Home too tight

RonDuch

I'm working on a mobile home with a neg. the pressure inside the home. Someone insulated the mobile home too much and now have a neg. the pressure inside when the furnace, bath fan and range hood are tested at the same time. The furnace is fairly new with an intake vent connected to the outside When all the things are going it brings in furnace fumes. I want to bring in makeup air somehow. I thought I would install an 8" duct with a hood on the outside and an inline damper inside it, not the kind that goes on a furnace tee but it's inline that you can adjust. But the owner is afraid that it will bring in cold air when the wind blows outside. Any recommendations will be taken. email me for more info. king9471@gmail.com.

ratio

The damper on the outside air could have an actuator installed on it, spring closed power open. Many furnaces have a line voltage EAC terminal, which is energized whenever the indoor fan is running. I'm not too familiar with mobile home equipment, but even if that terminal is not available, a current switch can be snapped around the blower wire to tell the damper when to open. Tie the outside air duct into the return plenum somewhere, the damper will only open when the fan is running, which usually means the unit is operating, which means that no unconditioned air is getting dumped into the space.

Jamie Hall

Classic. One should always shoot for two to four air changes per hour in a structure, and if one buttons it up too tight one will, indeed, have problems.

I would suggest that one good possibility would be to use a barometric damper -- such as is used on a boiler exhaust -- with an outside air intake and hooked into the return to the furnace (I presume that this is forced air -- anything else is vanishingly rare in mobile homes!). Set the damper so that it is very light -- that is, allows air in at the minimum possible negative pressure (you really want a slight positive pressure at all times inside, but that may not be possible). Make sure that the intake for that line is as far as possible from the kitchen and bath vents -- and certainly as far as possible from the furnace combustion vent. (I am also assuming that the intake vent to which you refer is for the furnace combustion air -- if not -- if the furnace combustion air comes from inside the mobile home -- you need to also provide an independent air intake for furnace combustion air).

If that doesn't do the job, I think you will have to put a fan on that outside air intake to which I refer above. Size it to two air changes per hour. If the place is really tight, you may even have to provide a dedicated exhaust vent somewhere -- but I would bet that the kitchen and bath fans leak enough to do the job.

Will it bring in cold air under some conditions? Indeed it will. But introducing that cold air into the furnace return will limit the impact.

I would not use a damper which the occupant can fiddle. They will, and you will be called back.

kcopp

"Classic. One should always shoot for two to four air changes per hour in a structure, and if one buttons it up too tight one will, indeed, have problems."

This is a misunderstood concept.
You truly want to make a homes envelope as tight as you can.
The saying " the house needs to breathe" is an old wives tale.
Air tight and vapor permeable.... that should be the goal.
Trapped moisture is what will give you the issues.
You will need to bring in a make up air but that will be clean air.
Erv/ hrv technology has come a long way. Some of the units install directly through the wall. 6" in diameter.
Appliances will need to be smaller and of sealed combustion.
.........

As to the original question.... You need to explain combustion air to the customer. Can you install a "fan-in-a-can?

Zman

@RonDuch
Could you provide some more details?
If your furnace is pulling it's own combustion air from the outside, how are the furnace fumes being drawn back in? It sounds like the furnace has been isolated.
There are several ways to deal with the hood and the bath fan. What are their ratings?
I agree with kcopp. "Build it tight and ventilate right"

Jamie Hall

I have no argument with the build it tight concept. None at all. Provided, however, that there is makeup air, as @kcopp mentions. In adequate volume -- which is two to four air changes per hour. If it can be brought in and exhausted through a sensible heat heat exchanger, so much the better (not a heat exchanger which includes latent heat). If one is using a heat exchanger, there is no need to heat (or cool, in warmer climates) the incoming air (although there may be condensation issues when cooling).

The biggest problem is that people do the build it or make it tight part of the equation -- and leave out or ignore the air exchange part, with the result that in many buildings I have looked at the indoor air quality is substantially worse than the outdoor air quality, particularly with regard to volatile organics (and sometimes communicable diseases!).

I first ran into this problem in the mid '70s (remember the Arab oil embargo?) when a lot of buildings I dealt with got themselves all sealed up -- and we started having a lot of health issues with the occupants (schools were among the worst affected).

ChrisJ

Jamie Hall said:

I have no argument with the build it tight concept. None at all. Provided, however, that there is makeup air, as @kcopp mentions. In adequate volume -- which is two to four air changes per hour. If it can be brought in and exhausted through a sensible heat heat exchanger, so much the better (not a heat exchanger which includes latent heat). If one is using a heat exchanger, there is no need to heat (or cool, in warmer climates) the incoming air (although there may be condensation issues when cooling).

The biggest problem is that people do the build it or make it tight part of the equation -- and leave out or ignore the air exchange part, with the result that in many buildings I have looked at the indoor air quality is substantially worse than the outdoor air quality, particularly with regard to volatile organics (and sometimes communicable diseases!).

I first ran into this problem in the mid '70s (remember the Arab oil embargo?) when a lot of buildings I dealt with got themselves all sealed up -- and we started having a lot of health issues with the occupants (schools were among the worst affected).

Forget about heating equipment for a second and just think, do you really want to be breathing everyone else's breath repeatedly? Or even your own for that matter? Let's not even think about what comes from the other end, and in what quantities.

I like a building that breaths.
Maybe not near as much as mine, but, I do like a good amount.

EBEBRATT-Ed

Others have made good points but no one has mentioned this:

Leaky ductwork.

maybe you have restricted returns with leaky supply ducts? That will make a building negative in the blink of an eye.

leaky ducts, disconnected or leaking supply duct

If not, bring OA into the RA plenum as others have mentioned mixed with the return air.

or use an ERV

Gordy

ChrisJ said:

Jamie Hall said:

I have no argument with the build it tight concept. None at all. Provided, however, that there is makeup air, as @kcopp mentions. In adequate volume -- which is two to four air changes per hour. If it can be brought in and exhausted through a sensible heat heat exchanger, so much the better (not a heat exchanger which includes latent heat). If one is using a heat exchanger, there is no need to heat (or cool, in warmer climates) the incoming air (although there may be condensation issues when cooling).

The biggest problem is that people do the build it or make it tight part of the equation -- and leave out or ignore the air exchange part, with the result that in many buildings I have looked at the indoor air quality is substantially worse than the outdoor air quality, particularly with regard to volatile organics (and sometimes communicable diseases!).

I first ran into this problem in the mid '70s (remember the Arab oil embargo?) when a lot of buildings I dealt with got themselves all sealed up -- and we started having a lot of health issues with the occupants (schools were among the worst affected).

Forget about heating equipment for a second and just think, do you really want to be breathing everyone else's breath repeatedly? Or even your own for that matter? Let's not even think about what comes from the other end, and in what quantities.

I like a building that breaths.
Maybe not near as much as mine, but, I do like a good amount.

The concept is to control the infiltration on your terms. To do this requires proper design, and mechanicals ERV, or HRV depending on climate. With proper design tight envelopes function very good, and are much more energy efficient. And no you won't be breathing everyone else's odors.

Zman

@RonDuch
Are you still there or a "One Post Wonder".
I don't think someone is going to email you as requested. It's just not the way forums work.

Gordy

What goes out must come in. What comes in should be conditioned to the space. Simple as that.

BillW

Bringing in outside air directly can lead to all sorts of problems. The only time it is acceptable to do that is when it is providing combustion air for a furnace, boiler or water heater. Then, the air is directly fed to the burner, or into a closed heating equipment compartment. I'm not sure if you are trying to get combustion air or ventilation. Negative pressures can be deadly, if they cause carbon monoxide to be pulled back into the house. The best way to provide ventilation air is by a heat recovery ventilator. They treat the outside air with a filter, then recover up to 80% of the sensible heat, providing ventilation and energy savings. They also tend to dry the space they are supplying. They must be mounted in a conditioned space, and must have a drain to dispose of the water from the defrost cycle.. They may be free standing, or ducted into a forced air system.

ChrisJ

Gordy said:

What goes out must come in. What comes in should be conditioned to the space. Simple as that.

I've had very bad luck explaining this to people.
I'd say, most people don't realize if you exhaust air, you must bring air in, some how, some where.

A co worker yesterday told me to put a fan in my attic to blow the hot air out. I said I had considered it, but was concerned about pulling moisture in. He said "oh no it won't do that, it just blows the heat out, it doesn't pull anything in".

I gave up.

Gordy

Depends on how well the attic is ventilated via soffit/gable, and roof venting, and if the attic floor to living space ceiling detail is sealed well.

It's better to have a cooler attic, but if the above mentioned is done poorly then yes you can put the living space under negative pressure drawing in high moisture content air putting more burden on the AC system among other unintended consequences.

If the above mentioned is done properly then there is no need for an attic exhaust fan. Natural convection draws air in through soffit venting, and exits roof venting. Still in doing so if the attic/living space ceiling detail is poorly sealed you can still have negative pressure impacting the conditioned space.

Plenty of visual stimulation on the web on how air based comfort systems, combustion appliances,wind etc effect pressure differences in homes. Even the location of FA systems ductwork .

ChrisJ

Gordy said:

Depends on how well the attic is ventilated via soffit/gable, and roof venting, and if the attic floor to living space ceiling detail is sealed well.

It's better to have a cooler attic, but if the above mentioned is done poorly then yes you can put the living space under negative pressure drawing in high moisture content air putting more burden on the AC system among other unintended consequences.

If the above mentioned is done properly then there is no need for an attic exhaust fan. Natural convection draws air in through soffit venting, and exits roof venting. Still in doing so if the attic/living space ceiling detail is poorly sealed you can still have negative pressure impacting the conditioned space.

Plenty of visual stimulation on the web on how air based comfort systems, combustion appliances,wind etc effect pressure differences in homes. Even the location of FA systems ductwork .

My concerns are also over moisture in the attic, not just the living space.

My attic is hot, damn hot, but it's dry. Moisture is a huge concern due to having ductwork up there, as well as the air handler, line set etc. Remember, my attic has no intentional ventilation at this point, though it originally had a slate roof which likely provided a good amount.

Right now, nothing, and no mold or moisture issues either which is why I'm very hesitant about making modifications. Such things need to be done with great care and consideration.

Heat..........while is bad, it's not as bad as condensation.

Gordy

I don't think it would be an issue. In your case. Winter time is the biggest issue with attic condensation. Warm moist interior space air getting pulled up into the unconditioned attic coming into contact with building materials.


Think of it like this. You run a higher risk of duct work condensation the hotter the attic. If at present you are not seeing condensation on, or inside your ducts with your hot attic. Doing something to bring that attic temp down will not hurt anything except bring in outside air to the living space. By venting the attic you help take the load off the ducts, and air handler. Decreasing the possibility of reaching the dew point inside the duct work.

ChrisJ

Gordy said:

I don't think it would be an issue. In your case. Winter time is the biggest issue with attic condensation. Warm moist interior space air getting pulled up into the unconditioned attic coming into contact with building materials.


Think of it like this. You run a higher risk of duct work condensation the hotter the attic. If at present you are not seeing condensation on, or inside your ducts with your hot attic. Doing something to bring that attic temp down will not hurt anything except bring in outside air to the living space. By venting the attic you help take the load off the ducts, and air handler. Decreasing the possibility of reaching the dew point inside the duct work.

That's incorrect.

You run a higher risk with duct work condensation with a lower dew point.

The attic primarily getting air from inside, means it's dew point is incredibly low when it's 125F up there. Much lower than if it had outside air coming in which is say, 85F and 65%.

You're right though, the big risk is in the winter, and I've never seen any signs of moisture up there.

Also, bringing in fresh air will in crease the chances of reaching dew point inside the duct work in the winter, that's bad.

Hot air in the summer isn't doing anything with the dew point inside the duct work, except lowering it, if anything.

I currently have little insulation, that air space trapped in the attic acts as insulation.



72 degrees at 45% warmed to 125 degrees gives a RH of 8% and a dew point of 48 degrees.

85 degrees at 65% is a dew point of 73 degrees. When raised to 125 degrees you have 20% RH. With a dew point of 73 degrees you are far more likely to condensate than you are at 48 degrees.

Gordy

Execute, and use what ever information you wish. There is a whole web full of it. Not just here.

Gordy

Apparently in your specific case from readings/comments your attic temp seems to be residually conditioned from the conditioned space. Seems I have seen you reporting humidity near occupied space conditions.However I also noticed wild temp swings. 111, 125 degrees during the day, and close to oat at night.

Jamie Hall

Couple of miscellaneous thoughts... first, unless I am mistaken, the dewpoint of an air mass does not change with the temperature (provided it stays above the dewpoint). For what it's worth.

On the subject of high temperatures in attic (or other) spaces, though, I do have a caution from some vaguely remembered material -- one needs to be a bit wary of some plywoods and their glues... but I don't recall the temperatures involved.

Gordy

What I have not seen is what the humidity is in the attic, verses out doors when the attic is 125 degrees.

ChrisJ

I cannot say because my wireless thermometer's hydrometer isn't accurate at 125F

But the attic's dew point matches indoors all other times, no reason to think it wouldn't at higher temps.

Gordy

In my above statement that you seem to find incorrect. There are assumptions being made.

Your ductwork is insulated to an r8, and has a vapor barrier.


If your fine with what you have leave it alone. 125 degrees does create more load on duct work and equipment in the attic. It's your cooling bill. I guess analyzing that after a reliable utilities bill appears, and compare to window shakers era. If it's cheaper than before leave it. Monitoring duct works interior, and exterior for condensation.

Many attic installs out there in which the attic is ventilated with zero issues. But then the attic, and conditioned space are separated with sealing, and decent insulation.

I will add that having equipment in the conditioned space is superior no doubt about it. However that is not what you have. More of a secondary conditioning that happens by accident, and is not deliberate nor consistent.

Big Ed_4

Anyone do an ventilation system to meet code in an smoking area ?