Have an oversized furnace

turfgrass

I had a new oil furnace installed at the end of 2007. About a month into operation I realized there were signs of problems. I went back and forth with the company to figure out some issues. The furnace does perform as expected. I figured the replacement furnace was oversized for my duct work. Below is what I've come up with on my own.

The furnace is noisy, short cycling and unfomfortable high and low room temps. There was also some kind of backdraft that left an oil smell. This was corrected by leaving the basement door open to allow for more return air to the furnace. I even found a 12x14 cutout in the return trunk, (masked by floor joists) to allow for more return air. I have a feeling that the original furnace was even oversized, because of the hole I found in the return trunk. Now it's twice as bad.

My current furnace is a Hallmark 115d Highboy. The BTUHs are 140,000 input and 114,000 output. Below are my results using a HVAC Calc program I purchased. Interested in knowing what people think now that the heat loss/gain calcs are complete.

The first calculations for the house had a heat loss of 50,240-52,119 BTUH for the basement, first and second floors. The difference in BTUH range was, whether I zoned the rooms or not, the heat gain ranged from 14,069-14,926. I included the basement, because there were two registers cut into the supply trunk that maintain a basement temp. of 52 degrees. Otherwise, the individual heat losses are- basement 17,377, 1sr floor 21,189 and 2nd floor 11,674 BTUHs.

I also made calculations that only took the first floor and basement into consideration. These calculations were if the future HVAC pro wanted to consider using two furnaces. One furnace properly sized in the basement and a new one in the attic. The results were basement and 1st floor at 38,185 BTUH losss and the second floor only at 22,119 BTUH loss. A second reason for the two furnaces, was the fact that the duct work may need to be redesigned and one second floor bedroom is colder than the others. It even is situated on the southside.

Look forward to any comments. Thank you for reading.

hvac122

Looks way oversized. What size was the old furnace?

turfgrass

No clue as to what size furnace. I just trusted the company, because they were decent for the regular maintenance and oil delivery. I assume that the older furnace was even a bit to large, because of the hole I found in the return trunk.

Looking for ideas as to switch out the furnace or have some duct work done.

jogr

I don't understand two things:
1. why in your first heat loss calculation does the 2nd floor have less heat loss than the main floor? The main floor has only the exterior walls exposed vs the 2nd floor having presumably a similar wall exposure plus the full attic exposure where heat loss tends to be the greatest. Is this because the 2nd floor is gaining heat from the first floor?

2. Why would the heat loss per floor be different if you used two furnaces vs 1?


It certainly appears that downsizing the furnace would address your problems. I'd find a different HVAC pro, have him run a heat loss and check out the ductwork and suggest an appropriate fix.

turfgrass

jogr-

1. The difference between the first floor and the second is: larger windows, fireplace, 4 doors and larger sq. footage because the 1st floor includes a breeze way to the garage so, more heat loss.

2. Not sure if I'm going to answer your question correctly, but I calculated the second floor as a seperate project. This project would include an HVAC located in the attic. This has to be a major heat loss compared to the one basement furnace that maintains a basement temp of 52 degrees.

I set up the program for diffferent jobs whether I had one or two furnaces. This allow for a seperate zone for the 2nd floor bedrooms and curing the problem of the single cold bedroom.

With that being said, how cold would the bedroom get if I stay with one furnace that is downsized to better match the duct system? Not sure how this should be treated.

Should I consider keeping the larger furnace and have the exposed supply and return trunks enlargened or, go with two smaller furnaces? I don't know which way would have the greater electric bill. Two properly sized furnaces or one large one with altered duct work. I kinda see the cold bedroom never being corrected with the larger furnace route.

turfgrass

Just got off the phone with the people at Boyertown Furnace Company, (Hallmark). They mentioned that there looking for a temperature rise of 70-75 dregrees. If my furnace BTUH output is twice as high as the load calculations for heat loss, where would the temperture rise fall?
Would the undersized return ducts cause a higher temperature and cause the short cycling?

I assume either more returns or suppy ducts would have to be added to release heat and become more balanced.
Hallmark mentioned that the furnace could also be downfired depending what I needed for a temperature rise. The setting is usually on high for a/c purposses.