I'm looking to convert my garage into a bedroom, but have some heating questions. First, currently the entire house is about 800 sq ft, the addition will about about 200sq ft. The furnace is centrally located inthe house and runs into the attic crawl space. Can I tap into the existing living room duct and run it across the crawl space, down the wall and into the new room? My space and area to work with is limited. I am planning to pruchase a new furnace later this year, and am not to concerned about the room gettng AC. What are my realistic options here? Thanks to anyone that can help.

 

Quite simply, no you can not tap into a duct that serves an existing room.

Reason is because the duct that supplies any given area normally only has enough airflow to satisfy that area.

The first thing you need to do is figure out if the existing furnace can handle the added space.
If it can you would need to run a properly sized duct right from the furnace plenum and then balance the airflow.
It would also be impossible to tell you what size to make the duct because we would need to first know what the heat loss is for the room in BTU's.
The proper way to do this is to do a heat loss/gain calculation for the room and only then properly size the duct.
Here is an online heat loss calculator if you want to diy:
http://www.hvac-calc.com/main.asp

It is quite common for garage conversions to have their own heat source because they are not usually as well insulated as the house.
You don't say where you live but in a cold climate there is considerable heat loss through the concrete floor unless you build it up and well insulate it.

If you search back through previous posts there are many from people who are having problems with cold garage conversions.
It is a common occurance.

 

Thank you for your response. I will check out that link. I live in NY state, and I will insulating the room with R-5 sheathing, r-19 for the floor, r-13 for the 2x4 walls, and r-30 in the ceiling. I will laying a new concrete pad with a 6mil barrier over the dirt floor. Being an older home and the previous owner did not invest into the property, this new room will be the best insulated rooom of the house. This is only an estimate, but the furnance is about 20-25 ft away from the garage, if that info is needed. As for the furnance, I plan on replacing it later this year. I already looked into one and received a quote and asked if what I would be getting would be adequate for the addition. The tech said yes. I really have no other options than to do this work myself because of $$$.

After some reading and searching, will I have to place a cold air return in the room? Any recommendations?

 

You are proposing an increase in the size of your house of approximately 25%. If your present furnace is vastly oversized (quite possibly) then you could do as Greg suggested.

You REALLY need to do a heatloss calculation to see what your requirements are now and what they will be after the new room and any upgrades to the original house. Only with this information can you make any attempt to properly size the replacement furnace.

Yes, a return-air duct is a necessity. The R-A duct should be 1-1/4 to 1-1/2 times the area of the supply duct. The size of the supply duct (and you probably will want more than one register in the room) is dependent upon the heating requirements of the room, the temperature of the air from the furnace, the insulation on the duct, the length of the duct and the acceptable noise level from the moving air.

 

Furd,

Thanks for the reply. I will running the heat loss calc once I get homeOne more question on the return air vent. Am I right to assume the return vent needs to be away from the supply vent? Also, where do I need to run the return to if the situation arises that I cannot reach the current one located in the next room over. How many twists and turns can it have? Unfortunately, I may only have enough room for one supply vent due to the layout and size of everything. I currently only have a small space to work with that is closest to the interior of the existing house. I could run the duct under the floor, but that would produce more area to travel and I would then have to support it over the cement pad in the crawl area.

I will post again once I have ran the heat loss calc. Thanks again for the help so far.

 

It's only a small clue but the run time and the off time of your furnace in the coldest weather would give you some idea as to how your current furnace relates to the size of the house and the heat loss.

When figuring all this out the specifications of your furnace in terms of capacity and airflow are quite important but this info should be easy to get.
You would especially need an airflow chart for the blower as many furnaces have optional blowers for different applications.

As far as recomended duct sizes go all sizing has to take distance and albows into consideration.
furd's recomendation of 1 1/2 times the supply is fairly accurate all things being equal.
If it were that the return had to be run for more than the usual distance or number of bends then it would have pot be bigger.

All these little details will make or break your project if not done correctly.

Like I said, if you read through past posts on this subject many questions you do not even have yet will be answered.

 

Thanks for the advice and tips. I should be home soon to have a better grasp of things and run the heat loss calcs. With a quick guess, since the extreme cold weather is over, maybe 5-8 min of run time when set at 72, and maybe 30-45 min to turn back on. But like I said, just quick guesses. I recently just replaced 4 out of 9 windows through the house. Of the remaining, the bath window will eventually be eliminated, as will 2 others when I enclose a small attached porch off the kitchen. The kitchen window and hallway window are the others that have yet to be replaced. Those 4 windows have drastically reduced the amount of heat I was losing. A new entry door was also recently done, and sealed up nicely.

The only question I have left at the moment is, where should I run the return air to? Access to the existing crawl space is limited. Can I loop the return through the attic and down into the room with the furnance and tap it into next to the filtered piece?

Again, thank you for my inexperience. I have read many posts, and the one I like the most said, "If you can plan it, you can build it." I already have my plans drawn up with window placement, door, flooring, and the new wall. All I have left is the heating and electrical to plot out. Different topic, but any advice on what I need to do for a electric inspector? Beside the rough in.

One more small question, will a ceiling fan help the heating issues at all?

 

I just finished the heat loss calc, but I used the trial version so it won't give me all the numbers. Is there a manual way to get the calculations? Or another program that would give me more information?

BTW, Currently the unit is 75,000 btu and 80% (when it was new, probably running at 60%, so they say), and is about 8 yrs old. The newer one I was estimated on was a 80% at 60,000 btu. Should I invest in some maintanance on my current unit or buy new?

Also, the duct that enters into the living room is about 24"x6". It also serves as the main duct that connects 2 bedrooms and the bathroom to the furnance. A total run of 16ft from end to end. The 2 bedroom vents are 10"x6", and bathroom is 10"x4". The kitchen vent is 10"x4" and is connected to what looks to be a 6" round duct, not flex. The cold air return is 32"x7", and I only have 1 located on the far wall/floor from the wall/ceiling grill in the house. The filter of the furnance is located in the hallway across from the bedrooms, and is 25"x16". If I am learning from what I am reading, that gives me 344 sq in of vent and 224 sq in of cold air return. Not correct, right? Would reducing the vent size in the living room be needed to balance things out?

I hope this additional info helps you guys help me out more. I know you guys say find out heat loss, but with my new windows, upcoming re-siding/insulation of the exterior, and I will be replacing the rear entry door, the places that lose heat currently will reduce by next snowfall. (when I say next snowfall, I mean the snow that falls in November or December, not April 15th!)

 

My roast is about done spinning on the bbq so all I can suggest right now is invest in the fifty bucks for the home version of HVAC-calc and get the complete picture!

You are going to spend a great deal of money and time on your reno and this is no time to be stingy.

 

There are many heatloss calculators available on the 'net. Do a Google search for heat loss calculation.

It appears that your present furnace IS vastly oversized if it runs only 25% of the time in the coldest weather. Ideally the furnace should run constantly during the coldest weather. If your present furnace is indeed an 80% model (does it have an induced draft fan?) then the only reason to replace it now would be to have two-stage heating and/or go to 90+% efficiency.

Yes, your return-air duct is too small. It needs to be no less than the area of the furnace inlet and depending on the length and number of turns it probably should be larger. Unfortunately, what you have is common. The fact that your R-A grille appears to have much more area is misleading as the louvers on that grille will significantly decrease the effective area of the return.

I am assuming that the filter is at the R-A grille and not at the furnace itself? You could run the new R-A from the new room directly to the furnace and use a filter grille in the new room. You would still have the problem of the R-A from the main part of the house being too small but the new room would at least be correct.

What you have is an all-too-typical installation. Builders (and the designers) all too often do not think of the mechanical systems when designing and building houses. The result is that oversized furnaces and undersized ductwork are placed where they will fit rather than where they belong. This will result in noisy and poorly operating systems. The builder doesn't care as he doesn't live in the house and he doesn't pay the ongoing costs of the poor installation.

 

The filter is not at the RA, but rather at the furnance. I'm guessing that the RA leads back into the furnance where the filter is? But then the filter is on the wall it itself, and nothing from the RA is being filtered. Is the filter opening acting as a RA, and the one in the living room a supplemental one? Then I should consider changing the grill cover to a filtered cover. If all things considered are true, and my furnance is oversized, all I need is the H/L calc to get the correct sizing of the duct work? Should I consider reducing the size of a portion of the ducting that leads into the living room? If I did that, would I be able to Y or T off the line and run the ducting to the new room?

Thanks again for the help, It's now near 6am EST and I am heading to work. I will be able to check in throughout the day to look for replies, but will have to wait until I get home to worry about the heat loss.

 

Just an update, after a quick calculation it says I have about 40K heat loss and 18.5K heat gain. I hope this helps!!

Thanks again, you guys have been great!!

Added: loss with addition is about 49.5K and gain is about 25.8k

Thanks

 

It's been more than twenty years since I did any duct sizing and back then I did it manually. It does require a bit more than just the total heat loss calc for the house.

Specifically, you need the heat loss for each room in which you are going to have a supply duct. You also need to know the output temperature of your furnace and the total air flow in CFM and all the other duct runs.

The bottom line is that you need to calculate the necessary airflow to a room to offset the heat loss and then you need to understand the siting requirements of the registers and therefore the required number of ducts for that space. To calculate the actual duct size you need to incorporate the flow rate through each duct, keeping in mind the velocity of the flowing air. Higher velocities will require smaller ducts for any given BTU flow but will also be noisier than will be larger ducts and lower velocities. Smaller ducts will require less insulation and will have lower heat losses than will larger ducts.

So you see it is an issue that has many compromises.

If you do a Google search for duct sizing programs there are a huge number of hits. Most of the available programs are not free but there are some companies that will take your measurements and do a calculation. You could also try your local library in the applied technology section and possibly find a book that details duct design.

It is a lot of work but if you do it all by the book you will be rewarded with a home that is comfortable and has a quiet system. Not too many of those being built these days.