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DIY Hydronic Air Handler

sefsef Member Posts: 4
I have a Boiler and mixed Hydronic system installed and running nicely in my house for a couple of years now. I have in-floor pex on a couple of floors, some rads and domestic HW coming from a new Natural Gas modulating Boiler, and an old 96% gas furnace (with Variable Speed motor) for some remaining forced air and AC in the summer. I installed a separate zone with pump for a future Air Handler during the time of the original set-up. I am currently wanting to de-commision the old gas furnace and set up a new Air Handler with Hydronic Coil and a new ACoil for summer AC.

My thought is this...

I might get a couple of hundred bucks for the 5 year old furnace and then spend 10 times that on a new Hydronic air handler, but could I not just remove the entire heat exchanger from this furnace and install a coil in its place? These coils seem to run a few hundred and I would be re-using a perfectly good sheet metal box and motor rather than (likely) sending it to the dump.

Obviously the control panel will be of no use but can I not set the fan speed to low and have it run at the same time the Pump controller starts the pump, when the thermostat calls for heat?

Has anyone done anything like this?


  • JUGHNEJUGHNE Member Posts: 7,183
    I did something like this years with the old Amana system that already had HW coils in the AH with AC coil above.
    Only issue was it needed a time delay to start the blower to avoid cold air initially. A strap on aquastat on the return pipe to start the fan might be in order, it would also milk heat out of the coil after the burn cycle.

    Your HW coil needs enough air flow thru it for the AC system.
    Also the HW coil needs much hotter water than your in-floor heating can stand.
    So some tempering/mix valve needed.
  • sefsef Member Posts: 4
    That is a fair point JUGHNE, I had assumed that all Water to Air coils are similar. I was looking at the typical 3 1/2" units commonly available for the outdoor furnace market. They use 1" connections that is what I have roughed in now.

    Also all my other zones have mix down valves to lower the temp coming off a primary loop while the rough in for the hydronic furnace line is a direct tap off the loop at it's maximum temperature. So my understanding is that I should be ok there.

    I see the point regarding fan lead in delay and lead out time. I will need to study that aquastat recommendation - Thanks.
  • Jon_blaneyJon_blaney Member Posts: 76
    The air from a HW coil is going to be cooler than that coming out of a furnace. Can you move enough cooler air to warm the house?
  • sefsef Member Posts: 4
    Well, I am working on the assumption that the basic principal is workable as far as BTU transfer, as the arrangement is fairly established. Viessmann (my Boiler), Rheem/Rudd, Goodman, and many others sell the systems - by that I mean Air handlers with hydronic coils meant as the primary forced air furnace. What I am thinking of doing is no different except by re-purposing my perfectly functioning gas furnace. My guess is that the biggest 'missing link' will be the logic board in the commonly available purpose built units.
  • EdTheHeaterManEdTheHeaterMan Member Posts: 798
    I think @Jon_blaney wants you to consider the fact that the furnace is providing air temps up above 140° but the fan coil with 180° water may only provide 120° to 130° air in the same duct system. I don't believe this will be an issue unless your ductwork is undersized. You won't notice the problem until the outdoor temps go below the design temp of your system. So for maybe 3 days every 5 years or so, you may have an issue. That is why they make blankets.

    If your furnace still cycled off during those unusual low-temperature days that happen only once every 10 years, then you don't have an issue.
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,307
    There is no reason that what you want to do will not work. You just need the correct sized hot water coil that you can match with the fan speed for the amount of air the fan can deliver.

    You need a control to start the fan and a strap on aquastat that makes (closes) on temperature rise will work but If you haven't piped the coil yet I would uses an imersion aquastat with a well instead of a strap on. Mount it on the return line from the coil and set it to about 120 degrees to make on rise in temp with a 10 degree differential for starters. If yor piping is small uses reducers on a larger tee so the aquastat well does not impead the water flow
  • hot_rodhot_rod Member Posts: 13,832
    There is a bit of math involved in properly sizing a hot water coil, the OWF suppliers often wrongly sell them by physical size only.

    The SWT temperature, flow rate, delta T,
    the air flow in CFM and required air side discharge temperature.

    Coil manufacturers play with fin size and spacing, the number of tube and row passes across the coil, and the actual dimension to match the coil to the application.
    Most of the coil manufacturers have free design software that allows you to enter your design parameters to get the correct size, or confirm the coil you are considering.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • TAGTAG Member Posts: 276
    Personally -- I would do some rethinking. Years ago with Cast Iron boilers pumping out 180 degree water every cycle people went the fan coil route for a variety of reasons. My brother outside of Boston used one for a new addition. His current boiler for the main house radiators being so oversized the addition was just another zone. HE did not want baseboard and radiant was too expensive for him at the time. My local oil dealer always recommended this route when I was doing a new project - often had the possibility of two systems -- this way each unit had a coil and the boiler was available for DHW and some radiant. Doing oil furnace on a second floor was not going to work well and they wanted to sell oil. JUGHNE -- brings back memories -- I had a project years ago that used an Amana HTM unit on borrowed time for a large addition. We piped over a zone from the boiler to keep the Amana unit w/ AC -- it provided the needed extra BTU's as the new retrofitted radiant was not enough on the coldest days.

    You have the best of both worlds now .... don't think the Viessmann will be condensing pumping at the temp water you need for the coil .. and my memory is it does not deliver 180 anyway. My current project has various radiant throughout -- Viessmann 200 boiler and 300 indirect ....but I also installed a propane modulating furnace in the ductwork system providing AC. Is your current unit modulating ? You will lose all the control going with the coil -- it becomes a dumb system. Years ago with a furnace being only on and off -- the switch and the control to fan coil was easy and straightforward and the results the same ... not true today.

    People don't install what you have today -- for cost reasons ... not because it is not better. The boiler not condensing will give you a lower efficiency.

    Are you using the ODR for the boiler? I did a big old house w/ radiators and used ODR ... it worked great as all the old radiators were oversized for the spaces anyway.
  • sefsef Member Posts: 4
    Ahh, thank you, now I see jon's point. All this is very useful advice and will help me as I move forward. Thank you all.

    I should add that the bulk of the house is now serviced by in-floor and water radiators fed from the boiler so the forced air is not really adding so much any more - but it comes into play as supplying 50% to 60% of the cooling in the summer - that is the biggest reason for keeping it. I have a Mits Mr.Slim on the top floor that is fantastic but was a lot of work concealing the wall unit, (keeping it fully functional and serviceable) and I have no way to conceal them on the main floor.
  • TAGTAG Member Posts: 276
    I'm doing three Mitsubishi mini-splits in my new project as well ..... could not get ductwork to work in the new addition or distant loft . They obviously make the concealed/ducted and you can recess the floor units ... but, the wall units IMO work the best. I have used the ceiling cassettes with good results when nothing else would fit/ work. With good placement they blend in more then people think they will ... too many times people retrofitting them don't go past the easiest/ cheapest install location. I have seen some wacky installs
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