Technical Document Condensing hot water boilers

Transcription

1 Condensing hot water boilers Key subjects in selecting a condensing hot water boiler. Efficiency Material of construction Design of construction Control capabilities Turndown Flow rate Installation Warranty Efficiency presently AHRI utilizes the BTS-2000 standard for determining how efficient a boiler is. However, The BTS-2000 testing standard does not incorporate real world partial load applications. Which have resulted in boiler manufacturers designing their boiler to the BTS-2000 testing standard. As well, it also has been proven that some of the manufacturers certified by AHRI maintain a higher thermal efficiency than their combustion efficiency. Below is the AHRI report for one particular manufacturer. Notice how the Gross Output has increased on the later report yet no change in combustion or thermal efficiencies have changed(this information was accessed from the AHRI website between Jan. and May of 2014) More so, this manufacturer states that their efficiencies are 92.7% leaving out the fact that their combustion efficiency is only 86%.

2 Below are some of the criteria for the BTS-2000 testing standard for condensing boilers. Although, this is only a small amount of the requirements for the testing, these are the key factors which identify that the tested boilers are not tested at different fire rates. Below was taken directly from the BTS2000 testing procedure. Section 5.1 Thermal Efficiency Test Shall consist of a test point conducted at 100% +/- 2% of the nameplate boiler input. The test shall yield a complete accounting of the energy input in terms of output and losses. Section 5.2 Combustion Efficiency Test Shall consist of a test point conducted at 100% +/- 2% of the input to the boiler and shall yield an accounting of energy input in terms of products of combustion only. Section Condensing Boilers For condensing boilers, the outlet temperature shall be 180F +/- 2 and the inlet temperature shall be 80F +/- 5F at all times during the test. As you can see the test is at only full fire rate and while maintaining a 100 degree delta T. Each boiler tested is commonly given a combustion efficiency and thermal efficiency. This info can be found at Actual efficiencies have been difficult to identify as boiler manufacturers are usually less than objectionable with the efficiency curves they give within their marketing material on their products. One common denominator would be combustion throughout the firing rate. As commonly, Condensing boilers with and without higher turndowns will require excess air into their combustion at lower firing rate to maintain a stable flame which equates to less efficient combustion and a lower return water temperature required to condense. This is found more so in higher turndown design. The indicator of this would be CO2 percentage loss, or O2 rise when turndown occurs or at lower firing rates. You can find this in the installation and operation manuals of the boilers. CSA or the (Canadian Standards Association) also third party certification group certifies efficiencies in both high and low fire which is a somewhat more reliable certification standard. Below is a combustion chart identifying the loss of CO2 when turndown occurs.

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4 Does this really make a difference in yearly cost to operate this boiler. YES IT DOES! How much savings can be obtained from a single percent of efficiency? If natural gas cost $.70 cents per Therm or 100,000btu s. Than 1,000,000btu s would cost $7.00 per hour that is, and at 100% efficiency. Now we need to know how many hours annually our 1,000,000btu s will be needed. A BIN data chart based on location can be used to identify annual run hours.

5 Wilkes-Barre Pennsylvania Technical Document AVG T Bin range TOT Jan Feb March April May June July Aug. Sept. Oct. Nov. Dec. % TOT %CUM %Load to to to to to to to to to to to to to to to to to to to to to to to to to to to to to to to to to to to to Total of TOT is 6,414 TOT = total of time in hours spent in the year at that temperature span. %TOT= represents the percent of time out of the year for that temperature span. %CUM = this is a percentage accumulation of TOT% as the accumulated and above are added to complete the total. %LOAD = percentage of design load required at that accumulated time. If we take the TOT and divide it by 100 then multiply it by %load per each average temperature then add them all together we get the required full fire rate hours annually which are hours So if 1,000,000btu s cost us $7.00 per hour x =$15, would be the annual cost at 100% or gross annual calorific $.70 per therm Now we need to add in the efficiency. 96% efficiency would cost $16, Or

6 $15, x 1.04 = $16, Now if we do the 95% efficiency we get $16, Subtract one from the other $ annually per 1 million btu s per 1% of efficiency. There are a few factors such as Boiler selection Gas price per therm That can affect this cost design. But $ per million btu s per % of efficiency annually is a good base calculation. Example: a school requiring a 6000MBH load design and the engineer specifies 96% Patterson Kelly Mach or Sonic boilers. There 5 year fuel cost would be $16, x 6 (for the load)= $97, x 5(for the 5 years) = $485, If the engineer agreed to the subject 92.7% efficient boiler the cost would be $15, x = $16, x 6 (for the load)= $100, x 5(for the 5 years) = $500, The difference being $15, in five years operation. Not a real big deal but as identified above the lesser boiler is only operation at 86% combustion efficiency $15, x 1.14 = $17, x 6 (for the load)= $106, x 5(for the 5 years) = $532, That would be a five year operational difference of $46, or an annual difference of $9, in fuel costs. Points to look for Confirm the AHRI or CSA certified efficiencies, both combustion and thermal. Found at or ask you local manufacturers Representative for the documentation for the CSA certification. Look into the manufacturers I/O manual to see if combustion is maintained throughout the firing rate to identify if a loss of efficiency will occur. See if Co2 drops at lower fire rate or O2 rises.

7 Technical Document Material of construction Material for heat exchangers within condensing boilers presently is: Stainless steel, Steel, Copper, Aluminum, and Cast Iron as listed certified by AHRI. A lot of manufacturers have model lines made from more than one of these metals For example, Buderus manufactures condensing model lines from Aluminum, steel and Stainless steel. There are also manufacturers whom use two heat exchangers in the same boiler with two different metals. However it falls down to material and how it can be utilized to construct the exchangers and how well it transfers heat. Casting can be done with Cast Iron and Aluminum while general constructions using welding or rolling would be used with Stainless steel, copper and steel. There are a lot of considerations when it comes to selecting a material. Thermal conductivity Chemical tolerance Productivity First Thermal conductivity - this is scientifically measured utilizing thickness and conductivity as K (the higher the K the better transfer of heat). Fourier s equation material K stainless steel 7 to 26 iron nodular 18 iron pure 42 wrought iron 34 copper pure 220 copper bronze 75/25 15 copper brass 70/30 64 carbon steel 25 aluminum 120 Ideally, copper pure(very expensive) and aluminum have the best capabilities for thermal transfer meaning they will allow the fires heat to pass to the water faster, being more efficient in heat transfer. As iron or cast iron is slow at this, it would be utilized to store heat and not to transfer it as in cast iron radiation. Above identifies cast iron

8 Technical Document requiring 3 times and Stainless Steel 6 times the energy needed to equal the thermal conductivity of Aluminum. Secondly material tolerance all of these materials deteriorate, and at different rates. The exchanger is prone to faster deterioration due to it being the highest point of heat transfer which attracts minerals from within the water. However, this is what we need to consider in this application. Correct water chemistry benefits both boiler and system, and its efficiencies. Without it or knowledge of it. Scale and corrosion are capable of reducing efficiencies and causing possible failure. Identified, aluminum deteriorates faster than the other materials (as seen below) if no action is taken when system water is out of tolerance. However, the water being out of tolerance will foul the other materials stainless, cast iron, ect leading to loss of heat transfer and efficiency and will create galvanic corrosion as well. Aluminum is more susceptible to galvanic corrosion but if you have galvanic corrosion all metals will deteriorate. Aluminum today is used in literally every cars engine made, from the block to the heads to the radiator. And under conditions above the temperatures seen within a heating system. Aluminum is a viable and very effective material to utilize for hot water exchanger construction.

9 You will find all condensing boiler manufacturers do require a system flush and proper water chemistry at some level or they will clearly identify within their warranty statement that warranty is not covered due to corrosion, erosion, lime buildup or scale. The chart above identifies how the metal will react within corrosion least noble will deteriorate faster than most noble. Each boilers I/O manual will have some criteria on water chemistry to maintain at or above/below certain chemical properties to prevent corrosion from being a problem. This usually consists of a solids particle count, PH, Hardness, CaCo3, and Chlorine percentages. On closed loop systems, commonly there are leaks. As initial treatment corrects the chemistry, a quarterly test for the first year of operation is a good practice to indicate of system make up water changing the chemistry and how much added chemical will be needed. A water meter is commonly used on the feed water line for the recording of system water loss so an automatic chemical feeder can feed the system as needed. A LITTLE PREVENTION CREATES A LOT OF BENEFIT. Water chemistry and condensing boilers The reason a condensing boiler is selected over a non-condensing boiler is to ascertain the savings of fuel cost. If a condensing boiler is installed and water chemistry is not maintained it is possible the efficiency gained by the use of a condensing boiler will most likely be nullified by the scale build up or fouling on the heat exchanger regardless of the material tolerance. As the U.S. department of Energy has found that 1/64 of scale will reduce boilers efficiencies by up to 4% making a 92.7% efficient boiler incapable of condensing if the water is not checked and or treated properly. As the exchanger being the location of highest heat transfer the unwanted minerals within the water will be attracted to that location. Fouling the heat transfer surface and reducing the transfer process.

10 Productivity cast or constructed When a section of a heat exchanger or an entire heat exchanger is cast the possibilities of designing flow, heating surface area, and thermal transfer are more robust. As the casting is performed throughout the section or complete heat exchanger at the same time the metallurgy remains consistent. Constructed or built exchangers with good design are also very capable but the consideration of welded or pressure stressed locality needs to be considered. And the restrictions of constructed exchangers are limited as design can t as easily direct flow or create more heating surface. Points to consider The level of thermal conductivity Water chemistry Cast or fabricated exchangers Design of construction condensing boilers have become confusing when it comes to design of construction. Some manufacturer s designs consist of more than one heat exchanger and/or multiple burners, gas valves, or blowers which adds more moving parts for more possibilities of failure. If the benefits of such design outweighed the added possibilities of failure it would be worth the consideration but presently they don t. Annual servicing of a boiler is a practice of maintenance as much as it is a practice in safety and when performed properly, will lengthen the life of the boiler. Less parts allows for less cost of annual maintenance and less possibility of failure. Points to consider How many exchangers or burners or blowers within the boiler design How easy is the annual maintenance to perform Control capabilities Some condensing boiler manufacturers have vastly enhanced boiler control capabilities. This can be difficult to identify which manufacturer can do what. One of the most important aspects of a control would be its capability to adjust to the site systems needs. If the plan is to run the boiler at condensing temperatures all the time you need to adjust the post purge time and cycle times to shed off the condensation from the exchanger or if the boiler is over sized for the system you would want to prevent short cycling. Best practice would be to contact the manufacturer or their representation to confirm as to what you need that specific control to be capable of. Otherwise you may

11 Technical Document have the added expense of an additional control within the boiler room to achieve the desired operation. Present boiler controls may or may not consist of Set point control Fire rate control Outdoor air set point control Multiple boiler control Multiple boiler design of operation Annunciation of operation BAS (building automated system) communication Remote operation and annunciation Dual set point control for both domestic and heating applications Flame safeguard and safety operations Operation of boiler pump either on/off or VFD Operation of domestic hot water pump Operation of sealed combustion damper or wall louver motor Operation of electric 3 way valve Allow external controlling of fire rate or set point via signal voltage Allow remote signals of alarm and/or flame If connecting to a building management system will you require a Protocol converter? With all of these considerations, the needs of the customer should be confirmed with the manufacturer as adding a control within the boiler room can cost a considerable amount. Points to consider Will you require an external control? Will you require a protocol converter? What is the designed application and can the control adjust for it? Turndown Most condensing boilers modulate meaning they adjust their fire rate to the heat load requirements. How far the modulation can go from full fire rate is determined by the turndown which is a fraction. Example: if a 2,000,000 btu boiler has a 5 to 1 turndown it is capable of modulating from 2,000,000btu to 400,000btu. The larger the turndown the closer the modulation can adjust to the required heat load however, flame stability becomes more difficult in lower turn down as less fuel and air within the same sized combustion chamber become less stable due to lowering the combustion chamber pressure. This is when some manufacturers will increase the excess air within the combustion to maintain stability. This leads to a less efficient combustion process and

12 lowering the dew point temperature at which condensing of the combustion gases can occur. The indicator to look for to see if this occurs on a boiler is within its I/O manual under combustion settings. If the combustion (CO2 and O2) is consistent at all firing rates, then you can determine the combustion efficiency and dew point of the combustion gases will remain consistent. However, a lot of manufacturers add excess air. A CO2 of 9% will condense at 130F degree s while a CO2 of 6% will condense at 116F degree s this is a lot of time throughout the year this will occur and with this difference can cost approximately 5% to 6% annually. See below. This loss in efficiency can also be caused in longer venting runs as some manufacturers can run venting for further runs but not without lowering their turndown by increasing low fire fan speeds. Points to consider Check with the manufacturer if the combustion remains consistent throughout the firing rate. If not, you can be losing up to 5% efficiency annually. If running long vent runs you may need to increase low fire fan speed causing reduced turndown and losing even more condensing capabilities and combustion efficiencies. Confirm vent design with the manufacturer to prevent the need of adding excess air. Flow rate Utilizing VFD pumping either on the primary (boiler loop) or secondary (system loop) can be very beneficial in efficiency and operation. However, when a boiler is designed

13 Technical Document for use with wide flow rate capabilities to gain these benefits it is necessary to allow the boiler control to operate the speed of the VFD pump or pumps or to have the BMS(building management system) to maintain differentials in the return and supply of the boilers water temperatures to maintain the benefits. If VFD s are used, it is most likely controlled by pressure differential and not heat transfer. It is unfortunate that this design is not being utilized and is being seen as an easy option for retrofit as it can accept a wider flow rate. This can and usually cause either short cycling or prevent the boiler from maintaining the system loads during high demand. When 180 degree s is the maximum temperature of the system design. With a 40 degree delta T you have the capabilities to condense 98% of the heating season considering the selected condensing boiler condenses at 130 degrees which means its combustion would need to maintain a 9% or higher CO2. Points to consider Is the system design to utilize VFD pumping via. Delta T in conjunction with the boiler or BMS operation? Wider flow tolerance causes less efficiency when the boilers delta T is not the source for directing the flow rate. Does the efficiency of the system design outweigh the boilers efficiency loss when utilizing pressure differential sourcing? Installation The ease of installation is another consideration. Does a wall have to be removed to install the boilers? Is a forklift required on site? Does the floor or roof at the point of installation have to be structurally supported prior to installation? How much floor space will need to be constructed to support the installation of multiple boilers? How high will the ceilings need to be within the boiler room to incorporate the boilers? IBC or international building codes require certain clearances. When deciding which boiler manufacturer to use on your installations it is important to take these factors into consideration as cost of installation is on average the same as the cost of the boiler.

14 Some present manufactures require mufflers on their exhaust, Some have to be assembled on site, and some weigh 3 times more than others with the same MBH output. Some come with wheels on the bottom and a handle allowing one ton of boiler or 4 million BTU s to be moved by one person. Some have larger water connections than others for the same output and others are certified with different venting materials allowing venting cost to be 50% less. Check with your Manufacturer to find out the options they provide to simplify installation and lower the installations cost. Points to consider Does the boiler manufacturer chosen come with added installation costs? Will your boiler selection require more building square footage to be constructed and paid for? Warranty Warranty varies from manufacturer to manufacturer and can be figured as a life expectancy or guarantee of such from the manufacturer. This is important when selecting a boiler as the life expectancy defines the value at which the boiler will have throughout its warranted life of operation. Some manufacturers will extend warranties for an added cost. Some will incorporate a LIMITED warranty. Example: you have an option of; 2 million btu boiler running at 85% efficiency with a 10 year warranty. Or 2 million btu boiler operating at 96% efficiency with a 5 year warranty. Which has more value??? This is dependent on fuel costs, owner s situation, and installation cost. Installation costs set aside as they would be common for both, and fuel costs being the decision maker. We can utilize BIN data, heat loss and fuel cost to estimate the operational expense for each to weigh in on the value of boiler life expectancy factored by the warranty.

15 Nat. gas Based on a cost of $9.87 Per 1 million btu/hr. 85% efficiency cost $11.35 per hour for 1 million btu s 96% efficiency cost $10.26 per hour for 1 million btu s A BIN data chart based on location can be used to identify annual run hours. Wilkes-Barre Pennsylvania AVG T Bin range TOT Jan Feb March April May June July Aug. Sept. Oct. Nov. Dec. % TOT %CUM %Load to to to to to to to to to to to to to to to to to to to to to to to to to to to to to to to to to to to to Total of TOT is 6,414 TOT = total of time in hours spent in the year at that temperature span. %TOT= represents the percent of time out of the year for that temperature span. %CUM = this is a percentage accumulation of TOT% as the accumulated and above are added to complete the total. %LOAD = percentage of design load required at that accumulated time. If we take the TOT and divide it by 100 then multiply it by %load per each average temperature then add them all together we get the required full fire rate hours annually which are hours

16 85% efficiency cost $11.35 per hour for 1 million btu s x = $25, % efficiency cost $10.26 per hour for 1 million btu s x = $22, A 11% difference in efficiency per million btu s annually is $2, If you purchased the 85% boiler the Operation after 5 years would be a cost of $126, per 1 million btu s If you purchased the 96% boiler the Operation after 5 years would be a cost of $114, per 1 million btu s This leaves an estimated savings on 5 years cost of $12, per 1 million btu s. Now double that as the boiler size is 2 million btu s and the 5 year savings would be $24, or the cost of another 96% boiler. Points to consider Do the savings in fuel cost outweigh the cost of a new boiler within the warranty period? Recap points to consider that effect value What is the certified efficiency rate and does the boiler maintain combustion throughout the firing rate? What is the level of thermal conductivity and the required water treatment? How many burners, gas valves, blowers are used and how easy will it be to perform annual maintenance? Does the boilers control allow for all of the designed needs? Is the boiler selected, designed to lose 3% to 5% efficiency due to adding excess air in the combustion while reducing condensing time? Does the system need a wide flow rate, and if so, is the flow rate being determined by the Delta T across the boiler? Choose a boiler manufacturer that has designed to reduce installation cost. Is the selected boiler beneficial, or will its lack of objectivity cost money throughout its life expectancy?

17 How Much savings will your decision create? This is dependent upon Peak load With replacement applications it Depends on the efficiency of the equipment being replaced A value can be determined on the differences between efficiencies as estimated, although not exact. Below is a chart defining saving between different efficiencies in different locations. This should help determine fuel cost savings. With all of this information you can make a good judgmental decision on which condensing boiler manufacturer works best for your application Respectfully, Eric Marchington President Green Products of NEPA Phone Cell e.marchington@greenproductsnepa.com