Location?....you have nat gas service available to your home?...if so, this is a no brainer....get it done now and start saving versus the more expensive fuel oil heat...

IMO

Location is MA. Both units already use gas for hot water and there are separate meters with spare lines that I had installed a while ago for this very eventuality. I'm looking at $15k for two gas furnaces vs maybe $9k-10k to put in one gas furnace on the tenant's side and replace my oil tank.

What do you pay for a gallon of oil, and what are your gas rates? How much do you spend on oil and maintenance each year?

You have to spend $2K for a new oil tank. so a gas furnace for your apartment is an additional $5K. If you provide the numbers, I then we can calculate that you can save $5K in heating costs and maintenance within 5-7 years. This assumes you will still be in the house.

The tenant's furnace is very old and wasting a lot of energy. It needs to be replaced. It makes no sense to install another oil furnace when you have a gas feed in the house.

The value of your house with two gas furnaces will be higher than the same house on your block with two oil furnaces. This is something to also consider as you make this investment.

I spend $225 a year on a service contract for my oil furnace. This pays for cleanings and small repairs and gets me a discount on parts and labor. I used around 400 gallons of #2 last year and it cost me $1k (average cost per gallon as $2.60)......so I'm not an enormous energy user. I'm also a low NG gas user. My last monthly bill was for $30 and I used 14 therms, but that includes a lot of delivery charges and adjustments so I'm not sure what will happen when my usage goes up.

Yes, the added value to the home is something I like about installing 2 gas furnaces. Also the convenience of not having to worry about oil delivery is nice as will be having 2 of the same system.

The gas furnace for the tenant is a no brainer....I'm not going to put in another oil furnace as the capital cost saving is small and I want to give my tenant the best heating possible and that includes the cost of fuel.

You need to study your bill and see what you pay for each therm of gas. You should not include the service charge. You paying that charge each month to supply the gas hot water heater, so don't include it as part of the gas costs for the furnace.

I am confident you can cut your fuel costs for heating in half by installing a 95%+ efficient gas furnace. Plus you won't need to pay for a $225 service contract for cleanings and small repairs. So that is about a $725 a year savings and a return on investment within 7 years.

Purchasing two furnaces at the same time may result in a small break on the price. If you are going to do this you need to move quickly before the first cold snap in order to get the best prices. Some cold weather is moving in. Hopefully you have another 2-3 weeks of mild weather.

I looked up some government statistics and last year heating a house in MA with oil cost 2.5 times the cost of using NG. So I think your numbers are pretty good and I can expect my annual heating cost to go from something like $1225 to $500 so that's a 7 year pay back. I will have more quotes next week and I'll update with those, but 2 NG systems seems like the thing to do.

I have one contractor that is recommending a 92% efficient Goodman furnace (Model # GMSS920804CNA). The stated reason is that even without the $600 rebate you get for a higher efficiency furnace the cost is still less and the 92% efficient systems are simpler, more reliable and will be less trouble to own and easier to maintain than a 96% efficient system. The cost is $7925.

Another quote is for a 96.5% efficient Carrier 59SP5A080E17 cost $9835.

Which way would you go?

Stay away from Goodman...go with minimum properly sized 2 stage var speed high eff nat gas furnace....Carrier/Bryant or Trane/AmStd...

IMO

How large is the house? What are the sizes of each apartment? Have improvements been made to the windows and insulation?

I doubt you need two 80K BTU furnaces. Try to find a contractor who will do load calculations for each furnace.

It is hard to comment on prices if they include the oil tank removals. I also would rather see you spend the money on something other than a Goodman furnace.

My unit is 1500 sqft, the rental unit is 750 sq ft. Both units have replacement double glazed windows and some circa 1990s blown in insulation. This is in Boston, MA

The quotes from both companies are for "soup to nuts" install and removal of oil tanks. Both contractors told me that the cost to remove each oil tank was about $1000, so take that off each one of the quotes to get the cost of just the furnace install. They are:

Goodman 92% efficient natural gas furnace install and oil tank removal

1500 sqft apartment 80k BTU = $7925

750 sqft apartment 60k BTU = $7725

Carrier 96.5% efficient natural gas furnace and oil tank removal

1500 sqft apartment 80k BTU = $9835

750 sqft apartment 60k BTU = $9620

Again, I would not have Goodman. And I question if you are being sized correctly. I would think a 60 K high eff for your side and a 40 K high eff for rental. Don't believe it? Make the sale contingent on a load calc for correct sizing. Your pricing is ridiculously high. You need more quotes as far as furnace pricing goes.

I am done here.

Good Luck!

IMO

Thanks for all the comments they have been very useful. I am getting more quotes, but a review of statistics on Angie's list shows that the quotes are not outrageous for Boston. I agree that at 40kBTU unit might be better for the first floor 750 sqft apartment, but 60 kBTU seems a bit small for my 1500 sq ft 2 level flat.

Your install costs (~$7000 on the Carriers) seem pretty high, I'd get a couple estimates despite the difficulty (knowing how hard it is to find competent and reliable HVAC in the Boston area). The benefit of going from a 95% furnace to 96.5% are fairly low while the cost can nearly double. Have a look at the Comfort 95 series from Carrier, a basic high efficiency furnace.

No Carrier Comfort series...look at the high efficient Performance series with high eff or var speed blowers...

IMO

Well the latest quote is in and it's the best so far. It's for two America Standard Freedom 95, two stage, variable speed furnaces. A 60k BTU system for the rental unit ( it gets a $600 rebate after installation) and a 100kBTU unit ($300 rebate) for the larger upstairs apartment. The quotes include everything; permits, thermostats, hard wired CO detectors, and removal of old oil tanks.

60k BTU cost $5500

100k BTU cost $5900

I will get $900 in rebates so the total cost for the install is $10500

I talked to the contractor as I believed that the units were oversized. The furnaces are purchased from the gas supplier and are the ones approved for rebates. He said a single stage 40k BTU unit will be fine for the rental apartment, but American Standard does not make a 40k BTU two stage unit and the 60k BTU unit will cost the same after the $600 rebate and it will be quieter, cheaper to run and will keep the tenants a bit more comfortable. I could get a 40k BTU Carrier Performance 96, there's a $300k rebate on that.

He says an 80k BTU unit would be ok for the upstairs two floor apartment as far as heating goes, but from experience he feels that the larger fan capacity of the 100 kBTU unit is needed to get the air up from the basement to the top floor.

What the contractor is saying makes no sense to me. The larger furnace will push more air through the duct work and will actually be more noisy that the smaller size. In addition it will be more expensive to run and more uncomfortable because it will short cycle and initially blast out more air on start up.

I have a 60K BTU furnace in my basement which services the second floor. There is no issue suppling air even at the lowest speed. The 80K BTU is problably over sized, so you know the 100K BTU is even worse.

"Location is MA. Both units already use gas for hot water and there are separate meters with spare lines that I had installed a while ago for this very eventuality. I'm looking at $15k for two gas furnaces vs maybe $9k-10k to put in one gas furnace on the tenant's side and replace my oil tank."

Do it ASAP; Seems a bit high for the furnaces compared to rural areas around here...

Bad sizing...a big mistake in the making...I never understand why homeowners come on this site wanting help and advice but have a hard time accepting it....issues will be upfront costs, noise, comfort, short cycling, and possible premature failure...live and learn...and why buy a two stage furnace and turn it into a single low stage model. Keep in mind this furnace requires a 2 stage thermostat. XT95 40 K single stage model for the btm unit.

IMO

Homeowners are hoping for a confirmation that their contractor isn't completely off the mark. When the forum advice differs from that of the contractor the homeowner is left with the problem of deciding which to go with. As the contractor has actually seen the job and hopefully is a professional with experience of the local housing there is a tendency to give that opinion more weight. But I will bring up the concerns about oversizing again and go over his heat loss calculations. For my apartment he comes up with 78kBTUs and for the first floor apartment 36kBTUs. The contractor is perfectly ok with installing the American Standard Silver 95 40kBTU single stage furnace rather than the 60k BTU two stage. But if the two stage furnace was installed would there be the same issues with short cycling and life time as you'd get by installing a single stage 60kBTU unit?

Your dealer is misleading you due to ignorance. Go ahead and take his advice and you will regret it...box filters to be installed?...2 stage thermostat to be installed for upstairs system...I am done here...good luck...OP will need it...

IMO

The contractor talked to me about needing 2 stage thermostats and they are included in his price. He has done the heat loss calculations and they agree with mine. I will probably go with the 95% efficient Am Std 40k BTU single stage unit that he offered to quote and will discuss the 80k BTU two stage unit for my apartment.

I spent this evening measuring my home and doing heat loss calculations. Part of the uncertainly I had were the R values for the walls and top floor ceiling, so I estimated them by measuring the temperature of an interior wall, an exterior wall/ceiling and the outside temperature.....it's 41F outside tonight. I get R values that range from 7 to 14, which seem consistent with wood framed walls filled with cellulose that has fallen down over the years. For the rental apartment my new heat loss number is 43kBTU for a 5F low design temp and for my apartment it is 79k BTU.