Cold air return vents

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I have a 1972 Newport 14X70 SW mobile home. The heating vents are in the floor (most are in the middle of the rooms)for my Miller furnace. I also have 6 "other" vents, three on either side of the MH against the outside walls. Are these cold air return vents or what? They seem to open into the sub-floor where the water pipes and heating ducts run above the belly insulation.

I read on this site that most mobile homes don't have any cold air return vents and that's why there are large gaps on the bottom of most interior doors. I have those large gaps on the bottom of my doors, but I also these odd vents. What purpose do these vents serve if they are only opening into the sealed belly area? Are they to augment the heating ductwork in keeping the water pipes that run under the floors unfrozen?

Should I keep them open, or seal them off with insulation for the winter. I hope someone has had experience with vents such as these and can tell me their purpose, as they don't seem to make a whole lot of sense to me. Thanks.

MobileWayne

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Wayne, with a 30+ year old home anything is possible. Can you see where the air intake to the furnace comes from? Many gust drop a duct down from the furnace through the underbelly. If you do not see one from under the home than it is POSSIBLE that over the years someone added cold air returns. Greg

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Hey Wayne
I don't have those vents in my Newport-granted mine is a slightly later model-but I'd probally agree with Greg that they are some jerry-rigged cold air returns put in by a former owner.
Take some pictures for me.
Good luck
brucetrout

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Well, I guess they must be cold air return vents and my furnace intake must be in the sub-floor. I've never noticed one while working down under near the furnace and I was under there quite a bit this summer. They look to be original and not a later add-on. Perhaps that is how this MH was built. It certainly doesn't make any sense to have these vents opening into the dead space between the floor and belly unless the intake is there.

Sure would love to get ahold of an old set of plans for my home, but we know how difficult to find those are. It doesn't explain the large gaps in the doors, but I doubt anyone would go to all the trouble to add those vents and move the furnace intake into the belly unless the furnace was replaced at one point. I was told by the seller that my Miller is the original furnace, but perhaps they were mistaken.

MobileWayne

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Hi,

What is your furnace model and serial numbers ?

Is it gas or electric ?

Are there any vents in ceiling ?

Those vents were an original install for return air back through the underbelly which was an early HVAC design in some MH's.

They had an upflow furnace with registers in the ceiling.

Does your furnace appear to be 35 years old ?

Thanks,
Robert

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Hi Robert,
It's a Miller MOC-80 (oil fired). Serial # plate says Lear Siegler 10B-1975. No registers in ceiling - they're in the floor. Certainly appears to be 35 years old. I was told the MH was built in 1972, but haven't seen anything in writing to nail down the exact year, as there is no manufacturer plate to the MH that I can find that has the build date. The park office records indicate it was sited on the lot in 1972. I originally had a problem getting insurance due to the lack of info on the home and used the park's records to prove it was built in 1972 (or possibly a bit later).

MobileWayne

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Hi Wayne,

It probably is original then. That system was also used with floor supply registers.

There was a connector duct/pipe from belly area to the furnace.

The return air would pass over the heat source and be delivered through the main duct trunkline.

Is there a return air grille in the furnace door, furnace closet door or in floor in front of furnace ?

Is there a filter to clean and/or replace and if so, where is it at ?

You'll have to closely inspect system to determine if it has been disconnected and unused or still in use.

It is best to discontinue use and seal up and create a new return path IF still being used.

Look down in register along wall closest to furnace and see if you see anything that the return air could flow through to furnace.

Thanks,
Robert

[Mark]

As someone who has worked on zillions of mobile homes, I can tell you that those vents were original. In some home, the cold-air vents are simply holes in the floor. Look down them and you see right into the belly of the home. The idea is that the furnace draws air thru the belly, pulls it back up just in front of the furnace (do you see a grate in the floor in front of the furnace?). If so, that's why. It's pulling the air thru the the belly via that grate. Not a very good system knowing what all can be in your belly, but that's how the design on many of those old home works.

Mark

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Hi Robert,
Yes, there is a return air grille in the furnace closet door and a foam filter (just replaced) in the furnace door itself. I just checked those floor vents with a stick of incense while the furnace was running and there is definitely air flow while the furnace is on, but the air seems to be coming into the room rather than being sucked back into the sub-floor/belly area. Shouldn't it be the other way around? Now that I have all the large holes in the belly from the previous owners repaired, I see why they paid almost $2000 (according to their fuel oil company) in heating bills last winter! Perhaps it's this unseasonably warm winter, but I'm still on the first tankful I purchased here last May and I don't skimp on the heat. Whatever this screwball setup is, it seems to be still functional as well as original.

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With return grille in closet door and filter in furnace door, they are not being used any longer.

I'm guessing this is original to home (the floor/belly return in 1972), then around 1975 a new furnace with return in furnace door was installed.

IF I'm right, those floor registers should have been closed off.

That is why they're blowing rather than pulling. Also by air blowing out, this indicates a leak in main trunkline.

Hope this makes sense.

Take care and best wishes,
Robert

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Hi Robert,
Hmmm...I'll try closing those old returns off for a while to see if it makes any difference. I inspected the sub-floor area last summer before sealing the belly completely and did not notice any disconnected ductwork and the floor registers all work extremely well when the furnace is operating. Even the ones farthest away from the unit, but it's always possible I missed seeing something.

At least now that I have the belly completely sealed, any leaked hot air from the duct work is just going into the sub-floor and keeping my pipes warm (and I'm not sure that's a bad thing given the single-digit temperatures this week.) At least I'm getting some of that heat back up into the home through the old return vents. Guess it's time for a little experimentation. Thanks much for the info. Living in this old MH is quite an adventure and I probably would've never figured any of this out myself.

MobileWayne

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Those systems are not good and it is strongly advised to seal off the vents AFTER it is certain that you have a return air path to and through the furnace.

The danger (health wise) is that you are breathing air that is blowing through the belly and over insulation and any rodent droppings that may be still in there.

The system is called a Cold Air Belly Return System (CABRS).

Hope all this helps.

Take care and best wishes,
Robert

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Hi Robert,
Well, I certainly see your point about those vents remaining open being a health hazard. Lord knows I've had my share or rodents in that space this winter. So you think the furnace was replaced at one point and the cold air return was added to the furnace closet door? I guess whoever did the job knew enough to plane the bottom of all of the interior doors to allow for air circulation to the new return. Once I'm 100% certain that was the case, I will seal the old return vents. Thanks again for the info. I was just about to install laminate flooring here and would've cut around those vents. If they are not being used and can be sealed it will make for a much neater install to lay laminate over them and make them disappear.

MobileWayne

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Hi,

The vents and return system would have been there since its manufacture.

I don't think they would have done that and not install that type furnace.

So, under the circumstances, I believe either it was changed within first 3 years or changed at beginning because someone didn't want that type system.

Whoever installed what is there now, would have known to cut the doors.

With that said, there wouldn't be two return set-ups. So, if you have a return grille at the closet door and a filter in furnace door and air is being drawn through there, that is a return system.

With that info plus the fact that the air at those vents is blowing out, not drawing back into belly, that return system is not operational.

Now, I'm not there, but with the info you've given, I'd say it is ok to seal them up.

Take care and best wishes,
Robert