[pwd77]

I live in very cold winter climate and I am looking for ways to reduce loss of warm air.

One obvious source is the over-the-range microwave vent. As far as I know, the only thing between the range vent and the outside is a chinzy little flapper on the microwave. It flaps in windy weather, which cannot be a good sign.

Is there a better way to seal/damper the vent to allow good flow out, but keep warm air in when not in use? I believe the duct is standard 3-1/4 x 10, which collects to a square (10x10"?) "hood" on the rooftop, and there is nothing else in the vent other than the little flapper.


Along these lines, the bathroom vents let in cold air as well. I assume they have the standard flapper as well, which does some, but not enough.

Specific links to products would be very helpful.

Thank you!

[hvac1000]

Basically the little flapper is all that is available and here is why.

The units microwave/bath fans etc do not have a powerful enough blower to operate a better sealing device. If you notice you can almost blow open one of the little damper doors with your breath. This is all so designed to allow the manufactures to offer a cheap/reasonable seal without going to more costly dampers and bigger motors. Another reason for not using bigger motors is that they would really waste energy because they would suck just that much more air you already paid to heat to the outdoors.

Are there ways around this?

Yes with the use of a proper make up air unit.
Any air that is exhaust from a home has to be replaced. Bath fans,microwave oven fans, clothes dryers, gas and oil furnaces,fireplaces, Jen Air style indoor grills, and many other appliances remove preheated air from the home and send it outside with your dollars you paid to preheat it with.

The reason you feel cold air flowing back into your home from various pipes/vents etc. is because your home is running on negative pressure and the home is always looking for any hole or opening be it ever so small to draw the air in it just got rid of by all the machines in your home so it can be balanced or have neutral pressure.

For the average person the cost of a air make up unit that really works is too much money and even the simple one I designed for my home still requires the air from the outside to be preheated which costs about as much to do as what is lost to start with. The big difference is comfort and never having to feel a cold draft. I keep my home just a slight bit on the positive pressure side by drawing outside air through a old oil furnace swing (barometric)damper and then into my return air to be heated. The damper allows for adjustment of the volume from outside and also closes the pipe off when not in use.

There are many parts to this situation and I have only touched on a few. I am sure the net is full of ideas you will just need to search under negative pressure.

[pwd77]

Thanks, HVAC100, this is good information.

I see your point about the cheap exhaust fans requiring minimal backpressure. And it is true that venting air out requires replacement air.

Clearly you have solved the issue by controlling the pressure and airflow in your house, and I think that is the way to go. I will look for more information on the net.

That said, I think leaky vents are not the way to go. That means there is cold air leaking in 24/7, PLUS the time the vent fans are on. I would prefer the airflow be going full blast for the few minutes a day I need it, then closed off.

I would like to systematically replace the cheap fans with more powerful ones, and the same time improve the vent control if possible.

Assuming I would like to upgrade my control over the airflow, do you have any other suggestions?

Thanks!

[hvac1000]

The reason I took the approach that I did was I figured it would be next to impossible to add expensive dampers and motors to all the appliances. So I took the whole house approach.
Other ideas below.

HE100 Air-to-Air Exchanger

Air Exchangers

[pwd77]

Again, I see your point. I am learning a lot from this! Controlling each vent could get expensive, but I would at least like to price it vs. doing the whole house approach.

Is your "slight positive pressure" actively controlled, or do you something simpler (like a small fan on all the time)?

[hvac1000]

Actually it has a active control of sorts. It uses a pressure balance sensor from a very old Honeywell damper system. As the air zone would call as in a ((multi zones duct system)) it would speed up the blower to allow the zone to have proper airflow. When the zone was satisfied it would drop the motor down a speed and naturally close the damper for that zone.

This is the kind of system that controls the blower on the main section of the house. It has been modified from Honeywells intent and I hope it never stops working because I will never be able to replace the sensor since it has been long out of production.

I figured that someday I would have to replace this setup so I have done a bit of research. I believe the below item will work for my purposes with modification but hopefully I will never have to change my design for lack of one part (pressure balance sensor)

IAQ Direct - ZoneHandler

[pwd77]

That looks like a good little device. I hope you never have to use it :)

I have learned a lot, thanks very much. I could go on and on down some of these tangents, but I shall control myself.

[hvac1000]

Glad to be of some help.

[alternatives]

For those who may be reading this (as this is a late reply) I solved this same situation by orering a motorized in-line damper unit (From WW Grainger) and wired it into the fan module on my broan unit. The only minute drawback was is that when the exhaust fan is switched on it takes the motorized controller approx 5-6 seconds to fully open, however I found this to be an excellent solution to an aggrivating problem.

M..
:cool:

[hvac1000]

Most motorized inline damper units have a end switch kit available and that will also not allow the Broan fan unit to operate until the damper is completely open.

The reason I like to control the exhaust fan motor is just in case the damper fails to open. This situation can cause damage to the motor if the motor is left running with the damper shut. The end switch locks out the unit until the damper is open saving it from damage.

[pwd77]

Nice suggestions, thank you.

At this point I think I'm trying to get Porsche performance from a Ford: over-range microwave vents are poor at best. I think I'll put my efforts into a better range hood. A stronger fan should allow a better damper.

That said, its great to know these things exist, and I may reconsider and see if it would work for me.

[smithbro]

Hhhhmmmmmm... Great idea guys. I have been struggling with the idea of some sort of damper in the range hood exhaust system for years (I have been using a an 8 in styrofoam cut out cramed in the opening). I had just about given up until reading your Q & A's. HVAC1000, do you think running a fresh air return from an outside source into my a/c system's return would do the same thing? Then would leaving the fan on circulation (Trane H/P) keep the pressure up when running? Just wonder why my HVAC contractor hasn't mentioned this fix from all the questions I've asked him...

[EyeSimp]

I installed an over-the-range microwave in Vermont and that first winter it was like having a door wide open all the time. I had replaced a plane old fashioned over-the-stove vent hood that never had a problem with air leaking and was totally surprised by the problem.

After having fits trying to sort out the problem this is what I found:
-- My inside vent damper was installed and working. It was being overwhelmed if there was any wind at all and it was always cold even without wind.
-- My outside vent damper was actually blowing open in a strong wind and allowing cold air to pour into the house. This was the original vertical damper on the outside wall, not the roof and it had never caused a problem with the old range hood.
-- The issue turned out to be that the critical distance between the height of the microwave and the height of the outside wall damper had never been adjusted at the time the microwave was installed. There should have been a 3' difference between the height of the internal damper and the height of the outside damper on the exterior wall but mine were level. As a result, high wind and cold air had a straight shot into my kitchen. If you vent to a wall rather than the roof you should check the heights of the dampers carefully and review the instructions for height information.
-- I finally built a wooden box over my original horizontal vent damper with an exit to the bottom. (Looks like a bird house on the wall now.) It did reduce the efficiency of the vent fan so I drilled a couple of 1/2" holes in the face of it. The extra cover provided some relief but cold air remains an issue. I believe the fix is to change the height of the external wall vent but I live in a condo and that's not allowed.

[Pjr3140]

I'm having a similar problem as described above. It appears the microwave damper is working, as I can hear it close when the fan is turned off. The venting runs in metal rectangular duct work through the back of an upper cabinet, into and through the roof into a pvc open ended pipe that ends vertically about a foot above the roof itself.
I can feel the cold air radiantly leaving the duct work in the upper cabinet as well as in and around the microwave, causing a temperature drop in the kitchen area. Is this a proper installation? I have used foam insulation all around the duct work entering the cabinet but without noticeable effect. Should the pvc pipe have a damper on it too? It is obvious that cold air is flowing backwards all the way to the microwave but how to stop that?