Replacing oil heater - sanity check

mass_biker

We're deep in renovation of our 100+ year old house in MD. As part of the reno, we've gutted the kitchen/back of the house, blown in new insulation, added batting, and are going to blow in insulation in the attic (which was woefully underinsulated). We are retaining as much of the radiant (hot water) heat as possible. In some of the newly redone/rezoned areas, we have added in forced hot air as a way to run that zone separately.



While the work is ongoing, I've asked the master plumber to look into the possibility of replacing the existing oil burner.



The old one is apparently ~ 80% efficiency. Unclear how much longer it has legs. It was sized to heat a big home (6000SF). The suggestion is to replace this with the following:



Buderus GB 162/80 @ 94% efficiency nat gas condensing boiler

Buderus AM 10 boiler header and outdoor reset package

Little Giant condensate pump



And all relevant flue/pipe/valve stuff.



The breakeven calculation goes like this:



Assume home uses 1000 gal. of fuel oil during season @ $3.20/gallon and 80% efficiency it will cost $3,200 to heat the home. 1000 gal of fuel oil has BTU content of 140mil BTUs so at 80% efficiency the oil boiler provides 112mil BTUs to the house.



Solving for the gas equivalent gets me to around 120m BTUs needed - or 1200 therms. Nat gas at $1/therm means that it will cost (cost of commodity) $1200 to heat the house.



Not including transmission/delivery of gas, the price differential is $3200-$1200 = $2000; assuming 1000 gallons. But you need to factor in all the extra delivery/transmission surcharges.



I will be contacting oil company to see what the actual billings last year were.

What else should I be looking at to see if this all makes sense.

It seems that most homes in my area have done the oil->gas conversion.

Is it warranted just to replace the old oil with a new oil furnace?

JeffM

conversion to gas

I think you're on the right track, but would advise doing a more conservative run of your calculations as well. You're working with today's fuel prices, but what if they change a year or two down the road? I'm currently thinking about converting from oil to gas myself in a house with roughly similar annual usage. At this year's costs it's an easy decision, but since the payback period is a few years, I also ran the math with natural gas costs 80% higher than they are now just to be safe. Gas is really cheap now, but in a five year window I expect that its cost will rise some. I have gas in my home already (stove), so I was able to factor in the transmission/delivery charges from my existing bill - maybe you've got a friend locally who can give you a peek at their bill for that?

I used a calculator that I found on-line to make things simpler to play with the costs and compare:

http://downloads.cas.psu.edu/energy/ENERGYCOSTS_09.XLS

LarryC

What is your heatloss?

Do you realize that your existing furnace was probably much too large to start with?  Typically, home furnances are twice as large as what the structure needs.  Now that you are adding insulation, you may find that the existing furnance may be way oversized.   



Find a HEATING PROFESSIONAL that will perform the heatloss calculations and determine an appropriate size furnance.  Don't be surprised if the replacement furnance is a third of the size of the existing system.



The heatloss calculations will need to measure each room, the type and amount of insulation in the walls and ceilings, window sizes, locations, and construction.  A proper heatloss calculation is not performed by reading the nameplate on the existing furnace and replacing it with the same size or next size larger.  It is also not done by walking the perimeter and multiply by some sort of BTU/square foot value.



Nothing against the plumber, but not all plumbers are heating pros just like not all heating pros are plumbers.  They both work with pipes and their fields overlap.  If your plumber does not design, install, and maintain heating systems as a major part of his business, I am suggesting to find a heating contractor for this part of your renovation.



I would recommend that you click on the find a contractor link at the top of the page to find a heating pro near you.



Good Luck.

Zman

80 % is not 80%

I like where you are heading and the quick look at your approach looks good.

We are consistently seeing 25%-50% savings with modulating boilers.

The reality is that your existing boiler is likely yielding 50%-70% efficiency. It is probably single stage, so it cycles often and significantly oversized (that's the way they used to design them). You also will never get 94% out of the mod/con. I would estimate 85%-90% depending on you return water temp.

Jean-David Beyer

You also will never get 94% out of the mod/con.

I am not a heating contractor, but I do have a mod-con with outdoor reset.



It seems to me as though for some installations, you could get 93% or so from a mod con, if you measure it in the usual way (in particular, not counting the electricity required to run the circulators, etc.). It would all come down to the return water temperature the boiler experiences. I never figured out by calculating my gas bills and my heat loss just what the efficiency might be.



I do know my return water temperatures, and for my largest heating zone (radiant slab at grade), the return temperature can go from a little below 75F up to about 112F and a chart in John Siegenthaler's big book suggests that that runs at around 97% down to 93%. My upstairs baseboard zone runs with return temperatures from about 110F up to about 130F for efficiencies from 93% down to about 87%. To get those temperatures, I had to set the reset curves on the boiler vary carefully; using the default values would never get these results.

Zman

Check This Out

This does a nice job of explaining.

www.pugetsoundashrae.org/PDF.../AshraeCondensingtechnology.ppt

BostonGTR

Link?

Is the above link working? 

mass_biker

Thanks all

Great replies - thank you. Called heating oil co. and I just about fell over. Usage in this house in 2011(Jan -> May) was 2019 gallons. The nice person on the other side of the phone said that they were coming out every ten or so days. That works out to over $6k in heating oil costs. Even if I assume the old boiler is efficient at 80% that means that a 94% efficient nat gas boiler would require about 2400 therms. At today's low nat gas prices the price differential year one is over $4k. Payback on the $13.5K conversion would be around 3 years. I will re-run at higher nat gas prices too, but this seems pretty obvious to me. I will see what the deal is on boiler sizing. Assuming the old one is not at 80%, I think the new one could be smaller and therm usage could be more reasonable (although we are putting in a new gas stove so maybe that's a push).

Steamhead

I bet you find

the oil boiler is way out of tune and has a lot of deposits in it.



Can you post some pics?

Jean-David Beyer

Is the above link working?

Not for me.

Zman

Sorry

Try this

bill_105

The sanity check

First of all, You're getting the best advice.

The sanity check deal troubles me.

What happens if something goes wrong?

What will you do? Plead insanity?

Now, with exposeing yourself to HH.com you've lost a barganing chip!

You are at the best spot!

Jean-David Beyer

Check This Out -- This does a nice job of explaining.

The final link, the one that works, is a really nice  set of view-graphs.



While they differ slightly from the conditions in my system, it gives a really nice overview of the differences between conventional and mod-con boilers and the systems they run in.