem. While it is true that a tankless water heater does not consume energy between demands it must also be understood that they require a tremendous amount of energy when they are working. By example, while a 50 or 60gal tank type water heater will typically have a 35,000btu burner a whole house natural gas fired water heater capable of supplying the required 2.5gpm @ 69degF differential will typically have a burner in the order of 190,000btu’s.

When determining the size of a gas line we must begin by adding up the BTU rating of all burners to determine the total BTU load for the structure. The codes recommend that we then contact the local natural gas supplier to find the BTU content of the gas that they actually supply but when that information is not available we may use the mean average of 1100btu per cubic foot of natural gas. Once we know the total cubic footage we must then compute the distance from the gas meter to the furthest fixture to determine the required size of gas line.

As an example, let us assume the structure originally has the following demand:

Gas furnace………………………100,000btu
Free standing gas kitchen range…. 65,000btu
Gas clothes dryer………………… 35,000btu
60gal gas water heater…………… 35,000btu
Total…….. 235,000btu

The total cubic gas demand in cubic feet per hour is then 235,000 / 1100 = 213cu.ft/hr

Consulting the gas line-sizing table we find that this could be supplied with a 1” line up to 80ft.

If we then replace the tank type water heater with a tankless our gas requirement is now:

Gas furnace………………………100,000btu
Free standing gas kitchen range…. 65,000btu
Gas clothes dryer………………… 35,000btu
Gas tankless water heater………. 199,000btu
Total 399,000btu

399,000btu / 1100btu/cu.ft = 363cu.ft/hr
The same 1” gas line could then only supply the demand up to 30’ so we would then need to change the entire gas service from the 1” line to a 1-1/4” line.

On the other hand we could opt for an electric tankless however an electric tankless capable of meeting the demand requires a 220v circuit that can supply 100 to 150amps of current. Many older homes only have a 100 or 150amp electrical service so in order to remove the tank type water heater and replace it with an electric tankless we would need to change the entire electrical service entrance equipment increasing it by another 100amps.