Cold Air Return Placement in Hearth Room

[lumbajac]

I am planning on using an RSF Opel3 zero-clearance fireplace along with a natural gas forced air system to heat my new home. I plan on burning the fireplace 24/7 for the entire heating season and would like the natural gas to kick in as little as possible.

The RSF Opel3 has an optional central heating kit that can tie in directly to the central heating system's ductwork. See pg. 21 of the RSF online brochure if unfamiliar with this... brochure can be found at: http://www.icc-rsf.com/c/icc/file_db/maindocs_e/RSF-CAT.pdf.

I visited a dealer this past week and got a good look at the stove. I am definitely going with the RSF Opel3, but am leary about spending the $500 plus for the central heating kit if I can find a better way to distribute heat, plus another $200 or so on other items that would be needed for this setup to work.

Instead of the optional central heating kit I am considering their gravity heat kit, which would allow air to naturally flow below the fire place unit itself through a louver, behind the fireplace and up, then out 2 separate gravity vents either on the front or on the sides of the fireplace... I'd then have a cold air return near each of the two gravity vents at ceiling level to capture a lot of this heat... having the variable speed fan on the natural gas furnace running very low all the time, on a timer to cycle on every so often, or on a thermostat to kick on when a certain temperature is reached would then distribute this heat to the whole house through the natural gas furnace's central heating ductwork. I'm thinking this option would save hundreds of dollars up front on optional kits, would rely less on potentially noisy add-on blowers, and would probably consume far less power given that the new variable speed blowers on natural gas furnaces draw very little power as compared to what I would expect an add-on blower from the fireplace manufacturer to consume. If not familiar with the setup I am speaking of, woodheat.org has a good article with a diagram related to this type of heat distribution at: http://www.woodheat.org/planning/heatdist.htm

Any thoughts on this setup would be much appreciated.

Thanks,
lumbajac

 

[Gooserider]

Lots of discussion about using the house HVAC system in the hearth room area, I'd suggest doing a search - general conclusion is that it doesn't work very well for most people.

1. Per code, you MAY NOT have an HVAC system return located within 10' of a stove or fireplace.

2. Your gas furnace and HVAC ducting are designed to circulate air that has been heated to around 120-140*F - the air coming off your fireplace vents is going to be closer to maybe 80*F if you are lucky, and will be diluted by air from any other returns in the system, and cooled by it's trip through the ductwork, and the typical 20-30% losses in most duct systems (worse if the ductwork travels in areas outside the heating envelope of the house) so that by the time it gets to the areas you want to heat, it's likely going to only be 70* or so - hardly worth the trouble...

If I were doing it, I'd go with the accessory kit to connect the Opel directly to the HVAC system, that way you will be distributing really hot air at a temperature the system was designed for, without it being diluted by cooler room air. Alternatively, I'd look at seeing what sort of natural convection circulation I could arrange to move the heat from the gravity setup around the house WITHOUT using the HVAC duct system - possibly helping it out with a few strategically placed air circulating fans.

Gooserider

 

[lumbajac]

Lots of discussion about using the house HVAC system in the hearth room area, I'd suggest doing a search - general conclusion is that it doesn't work very well for most people.

1. Per code, you MAY NOT have an HVAC system return located within 10' of a stove or fireplace.

2. Your gas furnace and HVAC ducting are designed to circulate air that has been heated to around 120-140*F - the air coming off your fireplace vents is going to be closer to maybe 80*F if you are lucky, and will be diluted by air from any other returns in the system, and cooled by it's trip through the ductwork, and the typical 20-30% losses in most duct systems (worse if the ductwork travels in areas outside the heating envelope of the house) so that by the time it gets to the areas you want to heat, it's likely going to only be 70* or so - hardly worth the trouble...

If I were doing it, I'd go with the accessory kit to connect the Opel directly to the HVAC system, that way you will be distributing really hot air at a temperature the system was designed for, without it being diluted by cooler room air. Alternatively, I'd look at seeing what sort of natural convection circulation I could arrange to move the heat from the gravity setup around the house WITHOUT using the HVAC duct system - possibly helping it out with a few strategically placed air circulating fans.

Gooserider

Thanks for the input - I've already reposted this in the hearth room area and have gotten a couple of replys. One member was concerned that if I used the optional central heating kit that the air would be too hot for the natural gas furnace plenum. Another member had cold air returns close to his woodstove that helped some, but eventually he knocked down a wall to get the air movement he needed to distribute the heat effectively to his 2700 sq.ft. home. I don't realy have that option with my floor plan, so I've really got to consider the central heating kit.

After thinking about it some more, I believe the central heating kit's blower would basically stop most air/heat flow from going in any direction until it kicked on per the thermostat. With that said, my thought would be that the central heating kit's blower would have to be wired in with the natural gas furnace's variable speed blower so that both would kick on together, effectively making sure that all hot air entering the plenum of the natural gas furnace via the fireplace would immediately be distributed to other areas of the house.

For all I know, the fireplace will do an okay job of distributing the heat on its own, but it would be quite difficult to add on the central heating kit after the fact, so I think I will spend the $500+ up front and have it taken care of. If its needed then I'm set.

Thanks again,
lumbajac

 

[Gooserider]

I don't know how much obstruction to airflow the heater kit's blower would offer, if it's a standard squirrel-cage type design, I would not expect it to offer that much resistance (unless there is some sort of flapper valve in it to close it off) I would expect that it would act about like a cold air return grille. However it would probably be a good idea to tie the furnace circulator and the heating kit blower together - presumably with some extra controls so that the furnace burner won't turn on if the fireplace is burning, or the heater kit blower if it isn't, etc. Hopefully the kit would have some documentation at least on how to do this, possibly even the right controls.

As to the plenum temps, that would be a manual and code type question, but I wouldn't expect it to be a big problem - however I'm sure RSF could answer it if the manual doesn't say... At worst I'd expect that you might have to add a bit extra clearance to combustibles near where the stove heat enters the plenum, but I'd be a bit surprised if that were the case, as I'd expect the heat coming off the stove into the kit to be about the same as what the gas burner would put out.

Definitely seems to me like you are doing the right thing to put the kit in.

Gooserider

 

[Redox]

Not wishing to start an argument here, but why is putting the stove closer than 10 feet a problem? I can understand that if the room had a door closed to the rest of the house that you could pull a vacuum in the room and possibly cause a downdraft, but if there are no doors, what's the problem? Why is it okay to duct the fireplace to the furnace when the fireplace IS the return? I have looked for this "code" and can't find it. Please enlighten me!

Chris

 

[begreen]

R602.8 Fireblocking required. Fireblocking shall be provided
to cut off all concealed draft openings (both vertical and horizontal)
and to form an effective fire barrier between stories, and
between a top story and the roof space. Fireblocking shall be
provided inwood-frame construction in the following locations.
1. In concealed spaces of stud walls and partitions, including
furred spaces and parallel rows of studs or staggered
studs; as follows:
1.. Vertically at the ceiling and floor levels.
2. At all interconnections between concealed vertical and
horizontal spaces such as occur at soffits, drop ceilings
and cove ceilings.
3. In concealed spaces between stair stringers at the top and
bottom of the run. Enclosed spaces under stairs shall
comply with Section R311.2.2.
4. At openings around vents, pipes, ducts, cables and wires
at ceiling and floor level, with an approved material to
resist the free passage of flame and products of combustion.
5. For the fireblocking of chimneys and fireplaces, see Section
R1003.19.

2.3. Return-air inlets shall not be located
within 10 feet (3048 mm) of any appliance
firebox or draft hood in the same
room or space.
3. Rooms or spaces containing solid-fuel burning
appliances, provided that return-air inlets are
located not less than 10 feet (3048 mm) from
the firebox of such appliances.

 

[lumbajac]

R602.8 Fireblocking required. Fireblocking shall be provided
to cut off all concealed draft openings (both vertical and horizontal)
and to form an effective fire barrier between stories, and
between a top story and the roof space. Fireblocking shall be
provided inwood-frame construction in the following locations.
1. In concealed spaces of stud walls and partitions, including
furred spaces and parallel rows of studs or staggered
studs; as follows:
1.. Vertically at the ceiling and floor levels.
2. At all interconnections between concealed vertical and
horizontal spaces such as occur at soffits, drop ceilings
and cove ceilings.
3. In concealed spaces between stair stringers at the top and
bottom of the run. Enclosed spaces under stairs shall
comply with Section R311.2.2.
4. At openings around vents, pipes, ducts, cables and wires
at ceiling and floor level, with an approved material to
resist the free passage of flame and products of combustion.
5. For the fireblocking of chimneys and fireplaces, see Section
R1003.19.

2.3. Return-air inlets shall not be located
within 10 feet (3048 mm) of any appliance
firebox or draft hood in the same
room or space.
3. Rooms or spaces containing solid-fuel burning
appliances, provided that return-air inlets are
located not less than 10 feet (3048 mm) from
the firebox of such appliances.

Given this code, my option of locating cold air returns anywhere near the fireplace is pretty much out of the question - the only internal wall near the hearth room is the back wall that the fireplace itself is located on. All other walls are continuous exterior walls with the kitchen, dining, and living areas being one big great room so to speak... I can't put cold air returns in exterior walls so I am left with having to go with the central heating kit offered by RSF if I wish to distribute some of this heat. My natural gas furnace will be located in the basement almost directly below the fireplace itself... with that said, it will be a short run of maybe 6' at most of insulated ducting from the fireplace down to the natural gas furnace plenum so heat loss should be minimal. I looked into RSF's optional zone heating setup - nice idea, but requires zone dampers and separate thermostats for each zone which in the end would cost even more than the central heating kit. Therefore, I'll go with the central heating kit and either one gravity kit or an upper louver.

Thanks,
lumbajac

 

[Redox]

Again, not wishing to start a war, but how is it that you can connect a fireplace to your ductwork? How long has this particular code been in effect? I know an entire neighborhood here that was built in the 60's that has the main return for the furnace right next to the fireplace. We used to live there...

Chris

 

[lumbajac]

I don't know how much obstruction to airflow the heater kit's blower would offer, if it's a standard squirrel-cage type design, I would not expect it to offer that much resistance (unless there is some sort of flapper valve in it to close it off) I would expect that it would act about like a cold air return grille. However it would probably be a good idea to tie the furnace circulator and the heating kit blower together - presumably with some extra controls so that the furnace burner won't turn on if the fireplace is burning, or the heater kit blower if it isn't, etc. Hopefully the kit would have some documentation at least on how to do this, possibly even the right controls.

As to the plenum temps, that would be a manual and code type question, but I wouldn't expect it to be a big problem - however I'm sure RSF could answer it if the manual doesn't say... At worst I'd expect that you might have to add a bit extra clearance to combustibles near where the stove heat enters the plenum, but I'd be a bit surprised if that were the case, as I'd expect the heat coming off the stove into the kit to be about the same as what the gas burner would put out.

Definitely seems to me like you are doing the right thing to put the kit in.

Gooserider

The central heating kit's blower does not look to be a standard squirell-cage type and the kit is listed as having a backdraft damper... see following text from the RSF website:

"Central Heating Blower
The Opel, Onyx and Delta fireplaces may be equipped with an
optional 635 cfm external blower. This blower can be
used to circulate the heat from the fireplace in any direction and
must be used if you wish the heat to be distributed to a lower
level than the fireplace. Kit includes two noise reduction collars,
variable speed control, a mounting bracket and a back draft
damper. Option FO-FDHB6-1."

-lumbajac

 

[lumbajac]

Again, not wishing to start a war, but how is it that you can connect a fireplace to your ductwork? How long has this particular code been in effect? I know an entire neighborhood here that was built in the 60's that has the main return for the furnace right next to the fireplace. We used to live there...

Chris

I guess to see how RSF shows that you can connect their Opel 2 or 3 fireplace to your central heating ductwork, see their online brochure at http://www.icc-rsf.com/c/icc/file_db/maindocs_e/RSF-CAT.pdf pages 21-22. I showed this to my local HVAC contractor and he saw no problem with this setup so long as it is done right... he did not recognize any codes that would be violated or any common sense that would suggest it should not be done.

As far as the code limiting the distance of a cold air return to a fireplace and no less than 10' - I hadn't mentioned this idea to my HVAC contractor so he has not yet had a say in this idea. The original person that suggested such a setup to me was the fireplace dealer where I plan to purchase the RSF Opel3 from. He did not mention any codes, but did say that his local HVAC contractors always get mad at him for suggesting this idea to his customers.

Thanks,
lumbajac

 

[Redox]

OK, i had to blow up the drawing to see it. It is pushing the air into the supply duct, which shouldn't be a problem. The regular furnace fan may overwhelm the fireplace's blower when it is running, though. I was concerned about running it into the furnace return which might overheat the furnace. Good luck with it!

Chris

 

[lumbajac]

OK, i had to blow up the drawing to see it. It is pushing the air into the supply duct, which shouldn't be a problem. The regular furnace fan may overwhelm the fireplace's blower when it is running, though. I was concerned about running it into the furnace return which might overheat the furnace. Good luck with it!

Chris

Good point - I will have the system designed so that the central heating kit's blower and the blower of the furnace itself turn on together when the hearth room gets too warm, say 74 F for example, to distribute some of the warm air from the fireplace to the rest of the house. Somehow one should be able to program the variable speed furnace fan to only blow at a low speed as not to overwhelm the blower fan of the central heating kit when they turn on in tandem.... will leave that one up to my HVAC contractor.

lumbajac

 

[Gooserider]

Again, not wishing to start a war, but how is it that you can connect a fireplace to your ductwork? How long has this particular code been in effect? I know an entire neighborhood here that was built in the 60's that has the main return for the furnace right next to the fireplace. We used to live there...

Chris

I don't know the history on when it was adopted, but my understanding is that it started after some people died (usual way codes happen) because they had some CO / smoke problems with their fireplace, and the too close return picked up the CO and distributed it to a sleeping area.... Ditto if there is a fire starting in the appliance area, the return could contribute to flame and smoke spread, reducing the escape time.

One of the things that is now being proposed in MA, and I think elsewhere, is a feature for new construction that I think is a good idea and which has minimum costs - namely to tie the (already required) smoke and CO detecors to the HVAC system so that an alarm condition automatically shuts off the blower. I believe this is already required in comercial structures, and I see no reason it wouldn't be a good idea in residential construction. Also, unlike many other code requirements, it is neither difficult nor expensive to implement - I'm told most furnaces already have the needed control circuitry, and the alarms have interconnect wiring, so all that is really needed is to add one wire to tie the systems together.

While as a hard core Libertarian, I am not a fan of gov't regulation, this one doesn't seem that obnoxious as such things go; if you are going to let the gov't tell you how to build your house in the first place, this isn't an onerous requirement, and it has a good potential for reducing harm.

I mention it here because lumbajac might want to talk to his electrcian / HVAC guys about adding this IMHO worthwhile safety feature while they are at it.

Gooserider