HVAC vs fireplace

[Cliffh]

Last spring we had the 4 ton HVAC replaced with a 5 ton unit.  It was a great improvement over the summer, the AC would cool the house quickly and would keep it cool without having to run all day; it'd normally run for ~10 minutes at a time.  And the electric bill went down by almost 1/3.

I'm having a bit of a problem now that it's getting colder.  I like to have a fire in the fireplace in the evenings, sometimes all day if it's cold enough.  Mainly the fireplace is meant to supplement the electric heater in the HVAC.  The actual problem is that the new unit draws a *lot* of air.  Without measurements I'd guess at just about twice what the old unit moved.  Now it's pulling so much that the smoke from the fireplace is drawn into the house instead of going up the chimney.  Not a good thing with two old folks who both have COPD.  Not to mention setting off the smoke alarms.

I've lessened the problem by re-installing the glass doors on the fireplace and closing the flue to ~half open.  Still isn't enough though.

I thought I'd pass it by y'all for ideas/possible solutions to discuss with the AC guy.

[Hawkmoon]

The answer may not lie with the a/c guy. How old is your house, and are the  fireplace and chimney masonry, or metal?

For a fair number of years now (I don't remember when it crept into the building codes), fireplaces have been required to have glass doors, and a duct or opening of some kind inside the firebox to admit fresh air directly to the fire so you don't suck heated air out of the house and send it up the chimney. The reason is energy conservation. Fireplaces are not a very good way to heat a house -- the fireplace only heats one room, but it pulls air back out of that room to feed the fire and then that air goes up the flue. And the warm air that goes up the flue is replaced by cold air that gets sucked into the house through cracks and loose-fitting doors and windows.

The idea behind the glass doors and the inlet duct is to make the fireplace into its own closed system. By drawing combustion air directly from outdoors, you avoid sucking heated air out of the house and replacing it with cold air.

So my suggestion would be to explore if there's a way you can create some sort of duct to bring make-up air directly to the fireplace. You should never close the damper, even part way, when there's a fire burning.

[Nick1911]

PM sent.

[Hawkmoon]

FYI: https://www.askthebuilder.com/fresh-air-intake-vents/

[K Frame]

In the interim, open a window in the room with the fireplace to equalize the pressure and give the fireplace some extra combustion air.

[Ron]

I would have a chimney sweep company come out and check inside the chimney for restrictions etc.

Replacing the air handler shouldn't create any change in the air pressure of the house. Furnaces up to 80% or so draw air from in the house and send it up the vent pipe as well as hot water heaters also, that can create a small negative pressure inside the house. An air handler without gas heat just moves air around and shouldn't create downdrafts.

Opening the flue up all the way and cracking a window in the room with the fireplace are both good ideas. Closing the flue and starving the fire of air will both create more soot.

Did you replace your windows/doors this year? New soffits and fascia?

It sounds like your fireplace either has a restriction in the flue or is starving for combustion air.

[Brad Johnson]

Your HVAC unit shouldn't be creating any draw at all unless you have a fresh air exchanger that's malfunctioning. A central HVAC system is enclosed in the housing envelope so any pressure deltas should be internal to the system, not relative to home's interior/exterior. If you turn the system on and it creates a noticeable pressure change, especially if it's enough to backdraft a fireplace chimney, something is wrong.

Brad

[Cliffh]

The fireplace insert and chimney are both metal - basically a metal box inset into the wall with a pipe going through the roof - in a '98 double wide.  It's a Coleman/Evcon 36ECMII  https://www.allpartsinc.com/36-wood-burning-fireplace-36ecmii-36ecmii.html  http://fmisupport.hcents.com/supportdocs/54024F.pdf

We had the original, drafty, windows replaced and a metal roof installed at approx. the same time as the new unit was installed.  As mentioned earlier, the new unit sucks - there's a draft if you stand in front of the air intake - definitely more than the old one.

 The glass doors are bi-fold, and there's about 1/4" gap between the each of the door panels and at the hinge sides.

  I know you're not supposed to shut the flue since the gases have to have an escape path, but the smoke was already coming into the room.  Closing the flue half way seemed to help in that there wasn't as much air being pulled down the chimney, across the fire and into the room.

  If I've got a good fire going it seems to generate enough heat/updraft to overcome the draw from the heater.  There's no problem with smoke entering the house if the heater isn't running, whether the fire's roaring or it's died down to just embers.

  If I have a fresh air intake installed in the firebox, seems to me that when the fire starts dying down for the night and the heater kicks in, it'll pull air down the chimney (which is clean btw) and in though the air intake, even with the fireplace doors closed, since the doors/front of the firebox isn't air tight.  If I open a window, it'll start pulling in cold air from the outside too.

[Cliffh]

Almost forgot to mention that the heater is electric.

[Jim147]

If they replaced the furnace with one that will move the air the 5 ton needs yeah I can see the problem. Not to make it bad but if they didn't change duct work that is maybe good for 3-3 1/2 ton of airflow.

That single point return air would probaly suck the crome off a trailer ball.

[Hawkmoon]

I don't understand how your heater works. If it's pulling air down the chimney when it runs, that means it's creating a vacuum inside the house. If it's creating a vacuum, it must be exhausting air from the house to the outdoors. Why is it exhausting the air you've just paid it to heat?

Doesn't make sense to me. If that's what it's doing, I don't think it's operating correctly.

[Ron]

I don't understand how your heater works. If it's pulling air down the chimney when it runs, that means it's creating a vacuum inside the house. If it's creating a vacuum, it must be exhausting air from the house to the outdoors. Why is it exhausting the air you've just paid it to heat?

Doesn't make sense to me. If that's what it's doing, I don't think it's operating correctly.

I'm not a residential HVAC guy but I'll hazard a potential scenario.

The room with the fireplace has insufficient supply ductwork. The room has sufficiently sized return ductwork.

If the air handler for the new A/C was replaced and has a higher HP drive than the ducts were spec'd for:

The room possibly could run in a negative pressure between the fireplace combustion exhaust and higher volume return air to the airhandler compared to the volume of air supplied to the room. Other rooms would have to be in a positive pressure then I guess.

Without being there it is all speculation of course.

Regardless he needs more combustion air.

[Hawkmoon]

I'm not a residential HVAC guy but I'll hazard a potential scenario.

The room with the fireplace has insufficient supply ductwork. The room has sufficiently sized return ductwork.

If the air handler for the new A/C was replaced and has a higher HP drive than the ducts were spec'd for:

The room possibly could run in a negative pressure between the fireplace combustion exhaust and higher volume return air to the airhandler compared to the volume of air supplied to the room. Other rooms would have to be in a positive pressure then I guess.

If that's what's happening, the system needs to be balanced.

Even so, there needs to be an isolated make-up air supply for the firebox if the plan is to use the fireplace. I love fireplaces, but my house was built in 1950, decades before anyone thought of making homes "tight" and providing make-up air to fireplaces. Plus, the thermostat is just outside the living room, and the entire house (a ranch, with a bedroom in the converted attic) only has one heating zone so heating the living room with the fireplace would effectively turn off the heat to the rest of the house. (Hot water baseboard heat, not forced air.)

[MechAg94]

I would have a chimney sweep company come out and check inside the chimney for restrictions etc.

Replacing the air handler shouldn't create any change in the air pressure of the house. Furnaces up to 80% or so draw air from in the house and send it up the vent pipe as well as hot water heaters also, that can create a small negative pressure inside the house. An air handler without gas heat just moves air around and shouldn't create downdrafts.

Opening the flue up all the way and cracking a window in the room with the fireplace are both good ideas. Closing the flue and starving the fire of air will both create more soot.

Did you replace your windows/doors this year? New soffits and fascia?

It sounds like your fireplace either has a restriction in the flue or is starving for combustion air.

No need for that.  Just pile a great big bunch of Christmas wrapping paper in the fireplace and light it off.  That ought to burn out any restrictions.   

(don't do that)

[Cliffh]

Got home the evening of the 9th, opened the laptop & received a message basically saying "I'm broke so bad you've got to take me to the repair shop".  Picked it, (and the broken HDD), up this morning - on the way home from the hospital where my surgery had been canceled.  (grumble) Got up early, skipped breakfast, drove 1 1/2 hours, only to be sent home 'cause the doc was "busy with a trauma patient" (/grumble)

Had a chat with Nick1911 where we determined that what's most likely happening is there's not enough air coming out of the floor registers to supply the air demands of the heater.  A couple possible reasons are that the supply ducting is way too small (my idea) or there's a leak in the supply ducting (Nick's idea, most likely the correct answer).

DW talked with the AC installer, his idea was that there's a problem with the chimney - that the chimney was blocked, not letting the exhaust out and it would backup into the room.  He recommended a chimney sweep, she called and made the appointment.  I talked with the sweep when he called to confirm, he didn't seem to hear me when I mentioned that when the heater's running you can feel the cold air coming out of the fireplace even when the fire's out, the damper's closed and the doors are shut.

The chimney is now clean, and we're still having the same problem.

The sweeps' suggestions were to a) don't use the fireplace b) open a hole in the wall to feed air into the room and/or c) open a window near the air return.

Inspecting the ducting would be a serious pain in the ass.  It runs under the house, with about 6" of fiberglass insulation held in place by underbelly wrap (basically a tarp) covering the entire underside of the house.  I am not in any shape for tearing all that apart.  

Next step will be calling around to see if there's an HVAC company around who can inspect the ducting, preferably with a camera.

Today we just shut off the heater & used the fireplace, we'll reverse that tonight.

ETA:  I did a quick & dirty test on the air flow from the floor registers.  Turned on the house fan and held a few squares of TP over each register.  The house is a rectangle, there are two supply lines running lengthwise, one on either side of center-line of the house.  The registers on the rear side of center-line are definitely putting out more air than the others.

[Cliffh]

One other suggestion had been to throttle back the air intake by blocking it with "something".  Not sure I like that "fix" either, seems as if it'd put more of a strain on the motor.

[Ron]

Does each room have a return register or is there a single return register (usually in a hallway)?

Do you have any rooms you rarely or never use where you might be able to close the supply registers?

[Cliffh]

All of the rooms have a floor register (outlet), including the bathrooms.  There's one intake for the HVAC, located directly above the unit, which is in the laundry room at one end of the house.

There are a couple of rooms we normally keep the doors to closed.  I don't know that closing the registers would allow more air to circulate through the house, seems as if it'd put more back-pressure on the system and potentially force more air out of any leaks in the ducting.

Thinking about the closed doors, the air supplied to those rooms can only exit those rooms through the small space under the doors.  That right there is going to cut back on the amount of air available to the intake, and will have be made up from another source - such as the chimney.  We'll have to run an experiment. 

[Jim147]

You're right. I know it's noisy but leaving the laundry room door open will also help.

[Cliffh]

And you're right too - it is noisy.  We rarely close that door.  The builder did take into account that someone might want to close that door while the unit's running, he/they put a grill in the lower part of the door that's the same size as the grill on the air intake for the unit.

[Ben]

Not to hijack the thread, but is there something wrong with closing the door where the HVAC is? Mine is in the laundry room as well and I always close that door at night since it's next to my bedroom and I don't want the extra noise at night. I don't have any kind of vent in the door.

[dogmush]

Where the unit is is less important then where the air return is.

Does your air handler have a ducted return, or does it just suck in from one end of the unit?

Mine is in the garage, and the returns are ducted to pull from the house.

[Ben]

Where the unit is is less important then where the air return is.

Does your air handler have a ducted return, or does it just suck in from one end of the unit?

Mine is in the garage, and the returns are ducted to pull from the house.

I don't completely understand my system. It is a dual system - heat pump for heat and cooling, and propane furnace for heating at below 26deg. When I bought the place, it was set up so there was a register with filter in the living room ceiling, a register with filter in the upstairs bonus room, ans a smaller register in a hallway connecting the two back bedrooms.

That system crapped out, and the new one has extra large ducting coming down from the ceiling to the furnace unit (but filtering for the heat pump as well) with one of those ginormous 8" thick air filters. They removed the filters from the LR and bonus room registers and told me to leave it that way.  I hear air being pulled from those registers, and there is no obvious place, like a register, for "air in" in the laundry room.

[Jim147]

You are ok closing the door. The other unit just sucks through the door when it is closed.

[Hawkmoon]

I don't completely understand my system. It is a dual system - heat pump for heat and cooling, and propane furnace for heating at below 26deg. When I bought the place, it was set up so there was a register with filter in the living room ceiling, a register with filter in the upstairs bonus room, ans a smaller register in a hallway connecting the two back bedrooms.

That system crapped out, and the new one has extra large ducting coming down from the ceiling to the furnace unit (but filtering for the heat pump as well) with one of those ginormous 8" thick air filters. They removed the filters from the LR and bonus room registers and told me to leave it that way.  I hear air being pulled from those registers, and there is no obvious place, like a register, for "air in" in the laundry room.

Your description seems to be incomplete. Do you have ducted supply and free return, ducted return but free supply (highly unlijkely), or both supply and return ducted?