I'm considering making the jump from my circa 1997 oil fired furnace (two of them, actually) to either an ASHP or a GSHP. As part of this, I've been doing ROI calculations (with much help from Dana1!), and I'm realizing that a lot depends on what my real efficiency is with my current oil furnace.

The oil company does yearly efficiency checks and writes them down on a little card hanging off the furnaces. Looking at the nearly 20 year history, the furnace hasn't varied much... 81% to 83% efficient. Not bad, I thought. But then I realized that the oil company has a conflicting interest here. They make more money the more oil I consume. So at the very least, they have no real incentive to accurately measure the efficiency of my furnaces.

So is it a realistic scenario to see an oil fired furnace - assuming it's well maintained - hold on to nearly it's day-1 efficiency ratings for nearly 2 decades? I'm getting a home energy audit from an independent company, so that will probably answer my question for certain, but I'm curious if anybody has experience with the reliability fo AFUE measurements from companies with conflicting incentives. :)

80-ish percent sounds about right for that vintage oil furnace. 83% is a 0.83 COP and you will see a 3-ish or higher COP with ASHP or GSHP. So you will see a very significant improvement in operational performance and associated operational cost. You can use this calculator to compare heat sources and estimate what your operational cost will be:

Borst Integrated Heating System Performance Calculator

The raw combustion efficiency of a hot air furnace isn't the same thing as the system efficiency. Where duct imbalances occur, excessive air-handler driven outdoor air infiltration can be happening any time the air handler is running. Duct imbalances can occur from both duct design faults (such as inadequate provision for return air paths) as well as implementation faults such as leaks in the ducts, duct boots, air handlers, etc. It's pretty rare to find ducted air systems that aren't sending more than 10% of the air somewhere unintended, and it's unfortunately common to find systems where the leakage is 25% or more. And that's on top of any air-handler driven air infiltration losses.

The combustion efficiency also doesn't address the (fairly substantial) electrical power used by air handlers in ducted air systems.

"... combustion efficiency also doesn't address the (fairly substantial) electrical power used by air handlers in ducted air systems." per Dana1

and one may want to consider trying to find the 'x' of @$ spent now heating a year with oil, noting years can vary by well over 15% .
then hours of operation at say 30-40 degrees, where the oil may average (too, all other items, per Dana1) just as low as (even in the 1990's) under 50% efficiency about using- and how much per hour, ? (that nozzle size and gallons posted would be easier for others to share real efficiencies 'on the dollar')

then too 20 and 10 and 0 and -5, etc.

Maybe jot down a graph on the efficiency being rather low at 40 degrees and up to only (barely) 75% if the oil runs say over 40-miinutes per hour in really colder days...

Oil 'COP' to calculations of 100% electrical (COP=1.0) at the wallet seems may want to be estimated, if not already done.

? what is the dollar to dollar if you have to pay for just 1.0 COP 100% electricity? all your bill fo a KWH rate+charges...
per hour, and then per hour of oil at dollars per gallon then 75% being 1.33 times the cost per gallon, on your billing, to 50% oil efficiencies on the dollar being 2x's the costs per gallon per hour that you have used

It would be interesting to see data on the savings gained from checking and correcting room-to-room pressure imbalances in a typical ducted house. Not to mention the moisture damage it can cause.

I'm having my ducts sealed this week. (Aeroseal.) I've got some pretty good data for the previous 12 months, and at this point, I doubt I'll be doing any improvements/changes over the next 12. So I should be able to provide some insight into the overall effects of duct leakage on energy costs. The Aeroseal folks also do full duct blasting tests before and after so we can see what the CFM leakage was before/after and use that to estimate the cost of leakage on a per-CFM basis.

Ducts are now Aerosealed! Took almost two days, as I have a large house with LOTS of registers. Total pre-sealing CFM lost across all the ducts was a whopping 495.1 CFM of leakage! Over 300 of that was in my attic, which has soffit vents. After sealing, I'm down to 83.6 CFM. That's a 83% reduction. Solid. Time will tell how quickly the $3300 it cost will pay for itself.

You are golden! You either had to fix your crappy ducts or move to different heating system that didn't use them. All way more financially sensible than going to GSHP, eh? Now you can use the Borst calculator to determine your house insulation performance.

http://www.borstengineeringconstruction.com/Existing_Building_Energy_Usage_Analysis_Calculator.html

Well, this is pre-sealing, but I get:

2370 BUT/Hour-DegF for total heat gain/loss and 118492 BTU/Hour.

That's pretty close to my calculations in my big spreadsheet, where I estimated 110k BTU/Hour

That's a great find and fix. You will likely find that you are about 400 Btu/h/deg F now that your duct work has been repaired and improved.

Posted By sailawayrb on 25 Sep 2017 09:32 PM
That's a great find and fix. You will likely find that you are about 400 Btu/h/deg F now that your duct work has been repaired and improved.

I assume you mean my usage will drop by 400 btu/h/deg f, right? Not that it will go from what it is now down to 400?

No, 400 Btu/h/deg F would be for a more typical 2200 sf house...like my little house! Going back to one of your posts in the Geothermal Heat Pumps section, I see your house square footage is about 4500 sf...or a little more than twice 2200 sf. So, in round numbers, I would expect that you should be able to achieve about 820 Btu/h/deg F...or 60,000 Btu/h...or 12 Btu/h/sf. To get more accurate building envelope performance numbers you would need to collect your fuel usage and HDD data for some period of time and use something like our software:

Borst Existing Building Energy Usage Analysis Software

And don’t forget, now that you have your ducts performing way better, there are still more operational cost savings that can be achieved by transitioning from your current 0.83 COP heat source to say mini splits having a 3.0+ COP. This would likely be an operational cost reduction of 1/3 or more from what you would pay from using your oil furnace with fixed ducts. So you have come a long way from where you started by fixing the building envelope parasitic heat losses (i.e., the ducts) and you can still go a long way more by improving the efficiency of the heat source if you so desire.