Hydro-air systems have been around for a long time…even hydro steam. I remember the
first time I saw a radiator mounted inside of a plenum and I thought “what the heck is
this?” Back in the day when fresh air was deemed a necessity (unlike today in our tight
energy efficient homes - ha!), duct work would draw air from the outside and heat it with
these duct mounted radiators before delivering it to the living space. Not very efficient
but fuel was a lot less costly back then and the owners of these homes weren’t shy of a
few pennies. Times have changed however. Today when we say hydro-air we think of air
handlers with internal coils for heating and/or cooling.For the most part, hydro-air is a product of the electric conversion market when electricity prices started to rise. Replacing that old electric furnace and installing a boiler with an air handler allowed versatility when it came to zoning and potable hot water. Indeed, the ability to run additional emitters off the boiler often led to systems with multiple heat delivery schemes such as warm air, convection and radiant. Entire sections of the home could be heated by a different medium e.g. warm air upstairs; radiant downstairs; baseboard in the basement. And all of this with air conditioning and endless hot water as well. The more recent decades have seen an increased demand for air conditioning and, for homes heated with water, hydro-air is a very convenient solution.
One of the major differences between air handlers and direct fired furnaces is the quality of the air delivered to the home. The heat exchangers in fuel fired furnaces are subjected to extremely high surface temperatures which tend to dry out the delivered air. Whereas, with an air handler, the heat exchanger never sees temperatures above 180°F resulting in a much more comfortable living environment.
The implementation of ECM technology in HVAC motors has had a significant impact on the efficiency of the equipment. We first began to see ECM motors in air handlers and condensing furnaces which resulted not just in increased motor efficiency but also enhanced the performance of the blower by compensating somewhat for inadequate duct design. ECM motors make a lot of sense for air handling equipment because of the larger size motors needed to move large volumes of air.
And more recently we have seen the introduction of ECM motors into the wet side of the equation. All of the leading pump manufacturers boast ECM circulators - which begs the question of how best to use this technology in a hydro air application.
Circulator motors are notoriously small (1/25 HP is typical) so electrical savings are minimal. On the other hand, blower motors will be anywhere from 1/3HP to ¾ HP so the electrical savings are significant when switching to an ECM.
In the case of a circulator, the true benefit of an ECM motor is the ability to affordably control the water flow and thus control the BTU supply to the air handler. But only if that circulator has the ability to track temperature modulation versus pressure drop. In other words – Delta-T versus Delta P. With a conventional hydro-air system we have a fixed BTU output from the boiler and a single speed circulator. The air delivery is most often controlled by either an aquastat which brings on the blower when hot water is sensed in the coil or a timer delay function. This arrangement becomes even more complex if outdoor reset is factored into the equation; with modulating supply temperatures and fixed blower speeds, the temperature of delivered air at the registers can vary significantly. The efficiency of the condensing boiler may also be impacted as the boiler control is often programmed to maintain a minimum of 130°F.
Now, imagine if we could program a fixed air delivery temperature at the registers…say 120°F? But this can be done, you say, by controlling the blower with pulse width modulation. How about a less costly but more efficient method? By locating a sensor in the output airstream of the air handler and programming the pump to maintain a desired delivery temperature, the pump will speed up or slow down as it sees fit. If the sensor detects an increase in output air temp, the pump will slow down and deliver less flow and fewer BTU’s to the coil. When the sensor detects a decrease in temperature due to lower return air temperature, the pump will speed up and deliver more flow and more BTU’s to the coil. The result will be a more even temperature at the registers. In effect, the pump is responding to real time conditions which can be affected by events such as doors opening and the introduction of heat from secondary sources such as stoves etc.
One pump that has this capability is the Bumblebee or HEC2 from Taco. This ECM circulator can be programmed to maintain either ΔT or a fixed set point. This pump, which has its own sensor inputs, is factory programmed to maintain a 20°F delta between the supply and the return. It takes just a few seconds to reprogram the circulator for a fixed supply temperature and by placing the supply sensor in the air stream, the pump will speed up or slow down based on conditions. No more fluctuating air temperatures at the registers.
Air handlers come in all shapes and sizes. But the heart of the appliance is the motor. While still available, the PSC motor is fast giving way to the ECM. This electrically commutated motor offers significant advantages over the split-phase motor in terms of efficiency and performance. The same logic applies to the wet side of the equation and when properly applied, both the cost of operation and overall comfort can be greatly enhanced.
Learn more about hydro air here.
