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Forced Air Radiant Heat system

ajw1980 Member Posts: 4

I have a house with a very unusual heating setup. It can basically be described as a forced air radiant heat system. The house is on a concrete slab with forced air supply and return ducts in the slab. The second floor is built from concrete flexicore. The furnace pumps hot air through the supply duct towards the middle of the house and there are a couple of places where that air goes upward and is forced through the hollows in the flexicore hollows. This provides a radiant heat effect that comes mostly from the walls and flexicore. The return air goes through the flexicore and the concrete return duct for the far end of the house. I've got some diagrams of the floor plans I will attach that hopefully make things a little more clear. I've highlighted the supply in red, the return in blue, and the air flow through the flexicore with green arrows.

This system works well to heat the house. The problem is there is a definite odor coming from the duct work. I suspect the return duct, but which one probably doesn't really matter. Both the supply and return are probably about 40-50 feet in length under the slab with no real way to access them besides the entry point in the floor near the furnace. I've had a company that uses "duct armor" out that can be used to coat the interior of the duct work. They said they wouldn't be able to have enough access to even look at it until the furnace is removed. That really kind of complicates things, since I don't know if they will be able to correct that smell until I take everything apart.

So I guess I'm looking for alternate options before I tear everything up in case I need to abandon that ductwork. We really like the radiant heat, so if there was a way to convert to water pipes in the flexicore that would be good but not sure if that is feasible in any way. It seems like other options may be to use ductless systems, which would really not be ideal.

I'm in South Chicagoland, so definitely a cold weather area. If any experts have some thoughts on what I could do, I would appreciate it.



  • STEVEusaPA
    STEVEusaPA Member Posts: 5,324
    edited May 2018
    Are you smelling mold/mildew? Maybe a dead mouse? How about running a camera thru it (like the kind sewer companies use) first, before you go an expensive route.
    I see no way of running piping to make a hydronic radiant heat system, without severe demo. Unless you are able to use radiant panels, and/or radiant ceiling (not cheap, gonna need a boiler).
  • ajw1980
    ajw1980 Member Posts: 4
    I would say the smell has a concentrated dirt like smell to it and it started some time in January. It has kind of come and gone, so best guess is a crack in the slab somewhere and smells worse based on temperature.

    The place that does the duct armor stuff would run a camera, but they said they wouldn't have enough room to work unless the current furnace and ductwork are out of the way. That's a 28 year old furnace, so once I start removing stuff it's just going to get replaced anyway.

    I think cost is going to be high no matter what at this point in time. The duct armor option with furnace replacement is going to be expensive. Redoing the entire heating system is probably going to be more expensive, but I definitely need to have a plan B in case the duct armor isn't able to fix the odor problem.
  • ratio
    ratio Member Posts: 2,936
    Judicious use of a hammer drill might give you a hole large enough to get a borescope into but still small enough to easily patch.

    What an amazing system. I hope you are able to repair it.

  • Zman
    Zman Member Posts: 7,005
    It is hard to read the details of your plan.
    What size are the plenums?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • clammy
    clammy Member Posts: 2,668
    I had ran into a similar system once oil fired ,it was not in a residence aside from regular service the real issues started as the home made duct duct work under the home started to degrade but larger issues occurred when the original belt drive furnace was replaced with a newer direct drive fan furnace . The unit constantly cycled on limits and would have to be manually reset in the blower compartment.The issue was not with returns being it had one large return amply sized but the restrictive supply duct that only had a few supplies .The system was designed and built by a Japanese artist supposedly in the late 50 as i was told and was suppose to be like a early roman heating system it also had some block flue wall wall designed to heat up .When ever i had to service it ,it was only burner related and i really didn' t get inside .It wasn' t until some else replaced the original unit that i saw the whole pic .I gave them a price on a wall hung and snaking pex and installing panel rads and i think they went w electric baseboard .Peac and ood luck clammy
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating
  • Jamie Hall
    Jamie Hall Member Posts: 17,152
    As @clammy says, it is a modern version of a Roman (and perhaps earlier!) design -- the principle is also used in some solar heated houses. It works superbly well, although it is even slower to respond than hydronic type radiant floors.

    If it were mine to play with, I'd pull the furnace and give the ducts are really thorough inspection and cleaning. Then use the duct armour stuff to make sure that the ducts are smooth and clean -- and to seal up any cracks which may have occurred.

    When you replace the furnace, if you go that route rather than putting the old one back, pay real attention to the air flow and heat output and temperature requirements. You can push pretty hot air through that system, but there's not much point in trying to push too many BTU through it -- enough to keep the place warm.

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    JUGHNE Member Posts: 8,980
    If you simply pull the lower blower and filter out of the furnace you should be able to see the return air "pit" below it.
    If this is where the RA cores are then one could camera into them.

    I saw one house, slab on grade, where there was a pipe tunnel down the center of the house. It was used as the return duct.
    A bad smell showed up that turned out to be a cracked sewer drain pipe. Easy fix once located.
  • ajw1980
    ajw1980 Member Posts: 4
    Well, I think maybe we've found the problem. Saturday, we had some clear water on the first floor bathroom coming from the toilet. Today, the drain in the utility completed backed up and spewed out a bunch of nastiness. So I think probably there was enough sewage water leaking into the concrete ducts that was causing the smell. Hopefully the smell will clear up after it airs out for a while, and I can just go forward with a replacement furnace.

    Like @Jamie Hall said, though, if we've solved the smell issue, I'm still concerned about what to replace the current furnace with. That furnace in the picture is a 175K BTU furnace with about 60% efficiency. The temp limit is 190 degrees. I've never seen signs of overheating (like turn off the burners and keep the fan running and then turn the burners back on) and it does heat the house pretty well, so I think this furnace runs this design pretty well.

    I know a modern furnace isn't going to work the same, so I do still wonder what to replace the current furnace with and still have it perform well. I've had some people out to give me a new furnace estimate and of course no one has ever seen anything like this, so it would be nice to have a solid understanding of what is really needed to make this system work before I buy something new.

    For @Zman, I don't think the diagram has the opening sizes in the floor for the supply and the return. But here are the details on the duct sizes:

    Start of the supply is 18" x 20"
    Small branch is 6" x 13"
    First upward supply is 11" x 20"
    After the upward supply, it appears the main supply is reducted to 10" x 18"
    There are 2 upward supplies at the end of the slab duct work that are both 10" x 10".

    The flexicore has 4" hollows. The supply headers I believe are also approximately 4" holes cut through the flexicore cross ways to create a channel.

    The return duct is 15"x16".
    The downward return (although labeled "up") at the far end of the house is 10" x 24"
  • ratio
    ratio Member Posts: 2,936
    If the current furnace performs adequately, you'll want to match it's performance as closely as possible, which will require a goodly number of measurements to find out exactly what it is. Best to have that done before the demo...

    Finding the right contractor is critical. This isn't anything like a standard residential install. Is the original builder &| installer still around? I'd look for someone with experience with this kind of system, if only for support from off-site.

  • Mark Eatherton
    Mark Eatherton Member Posts: 5,837
    You might be able to set an ozone generator in the duct work to oxidize anything that might smell, but I have to caution you that if you have any live rubber products, like scuba gear, to get it out of the house, because the ozone will cause it to fail (oxidation).

    As for sizing, you'd be wise to hire a test and balance contractor to determine exactly how much air your current system is moving, how may BTU's it is moving and match it as closely as you can.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • ajw1980
    ajw1980 Member Posts: 4
    This house was built in the 1940's, so the original designer is unavailable I would say. I called the same company that installed the current furnace, but they didn't seem to have any additional information or records about it. They quoted a fully modulating furnace with variable speed fan, but I'm a little skeptical about that option since that seems like a big design change.

    A test and balance contractor does seem like what I need. I was trying to look for something like that, but I didn't even know the name for it. I think at some point in time, there was a furnace that didn't work as well because the far bedrooms have electric baseboard heaters. We've never had to turn them on, but I assume they were put there for a reason. I definitely don't want to replace that furnace and then require some auxiliary heat in certain rooms.

  • Jon_blaney
    Jon_blaney Member Posts: 143
    Is this really a forced air system or is more a gravity system? I would look at using an air handler with a boiler. Might have better control of the air temp by controlling the water temp but then again it maybe need a big blast of hot air to get the circulation going. A variable output gas furnace maybe?
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,837
    The reason the far bedrooms are cool is because of their location in relationship to the thermostat. They are like appendages, hanging out there getting cold, hence need for baseboard heat. Not any easy fixes for that condition. If you move the thermostat to that location, then the core will over heat.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Larry Weingarten
    Larry Weingarten Member Posts: 2,294
    Hello, If you got someone with a flow hood to check what's coming out at each register, you might then be able to balance the system, assuming your registers are adjustable. With that, those cold "appendages" might be able to be brought in line with the rest of the house. B)

    Yours, Larry
    JUGHNE Member Posts: 8,980
    Perhaps a modulating furnace or 2 stage with variable speed blower.
    Running the blower on the off cycles, on low speed constantly may even out the temps. These are ECM motors that draw very little energy on lower speeds.