Baseboard in forced hot air duct, would this work?

[twitch]

I have a Tarm 30 on order and will be heating a double wide mobile home and garage with it. The boiler room will be in the garage. One concern I have is that the feeder duct from my furnace into the ductwork is only 11 1/2" X 11 1/2". I haven't been able to find an HX that size and I'm not sure it would be big enough to do the job. There are two main ducts that run the length of the house, one on either side and they are both 7" X 16". I was wondering if putting 1" baseboard finned piping down the center of the ductwork on both sides would work. The ducts are a straight shot from one end of the house to the other. If this seems a little far fetched, are there others here that have the same type of existing forced air setup that are using a boiler?

 

[Nofossil]

I have a Tarm 30 on order and will be heating a double wide mobile home and garage with it. The boiler room will be in the garage. One concern I have is that the feeder duct from my furnace into the ductwork is only 11 1/2" X 11 1/2". I haven't been able to find an HX that size and I'm not sure it would be big enough to do the job. There are two main ducts that run the length of the house, one on either side and they are both 7" X 16". I was wondering if putting 1" baseboard finned piping down the center of the ductwork on both sides would work. The ducts are a straight shot from one end of the house to the other. If this seems a little far fetched, are there others here that have the same type of existing forced air setup that are using a boiler?

I like it - that's a creative idea. Rather than down the sides, though, you might thinks about several short lengths, perhaps around 36" if you have room that stack on top of each other and go diagonally across the duct so that the air has to pass through the fins. No idea how to calculate heat transfer, but it would be a good deal higher than the normal figure of 600 BTU/ft.

 

[twitch]

I like it - that's a creative idea. Rather than down the sides, though, you might thinks about several short lengths, perhaps around 36" if you have room that stack on top of each other and go diagonally across the duct so that the air has to pass through the fins. No idea how to calculate heat transfer, but it would be a good deal higher than the normal figure of 600 BTU/ft.

I hadn't thought about trying that, it probably would have a lot more heat transfer. My thinking was that having the baseboard in the duct, it would almost act like a radiant heat, although I'd still have the thermostat connected to the furnace blower. Another advantage is that if I'm going to be gone for awhile in the winter, the hot air furnace would keep my piping from freezing

 

[atlarge54]

I wanted to do that several years ago and didn't persue it since I moved to a different home. I tried to purchase finned tubing from the local supply house and they acted like I was from a different planet. They did have some discontinued baseboard available for $10/ft. though. This summer I bought about 35' of 3/4 finned tubing plus a nice smaller heat exchange for $25 at the local scrap yard. Check with the local scrap yard and maybe bribe the guy who checks in the scrap with a case of beer.

 

[mtfallsmikey]

Wholesale price on BB here still at $6.50/ft.....why don't you use a cased coil under the floor or in the plenum in the bottom of the furnace? (kinda tricky with A.C. A-coil tho)

 

[free75degrees]

You might want to check craigslist for cheap BB too. I got the BB for my power outage overheat loop from a guy on craigslist. Unused, 28 feet for $100 ($3.57 / foot). There were other people trying to get rid of used BB so I got the feeling that it was fairly plentiful on there.

 

[pybyr]

If this seems a little far fetched, are there others here that have the same type of existing forced air setup that are using a boiler?

not far fetched at all; I am building something similar where I am putting a big water-air coil over my existing hor air oil furnace

you need to start your figuring-out -- assuming that you are using your existing furnace blower to push the air-- with finding out how many cubic feet per minuts (CFM) that blower moves, and what amount of "back pressure" it can take (which will be a decimal of an inch of water column). Your furnace manufacturer's technical folks should be able to give you that data, like mine gladly did when I called them with the model # for my unit. I'll come back to those variables of CFM and back pressure further below-

then do a heat loss analysis on your house to see how many BTUs/hr you need for peak heating load- of if you are lazy and also confident that your existing furnace is not oversized (as in, it runs pretty steadily for long, long times during your occurrences absolute coldest weather, and doesn't do a lot of little intermittent brief and separate bursts of heat even in not-so-cold conditions) use its BTU/hr rating

then -- you'll want to talk with someone who is WAY more competent than a regular salesperson at sizing a coil to move the right number of BTUs, at the temperature of the water you anticipate as "normal" at your coil, at the CFMs moved by your blower, and with no more back pressure added by the coil than your blower can handle (and you want to err on the side of the lowest possible back pressure through the coil, because your ducts are already creating their own back pressure through the coil.

the assumption as to the water temp at the coil gets more complex if you are considering using heat storage or ever adding it in the future, as then you will need a bigger heat exchanger in order to be able to still extract enough heat from the stored water as it gradually cools

here are some threads with a photo of my coil and links to the folks who helped me size it and procure it. in my case, I have a BIG old farmhouse, so you probably can get by with way less coil; my coil also could've been smaller if I'd been planning to run without storage and known that my water would always be around 180

https://www.hearth.com/talk/threads/25090/

https://www.hearth.com/talk/threads/24899/

if the coil size you need to move the BTUs is bigger than your ducts, you could "funnel up" from the ducts to the HX, and then funnel back down from the HX back to the duct- just make the transitions as gradual/ tapered as you can for good airflow.

if time is abundant, you're handy with things, and budget is thin, you might be able to do something with good-sized car or truck radiator with a box built to match it to your ducts... just be careful that they're rated for the pressure you may be running your boiler system at

you'll also want the water-air HX on the "outlet" side of your furnace, as the electrical parts in your furnace are unlikely to be rated for entry/ ambient are much over 100degrees F- so putting an HX on the inlet side of your furnace blower would cook your furnace.

I don't have credentials, but your intake grille sounds inefficiently small to me and your whole system may run better if you are able to make the intake side of things less restrictive if you can do that without major disruption or cost

hope that all helps

 

[Chris S]

At the supply house- ask to buy their dented baseboard- a lot of it gets damaged and put aside. I wanted some to run on an outside wall behind a whirlpool & they gave it to me for 75% off. Then throw the covers away oops we're green now so recycle them!!

 

[pybyr]

At the supply house- ask to buy their dented baseboard- a lot of it gets damaged and put aside. I wanted some to run on an outside wall behind a whirlpool & they gave it to me for 75% off. Then throw the covers away oops we're green now so recycle them!!

\

brilliant

[and thank you for sharing]

you just solved what I need to do for my basement ceiling power-failure dump zone. and with prices of steel, etc for scrap, the covers, that I don't need anyway, can then go in my metal pile that I will eventually truck to the salvage yard, where they'll help cut the costs even further

 

[Chief Ryan]

You would have to make sure the airflow is hitting the baseboard on the thin part of the fins. What you are talking about is a hot water coil in your ductwork or a re-heat coil. You can have coils made up to fit almost any size duct you like, they would make it thicker for the size of the load in the room.

I'm not sure i like the idea of jamming baseboard in your duct work. Is this duct connected to an A/C unit. If it is you cant restrict airflow too much or you can damage your compressor.

Do a little more research before you do it. PM me if you like.

 

[Chris S]

I did this dump zone thing once before. Keep in mind that fin tube without the covers, and without a circulator pushing the water will not remove the 600 or so btus it is rated for. On my old install, I had a large zone upstairs with some cast iron and it worked well with gravity flow.
I don't think fin tube around the ceiling will really cut it. In my new install, I'm using a 12v circulator and a deep cycle battery because the elevation I have will not work with gravity...but I'm hanging onto a 5' long cast iron radiator just in case.
I like the technolgy & engineering of my installation better than I'd like to see an old radiator up on the ceiling of my boiler room.

 

[Chief Ryan]

Airflow is extremely important with A/C and Hot air furnaces. Too little air with a furnace can cause cracked heat exchangers. Too little air with A/C can cause compressor failures.

 

[twitch]

Thanks for all the replies. My initial thought was to use an HX under the furnace (downflow), but the feeder duct from the furnace to the rest of the ducting is small (11 1/2" by 11 1/2"). The duct feeding the registers is also fairly small, 7" by 16". I'll try to attach a drawing of the ductwork layout.

I talked to the company I'm buying the boiler from yesterday afternoon, and they suggested putting an air handler with HX near the garage end of the house in the crawlspace (the house is on a slab with about a 2' crawlspace), and tapping into the ends of the existing ductwork. This approach may be more expensive, but I wouldn't have to worry about restricting air flow from the existing furnace, and I could size the blower and HX correctly.