Efficiency of forced air vs hydronic heat

[shobroco]

I have a mechanical contractor insisting to me he can save an owner a ton of money because his system of adding a hot water coil into an air handler is far more efficient than a high efficiency gas furnace. I say it is physically impossible to be more efficient with a burner of the same efficiency rating but two heat transfers in the hydronic (gas to water to air) and a circulating pump added in. The building is a church with in-floor hydronic heating and a separate ducted system for A/C and heat. The intent is to leave the hydronic system at 17-18 C & use the forced air to bring the temperature up to 21 shortly before occupancy or to use just forced air in spring & fall. Gas furnaces are cheaper to install than another hydronic loop, controls, pump, etc. I'm the dumb structural guy advising the building committee on various things.

 

[moltenmetal]

Unless what's being discussed is an alteration of load between the hydronic and forced air systems I would tend to agree with you. Heating the air directly should be no less efficient than using the existing hydronic system to heat the air. The existing (presumably) tubing-in-slab high mass hydronic heating system would be poor at rapidly responding to setpoint changes, and you need the forced air for A/C, so it's a natural thought to use a high efficiency forced air unit to do the heating season setpoint changes.

An air handler with a coil would make sense IF this were a system heated by a ground-source heat pump, for instance. If the source of heat is natural gas, it's a no-brainer since an air handler with TWO coils (one for heating, one for cooling) may be more expensive than the furnace simply because fewer of them are made/sold.

Hydronic systems can give equivalent comfort at lower air temperatures, though that is debatable as it is not observed in every installation. It certainly IS true in my own home. Perhaps this is what's confusing the local HVAC guy, who probably thinks (and sells people on the notion)that there's something magical about heating with water.

 

[IRstuff]

Seems to me that would depend on how hte hydronic system gets heated. If it's for free, then a separate gas burner is indeed more expensive.

Otherwise, how is your contractor getting his joules of heat into the water in the first place? Oh, let me guess, he has a gas water heater somewhere in his system.

In either scenario, X amount of joules must be supplied to the air. In either scenario, a significant chunk of energy goes up a B-vent, or equivalent. In one case, the water must be kept at temperature to minimize the system lag, which means that 24/7, the water is losing heat to the ambient, and must be made up by re-firing the water heater throughout the day. In the gas furnace, it's an on-demand system, and the lag is minimal, on the order of seconds, and the heater is off during the non-use times.

Seems to me that your contractor's approach is actually less efficient than a gas furnace, and probably has a longer heating lag.

TTFN

FAQ731-376

 

[shobroco]

Thanks, you guys confirmed what I already thought.

 

[quark]

I say it is physically impossible to be more efficient with a burner of the same efficiency rating but two heat transfers in the hydronic (gas to water to air) and a circulating pump added in.

That's it!!