efficiency of temperature settings for in floor heat system
kkn
| Posted in Energy, Heating & Insulation on
Hi I have a radiant floor heating system in a yoga studio. It is 3/4 pex imbedded in 1&1/2 inches of concrete on top of 2″ of foam insulation. the flooring is 3/4 ” plywood. The heat source is a propane water heater. My question is since I only use the room for a few hours a day I have been turning the thermostat down to 58 between classes and then turning it up to 75 about 12 hours before the next class (it takes that long to bring it up to temperature) is this the most energy efficient way to set the thermostat or is it more efficient to to keep it at 75 all the time??
Replies
What you are doing is more efficient, but it's hard to say how much more than leaving it turned up to 75 all the time.
Yeah, except when you're using a heat pump, it always saves SOMETHING to turn down the thermostat in "off" hours. How much it saves is the issue -- often not enough to worry about.
But particularly with radiant floors, it may be smart to turn down the heat in off hours so that the heat is still "recovering" when occupied. When the heat is on in a radiant floor it feels warmer than the air temp, so it may actually be more comfortable to have the temp at 68 and heating than at 75 and not heating.
A couple of things...
1) Is the water heater meant to condense and/or has been kitted out to handle low water temperatures? If this is a garden-variety storage water heater hooked up to deliver heat to a small space, I expect it to fail in short order. Look for rust around the draft hood, in the combustion chamber, etc. along with hissing/popping sounds as the room is heated - that is the water in the flue pipe running down and hitting the combustion chamber.
2) As you discovered, it takes a long time to heat all that mass. You are saving some energy for sure, but the lower the return temperatures and the longer the recovery, the more important it becomes for the heating system to be able to sustain the load.
We have radiant in slab
for 21 winters now. Our slab is 4" and to me, tho I've not got any backup on this theory...............
With our mass, to drop it down to 55 would be non productive. The energy to bring it up I think would outweigh the benefit of proposed savings. Of course, we do not leave for a couple days, then come back and want 70.
To complicate matters, we designed with passive solar in mind-floor heat on "regularly" at night (boiler and pumps working), kept to temp (indoor air temp) by the sun (unless cloudy). In the event of a very cold night and a blazing sunny day (yet still 18/20 outdoor temp), we might fly up to 76/77 easily. The floor shuts down of course and the slab cools. Now, the next night-has to come back up.
*we did not introduce and outdoor setback at the time.
However, in the fall and early spring we rely on a masonry heater-eventho radiant, much easier to gauge and adjust (via a weather report) to provide excellent lasting heat both day and night with one firing only (in the evening-2hr burn of mayby 15-18 lbs of wood).
The above has little to do with the original posters situation, but presented it as more info on radiant-primarily residential.
With only an inch and a half of mass (hard to figure it's concrete-perhaps gypcrete at that thickness) I'd almost expect a quicker recovery. Unless anyone has this application and can provide info-perhaps a radiant installer could impart a more studied detail on how low to go on the non inhabited setting..................I might lower it, but don't think I'd let it go down as far as the yoga school is setting it.
Seems I remember your name and heating, are you in the biz?
thanks.
The energy to bring it up I think would outweigh the benefit of proposed savings.
This is a common falacy.
What is?
my statement?
or it does take more energy in a radiant system to bring it back up to heat when it gets down to the stagnant temp?
speak to me dan, I'm not some genious.
The falacy is the belief that it takes more energy to bring the structure up to temp than it would take to maintain it at temp. This is only true when there's something like the "emergency heat" setup in a heat pump system. Otherwise you ALWAYS save energy using a "setback" (though how much energy is saved is always a good question).
you're suggesting that if set the heat in the slab, maintain it for a couple weeks 70.
I'll use the same energy or more than if I dropped it back say 12 hours every day to 55?
No variance for the time of day-the outside temp and those other sort of "constant" impacts on heating.
The heat retention and slow disbursement of the mass doesn't do a thing to increase the savings, when you consider the time it takes to make up the floor temp?
Then, apply it to the original post......................Eventho we don't know the time on/off in that yoga studio. Perhaps you're suggesting that there's no change no matter the conditions?
I've always leaned in the opinion that's considered a fallacy.
that is, if I understand you-which does happen sometimes.
thanks
Well, you've got me confused as to whether you're agreeing or disagreeing. But, assuming the cost per BTU of heat supplied to the system is constant, regardless of load, then it always pays (a little) to do setback, regardless of outside temp variations, thermal mass, etc. (One other caveat: When "recovering" from setback the user does not over-compensate and set the thermostat above "normal", nor does the thermostat itself or any associated equipment "overshoot" to any significant degree.)
This is easily shown because the heat loss from the system is a simple function of the "delta T", and reducing "delta T" will reduce heat loss.
ok
Whatever you said .
As far a "a little", that's enough for me.
No, I'm not agreeing. You tell me it's a fallacy, has to do with delta t and if that's some "law" then I guess I'm wrong.
thanks.
No, you WILL use more energy to maintain the temp vs. setting it back. Dan is right ... it is a fallacy: "that it takes more energy to reheat a radiant slab than to allow the setback during unoccupied periods". The trick is to be able to anticipate occupancy and when you then need to change to the occupied period. If your occupancy varies a lot, it makes it diffcult to set back.
Setback temps save energy ... conventional or radiant. The radiant system is a bit trickier because of the need to anticipate the startup temp.
As I said in another post ... you also would setback earlier (e.g. 5 hours before going to bed) to maximize the savings.
"Dan is right"
Can we frame
"Dan is right"
Can we frame that and hang it up somewhere around here??
Just make that your tag line, with the link to prove it.
I got some used toilet paper ... HA HA ... just kidding Dan!! ;)
The Wright Brothers were, too!
Clew
I just can't come to grips believing you and Dan, just because you said so.
Is there some authoritative study you can point me to, regarding radiant in the slab and set back, make up time, a bit of passive solar maybe.............
If the only benefit of radiant in the slab is that you don't have turn it up as high to feel comfortable..................
and it has nothing to do with energy efficiency v. forced air................
then I'm disappointed with the news, but comfortable as all get out with the gas bill and temp of this system here in NW Oh.
Remember, I'm just a dumb carpenter and I still haven't figured out what you two are.
thanks.
I'm an energy efficiency expert that has been in the industry for 30 years. Radiant slabs have to adhere to the laws of thermo like any other system. Thermal mass in and of itself will not magically create or cause additional energy consumption. Passive solar gain can result in higher efficiency for mass based systems because there is another source of 'free' energy and the mass is a way to store it.
But using mass for conventional heating systems (e.g. air or hydronic) will not magically reduce energy use just because of the mass. The benefit of radiant slabs as you elude to is ... that you can turn down your stat a few degrees and still be very comfortable. The primary reason that is, is that the floor is about the only part of a building the occupants routinely and consistently are in contact with. Warm floors can result in comfort conditions at cooler space temps. 90 deg floors sure are more comfort than the typical 55 deg floors that slabs normally are.
I think actual energy savings from radiant floor heating is often roughly in the 5-10% range. This will vary depending on the specifics of the house, construction, etc. CLAIMED savings are often in the 20-50% range claimed by marketers who really don't understand the science of radiant heat or thermal mass.
Many marketers of radiant slabs claim large savings ... for what reason ... not sure. They think that mass floors somehow magically create energy which reduces e.g. your nat gas consumption. That is simply BS. The savings is in the reduced space temp to have the same feeling of comfort.
As far as setback and setup temps, the slab system is much like any other heating system ... air, radiators, etc. They don't magically save or cause more energy use. But, as I pointed out, you do have to control the setback/setup functions in a much different way since the mass of the slab can cause a delay in getting the heat to the occupants.
These are not half bake theories or just Dan and my personal opinions. Common falacies abound around radiant slab energy efficiency and
By the way ... radiant floors is a bit of a misnomer. It's not truly radiant heat like the sun and earth. It is too low temp for that. However, there is a bit of a radiant component in that your body radiates less heat TO a warmer slab than a colder one (similar to your body radiating heat to a cold window wall).
This is the basic science of this. This isn't our opinion. People in the business realize this in spite of the continued misconceptions about how radiant floors work.
Now that is more like it.
Short answers while true don't provide diddly in the way of information. And I'm the kind of guy that needs simple explanations in order to come close to understanding. I also try not to take everything that falls outta someones mouth as gospel. The more info I get the better chance I have of perhaps of picking up on it.
Here's another one for you.
We also have a masonry heater. I think wood consumption is less than a good (real good) wood stove. I know the energy to operate it is definitely less. It is zero now. To heat this 2700 sf house I would probably need 3 burns of say 12 lbs of wood (with no sun and outside temp rising only to 12 deg. Normally in the 15-25 deg. day, two firings will be enough. Spring/fall, with lows in the 30's, one burn of 15lbs.
So here's the question. To maintain a consistent temp for 24hrs today (0-12F), I'd use 36.lbs of wood in the masonry heater. If I understand you right, I'd burn the same amount of wood in a stove if that stove was as efficient. The method of burn in the heater is quick and hot-no damping down to give a longer lasting burn. The damper is utilized to keep as much of the heat generated in the fire box, still allowing the flue temp to be very high.
In the wood stove, there'd be a good blaze going (not the well above 1500 deg's in the heater), constant feeding and then damping down at night for a slower burn. Having heated with wood in one form or another for 40 yrs I'm thinking there'd be a whole lot more smoke up the chimney with even the most efficient wood stove going.
If I'm thinking correctly, less wood use must only mean more efficiency as what energy source goes in, only allows so much energy heat out. That's it?
thanks.
Calvin, the main issue with your two different wood heaters is going to be flue temp -- the higher the flue temp, the lower the efficiency.
Beyond that you have the problem of establishing exactly what "equivalent heat" means between the two units since one will run the temp up high and quickly down while the other will produce more moderate temps. When the temp is higher than your "set point" then your losses are higher as well, plus, for equivalent comfort, you'd have to fire the "fast" unit more often.
Ultimately the issue is comfort, and if you can maintain the same comfort levels with lower average temps somehow (often circulating air to eliminate "cold spots' will do this, eg) then you can save energy on any system.
Dan
The higher flue temp on the masonry heater is not indicative of less efficiency. That is the way the system is designed to work. High firebox temp necessarily means higher flue temp. The benefit? Gases normally sent up the flue are burned-heat is recovered from that with the mass of the heater. No creosote-period, whether burning pine or ash. Less ash leftover per lb. of wood.
Now, the difference in the temperature between the firebox and flue relationship in each heater may be something you can use to back up your statement on efficiency. I still believe from experience that I use less wood to produce the same or better comfort, using the soapstone masonry heater.
But I must be wrong as energy in / energy out seems to rule the day (minus efficiency if your hypothesis is right)
Now, to sidetrack a bit.
Can we draw any conclusions re. heat by looking at electric illumination. In it's infancy, a lightbulb was a lightbulb. It used electricity and I suppose, higher watt lamp-higher energy usage, (not better reflection in the fixture). Would the better light and less energy used that we experience now, be merely a question of efficiency? And if so, would the different technology be equated with the delivery method-----incandescent v. sodium v halogen v LED?
thanks
Clewless reminded me that I forgot combustion efficiency. For most heating fuels in modern furnaces combustion efficiency is so close to 100% that it can be ignored, but with wood stoves the combustion efficiency probably ranges from 30% to 95% or so (thumb suck). This may well trump flue temp as the largest factor in determining total heat extraction from a cord of wood.
It's not clear exactly what you're getting at with regard to illumination. Definitely, the efficiency of a lamp is DEFINED as the light output divided by the energy input. A more efficient lamp will run produce less heat (wasted energy) for the same amount of light.
Eventually, it's always energy in = energy out. It's just a question of whether all of the "out" you get is usable or not -- does it heat the house or go up the flue? Does it light the room or only warm it a bit (and more expensively than most other heating methods).
What I was getting at is..............
It's not clear exactly what you're getting at with regard to illumination. Definitely, the efficiency of a lamp is DEFINED as the light output divided by the energy input.
That maybe, just maybe-efficiency of the burn or energy used is one thing, while the delivery system might just add something to the equation. Take that theory and apply it to radiant heat.................take that from wood fired masonry heat to in slab floor h2o radiant.
Less energy use to produce the same amount of light.
Less energy used to produce the same amount of heat.
That's why I would hope someone that is really conversant in hot water radiant in slab would come along and flesh that idea out. The common fallacy thing is based on the law-energy in/energy out. The only way to push that envelope is efficiency of fuel burned. But, cannot the delivery system give us another multiplier?
Better worded to side step the "law"- might be are there things in addition to the law that can produce more heat for energy used ?
You and Clew seem to be saying no, period. I'm not absolutely sure I can believe that just yet. If, efficiency encompasses all the "other" things involved that produces more comfort with less effort (or fuel), then I guess I'll have to live with that.
thanks.
And sorry to the original poster that hasn't returned anyway for taking this off in another, yet similar..................direction.
The first law of thermodynamics tells us you can never get ahead in the game. The second law says you can't even break even. The third law says that you can't get out of the game. (Well, actually that's a stretch on the third law, but it sounds good in physics lectures.)
That maybe, just maybe-efficiency of the burn or energy used is one thing, while the delivery system might just add something to the equation.
The delivery system might REMOVE something from the equation, but it can never add anything (second law). For instance, the electrical grid in some instances (and depending on how you "charge" the losses) often loses about half the input energy coming from the turbine or whatever by the time it reaches your toaster. These are losses in the generator, losses in transformers, losses in transmission lines, losses in the wires in your house.
It's important to understand that "heat", "energy", and even "mass" are essentially interchangeable terms. If you heat something up it will actually gain weight (when weighed in a vacuum) due to E=mc-squared. The amount of weight gained is imperceptable (not sure there's any scale in the world accurate enough to detect it), but the weight gain is real. If you burn something and collect all of the combustion products, then subtract out the weight of the oxygen used to do the burning, you'll come up (imperceptably) short, the difference being the amount of mass that was converted to heat and which subsequently escaped from the system.
It's not that energy (and remember, energy and mass are the same thing) is just "sorta" conserved, it's absolutely, totally, completely conserved.
So there are only three things (that I can think of) you can do:
Wring more of the energy out of the fuel, through better combustion and better heat transfer (which implies lower flue temps, etc).
Minimize the amount of energy that escapes from the system. (Lower transmission losses in electrical systems, better insulation in buildings, etc.)
Somehow create a perception that a system is delivering more heat than it is. (Cut back when unoccupied, use radiant (infrared) heat to create a perception of warmth in cool air, etc.)
I can appreciate 'your kinda guy'. Short answers don't do diddly when you want or need to understand something.
I started reading your new question .... 'if a freight train left NY traveling to Chicago going 85 MPH and a train left Chicago ...'
OK ... levity over.
Not sure what you mean by masonry heater. E.g. Russian fireplace? I'm assuming something like that.
The beauty of the mass type wood burner is that you burn all your fuel NOW ...burn it hot, efficient, and low smoke. Burn it now and save the Btus in the mass for later use. I assume there is a damper that closes between burns, right?
The conventional wood stove tends to be less efficient in the combustion process. That's often the way the fast, hot fire burn is controlled ... through some inefficiency.
GENERALLY, you will need the same amount of wood to meet a particular demand (OA temperature). However, there are differences in efficiency.
This is where it can get complex. It is difficult to pinpoint the efficiency of a masonry/mass heater. It's one thing to determine the combustion efficiency of the burning process. Quite another to determine the efficiency of the system. Similarly for wood stoves. You can determine efficiency at any point, but to do it across a typical burn pattern through a day is more tricky.
I'm not an expert in the modern wood burning stoves. I did talk a lot w/one of the manufacturers some time back. He was telling me about the extensive testing they went through to ensure the stove met emissions requirements (which is somewhat of an indicator of combustion efficiency).
I think for a 'conventional' modern wood stove to work right ... to hold a fire long term and do it efficiently, the combustion efficiency tends to be a little low. I remember actually building a wood burning stove when I was in college ... wow ... funny. I made it out of like 3/32 steel and cut it w/ a saber saw (one of my few tools). I even used at the time a 'modern' design. Amazing I didn't burn down my trailer I was living in!!
That's why wood pellets is a better product, concept ... the pieces are smaller and you can begin to regulate the fuel input much like you regulate your gas pedal in your car as the load increases (e.g. a hill), you increase gas input.
The answer? Not sure. But you are generally right. You require the same pounds of wood for either heater given a certain load condition. The difference is in how you burn it. My guess is that if the masonry heater is done right (i.e. for heat distributionj/release), it will use less fuel, because it is inherently somewhat more efficient. It burns relatively hot and fast (and efficiently) and stores that heat to be released/used later. The conventional wood stove might use more wood because the combustion efficiency is probably lower.
I'm not an expert in wood combustion appliances, though. Being primarily in the commercial sector, we don't have too many wood heated buidlings.
Passive solar heating works similar to the masonry heater. It takes advantage of the free and excess sunshine during a cold clear day and stores that energy up to be used later. In part that requires the space temp to be allowed to be elevated to accomplish this task. This stuff gets complicated fast (if you think about e.g. calculating where the Btus are and when), but the concepts are relatively simple and need to always follow the laws of thermo.
Clue
I assume there is a damper that closes between burns, right?
Yes, there's an ashbox vent on the bottom of the heater. Above, two smaller door vents. Both are adjustible. 6'6" up the chimney is the damper. A simple slide-in plate that cuts across the flue.
I'll give you an example of a typical burn. I load the small firebox with about 10lbs of forearm thickness wood. Bigger on bottom, smaller on top. I lay my kindling on top of that, a grumbled up section of the days paper above that.
Open the damper fully, same with all vents. Lite it up, wait maybe 10 minutes for full burn to achieve. Slide the damper closed about 2/3's to 3/4's closed. All vents still wide open. This allows the hot burn to continue, while cutting down on the chimney opening. A ventury effect I'd guess as the fire roars yet much of the burn heat is contained, while no real limit to the exhaust up the chimney.
1-1/2hrs later, down to nice bed of coals. These we want to extinquish quickly-don't want to waste heat going up the chimney waiting for the embers to die out enabling shutdown. I now open the damper to 1/2, shut the door vents so now only the ashbox vent is fully open. This makes quick work of completing the burn cycle. Once the embers are done-close everything down. Total burn time- 2 to 2-1/2 hrs.
The way the vents are situated (one below the fire, the other - 5" above the bottom of the firebox) gives air to the bottom of the fire while the door vents enter and tumble the air. This is soapstone-a firebox with grate.with an opening up top on each side, and air space for the heated smoke to fill , another enclosure of finished soapstone that has a bottom vent that connects to the masonry chimney.
You mention Russian Stove-similar, this idea comes from Finland. Much of Northern Europe and Asia have similar working ideas. Contraflow venting seems to be the common similarity. In Korea there are systems that burn the fire on one end of the room, the flue gases go under the floor across to the other side, connecting and exiting up the chimney. Spot on heating as well as radiant in the floor. The chimney sweeps job a bit harder? Maybe not as the burn is clean, residue minimal if at all. A more Complete Burn........................
which I guess gets me to the answer to my question. Might not be efficiency as much as a more complete burning of the fuel.
thanks.
Efficiency IS more complete burning of the fuel. A hot flue/burn also ensures that emissions are minimized.
Ok, thanks.
But why is dan hung up on the higher flue temp is a less complete burn, hence a poorer energy producer for lb. of wood?
I think the percentage of total heat produced vs. what goes up the chimney is more like it. The lower percentage, the less waste, the more complete combustion.
Whether or not e does equal the mc5.
I'm not interested in flue temp with regard to completeness of combustion, but rather heat transfer. The higher the flue temp, the more heat is going up the flue rather than heating the structure. Of course, lowering flue temp by mixing in more air doesn't improve efficiency (quite the opposite), but it's a simple thought experiment to see that if you take a flue that has, say, 400-degree gas flowing through it and lower that to 300 degrees using a post-combustion heat exchanger, that's getting more usable heat for the same inputs.
I think the percentage of total heat produced vs. what goes up the chimney is more like it. The lower percentage, the less waste, the more complete combustion.
The percentage of total energy INPUT vs what goes up the chimney is the issue. And for "what goes up the chimney" you have to consider both heat and unburned fuel -- both represent inefficiency.
Forget it dan
.
Hi Calvin,
I used to participate more actively here and at HeatingHelp. Two kids now divert more attention elsewhere. Never in the business except on the energy effieincy front, i.e. helping the government set new minimum energy effieincy standards for various appliances, including heating sysetms.
Anyhow, I agree that the system should be able to recover more quickly, but also consider the depth of the setback (i.e. the delta-T), the mass of the slab, and the heating output of the water heater. If this is a standard 40-gallon or 50-gallon water heater then you can expect something like 37kBTU of output, at most. HIgher input units exist, but most people don't use them, ditto for water heaters that are built for potential space heating also (i.e. Polaris, Voyager, etc.).
The use of a standard gas water heater (i.e. large storage tank, etc.) to heat a large space is IMO problematic. The appliance is not built for it, and once you retrofit the kind of protection that a standard gas water heater needs to survive sustained operation with low return water temperatures, you end up with a not so inexpensive appliance, crummy efficiency, no warranty, etc. Plus, the hot water storage battery (what a storage water heater relies on to overcome its input deficit) is puny compared to the heat storage capacity of a large slab.
Given the application, it would seem that a higher-output appliance like a wall-hung instantaneous water heater would make more sense. A wall-hung water heater can have a 8x+ higher input, no mass, direct vent, etc. and typically they offer kits to allow hydronic heating use (i.e. get a warranty). The marginal cost increase is pretty low for a properly-installed system and the performance should be much better.
Thank you
Our system uses a cast iron boiler. Other construction I have been involved in have used small wall hungs like the Munchkin. These heat sources I have a little experience in as to their operation etc. Never have come across the standard water heater method, but in a small space it seems like a moderately priced alternative. It certainly is mentioned as an alternative. You bring to light some limitations.
thanks.
Considering your expertise, where are we headed in the next 10 yrs on the energy efficiency requirement front?
if anywhere.
Thanks back...
We used to have a high-mass boiler when we were still using oil. Makes some sense when all one can pretty much get in this country is on/off burners for residential use. But, with all the gas being 'hydro-fracked' and the oil situation being what it is, we switched to gas heat instead. New low-mass, condensing, modulating boiler is a joy to stand next to and not to know if it's operating. Never mind the energy savings, which seem to be substantial. Just another indication of how AFUE is deeply flawed, I guess.
Our whole house is radiant, mostly under wood, though we have a big slab in the basement and multiple bathorooms kitted with the stuff also. The house is running off a reset curve right now, no thermostats in use other than to turn off the basement circuits as needed. I built some zone actuator interfaces with zero standby for our thermostats, am retrofitting them slowly (basement was guinea pig) to allow unused rooms to cool down. But in general I agree that deep setback with big slabs is a losing proposition unless the space is used on a very scheduled basis - the point of a home is to be comfortable.
Anyhow, as for where EE is going... my guess is up and up. There is a lot of savings potential in this country on account of the wasteful envelopes and heating systems. Not sustainable in the long run. How it all will be upgraded cost-effectively in the future remains to be seen. I'm just glad that I could get my home spray-foamed when I did, i.e. before oil hit $95 a barrel.
More efficient to set back. But you know the issue ... the reheat time and you seem to have a handle on that.
However, if you only use the studio for say 4 hours a day and it takes 12 to heat back up, you should also set it back e.g. THEORETICALLY 12 hrs before the end of your class/use. In other words, at a minimum, I'd consider setting back at the beggining of your use. The heat will stay in the floor ... where you likely most want it anyway.
Radiant systems are just like regular houses, really in terms of heat loss, setback, etc. Problem is the lag of the warm up period and getting a handle on it. Also if you happen to want to change schedules suddenly, a radiant slab isn't as responsive as a conventional gas furnace and ducted air system.