HVAC vs fireplace

[Ron]

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.  

Ironically, I was thinking that by restricting flow into unused rooms it might increase the systems static pressure and push more air into the room where you need it the most (fireplace room). If you are keeping some spare room doors closed you are for all practical purposes doing that already.

So my idea was not only wrong but in fact might be if not the main cause, at least a contributing factor to your problem.

Keep all the spare room doors open and make sure all the supply registers are fully open, see if that changes anything.

Ultimately though, you are sending air up through the chimney stack and out of the house.

That air has to be replaced from someplace. If your house is real tight there will be no way to make up the air being sent out the chimney. That's a recipe for venting problems.

[Hawkmoon]

Ironically, I was thinking that by restricting flow into unused rooms it might increase the systems static pressure and push more air into the room where you need it the most (fireplace room). If you are keeping some spare room doors closed you are for all practical purposes doing that already.

So my idea was not only wrong but in fact might be if not the main cause, at least a contributing factor to your problem.

Keep all the spare room doors open and make sure all the supply registers are fully open, see if that changes anything.

Ultimately though, you are sending air up through the chimney stack and out of the house.

That air has to be replaced from someplace. If your house is real tight there will be no way to make up the air being sent out the chimney. That's a recipe for venting problems.

In tight houses, fireplaces simply don't work, because after a short time there isn't enough air to send up the chimney due to the lower pressure iside the envelope compared to outdoors. This leads to smokey fireplaces and CO2 (or CO) contamination. That's why the building codes have, for a number of years, required a dedicated make-up air duct or opening to feed combustion air directly into the firebox.

[Cliffh]

Went to bed last night with the heater running & set at 72*.  Woke up this morning with the heater still running and the thermostat reading 68*.  Started checking the various registers, a couple had good airflow, some had very little.  Strange thing is that one of the registers with good airflow is also the one that's furthest away from the unit.  Put in another call to the original AC installer, I'll try describing the situation in different terms when he shows Monday.

I hadn't thought of the air loss through the chimney due to the fire/heat, and that having to be made up somehow.  That may end up being another problem once we get the (likely) duct problem fixed.  This old place isn't all that tight - it is tighter than it used to be - so there may be enough incoming through other leaks that it won't have to draw from the chimney.  Although it may still draw from the chimney once the fire starts dying.  Then again, there shouldn't be as much loss up the chimney once there's not as much heat...  Then again, it'll most likely draw from the area with the least resistance, which may end up being the chimney.....

[Hawkmoon]

Went to bed last night with the heater running & set at 72*.  Woke up this morning with the heater still running and the thermostat reading 68*.  Started checking the various registers, a couple had good airflow, some had very little.  Strange thing is that one of the registers with good airflow is also the one that's furthest away from the unit.  Put in another call to the original AC installer, I'll try describing the situation in different terms when he shows Monday.

I hadn't thought of the air loss through the chimney due to the fire/heat, and that having to be made up somehow.  That may end up being another problem once we get the (likely) duct problem fixed.  This old place isn't all that tight - it is tighter than it used to be - so there may be enough incoming through other leaks that it won't have to draw from the chimney.  Although it may still draw from the chimney once the fire starts dying.  Then again, there shouldn't be as much loss up the chimney once there's not as much heat...  Then again, it'll most likely draw from the area with the least resistance, which may end up being the chimney.....

You still haven't (IMHO) adequately described your heating system. Start with the basics -- ignore the fireplace and chimney, as if there's no fire and the damper is closed. How does the heating system work? Unless it's a fairly modern system that continually exhausts a percentage of warm air and brings in a corresponding amount of fresh, outdoor air to avoid "sick building syndrome," it should be a closed system. All it's doing is pushing air through the ducts and registers from one part of the house to another. As such, it's balanced. A closed system is always balanced. You can change the amount of heat one room gets compared to another by opening and partially closing registers, but the overall system is just pushing air around inside a closed box.

Now you open the damper and light a fire. The fire needs air to burn, and the hot air from the fire goes up the chimney and is exhausted. That air has to be replaced. In leaky old houses, the replacement air sneaked in through cracks around doors and windows. The term for this is "infiltration." Because you're sending heated air up the chimney and replacing it with cold, outdoor air, it wastes energy. That's why the building codes changed to mandate glass doors and make-up air inlets for the firebox. That allows the fire to pull air from outdoors to use as combustion air rather than using the air you already paid to have heated by the furnace. By using glass doors to separate the firebox from the house, you allow (in theory) the house to remain a closed, balanced system while the fireplace and chimney operate as a separate, flow-though system.

The question is why your heating system isn't functioning as a closed system.

[Cliffh]

It's a closed system as you described above, it's not designed to bring in outside air on purpose.

Right now, my money's on a leak in the ducting.  The closed system isn't - it's loosing air somewhere.  The fan's drawing the air from the house, pushing it out into the ducting, the air is not coming back into the house from the ducting (evidenced by the low/lack of flow at too many registers) - causing the need for supplemental air to be drawn in through *something*. 

[Hawkmoon]

I agree. If the system didn't have a leak it really couldn't pull air down the chimney.

The good news is that you don't have to worry about pipes freezing in the crawlspace. 

[Ron]

I agree. If the system didn't have a leak it really couldn't pull air down the chimney.

The good news is that you don't have to worry about pipes freezing in the crawlspace. 

Sure is looking that way.

Trying to fix things over the phone or internet frequently doesn't work out well.

Hopefully the probable causes have been narrowed down.

[Cliffh]

Discussing the problem here (and with Nick1911) has narrowed the probable causes & given me a direction to point the repair guys.

And pointed out I may have to do something about a fresh air intake for the fireplace.

Yeah, the pipes aren't going to freeze - if I can afford to keep heating the crawlspace.    :)

[Cliffh]

The HVAC guy just left with my check.  I could have fixed it myself, but a) he was here b) it didn't cost all that much and c) I'm getting too old & broken to be working overhead under the house if I don't absolutely have to.

There are two main runs to the 12" ducting paralleling the long walls and a couple of feet to either side of center-line.  There's a 12" flexible cross-over connecting the two main runs.  The crossover had come loose from one run.

I'll bet the critters under the house appreciated the heat. 

Since it's projected to be too warm the next couple of nights to test the fireplace with the heater running I may just turn the fan on with a fire going and see what happens.