Most Tankless Water Heaters Will Not Work With Hot Water
Circulating Pumps
Hot water circulating systems with some exceptions are not
compatible with most tankless water heaters. There are some tankless
manufactures making tankless units that are compatible with circulating pumps.
Why not?
Hot water circulating systems were designed to work with
storage type water heaters. You have a big tank of hot water, and you circulate
the hot water through the hot water piping. Once the piping material has come up
to temperature the temperature of the circulating hot water stabilizes and you
have piping hot water at all of your fixtures nearly instantaneously.
The tank full of hot water acts much like a flywheel in an
engine… it stabilizes things. Without the tank full of water the burners would
be cycling on and off constantly trying to maintain the temperature in the
piping. Since the piping is continuously losing heat the heat must be
replenished continuously.
Tankless water heaters are much more complex than a
traditional storage heater. The tankless heater has to put much more heat into
the water much more quickly than a storage unit in order to get the water up to
the desired temperature before it leaves the heater.
It sounds simple but it is not. It turns out that in order
to get all those Btu’s into the water that if for some reason the water stops
flowing the heat exchanger will quickly overheat and melt. That is why tankless
heaters need so many sensors. It monitors the gas supply, the exhaust, and the
inlet and outlet temperatures in part to shut down quickly if a problem develops
to prevent destroying the heat exchanger.
So here is where we have our first problem… the minimum
flow that tankless water heaters require is typically ½ to ¾ gallon per minute.
Most hot water circulating pumps will not produce enough flow through a normal
plumbing system and the heat exchanger of a tankless heater to turn on the
heater. That of course, is not a problem for the heater… just a problem for the
user.
Let’s suppose that we get a good big pump and pump enough
water to turn on the heater. With a tank type heater the outcome would be easy
to predict. When the hot water in the tank falls below the burner set point the
burners will come on and heat the water until it hits the upper set point, and
then the burners will shut off. The cycle will repeat over and over.
Not so with a tankless water heater. The tankless water
heater is trying to maintain a constant outlet temperature. It doesn’t just
turn the burners on and off, it regulates or modulates the gas flames (or
electric current) to try and maintain a very accurate outlet temperature. Some
heaters even modulate the flow of water through the unit.
Remember, the tankless heater is measuring things like
inlet water inlet temperature and exhaust temperature and taking appropriate
actions to keep the heater running correctly and keeping the outlet temperature
at the set point.
However, the tankless heater wasn’t designed to raise the
water temperature only a few degrees. When the inlet water gets up near the set
point, which it will rather quickly, then the heater won’t know quite what to
do. The Exhaust temperature might not ever stabilize, and thus could cause
headaches for the tankless electronics. Tankless heaters for the most part just
weren’t designed to work with circulating pumps. All sorts of strange
malfunctions could pop up with such an arrangement.
Demand hot water systems are an entirely different story.
A demand system is a system that acts like a circulating
pump, circulating water through the hot water piping and then through the cold
pipe back to the water heater. But a demand system shut the pump off when hot
water reaches the last fixture. Therefore no hot water ever gets sent to the
water heater inlet.
As far as the water heater is concerned it just thinks
someone turned on a faucet. As long as your demand system pump is capable of
creating enough flow to turn on the water heater it will work fine with a
tankless water heater.
Chilipepper Sales 1380 Greg St., # 221 Sparks Nevada, 89431
PH (775)-359-1223
