energy use: forced-air vs hydronic

ChrisJ

Voyager said:

ChrisJ said:

> @Voyager said:

> @ChrisJ

>

> I don’t like my bedroom 64 all the time, just when I am sleeping.

>

> I would not try to talk anyone who likes steam out of steam or who likes radiant floors out of that either. I just overall like FA better now that I have experienced a professionally designed and installed system.



So, you do have pipes to freeze?

Of course. With a log home, however, most pipes for domestic water are in interior walls and thus the house would have to cool a lot to freeze, and it is very easy to drain the pipes if the home needs to be winterized or the furnace fails. Winterizing a boiler is a much bigger issue than is shutting off the well pump, opening faucets and draining the water heater which is the low point in my system. And refilling is a snap. No need to purge zones and such, just turn the pump back on and go close the faucets once the air is out.

When our boiler failed at our church building, the heating pipes froze and one split long before there was any problem with the domestic water pipes as the heating pipes were at the perimeter of the crawl space and froze first.

With steam, you do not need to drain pipes, just the boiler and if it exists, the wet return.

Plenty of homes have piping in exterior walls. Should they? no, even my 1860s house that had plumbing added later doesn't except for the kitchen sink which was done in the 1980s. But plenty of modern homes do.

The ONLY reason forced air is popular is because it's cheap. That's it.

Voyager

Actually, a well-designed and installed FA system isn’t much cheaper than well-designed and installed hydronic. Cheap systems are cheap for a reason and that applies to all technologies.

FA systems are popular for many reasons. When I built my house, I originally planned to use hydronic radiant heat. However, after several discussions with the HVAC company designer and reviewing all of the attributes I desired: Heat, AC, fresh air and high quality air filtration, he convinced me that FA was the best system overall. After 19 years of living with the system, I have to agree.

My system was decidedly not inexpensive. And it was not cheap either.

The sad reality is that most FA systems are installed cheaply with undersized ducts and too few registers and returns which leads to air velocities that are far too high, noise, drafts, etc. It is not unlike a poorly designed steam system which water hammers and hisses. My FA system is quiet, has lots of registers, lots of returns (at least one in each room), low air velocities and holds setpoint dead nuts and quickly responds to thermostat changes. It has been nearly zero maintenance other than filter changes. I have three Lennox furnaces. One attic one has required zero repairs in 19 years. The other attic one had an early board failure (3 years of age) and has been problem free since. The main furnace had its first failure this past fall when the draft inducer motor failed. All three still have the original igniters, gas valves, etc.

Henry

This is one of my designs for a contactor/engineer friend of mine. he absolutely wanted geothermal in the design. The house has 23 zones of radiant. The forced air system has a hydronic heat exchanger. The house has air conditioning in the summer. It is noiseless and very comfortable. His wife praises me every time I go by. The house was completely rebuilt and a large addition was added to the back of the house. We removed the whole cast iron radiateur system and Viking Junior cast iron boiler. In my own house, I went condensing forced air with air conditioning and an electronic air filter. It is quiet and one does not smell my dog's odour.

Voyager

I had three electronic air filters also, but the main one died a few years back when water from a clogged AC drain leaked on it. I could not find replacement parts any longer so I switched to a conventional filter. The two attic filters are plugging away with just normal cleaning required. Not sure I would do electronic again given the ozone and other issues, but they did seem to work nice, but so does the 5” MERV 8 filter.

I could go for higher MERV level, but then you get more flow restriction generally and more frequent replacement. My house is pretty clean and I can get through an entire season on a filter generally.

ChrisJ

Voyager said:

I had three electronic air filters also, but the main one died a few years back when water from a clogged AC drain leaked on it. I could not find replacement parts any longer so I switched to a conventional filter. The two attic filters are plugging away with just normal cleaning required. Not sure I would do electronic again given the ozone and other issues, but they did seem to work nice, but so does the 5” MERV 8 filter.

I could go for higher MERV level, but then you get more flow restriction generally and more frequent replacement. My house is pretty clean and I can get through an entire season on a filter generally.

I run a 20x25x5" MERV 8 Airbear filter on my air conditioning. I run 1200CFM through it.

I haven't tried MERV 11 or 13 because they're "charged" filters and best I could tell, there were arguments regarding how long that charge actually lasts in normal use. I've been tempted to try an 11, but I don't know.

Electronic filters also seem to be something that was very questionable, but I honestly didn't put much effort into researching that.

gaabbee

So I just switched my older hydronic FA system over to radiant last week. My FA system was designed poorly and made the house very uncomfortable.

I looked into updating it and trying to make it better but ultimately decided on the radiant. I looked into variable speed fans, heat pumps, ect ect but they all ran into the same problems I was already having. In my setup I had hot water from the basement boiler being pumped to the attic hx then through ducts.

All I can say is the house feels brand new now. The bedrooms are using a "sandwich" install vs staple up and actually respond pretty quickly to setbacks. (I'm still playing around with this)

Another point I'm not sure was mentioned in this thread yet but when the FA system turns on the fan motor would use lets say 500 watts. If the system ran for maybe 30 mins per hour that's 15,000 watt hours vs the radiant which I am using 45 watts for the pump, thermostats and valves that run lets say 45 mins per hour (generous) @2025 watt hours.

When you add in the savings from running a lower swt, zoning, plus about 100 percent of the hot water going into the floors and not up to the attic and through the ducts first, I couldn't find a FA system that could compete in my situation. Just wanted to give an opinion since the differences are fresh in my head and I know everyone's got different setups!

SuperTech

Electronic air cleaners are garbage and I recommend installing a media filter in every one I see. EACs cause plugged coils, drains and blower wheels.

That's another plus for hydronics, no filthy ductwork and filters to deal with equals better indoor air quality. No need for filters, ERVs and humidifiers. All of these components can do good things but require additional energy to operate.

Voyager

SuperTech said:

Electronic air cleaners are garbage and I recommend installing a media filter in every one I see. EACs cause plugged coils, drains and blower wheels.



That's another plus for hydronics, no filthy ductwork and filters to deal with equals better indoor air quality. No need for filters, ERVs and humidifiers. All of these components can do good things but require additional energy to operate.

The “FA systems create dust” myth is almost as good as the “FA systems burn moisture out of the air drying it out” myth. I am surprised people still believe these myths.

SuperTech

They don't create dust, they circulate it through the air!

If your house is spotless it's no problem. Add in dogs, cats and kids then the problems start.

Voyager

If you have good filters, the dust can’t “circulate.” At the most it gets transported to the filter. And if the air velocity is low, as in a well-designed system, the air flow is pretty gentle. It isn’t like there are 10 MPH breezes to pick up dust and move it around.

Again, you are repeating myths and at best partial truths from poorly designed FA systems with poor filtration. And every tight house should have HRV/ERVs for better air quality. That has nothing to do with FA or hydronic, although FA inherently allows for better distribution of the fresh air. Same with humidifiation. No difference between FA and hydronic. Heating air, without adding or removing moisture, will lower the RH and make things drier. It matters not how you heat the air.

Gordy

Funny...... forced air humidity in house I’m in is 11% with out introducing humidification. My radiant home never fell below 30% with out humidification.

Myth? Every forced air home I ever owned was dry period. Hence “scorched air”

Canucker

> @Gordy said:
> Funny...... forced air humidity in house I’m in is 11% with out introducing humidification. My radiant home never fell below 30% with out humidification.
>
> Myth? Every forced air home I ever owned was dry period. Hence “scorched air”

What's driving the difference? Is the duct work depressurising the house? I always thought it was infiltration rates that drove the humidity levels in a structure. I'm not disagreeing, I've observed the same thing, but I never had a blower door test done to see if there was a difference in the air sealing levels

Voyager

Gordy said:

Funny...... forced air humidity in house I’m in is 11% with out introducing humidification. My radiant home never fell below 30% with out humidification.

Myth? Every forced air home I ever owned was dry period. Hence “scorched air”

Scorched air is a marketing pejorative, not a scientific descriptor.

So tell me, how does a forced air system make the water vapor magically disappear? Where does it put all of this water?

Voyager

https://www.energyvanguard.com/blog/73469/Does-a-Gas-Furnace-Dry-Out-the-Air-in-Your-Home

Solid_Fuel_Man

I think the building envelope is the major contributing factor to indoor humidity. The heating system should have no impact on indoor RH as long as it doesn't influence air infiltration.

Solid_Fuel_Man

> @gaabbee said:
> So I just switched my older hydronic FA system over to radiant last week. My FA system was designed poorly and made the house very uncomfortable.
>
> I looked into updating it and trying to make it better but ultimately decided on the radiant. I looked into variable speed fans, heat pumps, ect ect but they all ran into the same problems I was already having. In my setup I had hot water from the basement boiler being pumped to the attic hx then through ducts.
>
> All I can say is the house feels brand new now. The bedrooms are using a "sandwich" install vs staple up and actually respond pretty quickly to setbacks. (I'm still playing around with this)
>
> Another point I'm not sure was mentioned in this thread yet but when the FA system turns on the fan motor would use lets say 500 watts. If the system ran for maybe 30 mins per hour that's 15,000 watt hours vs the radiant which I am using 45 watts for the pump, thermostats and valves that run lets say 45 mins per hour (generous) @2025 watt hours.

500 watts is 1/2 a kWh. 30 min runtime @$0.11 that would cost 2.75cents to operate.
Although 500 watts is very low for a blower, even an ECM one.

45 watts is .045 kWh. 45 min runtime @$0.11 would cost 3.375cents to operate.
Although 45 watts is low too when you consider 1 ECM pump and a zone valve controller.

Just clearing up the numbers.

Gordy

Hmmm. The radiant home was 50’s construction. Well built, lots of glazing. 40% glazing to exterior wall space ratio. Multi light wooden double hugs with storms. 4 open hearth fire places which were well used in the winter. Atmospheric boiler.

Present home is 2017 construction. R-21 walls tyvek exterior. Slab foundation with ductwork in attic space. Thermopane windows. 80% furnace.

First to house I built were both energy efficient construction. Both had forced air, both had 90plus furnaces. Both were dry in the winter. Second one had a Aprile in the house.

https://basc.pnnl.gov/information/building-science-introduction-air-flow

Gordy

Again we go back to well designed, and implemented systems. Just saying my experience. We see plenty of steam, and hot water based systems that are heating hells, but being forced air is a dominate market there is far more forced air hells in circulation.

Gordy

I will say this. Of every home I have owned, in only one did guests want to know what the heating system was because of the comfort level. It was the radiant one. Maybe it was because they didn’t see floor, or ceiling vents in rooms

SuperTech

I'm not spreading myths. I'm commenting based on what I have seen in hundreds of homes during my career as a service technician.

Maybe somewhere someone has that perfect forced air system where IAQ is perfect and the ductwork stays clean. Maybe you have it @Voyager . I have never seen it. What I see is return ducts and grills covered in dust. Air filters neglected. Mouse droppings and dead mice in the ducts and blower sections of units. Even the high end stuff in Westchester county and Greenwich CT where housekeepers clean everyday have gross systems. Mold in air handlers. It's absolutely disgusting.
I've seen units with the Carrier infinity air purifiers (yeah right) still have disgusting evaporator coils, same for units with U.V lights. Most new construction homes have filthy systems when the owners move in due to the workers running them while sheetrocking. No media filter in the world will save a system from that abuse.

Can a scorched air system operate cleanly? Yeah sure but it's takes an endless amount of maintenance to do so. Radiators and baseboards? Not so much.

As far as humidity is concerned, I have never had a hydronic system owner complain about dryness and nosebleeds.

Paul Pollets

Some or maybe most, have not read or understand John Siegenthaler's articles on efficiency or reasons for comparison...
http://mesteksa.com/fileuploads/Literature/SpacePak/SpacePak Water Works John Siegenthaler.pdf

Voyager

Gordy said:

Again we go back to well designed, and implemented systems. Just saying my experience. We see plenty of steam, and hot water based systems that are heating hells, but being forced air is a dominate market there is far more forced air hells in circulation.

Certainly. Hydronic has about 11% market share so there are about 9 times more forced air systems.

Voyager

SuperTech said:

I'm not spreading myths. I'm commenting based on what I have seen in hundreds of homes during my career as a service technician.



Maybe somewhere someone has that perfect forced air system where IAQ is perfect and the ductwork stays clean. Maybe you have it @Voyager . I have never seen it. What I see is return ducts and grills covered in dust. Air filters neglected. Mouse droppings and dead mice in the ducts and blower sections of units. Even the high end stuff in Westchester county and Greenwich CT where housekeepers clean everyday have gross systems. Mold in air handlers. It's absolutely disgusting.

I've seen units with the Carrier infinity air purifiers (yeah right) still have disgusting evaporator coils, same for units with U.V lights. Most new construction homes have filthy systems when the owners move in due to the workers running them while sheetrocking. No media filter in the world will save a system from that abuse.



Can a scorched air system operate cleanly? Yeah sure but it's takes an endless amount of maintenance to do so. Radiators and baseboards? Not so much.



As far as humidity is concerned, I have never had a hydronic system owner complain about dryness and nosebleeds.

i understand that when your only experience is with poorly designed and/or poorly installed systems, you will have a distorted perspective. The only way a forced air system can change the humidity is if it is using inside air for combustion and thus creating negative pressure that pulls in more outside cold air which tends to be dry and gets drier when heated. However, the exact same thing will happen if a boiler is using inside air for combustion.

There is simply no way for an air to air heat exchanger to remove moisture from the air. The same amount of water vapor that enters the intake of the heat exchanger exits the other side and goes right back into the house.

The type of heat source simply cannot affect the moisture level in a closed system. The only way to change that is to bring air into the house from the outside and a furnace that uses outside air for combustion (sealed combustion) will draw in no more outside air into the house than will a boiler than uses outside air for combustion.

So, if you are comparing sealed combustion boilers against a forced air system that draws combustion air from within the house, then sure the FA furnace will dry the air. That has nothing to do with FA vs hydronic and everything to do with sealed vs open combustion.

Voyager

And much of it is marketing hype. If you tell people something often enough (scorched air, for example), some will begin to believe it. If you don’t believe me, look at the details that come with most of your prescription drugs, in particular the clinical trial results. Often those taking the placebo showed improvement just because they thought they were taking a drug that would address their problem.

Marketing can be a powerful thing, especially given the knowledge most homeowners have in regards to things HVAC.

Brewbeer

The dynamics of forcing air to move around your house in ducts by creating areas of higher and lower pressure, result in more infiltration (in lower pressure areas) and exfiltration (in higher pressure areas), than in houses heated passively by radiation or convection.

Voyager

That is an interesting theory. Do you have any references to studies on this? Seems pretty far-fetched.

HWBB would have the same problem though as it creates a low pressure area along the fin tube that draws in the cold air on the floor. This same force would draw air through the floor or bottom of the wall. Hard to to believe this is a significant factor in a well-designed system.

SuperTech

Well designed and installed systems of either type sadly are the exception rather than the norm. I do see some high quality work but the rest of the stuff is what keeps me busy every day.

psb75

Voyager asks:
So tell me, how does a forced air system make the water vapor magically disappear? Where does it put all of this water?

Ans: Up the chimney

KC_Jones

psb75 said:

Voyager asks:
So tell me, how does a forced air system make the water vapor magically disappear? Where does it put all of this water?

Ans: Up the chimney

That would require a hole in the heat exchanger in which case humidity is the least of your worries.

ChrisJ

KC_Jones said:

psb75 said:

Voyager asks:
So tell me, how does a forced air system make the water vapor magically disappear? Where does it put all of this water?

Ans: Up the chimney

That would require a hole in the heat exchanger in which case humidity is the least of your worries.

Unless it's an atmospheric furnace, but in that case normal boilers do the same thing.


Forced air, if incorrectly designed has a tendency to increase infiltration which means forcing air outdoors in rooms that do not have properly sized returns, or more often, no returns at all. This air is than pulled in through windows etc in rooms that do have a path to the return.


In order to push air into a room, that room must exhaust the same amount of air some way. Either under doors, or through windows, cracks etc.

But you knew that, I'm just posting for others reading this.

psb75

psb75 said:
Voyager asks:
So tell me, how does a forced air system make the water vapor magically disappear? Where does it put all of this water?

Ans: Up the chimney

That would require a hole in the heat exchanger in which case humidity is the least of your worries.


I was assuming it was an atmospheric hot air furnace using interior house air for its combustion. That would send moisture up the stack and get replaced by cold, low humidity outside air through infiltration. In sum, leading to low humidity in the house.

Gordy

I presume no one read the link to building science.

KC_Jones

psb75 said:

psb75 said:
Voyager asks:
So tell me, how does a forced air system make the water vapor magically disappear? Where does it put all of this water?

Ans: Up the chimney

That would require a hole in the heat exchanger in which case humidity is the least of your worries.


I was assuming it was an atmospheric hot air furnace using interior house air for its combustion. That would send moisture up the stack and get replaced by cold, low humidity outside air through infiltration. In sum, leading to low humidity in the house.

Then the problem there is atmospheric equipment not forced air, forced air has nothing to do with it.

psb75

Oh yes. Forced air has much to do with not very good quality of comfort. It pressurizes the house in the wrong places and depressurizes the house in different wrong places--if its not balanced well--which it often isn't. It also creates air currents that make humans feel cold. The air stratifies much more radically with forced air heat than w/ radiant heat. The heat ends up at the ceiling. You're not gonna get much "love" on this forum for FHA heat.

duerstad

I have the answer to the question I raised to begin this thread. There was a bug in the software (BEopt/OpenStudio/EnergyPlus) which showed up as I tried to investigate insulation around the slab of the house we plan. In case anyone is ever wondering about the validity of this software, the answer to the problem needs to be given. It wasn't prejudice against hydronic. It seems that if one is interested in slab insulation, one really needs to go to LoopCad. Otherwise BEopt, etc., is excellent for learning about efficiency and comfort. I only found out about the "bug" four months after asking the question, but that's not the worst possibility. What is worse are the expensive mistakes that I see people making in construction, perhaps especially in HVAC. Those rarely get figured out and I see no end to houses with such unfortunate features. If you are willing to work at finding out what pays off instead of engaging in expensive trial-and-error, I highly recommend this software. Our HVAC will be very different than what it would have been without learning from the experience that is built into BEopt, OpenStudio, and EnergyPlus.

Paul Pollets

From John Siegenthaler:
https://www.pmengineer.com/articles/93941-john-siegenthaler-the-overlooked-benefit-of-hydronics

The Steam Whisperer

What John points out there is something I've been talking about here for years and in my seminars. What's curious is that the distribution efficiency of steam heating systems is not included in the article. Steam systems are dramatically more efficient than hot water systems since for many systems there is no electricity at all used for distribution. Another related factor is that it appears the heat generated by the energy used for distribution is included in the heat output of the appliance for efficiency testing. This artificially boosts the thermal efficiency of the appliance. An 850 watt blower is adding about 3000 btu/hr to the output of the appliance. So in his example, the 80,000 btu/hr output hot air furnace, only 77,000 is from burning the fuel and 3,000 from the blower motor. This works out to about a 3.75% drop in efficiency of the furnace.
The drying out of the air is not caused directly by the heating of the air in the furnace, but by the action of the blower. The only way a forced air system can work is by pressure differential. Even a well designed system will create pressure differential within a room that can cause additional infiltration of outdoor air. With a very tight building envelop with a well designed ans well installed system ( sealed ductwork, properly balanced blow in each room, etc), these additional losses should be minimized, but are still there. How I view it, is that the looser the construction, the bigger the efficiency and comfort gains will be from using steam or hot water radiant system.
The most dramatic demonstration of the impact of forced air was in a smaller home we had installed radiant floor. The home was so tight, that they had issues with mold growing at the tops of the exterior walls and exterior ceiling edges (where the insulation was not quite as thick) when the radiant system was in operation with an atmospheric boiler. The boiler went down and they turned on the forced air system that was primarily installed for cooling, but had heat available and over the period of a single night, the home became so dry that the family woke up with bloody noses and was sick. Knowing how the home was built (using stub bays as return plenums that were not caulked at the top plates) I knew there would be performance differences, but even I was shocked.
To add to this was the testing done by the DOE on typical homes with typical forced air systems. When the furnace fans kicked on, the air leakage rate doubled. Even without the fan on, the air leakage rate was 10 to 15% higher that an identical construction building without ductwork.

Voyager

I would be curious to read this DOE study. I have never come across it and a search just now did not find anything like this. Can you provide a reference?

ChrisJ

The Steam Whisperer said:

What John points out there is something I've been talking about here for years and in my seminars. What's curious is that the distribution efficiency of steam heating systems is not included in the article. Steam systems are dramatically more efficient than hot water systems since for many systems there is no electricity at all used for distribution. Another related factor is that it appears the heat generated by the energy used for distribution is included in the heat output of the appliance for efficiency testing. This artificially boosts the thermal efficiency of the appliance. An 850 watt blower is adding about 3000 btu/hr to the output of the appliance. So in his example, the 80,000 btu/hr output hot air furnace, only 77,000 is from burning the fuel and 3,000 from the blower motor. This works out to about a 3.75% drop in efficiency of the furnace.
The drying out of the air is not caused directly by the heating of the air in the furnace, but by the action of the blower. The only way a forced air system can work is by pressure differential. Even a well designed system will create pressure differential within a room that can cause additional infiltration of outdoor air. With a very tight building envelop with a well designed ans well installed system ( sealed ductwork, properly balanced blow in each room, etc), these additional losses should be minimized, but are still there. How I view it, is that the looser the construction, the bigger the efficiency and comfort gains will be from using steam or hot water radiant system.
The most dramatic demonstration of the impact of forced air was in a smaller home we had installed radiant floor. The home was so tight, that they had issues with mold growing at the tops of the exterior walls and exterior ceiling edges (where the insulation was not quite as thick) when the radiant system was in operation with an atmospheric boiler. The boiler went down and they turned on the forced air system that was primarily installed for cooling, but had heat available and over the period of a single night, the home became so dry that the family woke up with bloody noses and was sick. Knowing how the home was built (using stub bays as return plenums that were not caulked at the top plates) I knew there would be performance differences, but even I was shocked.
To add to this was the testing done by the DOE on typical homes with typical forced air systems. When the furnace fans kicked on, the air leakage rate doubled. Even without the fan on, the air leakage rate was 10 to 15% higher that an identical construction building without ductwork.

What really bothers me is how the world seems to create the impression that heatpumps are modern and efficient and yet steam is old and inefficient. Meanwhile............they are both basically the same thing as far as how energy is transferred.

As far as I recall, phase change is 100%, or very very close to 100% efficient, no?

Though we do pay to generate what ever amount of pressure is necessary to distribute the steam. That isn't free, in theory. Even if the system is in a vacuum you still need a pressure differential between the source and the condenser.



If an 850 watt blower is around 3000 btu/h, how much heat are most of the ECM blowers generating? It must be much lower, no?

The Steam Whisperer

Voyager.... the studies I've seen were all done in the early 90's, I believe. It's been years since I've read them... I should do a little digging myself since I am working on some in depth seminars that involve that data.
Chris J.... I believe ECM's use about 1/2 the power at full speed, but additional savings are achieved due to operation at lower speeds. So probably for full load ratings they would generate about 1500 btu/hr. In hydronics, with a variable speed controller the savings are usually 60% or more in electrical usage, however, there is then some increase in fuel usage.