I recently retired after 30 years as a residential contractor.
My wife and I now live in a new home I built in Colorado. It's
high in the mountains (elevation: 10,000 feet; heating degree
days: 8,500) and has 2,000 square feet of conditioned space
heated by a radiant-floor heating system.
The author's Colorado home has 2,000
square feet of conditioned space. His combined average costs
for heat and hot water are only $20 per month, a figure he
credits to tight construction, programmed thermostats, and the
efficiency of his tankless water heater.
While most radiant systems get their hot water from either a
tank-type water heater or a cast-iron boiler, my home's heat
and domestic hot water (DHW) are both provided by a single
Rinnai tankless — or "on demand" — water heater.
The Rinnai uses considerably less energy than a tank-type
heater and costs less to install than a boiler with a heat
Tankless water heaters are a proven technology with a track
record of many years for heating DHW (see "Installing On-Demand
Water Heaters," 2/06). But since few contractors have
experience using them for radiant heat, information about
reliable system design can be hard to find. In my case,
perfecting a system took some trial and error.
I consider the effort worthwhile: During the two years I've had
the system, heat and hot-water costs for two people have
averaged less than $20 per month.
It's important to note, though, that my house is
super-insulated (R-35 walls, R-50 to R-60 in the roof) and
takes advantage of excellent solar exposure. If the sun is out,
which it often is in this area, we don't need heat at all
during the day. We also supplement our heating needs in the
evening with a wood stove, which we enjoy for both comfort and
Finally, we use a programmed thermostat that limits the
temperature to 62°F in the middle of the day and at night,
and 70°F from 6 to 8 a.m. and 6 to 10 p.m.
Tankless water heaters take up very little space (mine measures
13 1/2 inches wide by 23 inches high by 9 inches deep), can be
mounted on any exterior wall, and offer nearly endless hot
Despite the fact that the units are not commonly used for
radiant heat, they're an ideal heat source for this application
because their output temperatures can be easily matched to the
needs of the radiant system. My Rinnai provides water of up to
180°F, which is more than sufficient for the 110°F to
120°F needed to run my radiant system. (Many of the
tankless units for DHW have a maximum output temperature of
140°F, but those used for heating should have higher
The economics of using a tankless water heater to supply both
DHW and heating are also quite attractive. Total cost for my
system was around $5,000, not including labor: $1,100 for the
tankless heater, $1,400 for the PEX tubing, and $2,400 for the
various components, controls, and fittings.
Although I could have gotten a tank-type unit for as little as
$200, the extra cost of the tankless model is more than offset
by its energy savings. The energy efficiency of a heating
appliance is expressed as a decimal called its energy factor,
or EF. A tankless water heater has an EF between .82 and .87,
which means it converts 82 percent to 87 percent of its energy
input to heat. Tank-type units have an EF around .59, so they
use considerably more energy than their tankless
One exception is the Polaris by American Water Heater Co.,
which has an EF of approximately .95. However, its $3,000 cost
— almost three times as much as my Rinnai — was too
high for me to justify. I could also have used a cast-iron
boiler; its EF of .80 is close to that of a tankless, but the
cost was more than $3,000.
Selecting a Tankless Heater
A tankless water heater can be powered by electricity, natural
gas, or propane. I considered only propane-powered models
because I don't have natural gas where I live, and because the
price I pay for electricity didn't make an electric unit
My unit is a direct-vent model, which uses outside air for
combustion, an important feature in a tight, well-insulated
home. Theoretically, it can be installed on any outside wall,
and even outside in some warm climates. I installed mine in the
mudroom near the electric circuit panel (see Figure 1).
Figure 1.The compact Rinnai heater, pumps, mixing
valves, and controls fit neatly on a utility-room wall. The
wall-mounted box to the left of the unit is an LCD temperature
adjustment control that lets the user match the heater's output
to the needs of the house.
My only complaint is that the noise level of my heater can be
objectionable at times. The mudroom in my house is next to the
first-floor bedroom, and in times of heavy demand, the tankless
unit "surges," creating quite a loud noise. If I had to do it
over again, I would not place the unit next to a bedroom.
The heater includes a remote controller with a digital monitor
that lets me raise or lower the temperature of the water coming
out of the unit. I set it at 140°F, which is hot enough to
meet all my needs with wood and tile floors. But a plumber
friend who installed one in a home with carpeted floors had to
raise the output temperature to 170°F, as carpeted floors
are about the most difficult to heat with an in-floor radiant
The remote controller performs other functions as well. It can
display 12 or more error messages that alert you to operating
problems. It can also display the rate that water flows through
the heater, and the actual output temperature.
Figure 2.The author’s radiant distribution
system consists of a primary loop with a heat exchanger for
domestic hot water, and secondary loops for each heating zone.
One lesson he learned through trial and error was that the
system worked properly only if the heat exchanger was installed
in the primary loop.
The most important part of selecting a tankless heater is
making sure it's properly sized. Proper sizing requires that
you get an accurate picture of a particular home's heating
requirements. To do that, you need to run a heat-loss
Numerous sources on the Internet offer methods of calculating
heat losses — or software that can do it — either
for a nominal charge or for free.
Uponor (formerly Wirsbo), a supplier of polyethylene PEX
tubing, offers software at www.uponor.com that will not only
calculate your heat losses, but also help you design the
distribution system. The software doesn't specify a heat
source, but it will work as well for a tankless as for a
tank-type heater or a boiler. It's available free on a trial
basis, by which the company means 15 uses or design sessions.
If you find the software useful, you can buy a permanent
In general, a well-insulated new house shouldn't require much
more than 15 Btu per square foot per hour. For 2,000 square
feet, that's 30,000 Btu per hour.
Even a poorly insulated house of this size shouldn't need more
than 60,000 Btu per hour. Most of the units you should be
considering will have outputs of at least 150,000 Btu per hour;
in most cases sizing will be determined by the home's DHW
requirements (see "How to Size a Tankless Heater,").
to Size a Tankless Heater
Proper sizing requires that you know how much water the
home will use during peak periods, the approximate
temperature of the city water supply or well water, and
how much the heater needs to heat that water to reach
the needed temperature.
The table below shows sample flow rates for various
appliances and fixtures. The graph below it shows the
temperature flow curve for a sample heater, in this
case one with an output of 180,000 Btu. Determine how
many fixtures or appliances will be operated at the
same time, add up their flow rates, then look at the
curve to determine if that unit will meet the home's
Assume that the home will require a temperature rise of
75°F. (That's the difference in temperature between
the 40°F well water and the required output
temperature of 115°F.) The vertical 75°F line
meets the curve at a flow rate of about 4.5 gallons per
This unit would be sufficient to run a shower (2.5 gpm)
and a faucet (1.5 gpm) at the same time. However, it
would not provide enough hot water for two showers at 5
gpm each. In that case, the solution would be to
install two heaters, or to install one heater with a
Though they are sold primarily for domestic hot water,
tankless heaters are actually well-suited for radiant
heating, which operates at low temperatures compared
with, for example, hot-water baseboard. So using the
unit for radiant heating generally has little or no
effect on the sizing calculation.
Sample Temperature Curve for an On-Demand
The output in Btu of a heater large enough to supply a
home's domestic hot-water needs is typically sufficient
to meet its heating needs as well. If the heating
circulators happen to be running at the same time as
the domestic hot water, the temperature in the heating
loops may drop slightly. But because domestic hot water
is used only intermittently, this would rarely be a
For instance, in my radiant-floor system, the water
returning to the heating unit from the heating loops is
only 10°F to 20°F below the zone design output
temperature of 110°F to 120°F — much
higher than the incoming well water's 40°F. So once
the water in the heating loops has been initially
heated, the heating unit has to raise the return water
only about 10°F to 20°F.
Other systems may have a higher incoming water
temperature, a higher design temperature, or greater
heating demand. But in almost all cases, the radiant
floor water in the loops will never be lower than room
temperature. Be sure to consult a manufacturer's
representative before deciding which tankless heater to
Designing the Distribution
When I first started researching the use of tankless for
radiant heat, I found several sources on the Internet that
recommended using an open distribution system, in which the DHW
supply flows through the radiant loops. I don't like open
systems (in fact, some states prohibit them); stagnant areas
may develop in the heating loops, especially in the summer,
allowing the growth of bacteria — including those that
cause Legionnaires' disease.
I eliminated health concerns by using a closed system, in which
the DHW and heating loops are totally separate. Water heated by
the tankless heater flows directly through the floor tubing,
while a heat exchanger provides heat for the DHW supply (Figure
The domestic hot-water system. In my
DHW system, water from the well enters the heat exchanger and
then goes to the sinks, the showers, the dishwasher, and so on.
I bought my flat-plate heat exchanger from www.houseneeds.com
for $250 (such products are also available at plumbing supply
houses). Mine is rated at 240,000 Btu per hour — somewhat
lower than my calculations indicated I might need. Even so, I
can take a shower and run the dishwasher at the same time. I
have not tried running two showers simultaneously, but the main
bath, which we use frequently, has four wall-mounted jets, each
of which disperses nearly as much water as a shower head. We're
able to operate them with no problem.
In a well-designed closed system, the heat exchanger will be in
the primary loop, while each heating zone will be fed by its
own secondary loop, as in Figure 2.
I learned the hard way that the heat exchanger for the DHW must
be in the primary loop. I initially installed it in its own
secondary loop, but found that the heating zones would steal
enough water to keep the heat exchanger from delivering
sufficient hot water to the taps, and that this happened even
when the heat loops were not active. So I reconfigured the
piping, and now all the water in the system flows through the
heat exchanger before being sent to the secondary loops.
The primary loop needs a pump rated for at least 6 gallons per
minute at 30 feet of head, which is the measure of the pressure
created by a column of water 30 feet high. A pump rated at 30
feet of head will perform until that pressure is reached, at
which point it can no longer supply water.
Undersizing the primary pump will cause failure. In fact, I
tried to save money by using a lower-capacity pump on the
primary loop, but it wouldn't deliver enough hot water to the
taps; I eventually had to buy a larger-capacity pump.
You also need to know your hot-water needs before choosing a
heat exchanger. As I mentioned previously, my water heater's
output is set at 140°F. When supplied with the 140°F
water, my heat exchanger delivers water at a temperature of
about 130°F, which is high enough for showers and the
Heating zones. Each of my two heating
zones includes a mixing valve to throttle the temperature down
— to 120°F for the basement zone and 110°F for
the main-floor zone. (I wanted the main-floor temperature lower
to protect prefinished hardwood floor I installed in the living
Since the heat exchanger gets the hot water first, running the
radiant zones at the same time does not affect the DHW
temperature. It does cause a minor change in the radiant-floor
water temperature, but this is of little concern because the
DHW is rarely on for more than several minutes at a time.
Occasionally, both the zone thermostats and the DHW will call
for the water heater to be on, so it's important to keep all of
the electrical connections on one circuit. None of the
electrical components require much power.
If you install a system with a combination of radiant heat and
a tankless water heater, you will need a variety of pumps,
switches, and fittings (see table).
The heater should have an intake water-filter screen and the
necessary vent pipe for the combustion and exhaust air.
Rinnai sells an optional plumbing kit, which consists of two
boiler drains, a pressure-relief valve, and ball valves. I
bought it, and installed additional boiler drains to facilitate
purging the system of air. If you're using mixing valves, you
also need a temperature gauge downstream from each valve, so
that you can monitor the temperature of the water in the floor
Because of the slow response times for in-floor radiant heat,
it's essential to use programmable thermostats. For instance, I
like lower room temperatures at night. It takes about two hours
after the thermostat turns off the heat for the air in the room
to start to cool. In the morning, the heater warms up the floor
for about three hours before the room air reaches its design
temperature. My programmable thermostat takes these conditions
For my system to work properly, I've found that the thermostats
that activate the zone pumps also have to turn on the primary
loop pump. At first, I connected my thermostats to a pair of
relays that turned on the zone pumps but not the primary loop
pump. I ended up replacing them with a single Taco SR503-EXP
relay that connects to as many as three thermostats. Now, when
a thermostat calls for heat, the thermostat turns on the pump
for that zone as well as the primary loop pump.
I've seen some system designs that leave the primary pump on
constantly, but I consider that a waste of energy.
Flow switch for domestic hot water. Something to be aware of is
that most tankless heaters have an internal flow switch that
activates the burner at a flow rate of 1/2 gallon per minute
(gpm) or higher. This is fine for showering, but a continuous
low flow of hot water — such as you might use for rinsing
dishes or while shaving — will not cause the heater to
fire, which means the water will run cold after a short
To ensure that I can get a low flow of hot water, I added a
standard 120CV flow switch (a McDonnell & Miller Series
FS6, available from several sources on the Internet, including
State Supply Co., www.statesupply.com) that closes contacts at
0.12 gpm (Figure 3). The flow switch activates the primary
pump, which creates enough flow through the heater to cause it
to fire. This has worked well, guaranteeing that the water
stays hot even at a low flow rate.
Figure 3.Key components in the author’s
system include the flat-plate heat exchanger (at right in
photo), which heats the domestic hot water loop, and a flow
switch (at left), which ensures that the heater fires even at a
very low flow rate.
Aside from the location of the heater, I wouldn't change
anything about my system at this point. I really like the space
savings and the flexibility it gives me to adjust design
temperatures. And because I have a relatively small,
well-insulated house, the heater runs for only six to seven
hours per day — or less — on even the coldest days
of the year, which should yield a fairly long operating
Bob Gleason was trained
as a mechanical engineer and owned a general contracting
business for 30 years. Now retired, he lives in Twin Lakes,