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

 

[ChasBean1]

Gwo Sun -

I think the idea is possible but very inefficient. You noted that the central systems will serve 15% of the design service. Variable frequency drives will operate the fans and pump at a minimum 25% of their rated speed, which would still be high for this application. This boiler, even at low fire, would probably have excessive heat output causing over-cycling while the system is operating at steady state. Another potential problem is the flow through the boiler - this sort of reduction without a primary/secondary loop and decoupler could cause laminar boiler flow and even further reduced efficiency or localized boiling, steam pockets, and hammer. As far as efficiency, prepare to pay a whole lot for something that could much more easily be done with a new 220v feed and electric baseboard heat controlled by wall thermostats.

I heard you cringe when I said electric heat. Okay - I understand. I don't know the nature and future of your firm and I think that would be the easiest and most cost-effective solution. If electric is out, think about future expansion and where you might physically locate modular boilers. Start with one small, efficient modular hot water boiler and tee into the existing supply and return water lines. Use a constant flow pump sized as needed to serve the loads - install a loop bypass to open as system control valves close to prevent dead heading and flow obstruction.

With this setup, buy an 8-10,000 cfm packaged 'throw-away' AC unit with gas heat, return air, economizer, and some 20% minimum outside air. Tap this into the existing supply and return duct work; blank flange the inlets to the existing AC unit. Execute your fire damper and air blockage plan.

Keep in mind that the hot water reheat system will require a blockade similar to the air system. Hopefully in the unused floors this can be done by shutting isolation valves at the main taps off the riser. On the used floor there are hopefully maintenance isolation valves at each terminal unit that will allow shutoff.

You might get these rooms to feel comfortable without issue, but serving these rooms with the systems you mention is like firing up the local electric plant to heat your house, when you have a wood stove and plenty of firewood nearby. High operating cost means high overhead which isn't good for any business. The modular boiler could be one of several to add later to accommodate expansion.

Best of luck. -CB

 

[gwosun]

Yes, I did cringe at the mention of electric heat.

I have heard that VFDs cannot go below 25%, but my experience with VFDs on sump pumps is different. I have had no trouble running them from 5 to 15 HZ. I have been told that the minimum speed prevents the motor from overheating due to low flow over the motor. This does not seem to be a problem with sump pumps. The power to the motor required for low speeds is so low that it can be dissipated without any flow. I have been told that VFDs used for fans may require a small fan run at full speed over the motor to prevent overheating. Will this work, or am I missing something?

Now I must confess an ovesight on my part. I have scanned the plans, which cleans them up quite a bit, and have noticed that the plans clearly say "sectional boiler"

I have considered the electic heat route, and although I despise it thoroughly, it is the least expensive short run solution. I oppose it because it sets a bad example. I am always lecturing on the two costs for every energy consuming appliance; the price of buying it and the price of using it. The electric heater is my example of the device that can use more than 100 times its initial cost in electricity during its lifetime.

Since the offices to be used are on the first floor and there is a full height basement, I am considering hot water heated baseboards or European style radiators as an option. The floor can be core drilled, and the pipes run across the basement ceiling. However, this does not address ventilation. The first floor offices, when they have windows, have solid panes that do not open. Many interior offices are not located next to an outside wall.

I know that what I propose is a lot of trouble and expense, but I keep returning to it as the best option.

Thanks to everyone for your excellent advice.
My experience in HVAC is not very deep, so I am grateful to those can point out the pitfalls.

-Gwo Sun