Oil Fired Boiler Being Used as the Heating Source in Two Air Handlers of a Historic Home

I'm not sure this makes any difference, but I've only run into a system like this here, where I live on the Eastern Shore of Maryland. In fact, I've only run into this once before.

Essentially, this looks like any other split system A/C / Furnace, however, rather than having a gas-fired or electric heating element, there are copper water pipes running through the supply side of the system. I'm sure I don't need to tell anyone here, but just for good measure, the blower is moving the air over the pipes, heating them and sending them into the house (that air comes out HOT too).

The systems, both the two A/C units, Air Handlers, and the Boiler were all installed between 2001-2003, the boiler being the most recent.

I understand how the system works, but I don't know what to call it. Is there a name for this type of thing? I've tried to search for some information online, but I've made no progress.
Does anyone have any idea why someone would install this type of system? From my point of view, I see no real benefit, it just seems as though its an extra expense. The first time I ran into this, the home had belonged to the owner of a local Oil Distributor. I'm not sure of the background in this case however.

A little more background on the home: It was originally serviced by a boiler with radiators throughout the home, but I've got no information as to when they we're removed. They've scraped the material and plugged the holes. The home also has a propane tank in the backyard which serves the four fireplaces throughout the bedrooms and formal dining and living room. It has a gas oven, but that line has been capped. It appears to me that these copper lines for the air handler we're run fairly recently, around the early to mid 2000's. I'm only basing this idea off the condition of their insulation though. It is in good shape, while the older insulation covering the refrigerant lines of the A/C is in need of replacing.

Any help here would be greatly appreciated. If more information is needed, just let me know and I'll try to get it on here as quickly as possible.

Oh, the recommendation I'm considering to the homeowner is to replace the two A/C units and Air-Handlers with Split A/C and Propane 95+ furnace. She's going to be waiting until the units die, but I want her to have some answers in the meantime.

Thank you!

Tags: A/C, Boiler, Furnace, Historic, Hydronic, Water

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Yes, its a Hydro-air system. It uses one boiler to supply hot water to the coil in the supply side like you mentioned. The reason for this system is to allow multiple A/C zoned systems without adding a whole Heating/cooling system.

One boiler can serve many C/A units, providing them heat during the heating season, thereby zoning them instead of installing a heating/cooling system for each zone. So all you have to add for each zone is just a cooling system which lowers the cost of installment, while also providing zoning to the cooling system. Its very cost effective and may be more efficient.

You typically see them in large homes where you would need to add more than one cooling system, typically over 4K sf that have 2 floors and a finished basement.

I have seen homes where they have one boiler supplying heat to 4 C/A units and more.

As far as your recommendation of replacement, you may want to replace them with a ductless VRF (mini-split) system, since you don't know how efficient the duct system is.  Generally these are by far more efficient, and you can get up to 8 zones. 

Hope this helps!

Hi Rich,

Thank you for the response, that does make sense to me.  

As far as the SF goes, this home is roughly 2300sf, but, due to its inefficiencies its using energy like something larger.

I have run a duct-blaster on the home and the systems are leaking roughly 35% and 25% on the 1st and 2nd floor, respectively. Unfortunately, mini-splits are out of the equation on this house.  The homeowner is very serious about maintaining the historic aesthetic of this home and is against any head unit on the wall.  Too bad though, the first floor is so open that Mini-splits would work great and give us the room to encapsulate their crawlspace.  

Thanks again!


The mini-split systems now have options for the on-wall units. Its the major issue with clients. They have several options, including ducted ones, and even one that looks like a steam radiator.

Check them out.

As for the duct leakage, you can only seal the ones that are accessible and they may be difficult to seal, but there is the Aeroseal system that will work fine. Although expensive it works very well. You could look that up, or there might be someone on this thread that can help you.


We see different forms of these up in New York.  Either a hot water air handler or hot water furnace, I would say the actual control would define it beyond that. Oil fired furnaces have lots of complications so this is a common answer for replacing an oil fired furnace.  There are a bunch of good tools for improving the efficiency of the oil boiler.  I specifically like the load based resets like the energy-mizer.  Also, you can increase the run time by installing a lower flow oil nozzle.  A good oil guy can find a few places to squeeze.

For duct leakage, feeling for warm spots can help you find leakage, or IR camera.  Personally I find the greatest "crimes" on the return system.  The wall return registers with pan returns have so many leaks, like the transition between the wall and the floor. Also they are typically missing actual duct. Just holes back to the fan. I have a short podcast about boiler on sound cloud. user addingenergy 



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