Home>Discussions>INSULATION & HVAC>Replacing Old Furnace/hot Water
31 posts / 0 new
Last post
Replacing Old Furnace/hot Water

Hi!We have an original oil furnace/hot water system in our 65+ year old house.The hot water runs off the furnace as well as the hot water radiators. Given the crazy oil prices, we are definitely going to switch to a gas hot water tank, but are unsure about the heating system. We cannot switch to forced air because of the age of our house and we are very comfortable with our hot water radiators. So should we go with a gas heater for the current hot water heating system or go for a new efficient oil system? I've heard that oil is far superior at heating hot water than gas and until the recent oil increases,our total hot water and heating costs were ok. Thanks, Fay P.S. We do have a three story addition in the plans for the next year or so that would include a new bathroom and kitchen.

Re: Replacing Old Furnace/hot Water


Your system is known as an oil-fired boiler, not a furnace.

Given that, as you say in your post, it is that old, it was probably originally a coal-burner converted to oil.

Something that old is probably only 50% efficient--the newer units are 83% to 95% efficient, so you should see a 30% to 40% drop in fuel usage just by getting a new boiler in there.

Yes, I would recommend you choose between a new more efficient oil-fired boiler, or a new more efficient gas-fired boiler.

DO NOT get a separate gas heater for the domestic hot water---it's much more efficient to get an INDIRECT HOT WATER HEATER---this is a 40 gallon indirect tank that uses the hot boiler water to heat the domestic hot water---Google Triangle Tube Phase 3 or Crown Megastor to view examples of these indirect heaters.

Don't ignore installing any needed insulation in the exterior walls and attic---this should be done now, before you buy the new boiler.

Choosing between an oil-fired or gas-fired boiler would depend on doing a FUEL COST COMPARISON (below), which compares 100,000 btu of oil heat with 100,000 btu of gas heat---this will give you a comparable cost of what fuel is least expensive in your area---you'll have to call the local gas co. to find out what they charge for 100,000 btu of gas, or a therm, or 1000 cubic feet.

After any insulation is blown into the walls, the boiler installer should do a Manual J HEAT LOSS CALCULATION to determine what proper size boiler should be installed.

Some rudimentary free HLC sites below can help you calculate this; a very rough estimate of a building's heat loss can be done by multiplying the total square footage of the building by 40 btu/sq.ft.

Thus a 2000 sq.ft.building X 40 = 80,000 btu/hr--this is roughly the amount of heat needed to heat the building on a cold day, and the size the boiler has to be---naturally, the amount & condition of windows you have, your location & many other factors will determine your actual HLC.

It's tricky to assume you will "add an addition later" & buy a larger boiler than you need now---this will lead to more fuel being burned until the addition is actually built---but you can calculate a HLC if you know the dimensions, & you have FIRM plans to do the addition.

Some previous posts and links (below) will indicate the boilers recommended for you to consider.

Get at least 6 estimates from (Yellow Pages) heating contractors, & fuel oil dealers---fuel oil dealers are also licensed to install gas units.


Re: Replacing Old Furnace/hot Water


Thanks for your reply. We will do a comparison of gas to oil to decide which one to go with.I'm just wondering why an indirect water heater would be more efficient. I live in New Jersey where we don't need heat for at least five months out of the year and we do have a large family ,so we need to be able to run consecutive and/or simultaneous shower/dishwashers/washing machines. Would that change anything? Fay

Re: Replacing Old Furnace/hot Water


The indirect water heater is the way to go---the standard size for a family is 40 gallons, but they come in sizes much higher if needed (up to 120 gal).

I've been installing them for years up here in New England.

A separate gas-fired, oil-fired, or electric water heater is NEVER used in conjunction with a HW boiler by anyone knowledgeable with hot water boiler heating----all of these fuel-fired units either have high flue losses or high operational costs (elec) that make them less efficient than an IHWH.

Even though it would seem that you would burn "extra oil" or "extra gas" during the non-heating months, it actually takes very little fuel to keep 40 gal. of water hot in these heavily insulated tanks---not only that, but they never wear out---since there is no burner (gas or oil) inside them, they last for decades---sitting there quietly beside the boiler---so get a good one--most of the ones recommended have a stainless steel inner tank that is more efficient in heating the water & keeping it hot for many hours.

Whenever there is a call for hot water--the piping that connects the boiler to the IHWH sends a signal to circulate the boiler HW---since no other fuel is burned in the warm months, it takes very little fuel to keep the water hot in the IHWH.

Sizing your IHWH would depend on "peak usage", which means which part of the day is the most HW used---it's usually the morning rush hour when everyone is going to work & school---a site below discusses peak usage.

Also Google "hot water peak usage" (without the quotation marks).

Recommended IHWH's would be Triangle Tube Phase 3, Weil McLain, Peerless, HTP Superstor, Buderus, TFI Everhot, Amtrol Boilermate, Viessmann.

These units typically cost ~$700.


Re: Replacing Old Furnace/hot Water

We're "novice" and considering replacing the original gas boiler (now that I know the difference between a furnace and a boiler, thanx to postings) in our 1975 Cape, and the water heater (its 1991). My guess is that this is not an indirect water heater since we shut off the boiler in April, but that doesn't shut off the hot water. We had an energy audit last August and thought we'd see some savings after all the chalking and insulating, but the gas company guy suggested a new unit, especially if we were planning to remain in this house.

I've asked around @ work, and heard the term, "triple pass". Have also been told that boilers don't have has high an efficiency rating as furnaces - but we won't change from baseboard to forced air). We were thinking about a tankless water heater, but were told they don't provide as much as a tank. We've also been told that since 1/2 our basement is a finished rec room that we need a special fan/vent installed now to bring in fresh air.

We've heard of estimates from $7,000 to $15,000 for gas boiler and water heater. Wondered too what the resale value is - can one expect to recoup 100% if selling in a couple of years?

Last summer a friend of mine replaced their 40+ year old oil boiler. The company installed a new Crown boiler for $4,500. Granted prices have increased in the past year, but is there that much of a difference between gas and oil units?

Re: Replacing Old Furnace/hot Water


In the first place, it's a big mistake to limit yourself to getting a single quote for a new system from the gas co.

They often quote a very high price---also get estimates from (Yellow Pages) "Heating Contractors", and "Oil Dealers", "Oils-Fuel"---oil dealers are licensed to install gas-fired as well as oil-fired units & often charge much less for the job.

If your current boiler is that old (over 30 years), it's a good candidate for replacement---most old boilers are very inefficient in burning fuel & often 1/2 the heat produced is wasted & goes right up the chimney---such a boiler is said to be 50% AFUE efficient---as compared to 85% AFUE for even the least expensive $1500 boiler (Slant/fin) sold by Home Depot or Lowe's.

Boiler design & efficiency in the past 30 years has been greatly improved & are much more efficient in fuel usage.

It is absolutely untrue that forced air furnaces are more efficient than boilers---since boilers concentrate the heat energy in hot water distributed thru baseboard, tubing & radiators it is the boilers which are more efficient.

Using the Slant/fin as a base price =$1500 + $700 for Amtrol indirect hot water heater = $2200 + $1000 installation costs & removal of old boiler = $3200 as low-ball price for a new system.

The Slant/fin Liberty is a decent pin-type cast iron, wet base boiler that is of tried & true design that is still widely used in the heating industry.

A 2nd step up from cast iron-pin type would be a cast iron triple-pass boiler (88% AFUE efficiency) that has a more sophisticated combustion chamber that circulates the hot gases 3 times to absorb more heat from the flame before it is exhausted up the chimney--these are excellent boilers that may add $2k onto the price of the new install--$3200 + 2000 = $5200.

The 3rd step up would be for the latest technology of variable output, condensing gas-fired or oil-fired boilers which are the most efficient in fuel usage (95%-97% AFUE)---these usually have stainless steel extended combustion chambers that burn nearly all the fuel & produce very little exhaust waste--the drawback is that they are much more expensive & tend to have more maintenance/adjustment needs & may not last as long as a cast iron unit, which will last for decades---typical quote: $3200 + $5000 = $8200-$15000 (depending on the brand installed--German imports like Buderus & Viessmann tend to be the most costly).

If you expect to sell the house in a year or two, it makes little sense to
buy a top of the line condensing boiler---you won't recoup the costs from the oil saved over the existing unit.

Get rid of the separate HWH--it is much more efficient to use an indirect HWH, which is piped as a zone off the boiler.

Since IHWH have no burners of their own, they last for decades & are well-insulated--they are run thru the summer with a boiler wired for cold start---you will burn very little gas in the summer months.

If the boiler room is judged as a confined space, the installer may want to install a fan-driven draft-inducer unit to give the boiler enough air to burn the fuel properly (Tjernlund).

Get at least 6 estimates from diferent heating contractors & oil dealers--the price quotes & choice of equipment will vary considerably---the installer chosen should be enthusiastic, & knowledgeable of the terms & brand names used in these posts.

Gas-fired units would be favored over oil-fired units in most areas due to the world-wide pressure on crude prices---but natural gas prices are also expected to rise---do a FUEL COST COMPARISON (links below) & call your local gas & fuel oil dealers to get the price of a gallon of fuel oil & a therm of natural gas; the FCC will tell you which fuel is least expensive in your area.

There are previous posts below that list some of the recommended boilers and indirect water heaters---Crown makes excellent units, also Triangle Tube, Peerless, Utica, Burnham, Buderus, Viessmann, Munchkin, many others.

You should do your own HEAT LOSS CALCULATION (links) & have the installers do a HLC to determine the proper size boiler to install.

Glad to read you have put in a lot of insulation; if you need any additional insulation in your exterior walls (should be R19) or attic (should be R40), NOW is the time to do it before you have the new unit installed---the more insulation you put in, the less fuel you'll burn.

You should see a dramatic drop in fuel usage of 30% to 50% if you decide on a new install.


Re: Replacing Old Furnace/hot Water

>A 2nd step up from cast iron-pin type would be a cast iron triple-pass boiler (88% AFUE efficiency) that has a more sophisticated combustion chamber that circulates the hot gases 3 times to absorb more heat from the flame before it is exhausted up the chimney--these are excellent boilers that may add $2k onto the price of the new install--$3200 + 2000 = $5200.

Thanks for the suggestions and recap.
Your above option is much more in our $$ ballpark. But that price didn't include a new water heater, right? That's just a boiler and installation, right?

I made some calls during lunch hour and granted until they can get inside "to see everything", some have even suggested that the current "old" boiler was really too big for the 1,800 sq ft Cape - many knew the builder and what equipment was installed in this development. The fact that we have a 20 x 18 family room, addition on a slab, that never warms up (but has 3 walls w/radiators), leads me to believe there is something more wrong with current heating setup, than it just being an old unit. One mentioned that pipes weren't "bled out properly"???

One company recommended replacing old wiring, piping, fittings and replacing thermostats (there are 3 zones). Others said only the boiler needed to be replaced. Period. Nothing more.

Clearly researching "this" could become a part-time job, with so much to consider. Thankfully the old boiler hasn't "died" and the water heater hasn't sprung a leak.

Re: Replacing Old Furnace/hot Water

The quote in all cases was for the boiler AND the indirect hot water heater, and installation.

Yes, a first step no doubt should be to have a service tech come over to make sure the system has no minor problems.

Boiler systems are periodically prone to getting some air in the heating pipes and convectors--this would lower the heat output of the system.

There are usually BLEED VALVES on the baseboard & radiators that can be opened with a screwdriver to release the air.

The temperature gauge on the boiler should read ~180 degrees when the system is heating the house.

You can try touching the radiator and baseboard copper piping when the system is heating---it should be hot enough in all rooms so you can't hold your hand on the piping for more than a second.

If you multiply the total square footage of a room or the entire house by a heat factor of from 40 to 60, you should get the total heat output requirement.

Thus an 1800 sq.ft. house = 1800 X 40 = 72,000 btu/hr to heat the house.

A 20 X 18 room = 360 sq.ft. X 40 = 14,400 btu/hr to heat the room.

Click onto my name to get previous posts on how to calculate the output (EDR) of radiators and baseboard to see if there is enough baseboard/radiators in the room/house.

Baseboard outputs 580 btu/hr per foot of baseboard.

A big problem with slab extensions is the radiational cooling from winter frost---which makes them very hard to heat (assuming the rads are putting out adequate heat).

Thus 20 X 18 = 360 sq.ft. X 60 = 21,600 btu/hr (or more) possible heat need for family room.

Re: Replacing Old Furnace/hot Water

>The quote in all cases was for the boiler AND the indirect hot water heater, and installation.

A neighbor, who's a serviceman for local oil company, just stopped
by and quoted me $9800 for Buderus triple pass boiler ($7200) and Buderus ST 150 indirect water heater ($2200), plus will to re-do the "spider's nest" of piping for current system, along with installing 3 new thermostats. I know Buderus is on your list of reputable companies.

He would do for less if we went with single pass boiler. Includes 5 year warranty on parts and labor. 1 year "free" maintenance.

Guess I'll use that as a starting point.

Re: Replacing Old Furnace/hot Water

Was just pointed into the directions of a Munchkin, condensing
gas boiler. I went online to http://www.munchkinboiler.net/
and it would appear to be a more efficient boiler (rated in 90s)
than some of the others mentioned. Heard one doens't need to
vent through chimney stack. Sounds too good to be true.

Re: Replacing Old Furnace/hot Water

Can anyone shed light on the System 2000 oil heating system? We are replacing a 20 yr old Weil McLain that we are told is still operating at about 86% efficiency, but is rusting out badly. Salesman was here last night really hard-selling the System 2000, guaranteeing a savings of at least 30 percent less oil used. Price to replace old unit, including hot water tank, was $12500!!! Other estimates were for Weil McLain WGO with Beckett burner: $5635, or Buderus Logano G115 w/Riello burner: $5895, both without a new hot water tank, but does include 3 new circulator pumps. Buderus does NOT include the Logamatic Control System. If we choose Boderus, is the control system a MUST HAVE? Is the estimate for the System 2000 reasonable? House is super-insulated, and we use about 700 gallons of oil per year for heat and hot water. Any comments, suggestions or alternatives would be appreciated.


Sponsored Stories

TV Listings

Find TV listings for This Old House and Ask This Old House in your area.