Need advice on using HVAC system for part of a building

[gwosun]

We have a 3 story office building in Burlingame, California. I would like to activate the old (1968) HVAC system to heat (cooling is not too important in this location) 12 offices and a conference room. This is about 15 percent of the area served by the HVAC system. The system contains:

75 HP supply fan
30 HP return fan
5 HP boiler circulation pump
2,700,00 btu cast iron boiler
chiller and cooling tower

I am planning to do the following:

Put variable speed drives on the fans and pump.
Close the fire dampers to the unused floors
Seal the registers in the unused rooms on the used floor.
Replace the old boiler expansion tank (no diaphragm) with a new tank.
Run the system to supply the design CFMs to the occupied rooms.
Possibly put in a new boiler, if the efficiency of the one We have is too low.

Since I do not know how much of the building we will use in the future, I want to keep my options open for future expansion. I may put in a number of smaller boilers, so that I can match the boiler capacity to the heat load.

Your advice on this project will be greatly appreciated.
In a similar situation in another building, we had room to join three furnaces ino the supply and return ducts. We did that, and sealed the unused registers, getting satisfactory heating. There is no room to do that in this building without major remodeling.

Please let me know if I have left out any important facts that you need to evaluate this system. I just finished scanning the prints and can send them on CD if you would like to see them.

-Thanks

-Gwo Sun

 

[quark]

That is Ok. But few comments.

1. Check the leakage in the duct system and rectify them.
2. It is better to seal at duct branch off position rather than sealing registers.
3. When you don't know the demand factor using multiple equipment will be a better option. But check investment, operation and maintenance cost of multiple boilers to single boiler.
4. If the AHUs are not in use for longer time cleaning of heating coils will be beter.

 

[Jonkatz]

Hello,
A few thoughts you may want to consider:
1. Consider installaing VAV terminal boxes to control the variable speed supply fan. In this case, the VFD for the small Hot water pump won't be necessary. This may be a cheaper option and will probably result in MORE energy savings because 75 HP fan will be modulated by heating demand.

2. Regardless if VAV boxes are used or not, you should check that the outside air damper minimum setting is adequate. If the OA damper modulates with the return-mixing damper , there may be a situation where the ventilation air becomes to low when the supply fan speed is significantly reduced.

3. As far as boiler is concerned, usually 3 smaller boilers will have a better seasonal efficiency than 1 large one. If your future requirements are uncertain, look into getting a Cast Iron Sectional boiler that can have capacity added by addiding sections to the boiler.

Hope this helps you,
J. KAtz
P.s. this topic would probably get more input if postd to the HVAC discussion.