Radiant Heat Efficiency

Jonathan_2

The only good argument I've ever heard is that radiant heat saves a bit of energy because the floor is warmer than it is in a home that uses radiators (baseboard, cast-iron, etc.). That said, the actual setpoint on the thermostat can be a bit lower because people still feel comfortable when standing on a warm floor (many people feel cold in their feet first). So a person who feels comfy at 72F in a radiator-heated home might be okay at 68F in a radiant-heated home.

Otherwise, I can think of no good argument (from a thermodynamic point of view) as to why radiant is more efficient.

Radiant Wizard

Radiant Heat Efficiency

Ok guys. Time after time I continually see post concerning which type of heating "system" is the most efficient. There are also people who are looking for that magic "savings" number. Now, can anyone give me a good arguement why radiant is the most efficent heating "system" than any other? I have my own opinion to this ready for discussion and if you think about it it's pretty simple.

I'd like to hear other peoples thoughts on this.

Radiant Wizard

Radiant Heat Efficiency

Ok guys. Time after time I continually see post concerning which type of heating "system" is the most efficient. There are also people who are looking for that magic "savings" number. Now, can anyone give me a good arguement why radiant is the most efficent heating "system" than any other? I have my own opinion to this ready for discussion and if you think about it it's pretty simple.

I'd like to heat other peoples thoughts on this.

Uni R

Freudian pun?

"I'd like to heat other peoples thoughts on this."

Radiant Wizard

My only argument

The first step in designing any form of heating system. It's the obvious most important step done and there in itself tells the story. A radiant heatloss is generally 35 percent less than a typical baseboard or forced hot air system. So before anything is installed you need 35 percent less energy to heat the space with radiant. Our problem here in the US is that most, not all, installers don't utilize outdoor reset for the system, they still use pin-typed boilers (that's another issue for another day) and they don't understand the science of hydronics itself. Yes, they are good mechanics but they tend not to continually educate themselves on technology but rather just stay the norm. I'm getting side tracked a bit.

So when I see posts of this site concerning efficiency I laugh a little because everyone seems to miss the obvious. They get all caught up in controls, burners, and boilers when the answer is in the heatloss.

DIY Homeowner

I can

Heat (energy) "flows" from hot (higher energy) to cold (lower energy). Second law of thermodynamics. As an analogy, think of different temperatures as different elevations. The hotter, the higher the elevation. The larger the difference in the elevation of two points, the steeper the line between them, and the greater the tendancy from something at the higher elevation to get to the lower elevation. Think water flowing downhill. The greater the "head", the more the pressure. This also works for heat.

In a standard hot water system, heat from the flame flows through the heat exchanger to the water, usually at 180 degrees. In radiant, the temp of the water doesn't have to be 180 degrees, it can be much lower. Therefore, the lower temps of the radiant system, result in greater temperature difference between the flame and the water. Greater temp difference, more flow. Of course, to gain this benefit, the boiler must be designed to take advantage of this principal. This does not apply if the boiler is heating the water to 180, and then a mixing valve is mixing it back down to radiant temperatures.

Do you buy it?

Another thought: With baseboard, you are basically putting hot elements up against an outside wall, which means a greater temperature difference between the outside and that portion of the wall with the baseboard. Greater temp difference, more heat flow through the wall and out of the house. This is very visible if you use a thermal imaging camera and look at the wall of a house on a cold day. The section of wall with baseboard is visibly different than the rest of the wall. With radiant, you don't got this happening, so another efficiency gain for radiant, and you don't need a special boiler to take advantage of this efficiency gain.

Radiant Wizard

No. I don't buy it

I do like your second thought though. It goes right to my point it's in the heat loss.

DIY Homeowner

Experimentation required

I don't have the equipment to do this, but there may be a way to measure for this.

Start with a cold natural gas boiler (70F) and a temperature gage in the exhaust pipe. Fire up the boiler, and measure the resultant flue gas temps. I hypothesize that as the boiler heats up, the flue gasses will also become hotter. On a standard non-modulating boiler, the amount of natural gas being consumed is assumed to be constant. Therefore, the heat energy being put into the system is constant over time (unless you can provide a convincing evidence to the contrary). If the flue gasses are getting hotter as the boiler is getting hotter, the only reason I can think of to explain this would be that less of the heat of combustion is being transferred through the boiler heat exchanger to the water. If less heat is being exchanged, then you’ve got less efficiency.

It’s the same principal as heat transfer from radiators. The hotter a radiator, the more BTUs it will pump into the room at any given room temperature. The colder the room, the more BTUs a radiator will put out at any given radiator temperature. It is all a function of difference in temperature. The greater the difference in temp, the more heat energy will transfer from the high temp to the low temp. Why would this principal be any different inside a boiler?

I’m not a heating guy, but I watch this board a lot. One thing that I hear many of the pros here talk about is outdoor reset saves fuel (and money) by reducing the temp of the boiler when it doesn’t need to be running at 180F. Do you believe that outdoor reset does not reduce fuel consumption? If outdoor reset does reduce fuel consumption, WHY does it reduce fuel consumption? What is going on to result in less fuel being consumed?

Uni R

Radiant Heat Efficiency

The concentration of heat leading to envelope inefficiencies is an excellent point but I think it is quite unfair to infer that it is a fair comparison (baseboard vs radiant) due to a couple of aspects. The first is that you don't need 180° water to heat baseboard most of the heating season, so why would you compare radiant with outdoor reset to baseboard without reset? Baseboard systems can work with outdoor resets. Secondly, baseboard doesn't need to be designed for 180° or 200° maximums. It can be designed for lower maximum supply temps, increasing the radiation area which would more evenly spread the heat load within the building envelope minimizing those peaks and valleys quite a bit. The slab losses would be lower but it is hard to say by how much and no quick way of taking a thermal image photograph of that. You'd need tons of in ground sensors.


In floor radiant heating has the advantage of being a bigger radiator than anything you can put on your walls. Secondly, the feet are in direct contact with it so there is the possibility of using lower temperatures. Is there any definitive study on the exact difference in temperature sensation this makes? Combined this would give it an advantage in efficiency as long as the under slab losses are controlled.


I think both are viable and very efficient options depending on a variety of construction related factors and that the most important thing in either install is matching the heat loss with the outdoor temperatures through outdoor reset and modulation (including staged boilers if need be). Furthermore, for modulation to be really effective the boiler is going to have to be a condensing boiler.


Even more important still is the building envelope. If you can get the heatloss low enough you could heat the house with a couple of Newfoundland dogs. I'm not sure how many BTUs there are in a bag of IAM's dog food though...

Radiant Wizard

This is what I'm talking About

Everyone gets caught up in the boiler, the controls and so forth. The question is, Why is Radiant Heat more efficient than the other forms of heat?

IT'S IN THE HEAT LOSS!!!!!!!!!!!!!!!!!!!!! YOU NEED LESS ENERGY TO HEAT THE SAME EXACT SPACE. PLAIN AND SIMPLE. YOU CANNOT CHANGE THAT. NO MATTER WHAT YOU DO A RADIANT HEAT LOSS IS GENERALLY 35 PERCENT LESS THAN A BASEBOARD OR FORCED HOT AIR HEAT LOSS!!!!!!!

Steve Ebels

A couple thoughts

Regarding boiler efficiency at lower water temps. The second law of thermodynamics certainly DOES apply in the scenario mentioned on this post. That being the cooler the water temp in the boiler, the greater the efficiency becomes. Here are some specs from Viessmanns tech sheet on their commercial condensing boiler showing efficiency vs. water temp.

Boiler supply/return at 176*/140*F = 86.8%

Boiler supply/return at 158*/122*F = 90.2%

Boiler supply/return at 104*/86*F = 95.1%

These numbers are for a VSB-17, which is a smallish (638,000 btu) commercial boiler but they are representative of all sizes in this particular model. From them you can clearly see that using the same piece of equipment, at different temperatures, yields very different efficiencies.

Now, to take advantage of this boilers' capability to produce maximum efficiency you have to provide a heating system which will function at low water temps. That's a no brainer. The only system that can take full advantage of this boiler's capability is a radiant floor or wall. Or a system that by utilizing full direct reset, can run at low to moderate temps most of the heating season. Piping losses are less by at least 2X with the lower temp water also.

That is one side of the radiant efficiency coin. The other side has to do with the same thing only on the system to room end of the equation instead of the boiler to system heat flow. When high temp heat sources are used, much of the heat winds up unusable at the ceiling due to the increase in temperature stratification. I don't know about you but I live on the floor at my house. By it's very physical nature, a low temp surface (floor or wall) transmits less "heat" to the upper parts of a room. The air simply does not rise away from the floor as quickly because there is less differential. (Why do many forced air systems feel comfortable only with a constant fan?)
A radiant system puts the heat where you are thereby enabling you to be comfortable at the oft quoted 2-3* less thermostat setting. Heat losses are thereby reduced from the fact that.... A. You have the lower temp setting, B. Less stratification of heat and C. Less differential temperature means less heat loss from the structure.

Now......... All that being said, I have seen "radiant systems" that sucked down fuel like pigs. These were invariably run with high temp boilers with no reset. They were also run with on/off temperature controls and utilized a mixing valve or injection to knock down the water temp. (What's the sense in that??) The mixing device does nothing for your efficiency and is just there to protect a boiler which is really not suitable for the job it's asked to do in the first place. (Probably raise some feathers with that comment but it's true) A properly designed radiant system can also provide significant reductions in electrical consumption. We have a 5,800 sq ft home that shows a 2.2 amp draw with the whole house "on". That's not a bad thing in my book. I could go on but let me end by saying the keys to radiant system efficiency are designing and installing the right system and utilizing the correct equipment for each project.

heretic

A few ideas

This argument will never be settled. Too many factors.

I think the marketeers who claim 30% savings (or whatever) are stretching. Your results may vary!

I like the convective losses theory. This will vary dramatically from one structure to the next, but on the whole, the 'stack effect' is going to be more powerful with higher air temps. If air temps are lower in real world use with radiant (this has been debated), convective losses for the structure will be less.

Another theoretical for radiant is that one is less likely to wind up with very hot air pooling high in the room, which costs energy to produce, but does nothing for comfort. Huge variance here from one structure to the next. Cathedral ceilings com to mind...

Another factor to consider is wind chill. Depending upon the velocity and location of air ducts, the inhabitants may need to set a much higher air temp to 'feel' the same level of warmth.

Mike T., Swampeast MO

Just because a heat loss program geared to radiant panels (or even some specific radiant product) produces a lower number than a Manual-J heat loss doesn't mean that the structure is loosing that much less heat.

Heat loss calculations are not by any means perfect--particularly in the quantity of heat required by a space or structure. Carefully calculated though they DO give a very accurate portrayal of the proportionate heat loss in each space of a structure.

Conduction-based radiant floor systems have a unique requirement--that of really needing JUST the amount of heat coming in as is leaving the space. Why do I say this is "unique"? Because the surface temperature of the floor has a DEFINITE and VERY LOW limit on temperature.

It makes ZERO sense to size a boiler much larger than required to achieve the surface temp required at outdoor design temp as such would only increase the surface temp of the floor beyond what is comfortable. Oversizing OTHER types of heating systems however (including radiant panels like ceilings or walls) essentially results in faster response when raising temperature. Again, remember that with radiant floors in particular you want the floors producing JUST the amount of heat being lost at ALL times.

Consequently, proprietary heat loss calculations for radiant floors do their best to "trim the fat" from more common heat loss calculations as this "fat" has ZERO use to the system.

As far as I can tell a radiantly heated room will actually loose more heat than one heated by pure convection. Just don't get in an uproar until you read the qualifications.

The air temperatures would have to be IDENTICAL--FLOOR AND CEILING. The convective source would have to be SELF-CONTAINED within that room or at least "air in" would have to EXACTLY equal "air out".

If such conditions could actually be created, the mean radiant temperature of the radiantly heated space will be somewhat higher than with convection. Higher mean radiant temperature of the exposed surfaces means greater heat loss through them.

There actually are studies both concluded and in-process that are attempting to actually answer these questions. Contrary to what you may think however, they seem to be concluding that tall radiators at fairly low temperature placed on inside walls are the most efficient heat sources (to a space).

I proposed this a while back based on measurements and observations having zero knowledge of any research along these lines.

Again, before the knee-jerk reaction, remember that this is in a space where the actual room air conditions are kept as constant as possible between the sources and that subjective measurements of "comfort" are NOT considered. Nor is the efficiency of the system at actually delivering the heat to the device heating the space considered.

GMcD

Radiant heating efficiency

There are a number of factors. First a fundamental understanding of human comfort needs to be considered: see these weblinks - http://support.caed.asu.edu/radiant/
http://irc.nrc-cnrc.gc.ca/cbd/cbd102e.html
http://eande.lbl.gov/btp/papers/44032.pdf

The percieved efficiency of the radiant slab floor heating system comes from the high degree of thermal storage in a slab. The slab acts like a static heat pump which maintains very stable indoor temperatures provided it operates in a constant circulation, variable temperature scheme. The slab only needs "trickle charging" to have it maintain it's temperature using low temperature water, which leads to vey efficient operation of a properly selected boiler set. Another factor is the instantaneous self compensating effect of the infra-red energy transfer in the space. If there are no objects in the room at a different temperature than the slab/radiant surface, then there is no heat exchange and the slab maintains steady state conditions. If an obeject or person enters the room that is a different temperature than the slab, the heat exchange process takes place at the speed of light. The last thing that provides big energy efficiency is the fact that the "resultant temperature" in the room is what the slab provides. A radiant surface at a temperature of 80F can provide a "resultant comfort temperature" of 72-74F in an ambient air temperature of 66-68F. Thats a lot of energy you don't have to use on heating up outdoor air or room air to the conventional higher temperatures that "all-air" systems are designed for. And the biggest thing is to spend the money on good glazing in the first place to reduce extreme transient heat losses.

kf_2

Radiant Heat loss

Here is something interesting. Many people on this post have mentioned that adding baseboard would allow you to run cooler water temps and still heat the house. This is absolutely true and would also be more efficient than less baseboard. However, these systems still require air movement (convection) to work. HOT AIR RISES!!!!

The fact is that the larger the radiator surface the lower the water temps and we all agree that lower water temps translate to higher system efficiencies. This is because of a few things, lower stand-by losses from piping and boilers, lower stack temps, and of course this is all multiplied when using a condensing boiler.

However, while radiant heat takes advantage of the larger surface = lower temp principle this is not the only reason it is more efficient.

As was pointed out earlier, Radiant heated spaces require less heat. This is most prominent in spaces with high ceilings and no heated space above ie. second floor.

This is becasue there is no air movement needed in a radiant space to properly heat the space. Why? Becasue we are heating objects and not air, and those objects are giving up thier heat slowly to the slightly cooler air around them. So as the air becomes slightly warmer it begins to rise but can't completely because the air above it is cooler and heavier. This is why the air at the top of a radiant room is cooler than at the floor. The opposite outcome of convection.

Why does this reduce heat loss? Heat loss occurs because of the most important principle of heating (HEAT GOES TO COLD IN ANY DIRECTION). And the larger the difference the faster it travels in that direction. So if the temps are cooler at the top of a room there is less of a temperature difference between inside and outside causing the heat to move slower from hot to cold. Also if it moves slower this decreases infiltration losses (less air changes per hour).

In fact ASHRAE says that when doing a heat loss on a radiant heated room the room volume should be reduced by 15% and that the Ceiling R-Value can be increased by 1.5%

Obviously a smaller boiler uses less fuel.

The reason it is said that T-stats are typically lower is because there is no air movement to pull heat away from someones body.

kf

Unknown

Some facts to back that statement up

I work in Levittown , NY , where radiant was used en masse - no insulation under the slab or along the edges . Quite alot of the copper was pushed lower than the pour - right into the dirt .

I brought this subject up to my service manager , and he told me on the average when we have to convert the radiant to baseboard heat , the amount of fuel used to heat the home actually goes up . I will see if he can get me some more data on the subject .

Steve Eayrs

Interesting thoughts.....and of course, just like the rest of you I agree with some and disagree with others.

I can easily see why a baseboard heat system, when compared to a radiant floor system w/ no insulation under it, would be cheaper to operate, but find it hard to imagine why it would cost more to run the baseboards??? Must be fairly dry soil, and not conducting like most slabs would?

As far as Steve Ebels comments.....(I know you were hoping to raise feathers so here goes) about mixing controls not making any sense, as far as efficiency reasons, and "only being there to protect the boiler"........evidently you don't work in an oil fired market, where we need to run the boilers hotter than "just whats needed" to heat the house. In the real world, where dhw production is a concern, even your best condensating gas boilers run hot, and mix down for the dhw heat demand. You cannot make much dhw with a 120 deg. boiler.
Also I feel the mixing controls are a big help in efficiency, since the boiler can run nice long burn cycles, (regardless of gas or oil), and the mixing (vsi, or other) will take care of putting out the just right temp., as long as its operating on the I/O control.
I agree with others that said the biggest savings with radiant is due to having warm air closer to the floor where you need it, and not wasting it up at the ceiling, where this excess heat just contributes to higher losses, (less efficient).

I'm not sure exactly what the initial message is talking about, by saying the savings is in the reduced heat loss, with radiant floors. If you are talking about, being able to maintain a cooler room, (at the ceiling), and therefore have less heat loss, I agree with you, BUT if you are simply saying the heat loss calculations are lower, therefore you have higher savings, then you are just playing with numbers. The true heat loss of any building, has everything to do with its construction, and remains the same regardless of the heat source or type. right?

According to my understanding, its where you place each btu that counts as increased efficiency, and therefore allows for maybe a smaller heat load, but the heat loss of the building is the same regardless.

If this is true than the controls, (I/O resets, mixing, built-in boiler logics, etc.) are really part of what is making the difference in efficiency, since they are what places the btu's where you want them. To say the answer is heat loss, is about the same as saying weather is the reason for the rain. You still don't know why it rains.

Steve

frank s

Radiant and the mass

the mass being your floor,if designed properly there is no convection, because there is NO stratification,the upward and
downward negate each other. As someone mentioned, hotair rises....that's not the case here, we are heating objects and not the air, believe you me nobody beats the WIZ

Steve Ebels

RE: mixing controls

My point exactly. There are numerous boiler out on the market, both gas and oil fired that require 140*+ in order to eliminate condensation. These boilers lose, or maybe a better choice of words would be, thay can't achieve their maximum efficiency due to this fact. I think you would agree that the best case scenario is a boiler that is able to track or match the required temp all on its own. Right down to 70* shutdown temps without mixing or need of protection.

As far as condensing boilers running at low temps and the dhw concern, most of these come with a dhw priority control that shuts down the heating side of the system while it makes hot water, then ramps down to the desired heating temp. I'm sure you are familiar with these. I didn't include a tankless coil dhw system in my train of thought because I really don't feel these are the ultimate in efficiency. (Whew! more feathers raised on that one!!)

Bottom line, I sell radiant primarily on the comfort it provides. There is simply no type of heating that compares to it in that regard. The efficiency thing can get confusing to the average customer when he/she is talking with several different contractors, offering different types of heating options.

Radiant Wizard

Hey KF

You get the prize.

kf_2

Radiantwizard

What do I win!

kf