Adding heat to a home office


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Old 12-16-20, 07:16 AM
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Adding heat to a home office

Here is my situation:
I have a large garage (2500sqf) that has a gas forced hot air furnace. I rarely use the furnace as the garage stays right around 50F most of the winter which is fine for garage work. On especially cold days when I'm under a truck working on something etc. I'll splurge and start the furnace to get the temps to 60F or more.

I needed a home office, so I commandeered a section of garage and framed it in to create a finished room. Insulation, drywall, proper door and window etc. I need a heat source in this room for when the temps drop. I decided against electric radiant floor heating as the material costs just aren't worth it. The cheapest floor heat solution was still in the $800 range.

Here is my question:
There is an air duct that stop right outside of the now framed in room. I would like to place a register in the room and use the furnace to supply heated air to this room. I would most likely close all of the air valves in the rest of the system (there are only 3 or 4 total on a main trunk) as I don't need the entire garage heated all the time. Since this room is sealed or "zoned" off from the rest of the room, do I need to add a return?

Right now there is no real return in the system. Think of the garage as a large rectangle with the furnace at one end. The furnace sits on a pad close to floor level and the air intake is wide open. The air output goes up into a trunk the runs the length of the long edge of the rectangle. Along this main trunk are vents/registers to let heated air out. There are two branches off the main trunk. one to a room that doesn't need to be heated and the valve for that branch is closed. The other branch goes to right outside the newly framed room. I planned to plumb and terminate this branch to the room.

Will this work? I'm concerned that just putting air into the room while not providing a return won't really work. I also don't want to put a return in the room that will let warm air out? What else do I need to consider?

I do plan on putting a thermostat in this room so it can call for heat when needed. I'll most likely run it in parallel with the main thermostat and handle the zoning manually.

Thanks in advance!
 

Last edited by ethan169; 12-16-20 at 07:17 AM. Reason: subed
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Old 12-16-20, 09:30 AM
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A certain minimum amount of airflow is required to keep the furnace from overheating. You may not be able to close off all of the shop registers. You will also need a return inside the office although you'll still be heating cooler air from the shop area. Not a real win-win situation.

It would probably make more sense to use a small electric heater in the office.
 
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Old 12-16-20, 09:44 AM
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Great points. There are a few registers that cannot be closed off on the shop main trunk, so most likely there will be enough airflow regardless. The entire shop area does have fairly heavily insulated ceiling (R30 batts with foam board ontop) so heating the rest of the shop area slightly isn't a huge issue and would likely help keep the heat in the office area. The entire garage is 1/2 below grade and has concrete block walls which are not insulated.

What if I just put a cold air return in the office area that isn't necessarily plumbed to the intake on the furnace? It would just be an opening to let the air out. Would the warm air entering the room be enough to "push" the cold air out? Or do I need the cold air return to "suck" the cold air out?

When zoned forced hot air systems are implemented in a multi level house for example, are the cold air returns all wide open 24/7? Or are they controlled with airflow valves as well?
 
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Old 12-16-20, 10:22 AM
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Not a good idea to close off other registers & just leave the one in the office open. To do so would likely cause the furnace to overheat. Repeated overheating will cause the heat exchanger to fail.

In applications such as yours, I usually suggest a strip of electric baseboard for the office.
 
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Old 12-16-20, 11:21 AM
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Either electric baseboard or a minisplit heat pump.
 
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Old 12-16-20, 11:34 AM
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Just for reference the office area is 162sqf.

Understood on not closing all registers, I can certainly leave enough open to ensure adequate airflow through the furnace.

Gas is inexpensive in my area, electricity is not. The transmission costs are the killer from my utility, so regardless of the actual cost of the power, delivering it causes the bill to be very high. I can easily implement an electric baseboard heater(s), but I'm not sure it will be that economical to operate. I will run some calcs to see what the actual costs could be.

Let's say I leave enough registers in the shop area to allow adequate airflow through the furnace and heat some of the shop and the office. Would my proposed plumbing of the register and cold air return on the office just simply not work?

I haven't put the floor down quite yet, so if anyone has a good recommendation in radiant electric heat, I'm all ears. This is on a concrete slab that is on grade and dry. I plan to use DMX type underlayment and laminate flooring. Depending on the flooring brand I may add an extra layer of fabric type underlayment.

Thanks again!
 
 

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