Furnace - Question on where to start with replacement

[RaeSzafir]

Where do I begin? I have not been around for a while, I would say probably at least three years since my son was born. I was out of work for 2 1/2 of those years home with him. As a result, projects around the house were pretty much at a stand still. Anyway, its been fun lurking and catching up on everyones stories and pictures.

I am starting to think about replacing our heating system. Right now we have a furnace that is no doubt older than my 37 years. We have both natural gas and oil on site and we have duct work due to our current oil burner forced hot air system. I would like to install a heating/cooling system but honestly have no idea where to start, what questions to ask, what to look for etc. This replacement probably won't be feasible until next summer but would love to start researching and fact finding now.

Not to mention I would like to have a backup plan incase the green monster that resides in my basement decides to kick on the coldest day of this winter (which we all know is when it will die if it is going to). Anyone out there been through the process that can offer some pointers. It would be greatly appreciated.

[triguy128]

Sizing, sizing, sizing. Then quality of installation. What brand you choose is mostly irrelevant if it's installed correctly.

You want the smallest furnace that will heat your home at "Design temperature" This isn't the coldest it will ever get. IT's a statistical temperature for that location. For example in SE Iowa, it's -5F. IT usually gets colder than that most years, but only for a relatively short amount of time. The other 99.9% of the heating season, the furnace is essentially oversized.

A properly sized system is running continuously at design temperature.

For comfort, you want a 2 stage furnace or modulating furnace. For efficiency you want a 95% or higher efficiency furnace, unless you live further south and a variable speed fan that uses less electricity.

Oversized furnaces need larger ductwork and short cycle, which makes temperatures in a house uneven and the temperature swing widely.

You want natural gas over oil... it's a lot cheaper to operate.

TO properly size a furnace, the installer needs to do a load calculation and look at you ductwork to make sure it's large enough. Older less efficient furnaces ran at higher temperatures and had lower airflow than newer furnaces.

A good website for HVAC information is "HVAC-talk.com". GO to the residential section. Look at the threads about selecting a good contractor and so forth.

[RaeSzafir]

Triguy,

Thank you, have you have given me some great info and more importantly a great starting point for research. I apprecate the info and will check out the site you provided.

[Danno]

after having been through it, I think the hardest part of figuring out what size to get was not knowing how drafty our home can be. We got our new AC and Furnace in the summer when we FIRST moved in. that was a bad idea, because the numbers all looked great for the size of the furnace we got, but if there is any breeze at all then that throws the whole system out of whack. even with plastic on everything. I'm working on it, but we are talking about being uncomfortable at times for the past 6 years!

well, if I am ever able to completely seal up the house then I suppose it will be "just barely" big enough to keep the place at 72 all winter long. I offset most of the problems now by burning wood in my wood stove. I love doing that, anyway, but the more I seal up the less gas I use, it would be nice to heat most of the place with wood.

So bottom line is to pay close attention to what you know to be true for your house and it's draftiness, not just the load calculations you can get by entering in square feet or cubic feet, etc, or even calculations that look at insulation.

Best case scenario is to be able to seal up your house to the point you are happy with before buying the new furnace. don't know if any of this applies to you, but it sure did to me.

[triguy128]

Look at it this way. A bigger furnace would simply mean bigger bills. 72F? I don't want to know what my bill will be if we try and keep thsi new home that warm I'm shooting for 68-70. But we'll see what happens when th humidty drops. Until i get all teh strom windows installed, my humiity levels will be limited... sicne the temperature of the single pane glass will determine hte maximum indoor depoint I can maintain.

You are correc,t Knowing your rate of infiltration is crucaial for a proper load calculation. You can try ot guestimate it. But only a "blwoer door test" done as part of an energy audit will give you an actual value. In some cases, air leaks can be responsible for most of the heating load... not conduction through walls and windows.

Start with air leaks in ceiling penetrations, then upstairs windows and finally downstairs windows and make sure you chimney damper is sealed good. Believe it or not, natural draft fireplaces can actually create enough draft that you'll cool off your home. that's why dampers are adjustable. Poeple tended ot use hte minimum damper setting back in the day, but that's not terribly safe and you get some of soot and smoke in the house.


Mostly I absolluately shocked that a clocal contractor didn't oversize you by at least 20-30%. I haven't found one yet that offered ot do a real load calculation. You may even be undersized because they used square footage. Old homes vary widely in infiltration.


IN my Home Borders install massively oversized furnaces in 2004. I have a 100k BTU downstairs and 80k BTU upstairs. there's ductwork that's sized barely adequately on both units. I'm added 2 supply registers just so the upstairs unit doesn't sound liek a jet engie is firing up (well it's not that bad, but it was pretty noisy). Also, no consideration for room size in how many and how large the registers were. They even put 2 supply registers in the attached garage ! Hello CO poisoning and gasoline fumes anyone? I fixed that first thing. The PO's sadly were pretty ignorrant on HVAC.

In reality, I need a 80k BTU unit downstairs and a 40k BTU unit upstairs. A little smaller donwstairs if I didn't bother to heat the basement.


Honestly I'm not sure why they didn't just replace the boiler and install a single high velocity system for AC A single chase could have been run upstairs easily to the attic.... unless the boiler piping had a lot of leaks.

[jharkin]

I know a relative bit about boiler systems... but not much about furnaces.

But one piece of advice I could suggest - if you are going to do this replacement anyway, try and do whatever weatherization (insulation/sealing/etc) upgrades possible before the install. It will only help in the long run and allow them to size the new hvac system even smaller.


Since your in MA... Look up mass save (http://www.masssave.com/) . You can get an energy evaluation and a lot of the work done though them with big utility rebates... AND they will give you info on the rebates the utilities offer for gas furnace installs. AND if go through the program you can get zero percent financing for the out of pocket costs of the insulation work and the new furnace.