New house...need ideas

[GaryS]

I'm designing a new house that will begin construction this winter. It will have geo-thermal heating and cooling, and other energy efficient features, including plumbing. It will be a single story house of approximately 2800 sq ft.

My plan is to use a tankless, whole-house water heater, with the geo-thermal unit pre-heating the water.

Has anyone seen such a setup in actual use? It's my understanding that there is a method to move hot water to the far end of the house before turning on the tap, is that feasible?, or would I be smarter to install another tankless heater at that end?

[SSN596]

You have a source of geo-thermal water available?
Tankless performance can benefit from higher inlet temps because their capacity in gallon per minute is a function of the temp rise that is required.

Recirculation pumps circulate hot water to the far reaches of the piping system, through a loop made up of the hot supply and a separate return line. This system eliminates running many gallons of water down the drain waiting for the tap to run hot. There is energy consumed by heat loss in the recirc. piping. You do need to talk to the manufacturer of the tankless, because they can be touchy with recirc, and sometimes require a more sophisticated design.

[LazyPup]

Wow.. this is a series of tough questions to which there is no easy answer.

First of all, in order to achieve the instant on hot water you need a recirculation system and as was already pointed out, tankless water heaters are not generally considered compatible with recirc systems because the circulators do not move enough water to trigger the tankless to operate.

The next problem you would encounter is the geothermal and tankless combination. A geothermal heat recovery unit will only produce heat when the geothermal HVAC system is actually running and understanding that an HVAC unit cycles from the thermostat there is no way to insure the geothermal unit would be running at the same moment that you are taking your shower thus a Geothermal heat recovery unit requires a storage tank.

The proponents of Tankless heaters would have us believe they are much more fuel efficient because they have no standby losses from a storage tank. A decade or so ago that was true because water heaters only had a simple 3/4' layer of fiberglass insulation but the ASTM standard now requires all water heaters to have a minimum of R-5 insulation. In order to meet the new standard water heaters now use cast in foam insulations which in some cases are meeting or exceeding R-50, thus standby losses are negligible. Couple that with the fact that the modern tank type water heaters have burners approaching 90% efficiency, which is much higher than there tankless counterparts.

Tankless heaters make the claim that they can produce an unlimited supply of hot water, but the question then becomes, what is hot? While a tank type heater can produce hot water up to to 190degF at best a tankless can only produce water 70degF warmer than the cold water supply temperature. It must also be noted that in order to achieve a 70deg differential they must dramatically reduce the flow rate.

In the sunbelt and in northern climates in mid summer the average temperature of the incoming cold water is approximately 50 to 55degF. This means a tankless on full fire can produce 55deg + 70deg = 125degF hot water however in northern climates in mid winter the ground temperatures drop and the water lines are only 6" below frost line so the average winter cold water temp is 36 to 40degF. This means the tankless can only produce 106 to 110degF hot water on full fire and reduced flow rate. The reductions in flow rate can be very substantial, by example a whole house tankless is typically rated at 6gal/min however at full load differential that drops off to 2 to 2.5gal min at 110degF output which is barely enough to support a tepid shower allowing of course that there is no additional heat loss from the piping between the tankless and the shower location.

Now in regards to heat radiation losses from the piping. That remains a constant whether the water is heated by a tank type or tankless and the only variable here is whether or not the lines are insulated.

Having said all that, there is a much more efficient system that will permit nearly a limitless supply of hot water, instant on recirculation, geothermal heat recovery, maximum fuel efficiency and the initial cost is about 1/2 that of the tankless system you propose.

Under the current code restrictions the hot water distribution temperature may not exceed 125degF unless all tubs and showers are equipped with anti-scald valves. When anti-scald valves are present the temperature may be increased to 140degF. The common practice is to install the water heater and set it at the code max 125degF to meet the code standard with the least amount of effort.

We must understand that for each gallon of water we draw off a water heater tank it must then take in an equal amount of cold water. Even with the burner firing the first hour recovery rate is approximately 70% of the tank volume at 10degF below the thermostat set point. This means that if we have a 60 gal water heater set at 125degF we can rely upon 60gal x 70% = 42gal at 115degF or basically enogh to allow to ten minute showers per hour.

With the simple addition of a whole house tempering valve on the water heater output line we can then turn the water heater up to 180degF. The tempering valve would then take 180degF hot water from the tank and mix it with cold water to supply 125degF water to the distribution system.

By code a shower head is limited to 2.5gal/min therefore a ten minute shower consumes approximately 25 gallons of water. Where we previously drew the entire 25 gallons off the water heater, with the tempering valve a 25gallon shower only pulls about 15 gallon from the tank to make the mix. We must also understand that at 180dgF the first hour rate of the tank would be 42gal at 170 and tapering off slightly from there but the full 60 gal remains well above our demand temp. In fact, this is often done in commercial service where they expect 6 to 8 showers per hour from a 60gal tank.

With the tank type heater you could also run a heat recovery unit from an AC or geothermal system and typically a 3ton ac can produce 50gal per hour at 180degF so the water heater burner will barely ever need to run. (As a side note, while living in Florida I installed a heat recovery unit on my AC. Three months later while checking the system I realized that I had never lit the pilot light on my water heater).

Now let us examine the economics of a hot water recirculation system.

You stated that your house is 2800sqft so I am guessing it has approximately a 30' x 60' footprint. I am also guessing that you have multiple bathrooms and most likely your main hot water line is a 3/4" line. Allowing for a lateral run and risers on both ends we could then estimate the total developed lenghth of the main hot water line is about 80' to 90' which is about average for state of the art new constuction.

Volume of a pipe equals cross sectional area x length.

Area equals Pi x radius squared.
Radius equals 1/2 the diameter.

For a 3/4" line the radius is .75 /2 = 0.375
Area = 3.1416 x (0.375 x 0.375)
Area = 3.1416 x 0.140 = 0.44sq.in

Volume per foot then equals 0.44sq.in x 12" =5.28cu.in
For 90 feet of 3/4" pipe the volume is then 5.28 x 90 = 475.2cu.in.

One gallon of water occupies 231cu.in so the pipe holds 475 / 231 = 2.05gallon. For illustration let us use 2 gallon.

This means that every time you use the shower you must run 2 gallon of water down the drain while you patiently await hot water. This is 2 gallon of water that you purchase from your municipal supplier, then pay sewer tax on it as well. We must also keep in mind that you initially paid to heat that water.

One Brittish Thermal Unit will heat one pound of water one degree of fahrenheit. The water enters the water heater as cold water so you must heat it from 55degF to 125degF or 70degF differential.

Water weighs approximately 8lbs per gallon so the cost of heating that water was 8lbs x 70deg =560BTU's x 2 gallons equals 1120BTU's of energy.

The code requires that when we install a recirculation pump in new construction we must insulate all hot water and return lines.

When properly installed the pump should only run when the temperature of the water in the line falls below a set point which is typically 20degF below the supply water temp.

We then only need to move an amount of water suffiecent to refill the lines which in this case is 2 gallons. Normally the pump will only run about 2 to 3 minutes once or perhaps twice an hour.

The water returning to the water heater is only 20deg below the set point so we only need to add 20x 8BTU = 150BTU per gallon x 2 gallon = 320 BTU

As you can see, not only does the circulating pump add convenience it delivers an energy savings of 1120BTU - 320 BTU = 800 BTU.

[Snoonyb]

LAZYPUP:
FYI, "tankless water heaters are not generally considered compatible with recirc systems because the circulators do not move enough water to trigger the tankless to operate."

This does not hold true for the NORITZ.

I've installed several and the most recent on a similar footprint as the OP. The owner has the recirc. pump timed for 15min. before the HW is generally needed for the family during the weekday and every two hours on the weekend days.

There are two full baths both 50' as the crow flies, a kitchen, 1/2 bath and laundry.

There will be another full master suite added next year, finishing the "U" shape.

[GaryS]

What great information! Now I'll have to digest everything you provided, so I can ask some intelligent questions. Thanks guys.