RADIANT HEAT: MORE EFFICIENT?

Chris_82

Think of this stuff as another tool you can use to heat a home or any space where it is appliciable. Not so much as the end all to end alls, but options. It pays to have lots of options. Personally I haven't seen an underfloor job where at some point or another an additional heat source has been added. Especially when they start rolling out the rugs.

Bob Schalit

RADIANT HEAT: MORE EFFICIENT?

Hello All,

I just got back from a seminar on "green building" and the lecturer from Efficiency Vermont told our group of 500 attendees that radiant heat was NOT more efficient than its radiation/convection competitors because "the same number of Btus go into the home, no matter what the heat distribution system is." He also claimed that radiant heat is at a disadvantage when trying to keep pace with rapidly dropping outdoor temps.

I always thought radiant heat was 30 - 40% more efficient than traditional convection-delivered and radiation-dispersed heat delivery systems. Was the lecturer correct?

ALH_4

Hydronic advantages

He was incorrect. Radiant "feels" warmer than forced air because of the radiant component that forced air does not have. If the air in the room is warm and the objects are cool, you do not feel as warm as if the air is cool and the objects are warm.

Zoning is also a major advantage of hydronic heat.

The lecturer needs to look a little more deeply into the differences.

Mike T., Swampeast MO

No, I do not believe that the lecturer was correct, but no more incorrect than blanket statements like "radiant is 30% - 40% more efficient" than other forms of heat.

Reasons why radiant can be more efficient:

1) For the same comfort level, structures heated radiantly can almost always be kept a few degrees F lower in temperature. Lower indoor temp means less heat lost in otherwise identical circumstances.

2) Temperature stratification is generally VERY low in radiantly heated structures. e.g. the temp of the air near at the ceiling is very similar to that near the floor. Heating systems where convection dominates tend to have much higher air temp stratification with the temp near the ceiling (where heat losses are highest) significantly higher than near the floor or at the "breathing line" about 55"-60" above the floor. The higher the ceilings, the more pronounced the temperature stratification with convention-based heating while ceiling height is virtually immaterial with radiant.

3) Compared to forced air systems, transmission losses in any hydronic systems are typically MUCH lower. Also, it is utterly impossible for radiant systems to produce pressurization imbalances. Such happens in nearly every forced air system with some spaces relatively over-pressurized (increasing exfiltration) and others relatively under-pressurized (increasing infiltration).

4) With a condensing/modulating boiler is combined with radiant floors/walls/ceilings using highly a highly conductive heat transfer mechanism, system operating temperatures are typically VERY low compared to nearly any other form of heating system. Such allows the condensing/modulating boiler to produce those BTUs more efficiently. Since some form of reset is now a given with condensing/modulating boilers, nicely conductive radiant floors operate at a lower and shallower heating curve than any other form, so as it gets colder out the mod-con maintains more of its efficiency than with any other form of heat.

5) Radiant floors are known as "slow heat". They are however also known as VERY EVEN heat. No hot/cold spots. Since this was a "green building" seminar where I can only assume that nicely insulated/weatherized shells are a given, I'm rather shocked that the lecturer would say that the system would have a hard time "keeping pace with rapidly dropping temperatures". Why? Because the air temp in the structure won't want to drop ANYWHERE near as fast as the outside temp and there will be PLENTY of time to add the necessary heat in any reasonable circumstances. The "slow" nature of radiant heating systems is in itself an energy saver. Why? Because the heating curve can be adjusted quite closely to the minimum actually required for any given outside temp. Again, this allows a mod-con to produce those BTUs more efficiently.

----------------------------------------------

My guess is that the lecturer is:

1) Involved in the forced air industry.

and/or

2) Is not familiar with MODERN radiant systems designed for the highest possible efficiency.

and/or

3) Is only familiar with poor radiant systems (there's an "oh it's so easy" DIY radiant company in VT that's especially prone to this problem).

JohnNY

Touchy subject but....

....from my own installation experiences, I believe the radiant heating industry has been feeding us a lot of stuff that is quite hard to swallow.

I sell radiant heat to people that want warm floors. Period.
I do everything by the book, but I never commit to any reduction in fuel usage or costs.

When I started installing RFH, about 15 years ago, I went out to Minnesota to take all the classes and I generally jumped in with both feet.
At that time it was presented as the time-tested, proven way to heat a structure at the lowest operating cost possible...just ask EVERYONE in Europe. They've been doing it this way for 100 years!

If that was true then someone please tell me why the products and installation methods seems to change every three years or so.

I get the impression that an entire industry has been built on selling things to the less-than-brilliant-plumbing-force-with-deep-pockets out here.

I know this is a bit of a rant but when it comes to high efficiency, (RFH, tankless water heaters, etc) perhaps the emperor has no clothes after all.



In all fairness, I'm just a bit bitter today after installing a nice mixed-temp heating system with an $800 Caleffi hydraulic separator that the heated water skates right through; the condition being exactly as I was trying to prevent.

I'm THIS CLOSE (fingers illustrating a small space) to going back to the old school way of pump on the return and zone valves all around.



Just kidding....

or am I?













definitely venting















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Jed_2

\"radiation/convection competitors\"

Is that reference synonimous with force hot air? I took it to mean fin tube, convector, fan coil, radiator, hydronic.
Just trying to understand delivery terminology.

Mike T. is absolutely correct in that other forms of delivery create higher convection currents, and depenpending on building envelope, can require higher energy usage. Infiltration, crackage, pressure differentials. As I understand it, radiant heated homes or buildings do not result in these high convection currents, and the RMT is a result of the total mass radiation of the space. Not the same thing as "air".

However, if this "green" building included an envelope that was fomed, and the air delivery system was high velocity, I can attest to the amazing comfort level of the space, anywhere in the room. Aspiration has it's advantages when properly applied. But, I have no way of knowing what this presenter was talking about. A BTU is a BTU from a generation/requirement perspective. But what happens after it is generated is synergistic with the structure.

JMHO

Jed

Mike T., Swampeast MO

JohnNY: Good and understood rant. I'll quote you a couple times but not as "JohnNY" but as "anonymous ranter".

I sell radiant heat to people that want warm floors. Period

Then you must only work with radiant floor systems in fairly "high loss" structures and/or brutal invironments. A reasonably "green" shell in much of the U.S. will NOT REQUIRE WARM FLOORS! Design floor temp requirements of <80F can be expected. In most cases the floors will feel "neutral" or even "coolish" <I>without in any way sacrificing comfort unless you've told the customer to EXPECT warm floors nearly all the time!


about 15 years ago...I jumped in [radiant] with both feet. A time-tested, proven way...just ask EVERYONE in Europe. They've been doing it this way for 100 years! If...true...tell me why products/mothods change every 3 years or so.

Because the time-tested, proven methods of Europe 15 years ago ARE STILL FOREIGN TO MOST U.S. HEATING CONTRACTORS and more imporantly because the "best" heat sources Europe was using at the time are only now filtering into the American market! The current European state-of-the-hydronic-art is STILL YEARS AHEAD but the forward-thinking hydronic engineers/builders are YEARNING for the technology. Of couse the good hydronic/radiant engineers designed their years-old systems in ways that the latest European technology only improves them...

a bit bitter today after installing a nice mixed-temp heating system

Please give a bit more detail regarding the reasons why you needed a mixed-temp system.

I sure hope it didn't drastically vary heating method--say hydro-air mixed with radiant along with the temps. In general you should do everything in your power to avoid creating "mixed-temp" systems. When you must create such, and if you're using a mod-con, the "European solution" is independent reset curves under the full control of the boiler. If you're encountering problems in such a system I fail to see how a "hydronic separator" of any sort or any cost could help...

I firmly believe that the best solution is the most simple solution suited to the job. Such "simple" solutions ONLY come when you view the heating system as a WHOLE and how each and every component interacts. Fail to do so and you'll find yourself "looking the for solution" based on an entire industry built on selling things to the less-than-brilliant-plumbing-force-with-deep-pockets...

Mike T., Swampeast MO

EFFICIENT RADIANT COMFORT

Start work in the dark at 6:00 a.m. in mid-December.

Around freezing outside.

Office about 58F by a cast iron radiator on a "constantly" maintained TRV setting that averages about 62F during working hours. Dressed in trousers and light sweater. Make coffee.

Morning "constitutional" across the hall in a "can flush anything American Standard commercial, long bowl, rear exit toilet". MY READING ROOM

All rooms surrounding this bath no more than 53F. Thermometer in the bath reads 60F or so.

But guess what, that bath has a radiant floor! Simply connected to a true constant circulation system. To my hand or bare foot the floor feels a bit cool, but the room itself "feels" FAR warmer than the office and I'll often read/study for a long time in perfect comfort with my trousers at my ankles...

ALH_4

I think

I read too quickly and misunderstood the post. I now read it as hot water radiators versus radiant floors. In that case, efficiency-wise, there is almost no difference as long as the radiators are sized to run at relatively low temperatures and the boilers are the same. If the lecturer was talking about radiant in thick slabs his other comment is somewhat correct, assuming on/off control.

So now I think the lecturer is mostly right.

Thanks, Jed.

singh

Electrical usage

Don't over look what it takes in electrical energy to "blow" that air around. In let's say a 10x14 duct. A 48,000 furnasty may use 2kw/hr
Now compare that to small tubing able to carry the same amount of btu's with a standard circulator pump, using only 80 watts.

That alone probably 40% .
Wait till these low energy pumps catch on, even greater savings!!!!

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Mike T., Swampeast MO

Great point Andrew.

I too was thinking about non-hydronic vs. hydronic.

However, in a VERY "green" home I will state emphatically that the ultimate comes from emitters designed/oriented/sized to deliver as much of their output as possible via radiation.

I'm having to "design around" highly convective-based emitters (back-to-back Myson 2-row "pedestal" baseboard) in a shell perfectly suited to TINY
but "fairly hot", vertically oriented radiant panels.

Unknown

The last big project I did,,,

(9000+ sq ft)Uses 110* water throughout and so far the floors are set at 77* with no complaints. Agreed, if the floors "feel" warm in a tight house the space will overheat. I tell my potential customers that the floors won't feel warm, they JUST WON'T FEEL COLD! When I discuss the subject I almost immediately introduce the "warm floor" as a common misconception about radiant heat.

Aidan (UK)

No question in my mind

A few points of common ground that very few people will disagree with - including your speaker. Some of these, Mike T already made.

1st Radiant is the most comfortable heat there is. Why? The majority of the time, radiant heat is delivered through the floor. This is where the coldest part of the human body typically is - your feet. If the floor is warm, you are more likely to be comfortable . If your comfortable, you are less likely to turn the thermostat up - hence use less energy.

2nd Hot air rises. No debate here. Everybody knows that one. We don't want to pay money to heat the ceilings - we want the heat at floor level where we are.

3rd Laws of Physics Heat loss is a function of the difference in temperature of between two sides of a conducting medium. Ceiling surface temperatures are higher with convection systems - the heated air rises. The ceiling temps with convective heating can be 5, 10, 15 degrees higher than with radiant- hence more heat loss to the outside through ceilings - you burn more fuel.

4th Zoning usually can be easily achieved with radiant heating, which can result in substantial savings if utilized. If you lower the temperature in a given zone, for whatever time period, you are going to use less heat energy.

5th Wind-chill effect. Any movement of air, especially dry air, will create this condition. Here's a real easy demonstration. Assuming your comfortable, pick up a pencil, hold in front of you, pretend its a thermometer and guess the room temperature. Say it's 70. Now pick up a magazine or some paper and fan yourself. You feel cooler, but the temperature of the imaginary thermometer did not change ... but you feel colder. When you feel colder, you turn up the heat, and burn more energy.

The rate of lost heat energy from exposed skin is a function of both temperature and air velocity. Radiant heat creates virtually no air movement, hence no wind-chill effect.

In my mind, using less energy translates to higher efficiencies. Hard to think someone would argue against these points.

JohnNY

Just playing Devil's advocate here.....

1. "If the floor is warm, you are more likely to be comfortable ."
This is not always true. Especially for the elderly.

2 is the same as 3.

4. Keeping one zone cooler makes the other zones work harder.

5. Why would you advise someone to use a pencil for this test and not an actual thermometer?

I love radiant heat and hydronics of all kinds. And I think forced air is a horrible way to heat a residence, but I do believe that the radiant heating industry has pushed itself on the American market largely in part by making some claims that are not entirely true.
Efficiency may possibly be one of them.

When you first were told about radiant heat, you listened with an open mind.
I'm just saying do not close it now.


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Bob Schalit

YOUR COMMENTS ARE APPRECIATED!

Thanks for your most interesting and thoughtful replies. I'm a non-plumber/non-engineer and I've learned a good deal from you. I'll pass your posts on to the lecturer.

Glenn Sossin_2

Warm Floors the best

Are you seriously suggesting that there are occasions where cool floors are more comfortable for the eldery?? I really would like to understand the basis for that comment.

It's been my experience, on multiple occassions, where we designed homes/apartments with radiant heat, specificallyfor the elderly with circulatory problems.

??? "2the same as 3." Ok they address the same point.

?? Keeping one zone cooler makes the other zones work harder ?? Zones don't work hard - they release heat energy based on a multiple of variables - thermostat being the principal one. That logic is a wive's tale. If it were true, there would be no benefit to setback thermostats - instead, they would cost us money to use because they would make the heating system work harder to reheat the house. I'm sure you agree, that doesn't make sense. Keeping one or more zones cooler, because they are unoccupied reduces the delta T, thereby reducing the heat enegy sent/lost to that room.

The idea of simulating a thermometer is simply a convenience - especially during a converstation with a prospective customer asking about radiant heat. During the course of the converstation, they recognize the temperature is the same, but when first asked if it's cooler when fanning themselves, their first inclination is to say yes, but then they think further and realize they feel cooler but the imaginary thermometer didn't change - most of them can't explain why.

I use it in another example, both in person and on the phone to demonstrate radiant heat. I suggest the following : its 45F, no wind, they are standing in bright sunlight, wearing a black leather jacket, holding that imaginary thermometer - I ask them to imagine how they feel .... then I say take two steps to the right ... your now in the shade of a tree. What changed? Do you feel the same? Did the temperature change? They recognize the temperature didn't change, but they also recognize the effect of the radiant heat - point demonstrated.

I am very passionate about radiant heat- especially because there are too many bozo's out there that think because they can solder/crimp fittings to pex, they can design/install radiant heat. I've been to many jobs where the system isn't working properly because of mis-installation. I believe education is crucial to prevent this.

I don't think the industry has pushed itself on anyone. Generally, radiant costs more to install than other forms of heating. People will not pay for it if they don't percieve a benefit - either in comfort, fuel costs, or both. In fact, I think its been extremely slow in being adopted here in the US. It's the predominant heat method in Europe - there's a reason for it.

Back in the 80's, occasionaly, you found someone putting radiant in some higher end bathrooms. In the 90's, people started adding it to serveral baths and their kitchen floors. In the beginning of this decade, it was not uncommon to find the whole 1st floor of a home being done with radiant with forced air as a backup. Now, it can be the whole entire house with along with condensing boilers, indirects, and indoor/outdoor resets. My home is 100% radiant including the basement and garage.

With regards to efficiency, I'm sure you would agree conceptually, if we can heat a home with lower temperature water, it likely to be more efficient than if we use higher temperature water.

I'm curious - you referred to claims that are not entirely true - what claims and by who? I will agree that there has been evidence to suggest that fuel savings may be overstated on occassion. I believe they are a strong function of system design, equiptment, and how the end user operates the system.

I'm sure you'll agree setback thermostats can save 5% - 10% or more in the typical home ...... but thats only if they're set & used. Likewise, in my opinion, a home with multiple zones of radiant heat would not yield substantive fuel savings unless the homeowner lowered the thermostat in unoccupied zones and took advantage of the setback feature of the thermostats.

The heating/cooling/builidng industry is undergoing massive changes - for the better. My opionion, radiant heat is a big part of it, mainly because it is the most comfortable form of heat and, when properly designed/installed, the most efficient.