Altitude and Deration

Zman

When derating gas appliances for altitude (6,000 to 11,000 feet) I have been under the understanding that all you need to do is follow the manufactures recommendations. Usually atmospheric appliances are are 4% per 1,000 feet and appliances with forced combustion are 2% per 1,000 feet.I have observed the amount of time needed to raise the temp of a known quantity of water a certain amount and concluded that this deration method works quite well.



In Colorado the Natural gas supplied has a lower BTU content. Normally gas has somewhere around 100,000 BTU/CCF . Ours is claimed to be somewhere in the 750,000 to 800,000 range. The reason given for this is that it is makes appliances run more efficiently with minimal altitude adjustments. I have been under the impression that this lower BTU content does not have a negative impact on the appliances output (more than the altitude alone) as you just need a slightly richer fuel mixture to get the same amount of gas to the flame.



Recently I have heard some people claim that you just derate for the lower BTU content and not for the altitude. This makes zero sense to me as we still have the issue of lower atmospheric pressure thus less air to burn.

I have also heard folks say you should derate for both altitude and the gas.



Does anyone have any thoughts on this?



Carl

Spence

De-rating

Your furnace maker has gone through a lot of time and resources to have their models listed by universally recognized agencies. Following their instructions for altering the standard operation gives you documented performance, a high margin of safety, testing agency certification, and a defensible position in court.

RJ_4

derating

I was servicing my Lennox 100mbh nat gas furnace ( just bought the house in the colo mts  9000 ft .  the burner orfices looked to small,  the furnace previously was on propane, after researching with the local supply house we determined that the orfice size was correct derated  to about 70mbh, still wating to hear from someone at colo nat gas on there feedback.  Lennox had little info I could access as I am a retired fitter and don't have a contractor license.  Also interested in the final solution.  Most of my career was at sea level did not encounter these issues 

Zman

Orifice size

RJ,

Thanks for the response. I think the BTU content of the gas definitely effects the size orifice needed  to get proper combustion. If the gas high content gas was delivered to an appliance installed at 10,000 feet you would need a smaller orifice than you would with the lower content gas.



My question pertains to the output of the appliance after it has been properly set up and it is running at it's designed combustion numbers.



Carl

Zman

Nothing?

I am fairly confident I understand this correctly.

Non of the Guru's of high altitude want to chime in?

Plumdog_2

Well, I'm no Guru BUT....

 You need to know the Caloric content of the gas being supplied, as I believe that some Western slope gas is full value, and Denver area gas from Excel is running around 880 or so. Colo. nat. Gas comes from the same supply chain as Excel gas. This gas is formulated to work well at Denver altitudes or thereabouts, so that ranges, dryers, water heaters, etc will burn more or less correctly even though they were designed and manufactured for sea level. But to complicate the issue, some gas appliances are "high altitude" models, and are sold in places like Denver and Salt Lake City.

 So you can take your input rating and reduce it 4% per thousand ft. altitude, then divide by the actual caloric content which will raise the input back up some to compensate for the "weak" gas. By the way, I feel 4% per thousand is too steep for figuring atmospheric density losses at altitude, because the weight of a column of air is not linear because it compresses.

 From experience, conversation with Colorado Nat. Gas, and Excel, and the use of a combustion analyser, I have arrived at a figure of 20% input reduction for 8-9 thousand feet altitude using their gas (for old-school atmospheric draft appliances). Then you can adjust primary air and/or manifold pressure slightly.  

Plumdog_2

Further....

You can cover your butt by following EXACTLY the manufacturer's instructions....but until recent times, some of them had no real experience with the altitude thing, and they just copy rules of thumb from somebody else's playbook. And the learning is still going on.

Many furnaces and boilers were built for sea level, uncrated at 8000 feet, fired up and walked away from. The CO levels in the chimney may be 900 or 1200 ppm; and it could take years, but it will fill with soot and go south from there.   

Zman

Derate

Plumdog,



Thank you for your reply.

My experience with triangle tube boilers in particular is that the 2% per thousand works perfectly,even with the weaker gas. If you calculate how long it should take to heat a specific capacity indirect tank at full fire, the math works perfectly.



I have also noticed that the triangle tube needs very little combustion adjustment at altitude when burning propane. When the same boiler burning natural gas is setup with an analyzer, it always needs a richer mix. My theory is that no fuel deration is required at all because you are giving the boiler the fuel content it needs by adjusting the mix.



This is one of those things that everyone has a different idea about, yet no one has a great explanation.



One good test would be to clock the gas meter while simultaneously verifying a known heat load.



Thank you for your post,



Carl

Mark Eatherton

Back in the day...

Carl, I have always derated atmospherics at 4% per 1,000 feet. I then take into consideration the density and caloric content of the fuel and adjust the orifi' accordingly, and have never had any problem whatsoever. Most appliance manufacturers are unaware that Denver has diluted (they pump air into it at a compressor plant east of Denver) gas and are usually surprised when told about it.



As for blower equipped premix systems, I have found that the 2% per gets you real close, but have also been told by product engineers familiar with Denver's gas that the neg reg venturi orificed fuels systems already compensates for the lack of fuel and lack of atmospheric density. Now that statement didn't make any sense to me either, but the combustion numbers always worked out using the 2% formula without having to change orifices or offset pressure settings. I've clocked NG systems using this method, and like you, it comes in real close to expectations, but it is a completely different beast. The problem is with LP systems. I have yet to see a meter on the fuel supply side of an LP system, so there is really no way of confirming "exact" caloric input and net output of the appliance.



ME

icesailor

Anal Tool Weenies:

Well, I guess that if you are a really anal tool weenie, you'd be going out and buying a LP gas meter (they make them) so you could clock LP gas.

Progress, not perfection.



Necessity is the Mother on Invention. I had to repair and re-pipe an improperly installed 2: gas train on a 3 million BTU steam boiler. I didn't really want to test my work with live LP gas, so I took a LP 12" gas regulator, adapted it to air and connected it to Press-Taps with a blue refrigeration hose. Did wonders for my stress levels. No leaks.

Zman

Thank You

Mark thank you for the input. Truly appreciated.

I think I will have to leave propane for another post. Not a very regulated product which has left me scratching my head more than once.



Carl

direland

This is the correct way to derate an appliance. Tried to upload a picture i guess it didnt work.

direland

0-2000' low altitude no de-ration
2000'-4500' high altitude 10% de-ration necessary
Over 4500' high altitude 4% per 1000 or part there of.

This covers propane and natural gas. You do not want to run natural gas heavy to account for an orifice designed for propane. You want to change the orifice for the gas type.

ChrisJ

direland said:

This is the correct way to derate an appliance. Tried to upload a picture i guess it didnt work.

The last post is almost 3 years old?

Zman

direland said:

0-2000' low altitude no de-ration

2000'-4500' high altitude 10% de-ration necessary

Over 4500' high altitude 4% per 1000 or part there of.



This covers propane and natural gas. You do not want to run natural gas heavy to account for an orifice designed for propane. You want to change the orifice for the gas type.

@direland
This is a pretty old post which has already been answered.It was really about putting aside some myths that about 2 different derations, one for altitude and one for gas density..
I have no idea where you get your info, but I disagree. Those numbers may be close for some atmospheric boilers but would not apply to any product with a burner.

Most manufactures are closer to 2% per thousand on mod cons and commercial boilers. Manufactures recommendations should always be followed.

I have talked with some engineers that indicate that if you carefully size the burner to the boiler and have sufficient boiler surface area, you can set up a commercial boiler to run at altitude with little or no deration.

Welcome to heating help. The home of "It Depends"...

direland

Well ok it was answered wrong from what I read and of course I aced my gas test so there's that. 2% is incorrect per 1000 feet of elevation sorry. I was Googling some other stuff and this popped up so I decided to answer I realize people get offended and the defenses go up when other people point out something that is incorrect. What I posted above can de-rate any gas burning appliance to the inch if ya want to do the math. To the best of my knowledge It is always better to underfire a gas appliance than over fire which either crack the boiler and or burner.

Natural gas can be adjusted at the last stage regulator up to 7"? I don't have my orifice chart book on me at the moment. And yes it applys to every product with a burner? Don't most gas appliances and boilers have a burner?

FYI engineers don't repair broken boilers... plumbers do.

Gordy

You aced your gas test? Which gas.......

Paul48

@direland

I'm no engineer, but please "give the devil his due". From the time you wake up in the morning, everything you use or touch is the result of an engineers work. The things you so proudly do, and take credit for are done to engineering specs.

Zman

@direland
4% per thousand is often correct for atmospheric boilers
2% per thousand is common for boilers with fan assisted burners.
These are just general guidelines. Manufactures publish the exact derations in their installation manuals. Read a few and you will see what I mean.
As I mentioned earlier. With careful engineering it is possible to eliminate deration all together.
I still remember something an old timer mechanic told me when I was 17 and had screwed up some parts on my car. He said, " Sometimes a little knowledge is worse than none at all"
Keep studying and you will see what I mean. I live and work above 10,000 feet. I assure you I understand altitude deration.

hot_rod

Up around Hailey and Sun Valley a few years back they pulled the gas supply so low that a black sludge plugged many appliances. This was due to many large snow melt systems using large quantities of the gas, in a heavy snow melt conditions.

All snowmelt boilers now require dedicated gas meters that the utility can lock off when gas supplies are low.

I think more resort towns may look into prioritizing supplies with loads like whole town snowmelt

Zman

hot rod said:

Up around Hailey and Sun Valley a few years back they pulled the gas supply so low that a black sludge plugged many appliances. This was due to many large snow melt systems using large quantities of the gas, in a heavy snow melt conditions.

All snowmelt boilers now require dedicated gas meters that the utility can lock off when gas supplies are low.

I think more resort towns may look into prioritizing supplies with loads like whole town snowmelt

That is interesting. From the utility point of view, it makes a lot of sense.
The black sludge is an interesting one. We have seen a heavy smelly oil in our area. You can get so much of it that is causes a trap in the line will make the pressure fluctuate wildly.
The utility mostly wants to deny everything. If pressed they will install a trap and blow down at the meter.
My guess is that very fine oils in the gas turn to liquid as the gas cools and reduces pressure.Just a guess...

jumper

My brain isn't working today. Why can't I choose a jet and adjust the mixture so that burner provides rated output?

Zman

The problem is the air density. There just isn't as much oxygen to burn the fuel. Adding a power burner is like putting a turbo on it to give it extra air. Generally that gets you the 2% per thousand. If you put a big enough turbo and have enough surface area , you can about simulate sea level conditions.

jumper

How about installing a bigger burner? Of course it depends on appliance? For example with a furnace there is the issue that the less dense air on airside has to be hotter or be blown faster. But for a boiler....

Zman

On big cast iron boilers, I have seen clever engineers put bigger burners on and keep deration to a minimum. When you are dealing with pre engineered boilers, you just go with the manufactures data for derations.

Mark Eatherton

jumper said:

How about installing a bigger burner? Of course it depends on appliance? For example with a furnace there is the issue that the less dense air on airside has to be hotter or be blown faster. But for a boiler....

This IS the correct way of dealing with deration and combustion efficiency.

For example, if your load is 100,000 btuH at 5,000' ASL, with an 80% atmospheric appliance, you'd need to add 20% for combustion efficiency and 20% for altitudinal deration. Instead of running the 100,000 through two equations, I've always taken the two factor differentials (.8 X .8 = .64) and divided the base load by that amount to determine the size of appliance necessary to net 100,000 btuH. So, in this case, 100,000 divided by .64 = 156,250 sea level rated appliance. At 5,000' ASL, running 80% combustion efficiency, the appliance will put out 100,000 btuH.

Speaking of "things" in the natural gas, there are places in downtown Denver that have "dirt" in suspension in the gas. Those neg reg gas valves have a VERY fine screen on the inlet to the gas valve that catches this dust, causing the filter to plug and the appliance to starve for fuel. When queried, they admit that there is "dirt" in the gas, and if it causes problems (which it does with these high efficiency appliances) they will install a primary filter before their meter.

Our local utility (Excel) has a "Saver Switch" that they install on the compressor of your air conditioner, and during peak electrical demand, they can and do interrupt the operation of the compressor to shave peak demand and avoid brown outs. They use a radio frequency to activate the interrupt relay. I can see the same potential for SIM peak demands. With SIM loads typically being 5 times greater than space heating demand. None of the utilities took that into consideration during the design phase of their distribution systems...

To direlan, carrying a chip that large on your shoulders is going to wear you down early. Listen and learn. Passing a certification means you study well and absorb/recall information well. Nothing beats actual field experience. Good luck in your ventures.


ME

Tai

direland said:
0-2000' low altitude no de-ration

2000'-4500' high altitude 10% de-ration necessary

Over 4500' high altitude 4% per 1000 or part there of.

ChrisJ said:

direland said:

This is the correct way to derate an appliance. Tried to upload a picture i guess it didnt work.

The last post is almost 3 years old?

Gordy said:

You aced your gas test? Which gas.......</blockquote

The same Test I took for Gasfitter Class A and B in Canada. I had to make a account to post my input on your comment of others.

I also agree with other, that manufactures has their own procedures on de-rating an appliance or caloric value of a gas for high altitude, but on my provincial and my Red seal test, direland answer is the correct one.

LaminarDiabatic

So it's a long story but what I've found to be true ( imperically) and with alot of before and after repair success by measuring capacity. I was originally taught that technically need to clock the meter and replace the spuds if you have too . Then 100,000 BTU input at sealevel x 80% efficiency equals 80,000 BTU output at sealevel. Derate 4%per thousand ft. I live at 5,000 ft so 4x5000 eqauls another 20% less output making it a 60,000 BTU output at my altitude. Then utilizing the heat rise method , with the altitude air correction factor , divide my true Delta T into my BTU output equals CFM. Then once you know CFM you can do a little more measuring for the latent heat formula and thus capacity . I've heard before that I'm wrong but like I said I've proven it to be true myself in other ways and calculations.

Zman

It sounds like you are working with a traditional atmospheric forced air furnace that has been set up for proper combustion. If that is the case, I would say you are absolutely correct.

captainco

I can't believed I missed this first time around. Prior to 2006 no equipment was actually test at high altitudes. The derating was an assumed adjustment based on the air density. In 2006 ASHRAE actually did a study and found that derating new equipment up to 7000 feet was not necessary. Those that do combustion analysis and tune up by combustion have already figured that out especially on furnaces. Boilers and waters heaters might be slightly different because they do not come from the factory as underfired as furnaces do. 130 degrees is the same at 10,000 feet as it is at sea level. If a furnace is not delivering at least that temperature and the correct airflow it is underfired. 6% O2 is 6% O2 at any altitude, just thinner at higher altitudes. 30ppm of CO is the same at all altitudes.
If you know combustion you can set furnaces up so they run the best they can and provide the output closest to their rating. If you don't know anything about combustion follow manufacturers recommendations.

Zman

@captainco
Attached is a table showing the air density differences at altitude as well as the adjustments the utility companies have made to the gas in order to compensate for the air density.
Please post the 2006 study you refer to as well as the instructions for any atmospheric appliance that does not specify 4% per thousand deration at high altitude.

captainco

The altitude derating dates back to the 1930's. Do you think we might be smarter than that now? Obviously many are not!!
The gas derating chart proves that the standard derating is probably incorrect.

It is ASHRAE-RP-1182 2006

Zman

Thanks @captainco,
I will study this in depth.
Upon a quick read, all the appliances tested are fan assist which as noted earlier are derated differently than the atmospheric models at 4%. The other factor which needs to be carefully considered on furnaces is the ability of the circulating fan to remove the heat from the air side of the exchangers otherwise you can overheat the exchanger and melt the venting, particularly if it is PVC.

I believe that most manufactures have cleaned up their ratings. I know both Lochinvar and Viessmann test at 9,600 + and have very specific instructions for each model.

Here is what I posted earlier and I still stand by it:

4% per thousand is often correct for atmospheric boilers
2% per thousand is common for boilers with fan assisted burners.
These are just general guidelines. Manufactures publish the exact derations in their installation manuals. Read a few and you will see what I mean.
On big cast iron boilers, I have seen clever engineers put bigger burners on and keep deration to a minimum. When you are dealing with pre engineered boilers, you just go with the manufactures data for derations.

This thread has a quite a bit of info from folks with significant altitude experience. I feel your blanket statement was misleading and irresponsible.

captainco

I am pretty sure myself and my students have done more high altitude combustion testing and adjustments than most. My first combustion testing at higher altitudes was in 1991. I don't make blanket statements, just sheets of data.
If your temperatures are in range you are not over heating.


I have studied installation manuals from just about every manufacturer and have yet to find one that give the necessary information or adjustments to verify their equipment is operating safe, efficient or within their own ratings.

2% or 4% derating is more of a wild azz guess than anything. Combustion measures reality not theory!

Why is setting airflow in heating by measuring flue temperature the only accurate method to do it versus manufacturers specs?

Zman

Absolutely the combustion needs to be set correctly.
I order for someone to design a system for both heat loss and correct air flow, some assumptions need to be made. If we can't trust the engineers who designed the appliance, we don't have much to go on.....

Zman

captainco said:

I can't believed I missed this first time around. Prior to 2006 no equipment was actually test at high altitudes. The derating was an assumed adjustment based on the air density. In 2006 ASHRAE actually did a study and found that derating new equipment up to 7000 feet was not necessary. Those that do combustion analysis and tune up by combustion have already figured that out especially on furnaces. Boilers and waters heaters might be slightly different because they do not come from the factory as underfired as furnaces do. 130 degrees is the same at 10,000 feet as it is at sea level. If a furnace is not delivering at least that temperature and the correct airflow it is underfired. 6% O2 is 6% O2 at any altitude, just thinner at higher altitudes. 30ppm of CO is the same at all altitudes.
If you know combustion you can set furnaces up so they run the best they can and provide the output closest to their rating. If you don't know anything about combustion follow manufacturers recommendations.

This is what I meant by blanket statement. No consideration for Atmospheric vs fan assist, boiler vs furnace, engineered vs non.
I find that some manufactures and engineers are all over this and do a great job. Others are just WAGging it.

captainco

Zman do you work at high altitudes? If you check the ANSI standards for certifying equipment you would see that they are not checked for actual high altitude operation. They are certified based on high altitude theories and assumptions. Independence, Ohio where they are certified is not high altitude.

There is a specific range of O2, Flue Temperatures, CO and Delta T's that equipment should all operate, regardless of the altitude. These can only be attained properly with a combustion analyzer and whatever gas pressure and orifice size that produces these numbers. Any other method is at best, guess work.

Zman

Most of my work is between 9,000-11,000 feet.
Every job has a full combustion analysis.
I have attempted to verify manufactures derating assumptions by measuring flow rates and delta t as well as clocking meters.
This is where the above opinions were formed.

I certainly don't think any data on the subject is perfect. One needs to have reasonable estimate in order to properly size equipment.

captainco

Zman - Do you have any typical combustion readings you have recorded after derating? Also the typical Delta T you get on furnaces? I ask because no manufacturer lists this information so how would we know what numbers are good or bad?
Trying to offer a little combustion training based on my experiences and results.

Zman

So, I read most of the ASHRAE report done by a 3rd party contractor.
I have issues with the methodology. Their idea of overfiring and underfiring is to crank up and down the gas pressure. Then they decided that wasn't working so they started trying different orifices with no real plan.
Never was the actual volume of gas measured nor were the output BTU's or the flue gas temps. Perhaps I am missing something but it reads like a grad school thesis gone bad.

I still maintain that with reduced air density you will have to reduce the volume (or density) of fuel added in order to get correct combustion. This, in the end will reduce the output of the appliance. Forcing more air into the mix (like a turbo on a motor) will offset some of this effect. The only question becomes, how much? If you can't count on the manufactures engineers to estimate this, you need to find a new manufacture. Some manufactures do test at altitude and provide real data. I am visiting one such facility on Wednesday.

ChrisJ

I don't know about appliances, but it certainly seems to reduce the power output of internal combustion engines. As far as I'm aware, even 2000 feet will have a measurable effect on an engine vs sea level even with the proper mixture.

And the higher you go, the worse it gets which I believe was the original reason the military started using superchargers on air planes.


Now maybe that's a bad comparison because you're trying to get X amount of fuel into a combustion chamber, but it does seem similar.