Best take a look and see if they are the same circumstances. The forced air probably is using inside air for combustion. This causes a lot of air exchanges with the outside. The hydro unit does not exchange the air as much allowing the dwelling to retain a lot of the moisture released from cooking, showers, etc. I am assuming both do not have a humidifier.

In a gas fired furnace, oil fired too for that matter, the air passes over a heavy guage metal heat exchanger that is upwards of 500 degrees. That kind of temperature totally zaps every bit of moisture out of the air you breathe. The air on a hydro air system passes through a hot water coil whose temperature is 180 degrees tops. Quite a difference no?

The moisture has to go somewhere. Unless it's going up the chimney, why does it matter how hot it gets? Even if the RH of the air going over the heat exchanger drops to 1%, when it cools it will go back up.

The use of external air for combustion makes sense, but wouldn't a boiler consume just as much air for combustion to generate a given amount of heat (efficiency being equal)?

Yes, assuming that both the boiler and the forced air furnace are the same heating capacity and use outside air for combustion, they would both need the same combustion air, so would both dry the air out the same amount. I think what you do get with many of the hydronic systems, though, is some moisture being added to the air due to water vapor escaping from the system.

"I think what you do get with many of the hydronic systems, though, is some moisture being added to the air due to water vapor escaping from the system."
If your hydronic heating system is adding moisture to the air you'd best call a service company because that means you have a leak in your heating system. These kind of systems are totally sealed. What you're thinking of is a steam radiator which in this case is basically apples to oranges.

Thanks, I thought so.

I think if you look even at a "sealed" system, there are some opportunities for moisture to escape. Gases can get through at connection points that are too small to allow actual water to get out. As stated by others above, the basic principles should dictate that a hydronically-heated house is not any more damp than any other, yet you do tend to feel there is extra moisture in the air. It has to come from somewhere, so that was just a thought I had.

"I think if you look even at a "sealed" system, there are some opportunities for moisture to escape."

This amount can not possibly be significant even compared to a drop of water evaporating in the sink.

ky114, look I've got 30 years in the HVAC trade and extensive knowledge in practically ever aspect of hydronic heating. I'm telling you there is no moisture that escapes from a system that is going to increase the humidity level in a conditioned space! Gases? What gasses?

Aside from a steam system with an open receiver, open traps or overactive failing vents, I expect you are talking about a water boiler when you refer to hydronics.

An average size atmospheric cast iron boiler (125MBTUH) loses about a quart a month under operation to the ambient air through difusion of the cast iron block.

Anymore water than that and you have a leak or failed air vent.

Not enough to raise a measurable humidity in the conditioned space

I am just curious, zl700. I don't know much about how the boilers are designed. If I assume that almost all of the surface is facing the fire, that loss would be almost all up the chimney. That seems like a pretty valid assumption, but is it true?

My theory, which may not be correct:

I'm currently heating much of the house with a "coal room stove", where I normally have heated with an oil fired forced hot air system. Both systems leave the air very dry ... the coal more so, it seems.

I believe the reason is because, in both systems the "heated air" is raised to a very high temperature, in the appliance, to get the house to the desired temperature.

Whereas, with a hydronic system, the water is heated to a high temperature, however a large volume of air isn't.

Of course, the super heated air can not hold the water.

Ionized,

I would guess on a standard section cast iron boiler the difference between the water side and atmospheric/combustion/vent side is 35/65 or 40/60.

The majority of water loss from the closed system side mostly takes place during off cycles when the ambient air around the CI block drops below the temp of water inside as heat always wants to travel to cold, and with this travels the water via diffusion.

tmajor, the volume of heated air is the volume of the dwelling space so it is the same in both cases. Note that the hotter the air is the more water it will hold. The reason that you have drier air with the coal is probably because you are sucking in more outside air than with the oil-fired furnace.