Furnace or boiler? Depending on what model, 20 years may be nothing. Replacement with something more efficient depends on a lot of things. How long will you have the house? Assuming boiler/hydro-air, Is there excess capacity in the hydro-air so lower water temperatures can be used for a significant time? How large are the hydro-air units compared to the low end of a new modulating-condensing boiler? If you really want a condensing boiler, propane may price out better because they can be made more efficient, condense more.

Combination boilers (instant hot water + heating) tend to be larger and it is difficult to match them to smaller heating loads. (The zone is relevant here more than the whole house.) Compared to oil and propane, electric water heaters may cost you less in the long run. That would solve your leaking indirect tank problem cheaply too. If your boiler is not a modern, cold-start boiler, it will save you a lot in the summer too since you can shut the boiler off.

If I were in your shoes, I think that I'd hire a local energy rater to help you pick the low-hanging fruit with respect to saving your coins. You might be best off switching to electric hot water, and keeping the boiler for now while making improvements in the house envelope. In the intervening time, you can think about what to do about heating and hot water. Heat pumps, air sourced, are working in colder and colder weather all the time. Ground-sourced have been there for some time, but are a costly investment. You might be able to keep your boiler and use it for back-up for the next 30 years in that role.

Someone has to look at some numbers based on your fuel costs to make any rational decision.

Hydroair is usually a good system as you can zone areas of heating and control each AH separately. Your first decision is to look at oil vs propane. Right now oil is cheap. History tells me that over time I never expect it to stay that way. Russia get stupider, Isreal attacks Iran, etc. The way to make LP competitive is to buy an install a 1000 gallon tank. With sufficient storage you are not having to do all the mid-season fills. Hopefully the 1000 gallon carries you over the worst of the season.

To equal the hot water output of a 199,000 btu gas fired tankless you would need a 120 amp service to the tankless. Just not practical for a whole house. A part of any decision is to have your duct work pressure tested. Generally, according to the DOE, the average residential installation will loose between 18-42% of its energy in duct losses.

Where are you in CT? How big is your house?

Thanks guys. I'm not sure how I feel about propane though, just seems less stable to have in the basement. Right now we have a 250gallon tank that is behind the garage that isn't for the range in kitchen.

House is 3500sq ft. I'm in Greenwich. Any way it's worth doing even two electric tankless? Also right now we do not have a generator and there is antifreeze in the hydro air system. This has always bothered me, ami being ridiculous?

The reason to have antifreeze in the hydro-air might be in case the heat fails so it won't freeze. Either that or its pipes run in unheated spaces and might freeze. The trouble with antifreeze in there is that it needs to be checked and serviced because the glycol breaks down producing an acidic product. That will happen more slowly than in your car radiator, but it still needs to be checked.

In order to run two electric tankless heaters, you'd need a larger electric service than is generally installed in a single-family home. Jackfre has indicated that you probably won't even want to mess with one due to the power demands so why are you thinking of two? If you already have a "standard" 200 amp service to anywhere near its capacity (air conditioning) , you'll need to go to a size that very few homes have. That will require a new drop, new meter, new service panel costing thousands. Do you want to open that can of worms? For two tankless, the POCO might want to bring in three-phase.

Propane is heavy so it accumulates at the lowest point in a structure. Nat gas rises so it is less of a hazard, IMO. Modern gas detectors go a long way to protecting inhabitants, but the house can still blow up if no one is home to hear it. That is what insurance is for.

If you get a big enough propane tank, you can use it to run your genset.

If your ducts are all inside the house envelope, even substantial ducts leaks are not a big deal unless it causes pressurization/depressurization of individual rooms or excessively uneven heat.

Yes the antifreeze is in case we lose power for days and the pipes don't freeze. I see that as something unlikely as the pipes are all in interior walls and the attic is finished except for where the 3rd floor air handler is.

Good point on the tankless, but still against propane there. If I'm staying with oil, then the best bet is probably that Peerless PT80 water heater? Any other models you guys would recommend about 75-80 gallons?

Anti-freeze in heating systems must be maintained. It can sludge up an become acidic over time. I think that with some of the apps available today to monitor your homes condition even if you are on vacation. I would flush the system and go to water if it was me. Glycol has a higher viscosity and requires more power to circulate too.

I would ask the folks from Leahy Fuel's, in Danbury, one of my best customers when I was back there, to explain propane to you. You can get combustible gas detectors as well as a warning if you feel you must. You just have many more choices of high efficiency advanced technology with gas than you have with oil. That technology improves your comfort and makes up the difference between the heat energy of oil vs gas. Oil tanks leak too and make your home a superfund site as well. Certainly your choice of fuels, but you get the gas lines run by a pro and you will not have problems.

I always have preferred the Phase III indirects. A stainless tank in a stainless tank.

"Glycol has a higher viscosity." Not only that but it carries less heat than water so the heat transfer efficiency goes down.

I'm also in a similar situation but leaning towards propane hydro air. JSG26, what did your end up doing?