understanding boiler chart

[otofmyway6]

Hi there,

I am designing a basic perimeter fin tube system for an office. I am using two condensing boilers. I haven't done this type of system before and am not really understanding how to read the boiler charts. one of the boiler charts is

"Size of boiler vs Temp Rise and Flow" for the Water Flow Data, and the other chart is "Size of boiler vs Req Water Temp Rise and GPH Delivered" for Recovery Data. I am having a hard time grasping the meanings of these charts and how to use them to size my fin/tube. Any help you can give is greatly appreciated as soon as possible. Thanks!

 

[ChrisConley]

Can you upload what you're looking at? It sounds like a domestic hot water product.

Don't typically see fin-tube combined with condensing boilers. Fin-tube needs a typically higher mean temp to convect, it can be done, but not a typical installation.

Backing up, the first thing you need to do to pick boilers is determine the heat load of the building. Chose a system delta T which will drive the flowrate.

 

[otofmyway6]

The building load is calling for 2100MBH of heating. I have (2)1600Mbh condensing boilers called out. To be honest, the more I think about it, I realize even less what I want my system delta T to be. Do you mean the dT of the water? or of the space? Sorry, it's obvious I'm an RTard when it comes to this. The 2100mbh is at 70degrees in the winter. outside is 6 degrees. If you mean the water, I believe the incoming water is approximately 50 degrees.

 

[imok2]

Most systems are designed for 85 - 95°C (185 - 200°F) water
The flow rate should be sufficient to keep this temperature change within acceptable limits The higher the flow rate, the lower this change. This change should normally be 10 - 14°C (18 - 25°F) for
space heating, go here to calculate:

http://wea-inc.com/

 

[chicopee]

Tell us the manufacturer and model numbers of the boilers as we may be able to download the charts and walk you thru their meanings.

 

[DrRTU]

Is the total building load 2100 M or just the load on the perimeter radiation? If you have AHU coils it will affect the design. Are you off on the decimal point? On smaller systems, I start with a 20 F delta T. I see designs up to 40 F DT on perimeter radiation. If it’s just the perimeter, take the design condition of 2100M at 70 IAT @ 6 OAT and start with a design of 180 LWT and 160 RWT. Look at the table on the boilers for the efficiency and output at 160 EWT. This is about 220 GPM on the system flow system main @ 4”. Two condensing boilers at 1600 M should carry the load just fine. In this application you will want the units to be twinned to both run at the lowest firing rate to get the highest eff. Use close spaced tees or hydraulic separator on a system pump and each boiler should have its own pump. The system will operate on OA reset to lower system temp but you need to “fix” the design condition at system design. Use you B&G system syzer calculator to help with pipe runs and flows. Use 2 system pumps on VFDs and 2 way control valves at the perimeter zones. Size the length of the radation based upon the BTU / DT and you will get the flow.

 

[otofmyway6]

thank you guys so much for so much input over the weekend.

Below is the condensing boiler, LAARS Rheos+

http://www.laars.com/LinkClick.aspx?fileticket=KVZ1Idg5teE=&tabid=1644&mid=6152

The total load on the building is 2100mbh (2.1million btuh). Each workstation (per the client) has 500W of heat output and it is mostly open office...i guess they all have high power computers with 42" LCD monitors on their desks.

The project options were presented to the client before I joined the team, and the decision was to use purely perimeter radiation for heating and indoor air-cooled DX with VAV for distribution. I'm sure you can imagine the size of the louvers I have on the walls of this thing... For the VAV system I will have hot water coils just to temper the air when necessary.

Brief summary...50,000 sq ft office space, 2 floors. tall ceilings and lots of glass. Even the boiler rep suggested we use non-condensing boilers instead for this application, and I agree, however I also recently discovered they want to use condensing boilers for LEED.

DrRTU, I will try your method. I think condensing boilers will be effective if I can run them with a 40degree dT. At the moment I am using two boilers, two B&G pumps, and one expansion tank.
Thanks again guys, I appreciate all the help. Hopefully I can contribute something in the near future to help others.

Andy

 

[chicopee]

From what I see, the Recovery table is an extension of the water flow data table. Calculating the amount of BTU/HR required for flow rate (gpm and gph), Temperature increase (delta dF),and Cp=1 for hot water, the values will be around 2,185,000.

The water flow data for the hot water heater tank is based on water quality since such heater is indirectly heated by the boiler operating in a closed loop system whereas the hot water tank operate in an open loop configuration.

Obviously the Rheos boiler has varying firing rates but the sizing data table will show the max. and min. firing rates.
Go on line as Rheos has manuals publicly available from which I was able to deduce the info in the first paragraph.

 

[RossABQ]

On a side note, that 500 W/ws value sounds bad to me. I suspect the client is looking at the power supply nameplate on the CPUs, typically 300 - 350W, adding in for a monitor and maybe a small printer. Since that heat shows as a credit to your system, it's important to understand it. I think a much more common number for modern office equipment is 250 - 275W. LCD monitors are very low wattage, power supplies rarely have 100% diversity, and modern printers have efficient features to reduce power consumption.

 

[DrRTU]

I never take credit for plug load in a heating calculation. Most software packages like HAP do the same. The Rheos is a good boiler but I believe its not the best application in this case. The Rheos is hard to get a reset on and min. flow/temp is handeled with a mechanical linkage. I am concerned with low RWT with the copper heat exchanger. You only get that JCI fixed set-point controller or you have to take direct control with a DDC controller. I would use a couple of Lochinvair KBNs and let the boilers OA reset control water temp.

http://www.lochinvar.com/products/Default.aspx?type=ProductLine&lineID=10

 

[chicopee]

By the way, the tables that I used were for boiler size 2400.