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What is the most effective heating system for Schools & Intermittent use buildings? (17 Posts)
What is the most effective heating system for Schools & Intermittent use buildings?I am a Gas Fitter/Heating Tech for a School Board in Western Canada (basically responsible for keeping the heat on). We have a wide variety of schools from a age of school standpoint, some with original heating plants. We have many old, heritage (early 1900's) schools, and some brand new schools and everything in between. We have heating systems with old low pressure steam boilers & steam emitters, to some old cast iron or steel hot water boilers with univent & fan coil heat emitters, to newer plants with condensing boilers and/or cast iron boilers that heat with univents, fan coils, or radiant heat sources like panel rads and in the case of a couple brand new schools, radiant floors.
So there is a wide variety of heating plants to see.
Now, one of the of the common themes in schools with any heating plant is intermittent use. The schools are only in operation for the most part around 8 hours a day (with the exception perhaps of if someone is using the school for some night event but this is an exception). So naturally most school operations departments want to take advantage of night time & weekend set backs to save money on natural gas. They will run the schools to 20 degrees celsius (68 F) during working hours and would to set back as much as possible (typically to 10 degrees Celsius which is around 50 F) to save money due to reduced heat loss, but still protect the school from freezing and provide a reasonable temperature to bring the school back up to temp from on Monday morning.
So naturally we have a few issues here. The main one is flue gas condensation on non-condensing steam & hot water boilers, the other one is design of old systems versus design of new where the new ones don't have as much pick up or oversizing as the old ones - the old original steam boiler plants, of which we have some still around (some old 1940's HRT boilers, some old 1950-1960's cleaver-brooks or similar fire tube variants) typically have very little issue bringing the temp back up. There is only a fixed amount of water in a steam boiler that the burner has to heat up so condensing isn't an issue really as once that water is up to temp and producing steam everything is good. The steam boilers typically have oversized emitters and can bring a school back up fast, and due to this fast recovery, can be set back relatively low. They also don't have much in the way of pumps other then vacuum pumps and feedwater pumps, so very little electricity use compared to a hot water system. Hot water boilers, due to the large volume of hot water in most systems in comparison, condense for a longer period of time (dependant on plant size versus building heat requirements) and take longer to bring a school back up to temp. There is also higher electricity use due to the many pumps required. The new schools have very little pick up as the boilers seem to be sized more tightly to the heat loss. This allows less of a set back. I've always been a firm believer in low temp hot water heating and I think if operated more correctly using DDC can probably work out, but a lot of the guys in the heating department I'm in who have been here a lot longer than myself are much bigger fans of the steam heating systems. They pick up faster, have less pumps to maintain/fix, don't condense as long, and seem to last and last without issue - some of our fire tubes are over 50 years old and still have original tubes or have had very few vessel (tube) repairs/replacements. Sure steam traps need some love every now and then and the feed water/vacuum system needs some attention, but I'm wondering what your experience/preferences are? What have you seen?
What, from your experience, is the best heating plant/emitter combination to maximize fuel efficiency for buildings with intermittent use/operatin to take advantage of setback to some degree. Are high mass radiant heat emitters out?
Being a fan of low temp hot water heating my solution (if I were to design a school or design a new heating plant/emitter retrofit) would be to use condensing boilers with say 150% redundancy (100% to pick up the plan on the coldest day of the year, the other 50% for redundancy, and for pick up on monday morning when a big dump of BTU's are required), and then use panel rads as they are a low mass heat emitter that can pick up faster than a high mass radiant floor and still provide pretty good comfort.
Thoughts?Class 'A' Gas Fitter - Certified Hydronic Systems Designer - Journeyman Plumber
Deep setbacks and radiantrarely play well together, unless the system is oversized and the building envelope is dodgy. A few degrees overnight, a few more over a weekend is about what most can reliably deliver without sacrificing both comfort and fuel consumption.
If you have a way to monitor daily or even realtime fuel use, you can find this sweet spot for each system, then work to improve overall efficiency on multiple fronts. Is someone performing regular combustion testing on the various boilers? Draft and combustion air improvements with a bit of tuning can work wonders at times.
The next step for us is a re-evaluation of the entire system, starting with a room-by-room heat loss and radiation survey plus interviewing occupants and operators to determine real and/or perceived shortcomings.
Envelope improvements are generally the next place we look. On older buildings, caulk and thermal window coverings alone can often provide major improvements.
Recommissioning of older systems, along with strategic repairs, replacements, and de-knuckleheading along the way comes next -- especially if budgets are tight. Replacing oversized single-speed circulators with ECM pumps on hot water systems often pays back fast enough to get the CFO's attention. If budgets allow, then new controls and ODR come next.
Especially in your areasteam is very attractive- for all the reasons you mentioned, to which I'll add that if the system is designed properly there will be much, much less risk of freezing damage to the system. In a steam system, the pipes and heat emitters drain completely dry except for condensate tanks, wet returns and the boiler itself, so an extended power or fuel failure would not kill the entire system as it can with hot-water."Reducing our country's energy consumption, one system at a time"
Steam, Vapor & Hot-Water Heating Specialists
Oil & Gas Burner Service
Baltimore, MD (USA) and consulting anywhere.This post was edited by an admin on May 12, 2013 2:28 PM.
An interesting threadThe old monster of setback benefits, or penalties is surfacing here, and it will be an interesting discussion. I believe that it is likely that the fuel used to exit the short setback, will equal the fuel used to maintain the constant temperature; but how can it be proved one way or another?--NBC
Individual room controlI'd suggest that you look at Rinnai Energysavers in each room. Modulating gas valve, modulating blower, programmable stat built in, very durable. Each teacher controls the temp in his or her own space. Check it out.
DDCKeep in mind guys that pretty much every school for the most part is controlled by the DDC. Not always the case in the boiler plant but the rooms and set back, and water temperatures, are controlled by DDC by guys we have that deal with some DDC issues in our heating department. I sometimes also dabble in the DDC as well.Class 'A' Gas Fitter - Certified Hydronic Systems Designer - Journeyman Plumber
The Grand Master of Intermittent Heating is. . .steam.
Intermittent operation, as found in older schools or churches, is steam's best territory, IMO. Remember that heir typical set backs were not merely "deep," but "OFF." Coal went out, and would be refired early in the morning. A modern steam boiler plant should (and often does) do precisely the same thing, but with some options for freeze protection.
There's nothing like supplying NO heat to a building with NO occupancy!
In NE Ohio, where I am, most schools' replaced coal with oil and/or gas boilers and are intentionally operated the same way. Well, they should be. One I'm familiar with uses older Hayes-Cleveland controls to modulate the firing rates and stage the boilers based on steam pressure feedback from the system. Around here, the rooms are all controlled with pneumatic modulating room thermostats. And vacuum return pumps on the condensate side. So should yours.
Maintenance or custodial staff arrives and hits the "GO" button and turns on the pneumatic thermostat air pump, and the system ramps up and fires at a rate and number of boilers required to develop a steam pressure consistent with the design of the system. Once the minimum firing rate is held for a specified time, the whole shebang shuts down until someone hits the "GO" button again, usually the next morning. Shutdown could occur awhile before students leave.
But how can this be, if the radiators, convectors, and ventilator/convector units are not unusually large, and why is there no concern for freezing of the heating lines? And how can a building with so much thermal mass heat up in 45 minutes?
First thing to remember is that any medium with a phase change and a huge latent heat capacity is going to act totally differently than one that doesn't. Don't let the fact that HW and steam both have boilers and radiators of some sort to lead you to think about them in the same way. And the need to get an intermittently heated building up to temp FAST requires a very athletic approach to heat distribution giving advantage to something that is both lighter than air and very intense. Every medium has advantages and disadvantages no matter what anyone says. In this application steam has a leg up on the others. Know where its advantages are and utilize them. Otherwise you're wasting a lot of potential and fuel.
So. . .
There are a few things I've learned about steam heating over the years that may shed some much needed light on how it best works:
• Steam is a dynamic heating medium due to both the vast quantity of heat required for phase change (with a 0 degree delta T in the process) and the volumetric change associated with it (about 1700 times). Refrigeration isn't a bad background for a steam heating engineer or tech. It gives you an idea how radically the amount of total heat transferred can vary (without temperature change!) due to ambient conditions, unlike water, air, or a box of rocks that you can use to merely shuttle heat around by warming it "here" and moving it "there."
• Temperature is not the same thing as heat transferred. A steam rad, convector, fan coil, etc. can condense A LOT more steam (and transfer A LOT more heat) into a cold room than a warm one, even if there's no temperature difference between the radiators of the respective rooms. The volume of steam drawn into each will be radically different, however.
• As steam flows through the mains it will dynamically change distribution patterns to favor a heat emitter in colder room(s) and/or one with the greater CFM of air flow through it, WITHOUT THE AID OF ANY ACTIVE CONTROLS WHATSOEVER. You really have to witness it in a larger building to appreciate it.
• Steam will heat up piping and radiation to full saturation temperature on contact, releasing latent heat and then "disappear" as a medium, leaving in its wake a trace of water, nearly a thousand times the heat one would expect from that mass of water and a powerful localized vacuum at the point of condensation. The localized vacuum created by the collapse of steam literally draws more steam from the supply. And this internal dynamic effectively "aims" the heat directly at the inner walls of the exchanger for highly effective heat transfer.
• Steam will heat up piping and radiation sequentially with a clear temperature line of demarcation at the leading edge of the steam "front." Steam mains must thus be considered as true manifolds and be treated accordingly with very fast venting and good insulation if any hope of even and timely heat distribution is to be realized.
• A steam heat exchanger can maintain full saturation temperature across its entire surface with virtually no temperature drop across it. Therefore, nearly 100% of total heat delivered to steam heat exchanger is transferred to the room air AND a steam heat exchanger can deliver more heat than forced hot water at the same temperature.
• Since virtually all heat delivered to the radiator is in the form of latent heat, actual heat loss along return lines is extraordinarily small as a proportion of that delivered to the conditioned space, even if the condensate return temps approach the steam temperature.
• Steam boilers are simultaneously evaporators and compressors. Radiators are simultaneously condensers and vacuum pumps. Therefore, their capacities need to match, and to match dynamically throughout the heating cycle. Matching can be accomplished with either a powerful boiler or boilerss that can be proportionally throttled, or by proportionally throttling (orificed radiator inlets or calibrated vents) on the condenser end of the equation. Preferably both. Obviously, room thermostats will quickly alter demand seen by the boiler, Responsive firing rates are a great match.
• Steam piping need not be large and heavy any more than hot water heating lines need to be large or air ducts need to be large. Steam need not be delivered to large radiators any more than forced hot water needs to be delivered to large radiators. So if there's supposed to be a vacuum return pump on the system, use it.
• Steam radiators, convectors, etc. can be a small fraction of the size required for forced HW to achieve the same heat transfer and mean radiant temperature of an exterior wall, etc. With small piping and small internal volume heat exchangers, the I=B=R steam pick up factor is unnecessary. For example, the SelecTemp system of steam heating, which used small refrigeration copper throughout, as revitalized by Gerry Gill and a few others here as "mini tube" steam heating, the boiler can be sized for the heat loss of the structure and not the radiation.
Other considerations that affect total fuel use:
• Since temperature and heat are not the same things exactly, stack temps on a boiler generating a medium with tremendous latent heat density (steam from water) are not precisely equivalent indicators of systemic efficiency as with those reheating a static, non-phase changing heating medium.
• Stack losses on gas or oil boilers can be MUCH higher than expected if fed into a cavernous chimney designed for very high excess air coal burners. Therefore, direct vent devices very often exhibit lower fuel consumption that's mistakenly attributed to their DOE or AFUE ratings. Proper draft management is a must.
• DOE heating capacity or I=B=R steam ratings as related to fuel input does NOT prove that steam systems are therefore 50% efficient, as I've heard some say.
And finally :
• There's nothing "green" about heating unoccupied buildings!
So getting back to old steam heated schools. Yeah they have a team of big boilers, but they only need run simultaneously at the beginning of the morning. And the heat should be off for all but about 8 hours per school day. Literally shut them down for weekends and holidays. At 6:30 Monday morning start pneumatic compressor, initiate the firing sequence, make sure the vacuum return pumps are ready to go, etc. By 8:00 AM it'll be 72 degrees in the classrooms. The boilers should soon be down to one boiler modulating on demand. The system should be off by about 2:00 PM, a little sooner on moderate days, a little longer on cold days.
If it's so brutally cold then the system won't shut down an that's a good thing since it probably could never make up that kind of temperature swing. ODR steam controls for non-intermittently heated buildings predict this and cancel set backs in unusually severe conditions. But most of the time you will enjoy huge temperature recoveries in no time and remember, once the steam all condenses, there's no water to freeze in the ventilator coils. Sounds efficient to me.terry
Intermittent HeatingThanks, Terry. Makes a lot of sense, especially with those old leaky buildings.
As envelopes are improved, where does the crossover point (setback versus shutdown) end up? I know several residential steam systems which won't tolerate more than about 4F setbacks. If they had an array of boilers downstairs which could be marshaled every morning they could recover in time, but would it actually save fuel?This post was edited by an admin on May 13, 2013 1:20 PM.
same ?;different viewI have the same question. I'm soon to be installing two stage-fired SFTR50s. Will the extra capacity enable me to have a higher set back? This past winter when I brought the house up from @45-50degs, it did so in a little more than an hour. I was impressed. My house maintains the heat fairly well, but I hate hot rooms at night, so I'd like to be able to let the temp fall until morning.
StudiesDo you guys know of any studies that have been performed along these lines? Someone, somewhere, must have done something..Class 'A' Gas Fitter - Certified Hydronic Systems Designer - Journeyman Plumber
Here's a blog with linksI think this addresses it pretty well: http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/50152/If-You-Think-Thermostat-Setbacks-Don-t-Save-Energy-You-re-Wrong
Setback studiesrarely (if ever) consider radiant heat.
Many of us have tried setbacks. Few have stuck with them, at least to the depth with which we started.
The comments tell the rest of the story...This post was edited by an admin on May 18, 2013 1:05 AM.
perception of comfortone point to consider is the perception of comfort.
in a room whose walls have cooled down during the setback, the heating will have to be set higher to compensate; whereas if the walls are kept warm, the constant temperature can be lower, without making anyone feel cold.--nbc
Setback fuel consumptionThis may be a way to measure any savings which result from a setback.
1.turn off the setback feature on the thermostat.
2.set the thermostat to the normal high temperature you use, and read the gas meter, and write down the time. Let the boiler run normally for a couple of days.
2. read the gas meter, and note the time. Set the thermostat down to the lower setting, and let it run for 8 hours or so.
3.reset the thermostat to the higher setting, and as soon as the thermostat turns the boiler off, read the meter, and note the time. You have to be vigilant here to catch the exact moment.
4.calculate the cubic feet per hour of the first run at the higher temperature, and compare that figure with the cubic feet of gas used during the lower plus the recovery period. The consumption figures should show how much fuel was saved or not. Unfortunately I do not know how to make allowance for the vagaries of weather in this formula, but maybe someone else has an idea.--NBC
allowance for the vagaries of weatherWould calculate BTUs per degree-day over a period of roughly similar days. Data from http://www.degreedays.net/ using a nearby PWS is amazingly useful.
Degree daysMy suggested test is probably too short to use degree days, in which case it could be comparing a weeks run at constant temperature with a weeks worth of daily setbacks. Then the degree day information would make the correction.
I wonder what the temperature is in those Canadian school rooms on a Monday morning! It might require a higher thermostat setting for being comfortable in a room with cold walls, as I said.--NBC
But back to intermittently occupied institutional settingsSetback is controversial only because each person presents one hard and fast rule for absolutely every situation.
This is a perfect illustration as to why HVAC is so much more complicated than it first appears. Consider 1) every construction type, thermal mass, insulation, height of structure versus its footprint, air exchange rate, etc; 2) heating medium i.e., hot air, water low mass, water high mass, water low temp radiant, water high temp convective, steam high mass cast iron, steam low mass convector, steam unit heater with blower, steam unit heater with blower as preheat to fresh air; 3) radiant heat surface available (none for hot air, a lot for low temp HW), emitter surface temperature, mass of interior surfaces (hard plaster vs drywall, or wood vs concrete), and resulting Mean Radiant Temperature viewed from the likely location of occupants, 4) heating system heat supply characteristics, i.e., single stage, multistage, modulating, multiple heat source (e.g. heat pumps with auxiliary heating elements); 5) number of zones involved; 6) occupancy characteristics.
7) Add to this the fact that assuming that it is statistically impossible to state that when a space has more than one occupant, that all occupants will perceive the same level of comfort!
So what specific setback temperature should be used?
Steam in an older structure that is of limited and predictable occupancy, is still the grand master of extraordinarily deep setbacks so long as boiler output can be matched to the real demands placed on radiators and convectors situated in a very cold space. Steam load under these conditions can be much, much greater than you could ever expect, which is precisely why it can recover so fast in institutional settings where boiler redundancy is common and very, very useful under these conditions. Besides, if there is a slight lag in Mean Radiant Temperature as people are arriving from outdoors for the school day, I've never noticed it because of the contrast of being outdoors first. Regardless, steam was made for just this kind of situation.
This is not the same as waking up from a deep slumber waiting for the house to warm up, where a cold toilet seat becomes an unforgivable shock to the senses! I'm sticking to intermittent institutional occupancy on this one.
Every other situation? I just tell people to take about a 25 or 30 degree day (or maybe 1/3 to 1/2 design temp in your area), turn the heat way down, and wait for an hour or so. In most older residences, you'll find that there's a sharp drop in temp first then a gradual drop thereafter, and even a point where the temp holds steady for a long time before dropping again.
The sharp drop is typical convective and radiant heat loss being uncompensated for. But that long period of steady temp is where your thermal mass comes into play. You probably don't want to loose that in a home with typical residential heating systems. So that becomes your starting point setback temp. Most any system should be able to recover from that without heroics. You conserve the heat in the thermal mass, but stop replenishing the heat that escapes quickly.
This is the best method I've found for determining a "coarse" temperature setback. It tells you a lot about the building. You fine tune from there based on the heating system's characteristics. And for temperature extremes? Especially those beyond design temp? For garden variety thermostats, I personally just cancel the setback and hold the temp a couple degrees below the normal occupied setpoint to limit the temp differential and then wait for that weather front to pass!terryThis post was edited by an admin on May 19, 2013 4:53 PM.