Boiler sizing

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Old 01-05-10, 08:56 AM
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Boiler sizing

I designed and built my original house, did the heat load calculations, sized the original boiler and installed the heating system (along with the other plumbing, wiring, framing, contrete, etc). A few years ago we had a contractor add a couple of rooms and do some general remodeling. I didn't pay much attention to the heat load or boiler changes.

I recall hearing, however, that the plumbing sub seemingly arbitrarily rejetted the boiler to increase its output to 200k BTU. My original calcs came in at about 40k but I ended up with a 75k boiler in that it was smallest available. (the original house was about 2000 sf, tight with a lot of south facing glazing. The heating bill was very low (~$100 per year) so things worked well.)

My current concern is the boiler output may be much too high and, therefore, inefficient. I also notice during a recent burner box cleaning that the water temp was around 160F max. I recall it should reach 180F or so for baseboard to work as designed. Perhaps the plumbing sub turned the temperature down as well.

It seems I can back into the total head load requirements and required boiler size by simply using the total number of feet of baseboard in the current house. It seems the current footage and location is adequate to heat the house even on the coldest winter days so what could be a better indicator of heat load needs?

So my question is do I need to adjust the gas jets? If use the current baseboard and multiply by a factor for BTU per foot (assuming a water temperture of X degrees F) I will arrive at a required net boiler output. My boiler is a Burnham XG2000 (induced draft high efficiency) so I get some loss due to efficiency. I figured I should also add an additional fudge factor of some amount. Is this a reasonable approach to check where I'm at and to drive any changes I may need to make? If so, what about factors to get from net to gross?

Also, what about the water temperture? It seems this needs to be right or the stuff above is moot. Should the water temp be 180F and if mine is not, how is the adjustment made to correct it?

Thanks for your help.
 
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Old 01-05-10, 01:33 PM
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First off it is all about heat loss. The connected load has nothing to do with boiler size only heating the home. If there is not enough baseboard to heat the home when the outside temp gets near design the house will not heat. If there is enough or excess baseboard the home will heat to outdoor design providing the boiler is big enough for the heat loss. Excess radiation on a properly sized boiler is not a problem but a larger boiler on less than required baseboard does not get more heat.
In other words if the boiler is 200k but the radiation is rated at 100k you only get 100k of heat. The boiler will just short cycle due to making heat faster than the system will get rid of it. Radiation output is a relationship of gpm flow and water temperature. The maximum radiation output does not change with a boiler larger than the connected load.
You need to redo the heat loss and check boiler size.
To up-size the orifices is not a good thing. The boiler is designed to handle so much product of combustion. To input more gas then the boiler is designed for is against code. The code states the boiler input may never be more than 100% of rated input or less than 98% of rated input.
 
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Old 01-05-10, 02:08 PM
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rbeck,

Yes I understand and I totally agree, looking at the problem from the front end (new house) it starts with heat loss. But you'd agree that any heat load calculation is an educated guess based on many varibles, the most uncertain (and, unfortunately, the most critical) is the air changes/air infiltration.

My assumption is the current amount of baseboard is adequate to heat the house even at the coldest days (design temp). Using this assumption it seems the boiler size could be based on the currrent baseboard - sort of backing into the boiler size based on what is actually working within the actual environment (i.e the actual air infiltration (not a guess), the effective insulation rate (not a guess), etc.)

But I understand from your comments is having a boiler too big is not inefficient but that it will have shorter cycle times. My question about the boiler size and if it was too big was bcause I am concerned about efficiency. I understand that the amount of energy provided to heat the house is determined by the radiation (assuming the boiler is large enough to handle the radiation.)

Also, regarding changing orfices, it was my understanding that my current boiler had several output ratings depending on which orfices were installed. At the factory, for my boiler, the smallest orfice was installed resulting in the smallest (75k BTU) boiler. But if I had ordered a 125k boiler, the burners, sheetmetal, etc would be identical with the only difference being the orfices. I may have understood this incorrectly.

But I'm also wondering about the water temperature. I found another thread on this site where the 180F water temp was not being reached and the issue was discussed. Is this temperature typically adjustable or is it a hardwired themostat that may degrade over time? I don't if or why the plumbing sub would have turned it down so it may be that the aquastat is just failing being that the boiler has been in service for over 25 years.
 
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Old 01-05-10, 02:59 PM
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An over sized boiler is very inefficient. The short the run cycle the less efficient it becomes. It is better to have a smaller boiler than the radiation as long as the boiler matches the heat loss. Never add anything to boiler sizes from the heat loss. Use the DOE output, sometimes referred as the gross output, not net output. There is always some fudge built into the heat loss program so you can get pretty close. Really beets rule of thumb sizings.
If you can heat with temperatures less than 180ş at design temperature that means you have way too much baseboard which is not a bad thing.
If the orifices for the boiler was up sized it is not the right orifice. That XG-2000 series boiler came in models XG2004 thru XG2006. The last number means the number of cast iron sections the boiler had. So it is not the same boiler with three different orifice sizes but three boiler sizes with different orifices.
 
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Old 01-05-10, 03:22 PM
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rbeck,

Thanks again.

If the water temp is not reaching 180F should I correct the problem that is causing this? I understand I may have too much baseboard (not a bad thing) but if it is failing stat (aquastat?) then I'm worried it may get progressively worse and totally fail at the completely wrong time.

I have the XG2004 with a DOE output of 75k BTU stamped on the label. Is there a way I can confirm the current orifices have been changed. If I knew the orifice size I may be able to find some orfice gauges to check.

But maybe, beings that the boiler is 27 years old, it may be high time to change to a new boiler before it fails. A few weeks ago I had to replace the controller board and the ignitor (outside temp at the time was zero or colder and I was without heat or domestic hotwater. My system uses a Amtrol heat maker. I love it.)
 
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Old 01-05-10, 10:15 PM
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So tonight I did some homework. After checking the boiler hot, it was at or above 180F so that isn't an issue. But I'm still concerned about the orifice change.

My boiler is a XG2004 which has an DOE heat capacity of 81k at 2,000 feet above sea level. The house has 96 lineal feet of slant fin. Using 580 BTU per foot I'd need a boiler of 55,680. If I use 650 BTU per foot I'm at 62,400. So even 5,000 feet where I'm at, my stock boiler should have been fine. The question I have are the orifices in the boiler the same ones that was in it when it shipped from the factory. As I said below, I recall hearing the plumbing sub say he was going to change them to accomodate the additional baseboard from the re-model.

What got me thinking about this was a few weeks ago when the boiler broke down and I spent some time looking it over. The burners had never been cleaned so I pulled those out, brushed them off and vaccumed the burner box. It was pretty dirty. The manual also mentions taking the cover off the heat exchanger can brushing the cast iron. This has never been done in close to 30 years. But with natural gas, is this necessary?

After working and cleaning and I got the thing fired up, I took a peek at the flame. The manual indicates there should be no yellow peaking on the flame but in my case I clearly have yellow tips. It's not the hard blue flame I figured I'd see. This got me wondering about the efficiency and if the orifices were oversized to the point things were totally out of balance.

So I'm looking for some guidance. How can I confirm the orifices are the stock size. Can I get this information from Burnham somehow? Also, about the cleaning, should I pull the top off the heat exchanger and brush down the cast iron? And what about the flame color? Is what I'm seeing normal or should the flame be a nice hard blue? I recall my neighbor's hot water tank flame after we installed it new a couple of weeks ago. It has what I'd call a nice hard blue flame. The flame in my boiler is sort of lazy with yellow patches.

Thanks for any help or insight you can provide.
 
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Old 01-06-10, 05:54 AM
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Originally Posted by dbsoccer View Post
So I'm looking for some guidance. How can I confirm the orifices are the stock size. Can I get this information from Burnham somehow? Also, about the cleaning, should I pull the top off the heat exchanger and brush down the cast iron? And what about the flame color? Is what I'm seeing normal or should the flame be a nice hard blue? I recall my neighbor's hot water tank flame after we installed it new a couple of weeks ago. It has what I'd call a nice hard blue flame. The flame in my boiler is sort of lazy with yellow patches.

Thanks for any help or insight you can provide.
Natural gas pr propane, you need to clean the heat exchanger.
A proper cleaning involves this, just not what some do and vacumn out the the combustion area.
The burners still may be a bit dirty, but the flames would start well off the burners if your boiler was firing at over twice the firing rate.

This is a serious matter as you are exceeding lots of design specs by over firing the boiler (if in fact you are).
The very first call I would be making is to burnham to find out what orfice / jet / manifold pressure was factory set up.
 
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Old 01-06-10, 08:37 AM
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Thank you TOHeating,

I just checked with the plumbing contractor who did the re-model. While he can't remember my specific job, he said changing orifices is something he would never do to increase output. He never touches what comes from the factory. He is concerned, as you and I are, about the yellow tipping on the flames. He mentioned the yellow flame indicates poor combustion most likely caused by air flow. The poor combustion can lead to soot build up on the cast iron which can lead to more problems. So my quest is now to figure out why I'm getting poor combustion.

This is an induced draft boiler. The controller board checks for air flow before allowing the firing sequence to proceed but I don't beliee there is volumetric verification of air flow.

I will be ordering a seal kit so I can pop the lid off of the boiler and clean the cast iron.

But my focus now is on combustion efficiency and the yellow flame. I wish I had something to compare to - like a picture of what it should look like. But I think, based on looking at the flame on my neighbor's hot water tank, I do have an issue. The question then becomes, what are possible causes of my yellow flame? My local plumbing supply told me they sell complete burner, orifice manifold assemblies from time to time. This was a bit of surprise in that the manifold assembly has no moving parts so what is there to wear out? Apparently our natural gas is considered dirty.

Any ideas to fix my yellow tipping would be appreciated.

Thanks.
 
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Old 01-06-10, 04:29 PM
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There are MANY factors in proper combustion, that is what we go to school for.
Primary air, ,secondary air it all plays a role in the combustion process.
The air proving switches do little more than prove that the fan is creating pressure. If the vent is blocked, it will still build pressure. If the heat exchanger is blocked and you have dilution air, you will still build pressure.
A very badly blocked heat exchanger will roll out the flames, it may overheat a spill switch (if equipped) etc.
Your manual is THE VERY BEST place to look for the answers.

A good cleaning of the heat exchanger, followed by burner cleaning and then tidy up all the soot and crap that comes down and blow out the burners with compressed air.

One of my techs checked a cast iron boiler that was spewing out about 250 PPM of CO, he did a proper thoughough cleaning with brushes and the same boiler barely produced 5 PPM after the cleaning.
 
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Old 01-06-10, 04:46 PM
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Thanks TOHeating,

I will get the seal kit in hand and get things cleaned up ASAP. The temperature is back near single digits here in Colorado so I need to pick my spot in time. And, in that I believe I now have lines and pipes etc running over the top of the sheetmetal, gaining access to the top of the exchanger may be a trick.

I fear I also need to look at other maintanence areas as well. The supply lines have at least two air purge valves that appear to spu rusty water at times. One is on the large cast iron device that eliminates air from the lines and the other appears to be piped directly into the top of the exchanger. I read somewhere where these items have a maintanence schedule. While I've got the thing off line, what other things should I be considering?
 
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Old 01-06-10, 05:04 PM
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Are there brushes specifically made for cleaning heat exchangers, or is it one of this whatever fits in there deals?
 
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Old 01-07-10, 08:15 AM
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The orifices are stamped with the size of the orifice. Pull one and if this is natural gas it should be a #45. If you pull one and it is stamped #45 it is correct.
The brush is what fits. Probably 1 1/2" x 1-1/2" brush will work.
 
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Old 01-07-10, 08:29 AM
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I've ordered my sealing kit but I have plumbing in the way of getting at the top of the exchanger. Hopefully the plumber who put the plumbing in the way will move it but that's my issue.

I was reading the manual last night. In the area of gas pressure and flow, it indicated to check the pressure at a manifold access port and adjust the pressure regulator as required to hit the spec. It also discussed a volume check using the gas meter timed for 3 minutes. If the volume was not enough it said to increase the pressure to a point. But beyond that point it said to drill out the orifices as to get the volume but at a lower pressure. Interesting......

Anyway, I need to get things cleaned up. My flames are not yellow tipping, the yellow orginates at the burner level. Maybe this is the same thing in that I have no baseline to compare. I do have a CO meter in the same small room as the boiler and it reads 0 which makes me feel good. The horizontal run of the exhaust vent is shiny and clean so it doesn't appear plugging is an issue. There is nothing restricting air flow into the burners.

Again, I'll start with the cleaning. TOHeating, you mentioned your son measured a drastic drop in CO (which I read as an increase in burn efficiency) by cleaning the exchanger. How can cleaning do this? Is it simply a matter of improving the air flow?
 
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Old 01-07-10, 09:14 AM
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Gaining access to the heat exchanger

On my Burham boiler, the canopy top is, apparently, attached to the top of the exchanger with several fastners. Is it the experience of others, are these fastners easy to remove or are they normally corroded such that they break off and need extraction tools or other methods to get the cover properly re-attached?

The boiler is 25+ years old and this cover has never been removed.
 
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Old 01-07-10, 08:55 PM
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I ordered a sealing kit. It is two weeks out. My exhaust vent is shiny clean and I don't see any way to adjust primary air on this boiler pointing to the flues being so plugged my air flow is disrupted causing the inefficient buring/yellow flames.

I'm still wordered about getting the canopy off the exchanger only to find I have to break off corroded bolts to do so. If they break off it seems like things could get pretty ugly. Is there a way to seal the canopy back down without the fasteners? Even if I get my piping moved I'm not going to have much room to work in that my hotwater maker sits on a platform directly above the boiler. Things are crowded. I could take the maker down but would prefer not to have to do that.
 
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Old 01-08-10, 10:11 AM
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rbeck, below you indicate that I should have a #45 orifice. I have some hole gauges that I can check the diameter. I am concerened that even though they may be stamped #45 they may have been drilled out. How did you know that I should have a #45? Is this from a Burnham reference or some other reference/knowledge.

I understand that orifice size can be adjusted for high altitude. I'm a 5000 feet. A local burner orfice firm indicate a #45 should be derated to a #47.
 
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