exact meaning of \"design temp\"?

Gordy

If you design for 5* and it gets -30 or -40 you will not be alone in court. There Will be the masses of other installers in the court system to back up the design criteria that was used.

Gordy

John Ketterman_2

It's cold here today, and I just realized I don't know what design temp means.

If it just hits 0F for an hour or two in the middle of the night, a house will not really feel anything because of its high thermal mass. But if it holds at 0F for a day or two or three, that's a different matter.

So my question is, what is the design temperature? Does 0F design temp mean the boiler must handle day after day with 65 degree-days each? Or does it just mean the boiler is designed for the season's minimum temperature to hit 0F at night?

Unknown

Design Temperature

This may not be the exact definition of "Design Temperature" but its the one I know and abide by.

Based on the Heat Loss of the structure, your geographical location and the desired temperature you wish to maintain inside the structure, design temperature would be the difference between the extreme coldest outdoor temperature you may encounter and the temperature you wish to maintain indoors. The standard in most homes for Indoor Temperature is 70°F and if it drops to -5°F overnight then your design temperature would be 75°F. If the desired indoor temperature were higher such as we encounter with Elderly Housing situations, then the design temperature would increase accordingly.

Even though the "extreme" outdoor conditions are with us approximate 6-10 days out of the year in most localities, the system really should be sized to those extremes. Doing so will not affect "Steady State Efficiency" that much provided the system is not oversized. Here is a graph from the Bureau of Standards showing Equipment Loads and resultant Steady State Efficiencies.

Glenn Stanton

Manager of Technical Development

Burnham Hydronics

U.S. Boiler Co., Inc.

Brad White

That depends.....

Not to be evasive, but it is whatever outdoor temperature was used to design the system. It can be anything within reason. Some "mission critical" systems take near-record cold but your average construction is often dictated by code for energy conservation.

Here in Boston, our Massachusetts Energy Code dictates 7 degrees F. despite the fact that it can get much colder than that. This corresponds to the 99.6% column of occurence. This means that it will get colder than that temperature 0.4% of all annual hours (35 hours in this case).

Absent code dictates, the 99.6% column is typically what most engineers use for heating. The "Median of Extreme Lows" is another. (In Boston, that is 1 degree F. by comparison.)

Key is, what is the coldest you would normally expect the temperature to be in any given year? Not the record breakers but rather, the coldest routine temperature that shows up year after year without an "OMG!" and dogs frozen to hydrants? That is the most conservative I would use designing a house, unless the code said I had to use a higher temperature, which it does.

As you note, the typical house can tolerate a cold snap of a few hours or even days...If the temperature drops a few degrees inside your house with your boiler cranking at 100%? That means you have a well-sized boiler. Traditionally, most boilers now installed would cycle even then...

Perry_3

Hmmm

I'm not sure that I agree with the concept of the coldest temperature that shows up year after year after year...

Last year was warm... and would provide a totally false reading.

Our design temperature here is -10 F. And we have been running arround that recenty (tonight I think they are projecting -13).

Now does -10 occur every year, or even the vast majority of years. No. But I think it is safe to say that -10 occurs 50% of the time.

Of course, I remember -30 from years ago perhaps ever 3rd year for a day or two. So, residential heating is not designed for the worst cases - just what the somewhat normal cold for a winter is (and is somewhat flexible).

Of course, We have some areas in the plant where they designed the heating system for -30 F.

Perry

McKern

Glenn, you might want to add...

Glenn, you might want to add "or what city hall tells you it is" because they want to see their design day temp used on your heatloss if you're filing for a building permit.

Brad White

But Perry

The 99.6% column most commonly used for design outdoor temperatures is the coldest common temperature that shows up year after year....by definition.

It specifically is not supposed to be near-record but is gleaned from approximate 30-year averages, specifically of what temperatures are common to any given year.

Remember, we are not talking just energy monitoring here but the design basis for boilers and radiation which will last the system's and home's lifetime.

I mean, here in Boston, with an outdoor design temperature of 7 degrees (it currenly is 8 at my house) has a record low of -18 F. No one is suggesting that one design for that.

Weezbo

True *~/:)

Yet, it Might be a good idea to check current Trends (last ten years) against 23, 16 , 100 year anomalies:)

Unknown

Efficiency of oversized boiler

Glen, thanks for providing the graph. It is generally accepted that an oversized boiler is less efficient than a properly sized one, but I have always wondered about the reason. My guess is that the larger boiler runs less of the time, and therefore has a greater standby loss. Do you have any info on the assumptions used in calculating the graph? Do they assume that the boiler is maintained at temperature during standby periods?

Without including standby loss I wouldnt think boiler size would make much of a difference. As an example, lets take an electric boiler installed in the heated space so there is no jacket or flue loss, all of the heat is retained. To maintain temperature in this house with a given heat loss you would have to input a certain amount of energy, say 50k BTU/Hr. In this situation there would be no difference in efficiency between a 50k BTU/Hr boiler running 100% of the time or a 200k BTU/Hr boiler running 25% of the time. The net energy input, output and efficiency would be the same, only the duty cycle of operation would be different.

Is this analysis correct, or am I missing something?

joel_19

design temp

Ok so this brings up a good point you desin for say 5 f and then we get several nights bellow 0 in a row or windy. Then you get the call "My house is 67f not 72f at 4 A.M. and I'm going to SUE you. Can you then point to the design temp as requested by th energy code and the judge and jury will let you off the hook??? Doubt it.

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jp_2

fudge factor

I think thats why there are such large fudge factors in the calcs, OR the AFUE rating???????????????????

Brad White

Due Diligence and The Standard of Care

So long as you state the parameters up front I do not think anyone litigious has much to stand on. I always set forth the design parameters. Most of the time the system has a 10% safety factor in it plus we all know where the bodies are buried....:)

If the system you designed is that close, bravo! I would not worry about wind too much for most heating systems and infiltration are designed around a 15 MPH wind. A little more wind means next to nothing on transmission.

Infiltration, if taken as the sum of room's has a built-in factor of safety because wind differential air only leaks in on two sides at a time, not all sides at once. (The rooms where it leaks out, -exfiltration-, sees conditioned air coming in from elsewhere in the house, not raw outside air coming in.)


If I were to design a house for the worst weather, the record cold, without a good reason (such as a hospital or prison where you cannot send folks home), I would be guilty of energy waste and oversizing a system for the life of the system. All for a fraction of 1% of annual hours, I penalize the entire system...One could quantify those dollars and go after you. I would also be in violation of the code (780 CMR Ch. 13 here in MA) and would be in trouble on several fronts.

So long as one designs to a reasonable standard of care (and following the code falls into that certainly for what is the alternative?) one is held harmless.

Not so say that you cannot be dragged into court for anything, but by the time one gets there, it is summer and the jury is very annoyed at some wus who cannot find another sweater...

Unknown

Take my word for it John,

Brad would know, a distant second may be UniR.

Dave

Perry_3

Brad - you missed how the \"il\" legal system works..

The case will be tranfered to another jurisdiction such as International Falls Minnesota - and will occur in mid winter. The prosecution will control that...

So you will be left explaining why you designed a building with a 5 F design day when its -30 outside (or colder) and it always gets to -40 almost every year.

The prosecution will appeal to emotion on who in the world would design a building for 5 F when it gets cold out there in the winter...

If you want to have any chance of stopping that you probably need a $500 per hour lawyer...

Perry

Brad White_9

Then I would have

Global Warming advocates in as co-defendants to explain why the historic design temperatures are inadequate...

Mark Hunt

OHHHHHHH SNAP!!!!!!!!!!!!


Mark H

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Gordy

Design Criteria

To give an example of over design lets use my present set up.

We have been down to design temp-9* in my area past to nights, Highs of 0*.

House was built in 52 with ceiling radiant. Knuckle head replaces original boiler with a WM cgm 7 210000 input 172000 output 150000 IBR.

According to my T-stat the total call for heat time was 7hours and 36min each of these two days.

Knowing that the total call time for heat does not include circulator pre purge, and high limit shut down to refire at low limit. The actual burn time of the boiler is even less than the 7hours and 36min say 7hours and 15min.

Doing some reverse engineering using the boiler input of 210000 btu HR. I burned 1522500 btus for the 7hour and 15min. In doing several conservative heat loss calcs using different programs I came up with 73000 btu hr for the structure. A close match mod/con of 80000 btu hr input would have yielded 580000 btu hr, with the same results to the radiant panels for temperature.

So that would mean my boiler at present burned about 150% more NG than a properly sized mod/con, or even CI boiler to keep my structure at the same indoor temp. Thankyou mister eyeball installer.



Now you see how over design can add up over the life of the boiler, affecting the pocket book of the homeowner, and the carbon foot print to the enviroment. I could heat two houses with room to spare with my present boiler.



Gordy

hr

It's all some what arbitrary

you chose the design temperature, along with wind conditions to arrive at the heatload. The wind (infiltration)strips heat quicker than just extreme cold in my opinion.

Probably even more arbitrary is the heat load calc used to size the equipment. Building materials, quality of product and workmanship are huge factors.

Usually there is a fudge factor in the load programs, and usually their is some room to tweak and increase output if the design is "in the ballpark"

Extended periods of below design days by say 10- 15 degrees may require non stop running and possibly some cooler indoor temperatures.

I have rarely seen a system boiler run non stop. The constipation is usually in the emitters or distribution system. Which may or may not be easy to adjust

hot rod

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Weezbo

That is Beautiful *~/:)

if anyone fully appreciated that statement they indeed would be experienced:) "may or may not be easy to adjust"...:) I am off to another fine day in the wilderness:)That is a dynamite statement Hot Rod

Unknown

I dont quite see how you are coming up with that conclusion.
If your boiler ran for 7.5 hours per day, it should have put 1,290,000 BTU into the house ( 172,000 BTU net X 7.5 hours) per day, which would indicate an actual heat loss of 53,750 BTU/ Hr.

If the mod con had an input of 80,000 BTU/Hr and an assumed efficiency of 95% it would be able to put out 74,400 BTU/Hr maximum, to meet the same required 1,290,000 BTU per day into the house. The modcon would have to run for 17.34 hours at full output to put the same amount of heat energy into the house as your old boiler did in 7.5 hours, using 1,387,000 BTU/Day input. Or if it was set up to modulate, it would be running continuously for 24 hours at the required 53,750 BTU/Hr out to make up the heat loss. Either way, the modcon would require 1,387,000 BTU input to keep the house at constant temperature. 1,522,500 - 1,387,000 divided by 1,387,000 is about .10 or about 10% more. So how do you come up with 150% more fuel usage? I think you might be forgetting that the smaller boiler will have to run longer to put the same amount of heat energy into the house as the old boiler did in 7.5 hours.

Unknown

Glenn, that's a strange statement. Of course your "steady state" efficiency is not affected by oversizing. The problem is, you hit your "steady state" a lot less often if your equipment is oversized. So that's kind of misleading. Also your graph seems to indicate you can hit 100% efficiency with an atmostpheric boiler.. is that just talking about percentage of its optimal efficiency, I presume?

Need to be careful with this stuff. Given the nature of boiler sizing increments, most properly sized boilers are already oversized to some degree. tweak your numbers too much and you step up to one that is very much oversized without trying too hard.

Unknown

Mike, he's noting that his calculated heat load is much lower than the boiler burned. Thus, the fuel it burned did not just heat the house.. a significant portion of it was wasted. You are assuming it all went into the house. The actual efficiency of his boiler is thus lower than you would think.

The mod/con could, on the other hand, run 100% of the time at the ACTUAL load (well.. close, anyway) of the house at 95% true efficiency. That is, much less being wasted.

Gordy

Using total input not output

Mike I'm using total input of both boilers as an example.

The total input is the fuel actually being consumed by the boilers. I'm burning 210000 btus an hour with my cgm7 highly oversized, versus 80000 with an ultra a close match when I swap out. Using outputs with efficiency ratings can get confusing besides input is what the gas company charges ne for.

So in a given hour of burn time there is quite a difference between the two. The mod/con is going to get more heat out of the NG verses the cgm 7, due to the difference in efficiencies.


Now my scenerio is quite skewed due to the gross over sizing of my present boiler. My heatloss does not change but how you compensate for it in your boiler sizing is very important.

My point is Mike that I did not need all of the btus my present boiler is producing, Nor would I have if it were a mod/con of 80,000 btus. A heatloss for the design temp is 73000 btus hr, I'm burning 210000 btus hr input.



If you want to go the output route then at 172000 btu hr (7.25 hours not 7.5, allowing for times burner is not firing during heat call at t-stat) I'm producing 1247000 btus hr, verses a mod/con at 73000 outbut btu hr producing 529250 btu hr. My structure has a heatloss of 73000 btus hr. so regardless of which boiler you use the heatloss is the same 73000.

Gordy

Gordy

Using total input not output

Mike I'm using total input of both boilers as an example.

The total input is the fuel actually being consumed by the boilers. I'm burning 210000 btus an hour with my cgm7 highly oversized, versus 80000 with an ultra a close match when I swap out. Using outputs with efficiency ratings can get confusing besides input is what the gas company charges ne for.

So in a given hour of burn time there is quite a difference between the two. The mod/con is going to get more heat out of the NG verses the cgm 7, due to the difference in efficiencies.


Now my scenerio is quite skewed due to the gross over sizing of my present boiler. My heatloss does not change but how you compensate for it in your boiler sizing is very important.

My point is Mike that a heatloss for the design temp is 73000 btus hr, I'm burning 210000 btus hr input.



If you want to go the output route then at 172000 btu hr (7.25 hours not 7.5, allowing for times burner is not firing during heat call at t-stat) I'm producing 1247000 btus hr, verses a mod/con at 73000 outbut btu hr producing 529250 btu hr. My structure has a heatloss of 73000 btus hr. so regardless of which boiler you use the heatloss is the same 73000.

Gordy

Unknown

Gordy, I understand what you are saying that the modcon is more efficient and your boiler is oversized, but I dont think the inefficiency is as large as you think. Lets agree that your heat loss is 73000 BTU/Hr. With the modcon, it will have to run continuously at an output of 73000 BTU/Hr to maintain your house temperature at 70. That means it will run 24 hours per day and require an input of probably 80,000 BTU/Hr to do it.

Your big boiler can put out more than twice as much. But to satisfy the daily BTU requirement it needs to run for only 7.25 hours. The rest of the day it doesnt use any fuel at all while the modcon is continuously using fuel for 24 hours, but at a smaller rate. In the end, the modcon will use less fuel because it is more efficient and it is running continuously, eliminating standby loss.

Gordy

Forgetting one thing

Mike you are forgetting, and maybe I did not clarify, That design temp was not for a 24hour period only maybe 3 hours the rest of the 24 hour period the structure heatloss would not see the full 73000 btu loss.

Maybe my grossly oversized boiler is skewing things a bit.

Lets say the installer did an appropriate heatloss and sized the boiler correctly. A closer match would have been a WM cgm 4 at the time with an output of 86000 btu hr. Still over sized but as close as you could have gotten at the time.

This boiler would have used 623500 btu hr verses 539400 for a mod/con. I'm still using more fuel to get my heatload satisfied which is 529250 for the 7.25 hour period,thats assumming all the 7.25 hours was at design temp of -9 which it was not. So I still would have used an extra 84100 btu hr worth of fuel that went up in smoke as Chong would say.

Plus the fact that the mod/con would have been modulating during the above design temp conditions to further save money.



Concentrate on the 7.25 hour period that is the amount of time the boiler has run to heat the structure for the 24 hour period.

If both boilers run for 24 hours

boiler 1 oversized CI 172000 output would use 4128000 btus

boiler 2 properly sized CI 86000 btu output 2064000 btus

boiler 3 properly sized mod/con 74400 output 1785600 btus

The structure would use if at design temp of minus 9 for 24 hours 73000 btu hr. 1752000 btus.

So we would have burned 2376000 more btus than needed to heat the structure with boiler 1.

312000 more btus than needed with boiler 2.

33600 more btus than needed with boiler 3.

Now which gas bill would you like to pay?



One thing to remember to is that when the boiler is in the burn cycle no matter what size the burner is as long as it is big enough the HX is only going to transfer so much heat the rest of it goes up the chimmney, forget the standby losses that is minor compared to using 210000 btus to heat 3 gals of water content in the boiler verses 80000.



Gordy