Radiant vs Efficient Furnace

kinglerch

Maybe this is a silly question, but what is the cost difference between using an efficient hot water heater with radiant heating and an efficient furnace? Right now there are 95% efficient water heaters and 95% efficient furnaces. If they are both fueled by natural gas, does this mean both solutions would cost the same to heat the house?

Jean-David Beyer

Too many variables.

Generally speaking, a boiler is more efficient than a hot water heater; these devices both heat water. A furnace is used to heat hot air. So really, your question comes in three parts.



1.) Is a boiler more efficient than a hot water heater? Generally yes, but if your heating load on the coldest day is very low, you could get a domestic hot water heater that is less oversized than the smallest boiler I know of. However, hot water heaters are normally operated to supply constant temperature at any time and therefore it is on all the time. A boiler, on the other hand, can be let to cool off between demands for heat. Furthermore, they can be modulating; i.e., they can put out different temperatures of hot water as the need arises (e.g., with outdoor reset), and as the various zones in your house require. Although I am not a heating professional, my impression is that in heating service, a boiler should last longer than a hot water heater. I do not know how long a hot water heater would last if it needed to put out 180F water to a baseboard heating system. I also do not know if the water heaters meet code requirements for home heating service.



2.) Is a boiler more efficient than a hot air furnace? If efficiency is measured at the output of the boiler or furnace, they may be pretty much the same. I am unaware of modulating furnaces with outdoor reset, but they could exist. The trouble with hot air is that it is the comfort of the house you want to obtain, not necessarily the efficiency at the output of the heating device. One problem with hot air systems is that the rooms that are heated run at slightly elevated pressures, and the pressure will find a way for you heated air to get outside. This does not happen with hot water heat. If you use a hot water radiant heating system, it may be that you can heat the rooms to lower temperatures (a few degrees) and still be comfortable. Also, it is more difficult to run large air ducts around a house than small-diameter tubing, especially if you need to insulate them.



3.) Is a hot water heater more efficient than a hot air furnace? If you are counting useful BTU/hr delivered to the house, my guess is that it is not. Both boilers and furnaces are designed for this sort of task, and have the necessary controls and safeguards. A hot water does not.

kinglerch

my question

didn't distinguish a hot water heater from boiler, although maybe it should have. My question was more if a hot water heating device (boiler) costs more or less to heat a house than the same efficiency hot air heating device (furnace).





But if I understand your answer, then I think I got what I was looking for:



- The general creation of heat from gas should cost roughly the same between a 95% efficient boiler and a 95% efficient furnace



- The extremely hot air from a furnace may not all end up being usable, going right up to the ceiling, and finding it's way to the outside.



- There are more factors with a boiler such as standby losses, that are not part of the "95% efficient equation". Essentially there is hot water in the boiler that is not always in use, which does not happen with a furnace.



- The mildly hot air from a radiant system would tend to stick in the usable areas of the room and (my assumption) what you are radiantly heating (copper pipe, cement, etc) would hold that heat and deliver more of it longer, even after the boiler has turned off, making the heating process more efficient.





So if I have understood this correctly, radiant heat from a boiler would normally cost less to operate than the same efficiency furnace, although you are sacrificing being able to heat the house quickly.



I have a radiant heat system and after a vacation when the heat was turned way down, it takes hours and hours to get up to temperature...but often that's the price of efficiency.

Jean-David Beyer

There are more factors

"- There are more factors with a boiler such as standby losses, that are

not part of the "95% efficient equation". Essentially there is hot

water in the boiler that is not always in use, which does not happen

with a furnace."



It depends on the boiler. My W-M Ultra 3 has only three quarts of water in the boiler, and the circulator runs (by default: I have raised it) 30 seconds after the call for heat ends and the fire goes out. This moves the hot water (all three quarts of it) into the slab where it will be used. I do not imagine W-M are the only manufacturer who do this.







"- The mildly hot air from a radiant system would tend to stick in the

usable areas of the room and (my assumption) what you are radiantly

heating (copper pipe, cement, etc) would hold that heat and deliver

more of it longer, even after the boiler has turned off, making the

heating process more efficient."



With radiant heat, it is not so much what the air does. With hot air heating, it is.

What really matters is the temperature of the objects around the people that matters. Radiant heat heats the objects around you (by radiation). When surrounded by warm objects you feel warm. For hot air, you have to heat the air first, then it heats the objects, then you feel warm. But those objects take a long time to warm up because the specific heat of the air is so low (compared with water). So to feel warm, the air temperature in a hot air system must be hotter than the air needs to be in a radiant heating system.



Right now the outside temperature is only about 58F. So the hot water being circulated to my finned tube baseboard upstairs is only about 82F to maintain 70F up there. Downstairs, it was circulating about 76F water in the slab to also maintain 70F. 82F air is more likely to drift up to the ceiling than 76F air is. These are limits. The slab never heats the air over the setpoint of the downstairs thermostat, since the thermostat turns off the water in that case. Upstairs, since the radiant area of the baseboard is much less, it must be hotter to get the required heat into the room. So the air coming out of the baseboard is closer to 82F.



And when it really gets cold (10F around here), the baseboard will go up to 140F but the slab will go only to 120F. Since this will be my first winter with this system, I may have to diddle these setpoints a bit for best results, but my guess is that they will be pretty close to what I need.

Devan

parasitic losses

You may also want to factor in the electric use to run a blower in a furnace to deliver hot air vs. a circulator pump to deliver hot water. The latter consuming less watts typically.

Jean-David Beyer

I forgot about that.

My church has two air scorchers to heat the place. It is bigger than my house (by far). Those units are 125,000 BTU/hr each. Each has IIRC a 1/2 horsepower motor and they use lots of electricity.



By contrast, my 80,000 BTU/hr boiler has 4 Taco 007s that are rated at 1/25 horsepower to circulate (not pump) the water around the house.

Unknown

whats

more important than the boiler being efficient in my opinion is the system. Any hydronic system can hold heat for a considerable amount of time. The forced air stops making heat when the blower shuts off. After that there's nothing to keep the home warm until the blower comes on again. Who cares if the furnace is more efficient if you have to run it more anyhow. How efficient is it then ?

kinglerch

that's the point, really

the total cost of ownership, for lack of a better term. The efficiencies of the products are the same...meaning 95% of your gas is turned into heat, but the efficiency of the technology may be very different. And the cost to run the two products is thereby very different.

Interesting information, thanks.

Jean-David Beyer

An afterthought.

"I have a radiant heat system and after a vacation when the heat was

turned way down, it takes hours and hours to get up to

temperature...but often that's the price of efficiency."



Not necessarily. I have a fancy setback thermostat that is not really appropriate for radiant heating in a slab. But it has one feature that is very useful for the problem you cite.



I can set it to hold any temperature I like, and have it switch back to the normal program after that time interval has passed. So if I go on vacation for exactly a week, and it takes 8 hours to bring my house back up to temperature, I would tell it to hold for 6 days and 16 hours. That way it would switch back to normal 8 hours before I get home.



Actually, mine must be set for 6 days so it turns on a bit early, but you get the idea.

CMadatMe

Here is the laymens anwser

Your question has 2 different anwsers. First, when looking at AFUE you are looking at that piece of equipments efficiency not the heating systems efficiency so I'm going to give you the simple anwser.

You need 35% less energy to heat a space radiantly versus forced hot air. If you do a heat loss for both types of systems you will find you need a smaller heating appliance with radiant then with forced hot air. Whether they both are 95% AFUE is not dictating how much fuel you will burn with each system. If my heat loss is less with radiant then I burn less energy becuase my required btu design load is much less.

Now, with that said, there are numerous ways with radiant that I can make my "system efficiency" (how I use the btu's produced by the heating equipment) more efficient where with forced hot air there really aren't.

Everyone get's caught up in AFUE on equipment. That is only one part of a heating system. How you get the heat, deliver the heat and hold the heat are just as and maybe more important than the equipment itself.

Radiant heat is just a term describing a type of heat and every job can be designed in many ways depending on budget and what the customer/contractor are trying to get out of the system. I hate the umbrella term "radiant heat."

Me and you could both have radiant heat but mine could be a much different type of system than yours. Where as with forced hot air every system is pretty much the same.

NRT_Rob

that's a gross overstatement

a well designed air system with a 95% eff. furnace will NOT require 35% more energy than a radiant system in most cases. no way.



that may be true for some systems, but there is nothing inherent in the use of radiant heat that makes that extreme statement true.

brad_14

Rob said it right "Well Designed"

A well designed radiant system is not just about the $ saved on energy usage whatever that # might be.

CMadatMe

Do the Heat Loss

Take the last job you just did and do a radiant loss vs a hot air loss. I never said AFUE. I have less of a heat loss for radiant period. The infiltration factor for forced hot air increases due to the amount of convection that is produced with forced air. This convection promotes a faster loss of energy through outside panels, windows or doors.

In radiant we have little to zero convection thus slowing our infiltration. This gives us a lesser required heat loss to overcome. So we need a smaller heating plant to overcome our loss versus yours. Meaning we burn less energy.

The heat loss difference is 35%. Let me ask you this. Take a 3,000 sqft house, well insulated, good windows and doors. Nothing fancy just typical. Generally what size furnace would you put in?