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38 Ideas on How and When to Use Hydro-Air Systems, and How to Size and Control Them HeatingHelp.com Bright Ideas! List # 310
38 Ideas on How and When to Use Hydro-Air
Systems, and How to Size and Control Them
During the week of May 6, 2002, Dan Holohan posed this question to the
HeatingHelp.com emailing list:
A customer asks you what you think about "hydro-air," a system that
uses hydronic coils inside ductwork to heat a home. Would you
recommend this system? And if so, how would you size it? What
equipment would you use (what works and what doesn't?), and how
would you install and control the whole works?
Here are the ideas:
1. A friend of mine just moved into a condo that uses a hydro-air system.
Theyre using a gas-fired water heater as the heat source and they piped it into a
heat exchanger inside the ductwork. The water heater maintains a 120-degree
F. temperature for the domestic hot water, and when there is a call for heat, the
pump comes on and runs water through the heat exchanger. I didn't think it
would work very well, however, as I understand it, the tenants are happy.

2. Ah yes, hydro-air. A nifty-sounding name for hot air with another exchange.
As with money, you lose each time you make a transaction. The plus side is that
a boiler is a better heat source than a furnace. You also can combine fresh air,
humidification/dehumidification and air conditioning with the ductwork. You also
need it more than if you had a radiant hydronic system.

Without getting into all the options and compromises inherent with a
heating/cooling system, such as register placement, noise and space
considerations Ill keep it simple. Let's say we are in northern New England,
primarily a heating climate (but changing fast because we can buy our SUVs
now). We can go with small high-velocity ducts for space considerations, and we
also want A/C. I'd use the manufacturers recommendation for sizing the whole
system. And I'd also be prepared to have supplemental heating, such as a
radiant floor or radiators because well probably need them. So, for our area, it
might be rather pointless, and that is why we don't do it too much around here

3. I have, and would, use hydro-air systems. I believe they are much better than
direct-fired, warm-air units, but not as good as radiant systems. I always try to
put the coil on the outlet side of the air handler. You should include a good air
filter, and make sure the customer knows how and when to change it.
I have set these systems up using outdoor-air reset controls, but you will need to
set your system minimum higher than for a baseboard or radiator system. In my
area, these systems are popular, especially in high-end homes. The problems
start when the different trades (plumber, electrician, and A/C technicians) don't
coordinate. That's where a guy who is familiar with all aspects of the system can
really shine.

4. How timely! I, having to address this situation next week. A customer wants
a quote to add hydronic coils into existing forced air ducts as part of a new
snowmelt system we're designing for her. The furnace is being removed, as is
the 75-gallon, 26-year-old water heater. My challenge is to integrate and
prioritize the domestic hot water and the indoor heat with the snowmelt boiler. I
don't have any answers yet, but I'm submitting so that I can tap into everyone
else's general fan-coil knowledge.

5. It all depends on what the customer wants. A hydro-air system can combine
the benefits of both ducted and hydronic systems. The boiler can provide hot
water for the air coil, radiant floor heating and domestic water heating all at the
same time, while the ducted system can provide A/C, humidification, ventilation
and filtration. If the owner requires the right combination of these items and is
willing to pay for them, then I would recommend it.

I would first size the fan-coil unit to match the heating load of the system,
including any ventilation air. I would then size the cooling coil (if required) to
match the cooling load of the system, including ventilation air. The boiler would
then be sized for the coil load as well as any other zones such as radiant floor or
domestic water, including pick-up losses.

I would lay out the wet side of the system as a primary-secondary system with a
reset schedule on each hot water zone, as required. This makes it easy to
ensure that the boiler gets what it wants, and each zone gets what it wants, even
if everybody wants something different.

The ducted system would be controlled on a first-come, first-served basis. The
first air zone to call for heating or cooling would operate the system; the opposite
function would be locked out. Once the initial zone is satisfied, the next zone will
determine the mode of the system. Each zone will have a zone damper that will
modulate open or closed, based on system operation and what the thermostat
wants. For example, if the system is in the heating mode and zone two wants
cooling, the damper will close to block the flow of hot air to that zone. A bypass
or "dumper" damper may be required.

The zone pump to the coil will operate when the system is in heating mode.
Since the pump is either on or off, a reset schedule should be used to prevent
over heating and short cycling.
6. We do quite a bit of hydro-air work. Here in Maine, it's a great upgrade to
get rid of an old warm air furnace and reuse the existing ductwork. By
installing a boiler, indirect hot water heater and a hydro-air box, we can
eliminate the old furnace and the electric hot water heater.

It's simply a matter of sizing the coil/fan unit to the CFM/BTU requirement of the
structure (we always check duct sizing and load first for a good match). The coil
is set up like any zone. A call for heat fires the boiler and circulates hot water to
the coil. An in-line aquastat prevents "cold starts" in the ductwork. We don't do a
lot of residential A/C here so duct sizing is usually not an issue. The boiler gives
us the ability to make inexpensive hot water and also get some hydronic heat to
some of the problem areas that usually plague these old systems (not to mention
radiant heat applications).

The only real problem has been finding hydro units with a low enough CFM
rating. Sometimes they're just too strong and it's difficult to control noise.

We've used hydro-air in garages and large spaces with much success. Again,
as a retrofit to warm air furnaces and existing duct systems.

7. Yes, I would recommend this system. I have seen them before and they
provide the best of both worlds. Wonderfully slow moving scorched air (big
pulley on belt drive blower) and humidification, plus electronic air cleaning. And
wonderfully cool central air in the summer (small pulley on belt drive blower, also
utilizing the electronic air cleaner. The boiler had a domestic coil to maintain a
nice dry and rust-free boiler in the summer.

Nowadays, you would use a low-mass boiler and an indirect storage tank. This
particular system was duct sized around the cooling load (bigger duct sizes) and
required manually switching the pulleys. They ran the system fans continuously
to better climatize the structure and to clean or humidify the air more effectively.
The trick for the winter was to slow the fan down so that the air just slowly wafted
through the rooms. I think they might have had Palm Beaches on there for a
while. Designed and installed by the local tin wizard, this system was a satisfier
for the owners!

8. The hydro-air dilemma is simplified somewhat if you have a customer who
wants high efficiency, high capacity domestic hot water and air conditioning. As I
have mentioned, I am working on an installation using the Lennox CompleteHeat
system for multifamily where they want individual control of the unit utility costs
and high efficiency everything in order to become LEED certified-on very large
homes (dare I call them McMansions) when you have multiple zones and far too
much load and duct run for a single system this makes for a good solution,
especially when you factor in the extra heating demands of hot tubs, lap pools,
and monster kitchens. I am working to compact this agenda for an integrated
domestic hot water, hydronic forced hot air, exhaust/ventilation heat recovery scheme for the modulars. I am also looking into small chillers since we can get
better load adjustment and dehumidification with no coil freeze up. I would be
interested in hearing