Easy boiler sizing using actual building data

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Old 03-26-16, 08:55 AM
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Easy boiler sizing using actual building data

There are many threads about boiler sizing and heat load calculations. Here is a simple, alternative way.

Collect data for a number of days on burner run-time, oil consumption and degree days. For “time” connect a 120 VAC hour meter counter into the burner oil solenoid circuit.. See typical ones at: http://www.ebay.comitm/221521766410?...%3AMEBIDX%3AIT

Data collected for a period can be used to derive all kinds of information: actual nozzle GPM, degree days/gallon, run time/dd, btu/hour, etc.. While it varies slightly use 140,000 BTU/gal for oil. If actual efficiency data is not available try 80% to 85%.

If boiler is also used for domestic hot water, some adjustment should be used for figuring building heat load data. Based on measured data here I use 1 gallon oil per week for DHW. Some sources say DWH is up to 20%. of load.

The most surprising thing is low number of hours boiler runs on very cold days. Some source show average US burners runs 20% to 25% of the time. At design temp of 0F my burner runs 8 hours/day or 33% (fired at 50% of rated capacity). One widely circulated figure is 85% of US boilers are twice the capacity needed.

Gathering and crunching numbers for building heat load calculations, is just that a calculation. In the real world there are many variables that skew results. That is why labs exist … to get actual data from a product or system. Usage data collected from a building avoids issues from construction, materials and other variables.

When boiler selected has more BTU's than needed, efficiency can be increased by firing it at less than rated capacity. Lowering the firing rate increases the efficiency. Using the data collected from the first part of this post it is easy to figure run time at lowest outside design temp for a given firing rate. Design in some over capacity for morning startups and other reasons.
 

Last edited by doughess; 03-26-16 at 10:37 AM.
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Old 03-26-16, 06:15 PM
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Your approach is valid!

There is one additional issue when replacing an old large-volume, hot-water boiler, particularly warm-start and cast-iron heat emitters. With a physically smaller replacement boiler, when there is a call for heat, the water temp will drop precipitously, probably below the condensation temp, before the water temp recovers. Either install a larger boiler or add a heat storage tank.
 
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Old 03-26-16, 07:04 PM
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Good point. Even with a hi mass system on cold mornings I have a problem with water temp dropping on morning step up.

To address that I use a PID controller alarm channel to prevent circulator operation below 135F. On cold mornings it will briefly activate during the initial cycles.

The a second PID alarm channel is used for boiler over temp protection to cut off burner at 180F. Is backup if the outdoor reset/aquastat system malfunctions or whatever.
 

Last edited by doughess; 03-26-16 at 08:21 PM.
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Old 03-26-16, 08:55 PM
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Based upon the issue raised about a high volume water system vs small(er) boiler, can't a boiler bypass be installed to deal with that issue? It worked for me.
 
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Old 03-27-16, 08:08 AM
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Knowing actual boiler operation data has many uses besides sizing a new one. I started with the clock to figure out how much the firing rate could be reduced without risking waking up to a cold house at design temp.

My 60 year old Weil-McLain came with a Honeywell low temp circulator cut off control. The $20 PID was a better and easier way to deal with the issue than boiler bypass and other approaches. A programed smart thermostat automatically compensates to reach target temp at set time. Boiler over temp protection was a free safety feature.

I use another PID with a $8 stainless steel probe to monitor stack temp. The temperature after cleaning is a benchmark. After a few months if the stack temp goes up, open the doors and give boiler a quick brushing.

There are many ways to improve system operation and efficiency without replacing the whole boiler package. A big advantage of old boilers is they are very easy to clean. Because they are so oversized,cutting firing rates can make them more efficient than new ones. Efficiency of 85% is do able. Boiler manufacturer ratings/marketing data and pricing are driven by profits. My car has never matched the sticker miles per gallon and furnaces do not come with any means to gauge their performance.
 

Last edited by doughess; 03-27-16 at 09:02 AM.
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Old 03-27-16, 04:28 PM
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I use a PID controller alarm channel to prevent circulator operation below 135F. On cold mornings it will briefly activate during the initial cycles.
Is something like that available commercially to install on a older boiler?
 
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Old 03-27-16, 05:42 PM
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Yes, PID's are a universal temperature control. Just stick or clamp the probe to something you want to control the temp of.

They are widely available on web, (ebay etc) with probes starting around $20. Just search for "PID Temperature Controller". Make sure it has Fahrenheit, not Celsius only. Some are sold as a package with probe and 20 amp solid state relay (SSR). Various types of probes are just a few dollars. Even 9" stainless steel ones for measuring stack temperature are less than $10. See: Digital PID Rex C100 Temperature Controller Max 40A SSR K Thermocouple Probe | eBay

Worldwide PID's are used by the millions in industrial applications. After buying one and downloading the manual I would recommend playing with it for a few days on the bench with simple test lights connected. They have a zillon features that can over whelm a new comer.

On the bench, set it up as a simple on-off temperature control with some delta T. PID's are like cars, many makes but once you learn to drive one the others are easy. Worse case use it as an accurate digital $20 thermometer.
 

Last edited by doughess; 03-27-16 at 06:02 PM.
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Old 03-27-16, 08:31 PM
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I have read this before I believe on another website and still don't buy it as an option for me. Others use your own judgement.

If actual efficiency data is not available try 80% to 85%.
Many older units are operating in efficiency's lower than 80%. That would change the calculation. If efficiency is not known how much emphasis can you put on an accurate boiler size.

When boiler selected has more BTU's than needed, efficiency can be increased by firing it at less than rated capacity. Lowering the firing rate increases the efficiency.
First off I like to follow code and not fly by the seat of my pants. Code states a max of a 20% de-rate. They do this for a reason, to assist in controlling flue gas condensation and safe operations.
There are ratios of fire side heating surface and heat input that must be followed during boiler design. When these ratios are changed you can accumulate more build-up of dirt on the fireside surfaces. This will reduce the thermal transfer over time.
There is a lot of effort goes into nozzle sizing. for a boiler. It takes a few months of testing to choose a proper nozzle for the highest efficiency. Not just dropping it and hope it works. I don't see where you are checking CO out of the boiler into the vent.
When a boiler is designed the operation will fill the flue passes with flue gasses. This slows down the flue gas passing through the flue pass. When de-rated the flue gasses move faster through the flue pass and reduce thermal transfer. The vent temperature will decrease and will drive up the efficiency on paper but not a true efficiency increase. The AFUE actually goes down.
Some things that change is standby losses which go up dramatically the more oversized the boiler. This will make the new boiler may be smaller but it will also be oversized.
Gilmore
Either install a larger boiler
I would never install a larger boiler than needed. Once in a while a buffer tank if needed but seldom needed with properly sized boilers.
Remember the things that don't matter when sizing a hot water boiler.
1. Amount of radiation
2. Type of radiation
3. System pipe size
4. Water volume

Doughess
To address that I use a PID controller alarm channel to prevent circulator operation below 135F. On cold mornings it will briefly activate during the initial cycles.
I assume we are talking about cast iron boilers here. Only do cold start, start the pump on the thermostat call and shut down the circulator when the demand goes away. To stop and start the circulator with a temp of 135f is not good for cast iron boilers. Warm the boiler up cool it off quickly, warm the boiler up, cool it off quickly. Excessive wear on the gaskets or nipples.
Better operation, turn on circulator warm boiler up gradually, slower expansion. Not warm cold, warm cold, just wwwwaaaarrrmmm up sllllooooowwwly.

Zack1978
can't a boiler bypass be installed
Yes, this will allow the boiler to warm up at he proper rate and avoid short cycling which will stop the wear and tear on the gaskets and nipples. Also stops flue gas condensation, raises comfort levels in the home, stops system noises and protects the boiler.

If something works in an application I would be hesitant to to encourage others to follow without testing in different applications, air temperature's and oil temperatures. I can take you to 3 different home within 15 miles of my house on any given day in the winter and they have different combustion air temperature and oil temperatures vary a lot. With reduced nozzle size it can make a big difference in combustion as outside conditions and draft changes.

Many fall into the return temperature scare. It is more important to control boiler rise than return temperature. To quote the late Gil Carlson from B&G " flue gas condensation and thermal stress are a result of extremely cold water entering the boiler or cool water at a high flow rate. Control the flow rate with a boiler bypass and all is good. Bypass of course just one way of protecting the cast iron boiler.
 
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Old 03-28-16, 08:50 AM
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Any heat that goes up the chimney above room temperature is wasted. Look at data on efficiency vs stack temperature. As stack temps drop efficiency rises. That is what high efficiency condensing systems are based on.

Boilers are heat exchangers that transfer the heat of combustion gases to water, steam or hot air. The higher the firing rate of a given boiler the lower the efficiency.

Over the years many things were done to lower building heat loss used to size the original boiler. Even on the coldest days, run time is a small part of the day.

That adds up to an opportunity to simply save money on systems and operation. If you want to buy a new boiler and fire it at rated capacity be my guest. Just factor it into cost of owner ship.

My whole point about using actual system data is that is real, not calculated. You can crunch all the numbers and follow the codes, but when you turn it on that is reality.

This has been mostly about an tweaking and optimizing an existing system. There are points raised that are important in system design that cannot be implemented here i.e. No way is my high mass system going to be run cold start.

I have a well instrumented system and all of the data seem to track. The savings have been more than the price of this house! That is REAL data and money.
 
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Old 03-28-16, 05:48 PM
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I cannot disagree with much of what you say but we must think about safety and longevity.

Any heat that goes up the chimney above room temperature is wasted. Look at data on efficiency vs stack temperature. As stack temps drop efficiency rises. That is what high efficiency condensing systems are based on
I agree 100%, no argument.

Boilers are heat exchangers that transfer the heat of combustion gases to water, steam or hot air. The higher the firing rate of a given boiler the lower the efficiency.
I again agree but I will add what are the consequences? Code's need to be followed

Over the years many things were done to lower building heat loss used to size the original boiler. Even on the coldest days, run time is a small part of the day.
Again I concur

That adds up to an opportunity to simply save money on systems and operation. If you want to buy a new boiler and fire it at rated capacity be my guest. Just factor it into cost of owner ship.
I have been in this industry for 40 years. I have worked at levels from tech, installation team leader, service manager, to the manufacturer level. I have seen many problems of changing the ratio and blocking boiler passageways. Houses have been sooted badly, fires caused by condensation causing the combustion chambers to fall in, people have been hospitalized and died from CO due to flue gas condensation by people dropping the firing rate too far. I would not be interested enough in saving money but putting safety aside. Who accepts the responsibility?

No way is my high mass system going to be run cold start.
Why not? It is done perfectly safe everyday with proper piping, pipe sizing and adjustment. Most every manufacture shows it in their manual and it has been over 50 years since Gil Carlson explained how to apply the bypass pipe to your system be it high mass or not.This keeps the boilers above condensing temps and creates more system comfort and higher system efficiency.

I have a well instrumented system and all of the data seem to track. The savings have been more than the price of this house! That is REAL data and money.
Number 1 your house must not have cost too much, and that makes you pretty lucky. This is tested at your home and you sound like a pretty smart guy and it may work for you. I realize that as you explain to people how to do this and they do not have your abilities or maybe don't monitor as you do and there is a situation then what??
 
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Old 03-29-16, 11:51 AM
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I have seen a lot of crazy boiler set ups too. DYI posts are full of them. Without going into boring details I am very safety conscious. Even keep a draft gauge on as a safety check?

Been using ASHRE and other technical data sources for years.

Have found the heating people use a lot of old wife tales that are not validated by data. My favorite one is “short cycling”. Data shows efficiency decreasing with run time, yet 4 minute minimum, 7 minute targets are commonly recommended. Taco even builds 4 minutes into some of their controls!
Tekmar creates issues factoring it into their out door resets. If my system runs 2 or 3 minuets at 85% I am happier than 7 minutes at 80%. Have never seen any data correlating MTBF and number of cycles.

Data on advantages of hi mass, cold start is hard to come by. It is not something that can be easily measured but the energy to maintain a hi mass at temp for a period of time versus cold start might not be very different. With cold start the lag time in room temperature at morning step up would be greater. Here, in a well insulated house with 1” thick, (plaster over sheet rock) walls, thermal inertia is a noticeable issue. Repeated thermal cycling of and old 50 year old cast iron boiler at minimum would add gray hairs.

With my out door reset/aquastat keeping water temp at 140F to 180F and PID disabling circulator below 135F there is no condensation issue here. Had never had galvanized stack pipes rust out. Title lined chimney has never had issue.

Other than smaller nozzles, only basic change here has been reducing stack diameter from 8” to 6” although firing at .79 gph even 5” is adequate. From old water heaters got the spiral steel liners from their stacks. Used them in boiler upper passages to promote turbulence without impacting flow. In an earlier life had designed systems with heat exchangers so appreciate the issues.
 
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