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fresh air intake for hvac

burbo | Posted in Energy, Heating & Insulation on September 3, 2008 01:17am

This is probably a long rambling question that has been answered before. Here goes. I want ( I think) to add a fresh air intake to my hvac system. It is a 90%+ efficient Trane propane unit. If I add a damper that is controlled when the unit comes on, how do I size it? Some people say 4 inch is good?? Also how would I connect the electrical connection? It has a 120V connection for a humidifier and a 120V connection for an electronic air cleaner. Each output can provide 1 amp. Connect one of these to a 24V transformer for a damper with 24V control?

 

Next question. I want to change to a heat pump. Can a heat pump be added to my existing gas furnace to provide the normal heat needed ? Instead of the usual heat strips the gas unit would heat at low temperatures  With propane at about $2.85 a gallon now, I think a heat pump would be more economical. Cooling here in  North Texas costs about $200.00 a month in electricity. My last propane bill was about $600.00 and only lasted about a month.

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Replies

  1. ronbudgell | Sep 03, 2008 01:39am | #1

    burbo

    One possibility is to control a 110 volt powered damper with a line voltage humidistat. Easy. Direct connections. No relays.

    Ron

  2. User avater
    Jeff_Clarke | Sep 03, 2008 05:02am | #2

    The Honeywell EARD series is usually what I use - http://www.grainger.com/Grainger/items/2GZD3?cm_mmc=Google%20Base-_-Controls-_-HONEYWELL-_-HONEYWELL%20Round%20Damper,%20Automatic,%20Dia%205%20In,%20Voltage%2024%20AC,%20Height%206%201%2F2%20In,%20Out these are usually set up as 24 VAC open and spring close (field reversible).   For a residence I would think that a 5" or 6" diameter would be adequate.

    Honeywell makes an appropriate 24 VAC transformer (about $30) if your system doesn't have a 24 VAC circuit.

    If you need to control via line voltage, you can use a voltage sensing relay to activate - they're pretty available at HVAC supply houses.  

    Jeff



    Edited 9/2/2008 10:42 pm ET by Jeff_Clarke

  3. Clewless1 | Sep 03, 2008 05:14am | #3

    Technically, you have to size your combustion air intake ... assuming that is what you are referring to. You size it based on the Btu rating of the furnace and you have to provide a hi/lo intake into your mech. room. If it is only for occupant ventilation, you size it for the number of occupants @ say 15 cfm per occupant. 4" duct may handle maybe what 6+ occupants. If it is a short duct run, you can move a fair amount of air through it, so 4 inch would get you a long ways to where you want.

    If this is a combustion duct, I'd talk to the BO ... if you are dealing w/ propane, I'd make SURE you went to the BO.

    Also, the furnace MAY have to make the damper open before it fires ... that is guarantee that that damper is in fact open before it opens a gas valve and fires off the electronic ignition. In commercial applications this is normally a definate requirement.

    If you already have central air conditioning ... the heat pump should be a no brainer to add as the coil is already there and the system is designed to accomodate the air flow of the coil ... unless the heating mode needs a bigger coil ... guessing that would be 'no' in Texas.

    Food for thought ...

    1. User avater
      Jeff_Clarke | Sep 03, 2008 05:41am | #4

      He's not talking about a combustion air intake.   With a high-efficiency unit (like direct vent) this is accomodated in concentric venting - usually.

      He's talking, literally, about bringing fresh air into a return duct (for tempering) as you would in a commercial setting.

      Jeff

      Edited 9/2/2008 10:45 pm ET by Jeff_Clarke

      1. User avater
        BillHartmann | Sep 03, 2008 06:25am | #5

        "He's not talking about a combustion air intake. With a high-efficiency unit (like direct vent) this is accomodated in concentric venting - usually."Not necessarilyWhile not the best installation many will allow you to use internal air for combustion.And if external air is used it is not alwasy concentric. Often 2 separate pipes a given distance apart..
        .
        A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.

        1. peteshlagor | Sep 06, 2008 08:58pm | #8

          Don't all 90+ efficient units have a built in supply air pipe of some type?  They can't be 90+ without it, can they?

           

          1. User avater
            BillHartmann | Sep 06, 2008 09:27pm | #9

            First of all I don't know the details of how they measure the efficiency. I am under the impression from what I have heard is that it only measure the gas in and the heat out. And does not account any heat that is in internal air that is sent out the flue.And comparing EF on gas water heater between the direct vent and the normal atmospheric vented units it appears that internal air is not counted for.So many message go by, but I think that I have seen that verified by a WH engineer that post her from time to time (and on another forum that I frequent)."Don't all 90+ efficient units have a built in supply air pipe of some type? They can't be 90+ without it, can they?"The basic way that you get the 90+ eff is by having secondary heat exhcnager where the coolor return air passes over the exhaust from the primary heat exchanger. And that cools it enough to get condensation and extracts more energy.Now it is not uncommon for a furnaces to be mounted in unconditioned space (garage or attic) in some areas. So in those there is no value at all in bring in outside air.A friend of mine was looking at repo semi-remudled home. It had a realatively new 90+ in the bedroom.Now I don't now where I got this from. I don't recall if it was from lables and/or looked up the manual, but the best of my recollection is that while it had an intake connection it was optional as to whether it was hooked up or not.And from the code the furnace in the BR was OK, if it did not draw combustion are from the room..
            .
            A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.

          2. danski0224 | Sep 07, 2008 06:05pm | #10

            The supply pipe is for (outdoor) combustion air.

            It has nothing to do with the efficiency of the furnace.

            A few manufacturers offered 80% models with piped in combustion air and a regular metal vent flue.

          3. rich1 | Sep 07, 2008 07:12pm | #11

            The supply pipe is for (outdoor) combustion air.

            It has nothing to do with the efficiency of the furnace.

            Well just to nitpick slightly, the colder the combustion air, the lower the efficiency of the appliance.  :)

             

            And just to stop any arguements from anyone else, generally speaking, a two pipe system works better and is safer than a one pipe system.

            Edited 9/7/2008 12:13 pm ET by rich1

          4. danski0224 | Sep 07, 2008 07:28pm | #12

            To nitpick further :)

            Where are the largest efficiency losses?

            Using colder outdoor air for combustion, piped to the unit as the manufacturer designed

            Using heated indoor air for combustion and allowing cold air to enter the conditioned space uncontrolled around windows and doors?

            Considering that it takes around 14 cubic feet of air to burn 1 cubic foot of natural gas... and your average 100,000 btu furnace is going to bring in 1,400 cubic feet of unconditioned outdoor air for each hour that it operates... and all that cold air needs to be conditioned, I'm guessing that #2 above is much worse.

            None of that is factored into the efficiency ratings, BTW.

            Saving money by taking the low bid single pipe system is not too smart :)

            Edited some math issues :)

            Edited 9/7/2008 10:11 pm ET by danski0224

          5. ronbudgell | Sep 08, 2008 02:49am | #21

            danski

            "your average 100,000 btu furnace is going to bring in 1,400,000 cubic feet of unconditioned outdoor air for each hour that it operates."

            I think your decimal point might be off by about four places and, in any case, it might take four or five or more hours of elapsed time to roll up an hour of burner time.

            Your point still stands, though. This is too much air to be coming into your house uninvited and uncontrolled.

            Ron

          6. danski0224 | Sep 08, 2008 05:09am | #22

            Math was never my strong point :)

          7. rich1 | Sep 08, 2008 08:37am | #23

            Won't argue about a two pipe being better.

            I was taught that a standard furnace needed 30 cubic feet of air when you factored in dilution air for the draft divertor.

            And when you factor in a VS motor often draws less than 1 amp and a belted motor can draw 6 or more amps, a 90%+ furnace should be a no brainer.

            When I get some time, I'll have to check if the proposed new AFUE testing standards account for cost of tempered air.  It's about a hundred pages long.

          8. User avater
            BillHartmann | Sep 07, 2008 07:39pm | #13

            "A few manufacturers offered 80% models with piped in combustion air and a regular metal vent flue."Do you have any idea of who offers these.I did not realize that there where any. I might be interested in that setup.Specially if I go with a dual fuel heat pump as the gas would only be a used a small percentage of time..
            .
            A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.

          9. danski0224 | Sep 07, 2008 07:48pm | #14

            I don't think any are available now. Last one I did was about 5/6 years ago... Armstrong/AirEase. I know Armstrong no longer offered one as of 2/3 years ago.

            They were more money than the regular 80%, and they came with 2 stage gas valves. People did not want to pay the extra cost of the unit + the cost of the PVC pipe.

            If you get the combustion air piping near the appliance, it is almost as good.

            They were marketed as sealed combustion 80% furnaces. Quiet, too, with the burners in a box.

          10. User avater
            BillHartmann | Sep 07, 2008 08:57pm | #15

            I built my house in 79 and tried to be energy efficient, but without being some of the alternate design houses of the time. And without a lot of knowledge and of course no internet.At that time there some electric ignition furnaces with motorized dampners. I like the idea, but was concerned about how reliable that system was.Also my house was built into a hillside and the street enterance is at the garage level, 1/2 flight to 2nd story (all bedrooms) 1/2 flight to the basement under the grage, 1/2 flight to the "main" level.And more 50% is at ground level with large amount of windows on the west side (lake view) and south.All of the HVAC people said "and we will stick a couple of vents in the ceiling for the "basement". I could not convince then that it was not a basement, but had large exterior exposure and needed full conditioning. I knew "a couple of vents in the ceiling" would be cold. There where a couple of house here just like that.FINALY found someone that would put in 2 systems with the first floor downdraft to under slab ducts and floor registers. Also I only installed AC on the 2nd floor system.This basically worked well, but the both furnaces where drastically oversized. So I ended up with a utility closet with 2 furnaces and a gas WH and vents for outside combustion air.And that was not a bad idea. But the detials where not done right and it brings in a lot of cold air and it communicate that with the rest of the house.So I would like to get rid fo the vents and use a direct vent WH, and outside combustion air for the furnace(s).The upgrade from 80 to 90+ is not cheap and there is also the question of trying to place the inlets adn exhaust with the needed clearance between all 3 and windows and walls.And if I kept the 2 systems then that much more expensive.So my concept is to go with one 90+ and heatpump with manual dampeners to distribute the heat up and down.But I have not shopped for it yet..
            .
            A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.

          11. danski0224 | Sep 07, 2008 09:29pm | #16

            If you have a "direct vent" installation (intake and exhaust to the outdoors) your vent termination can be within 12" of a window or door.

            An "indirect vent" (exhaust only) you must be about 4' away (can't remember specifics).

            If you go to the PVC vented water heaters, that is where you will run into venting issues to the doors/windows because they do not have an intake pipe on the residential stuff yet.

            If your equipment is 79 vintage, then the upgrade to 90% may be worth it.

            If you do the heatpump thing, get the variable speed motor on the furnace.

          12. User avater
            BillHartmann | Sep 07, 2008 09:42pm | #17

            "f you go to the PVC vented water heaters, that is where you will run into venting issues to the doors/windows because they do not have an intake pipe on the residential stuff yet."It is right next to the exterior wall so I wanted to go with direct vent. Just an elbow and termination.The PVC are power vented and there are power vents that are also sealed and use outdoor combustion air..
            .
            A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.

          13. danski0224 | Sep 07, 2008 10:17pm | #18

            The PVC are power vented and there are power vents that are also sealed and use outdoor combustion air.

            Haven't seen those yet for resi use. Bout time, I guess.

      2. Clewless1 | Sep 03, 2008 02:40pm | #6

        Maybe the OP can clarify for us? We can't help him if he doesn't help us first  ;)

  4. danski0224 | Sep 06, 2008 05:07pm | #7

    Fresh air intake:

    Simply adding something like a 4" duct to the outdoors connected to the return duct on a typical residential system is a bad idea. The systems are not sized properly for the additional load. The 4" duct into the return is usually ok in heating season, but it will overload the AC in cooling season.

    If you want added fresh air, go with an ERV. Those temper the air somewhat before adding it to your home.

    Heat pump:

    Yes, you can add one. It is just an air conditioner that works in reverse.

    Odds are, you will also need a new indoor coil and a new refrigerant lineset... plus added work because the new 13 SEER coils are larger physically than the 10 SEER stuff.

    It will have to be installed properly, and you will need new multistage controls (thermostat and outdoor temp sensor) to make it work.

    If you need heat one month out of the year, adding a heat pump is not a very economical choice... much like buying a new car for 10 more MPG when your old one is paid off. Maybe your new furnace was not sized properly, or the gas pressure wasn't set at the gas valve.

    Hopefully they installed an intake and exhaust pipe for your furnace.

    If your AC system is less than 13 SEER, and you are considering replacement, then it is a better option. Make sure you review the heat pump specs on the condensers. 



    Edited 9/6/2008 10:12 am ET by danski0224

    1. User avater
      jonblakemore | Sep 07, 2008 10:29pm | #19

      "If you want added fresh air, go with an ERV. Those temper the air somewhat before adding it to your home."

      I don't think you can make that blanket statement. Some systems will need an ERV, but some do well with just a fresh air intake.

      We just built a house that has a pretty low air infiltration rate (~.14 CFM/SFBE at 50pa) and all the calculations pointed to a Honeywell unit as being the most cost effective solution.

      Yes, ERV's will temper the incoming air, but so will the coils on your air handler. In my case, with efficient equipment and a good thermal envelope, the break even point on the extra money for an HRV or ERV was something like 15 years.

      Of course, to make a really informed decision, one would have to know all about the house and climate in question. 

      Jon Blakemore RappahannockINC.com Fredericksburg, VA

  5. IdahoDon | Sep 07, 2008 11:00pm | #20

    If you are looking for efficency and a lot of fresh outside air, a heat recovery ventilator will save a bit of money each month over simply pulling in outside air (and forcing out inside air that you've paid to heat/cool. 

     

     

    Beer was created so carpenters wouldn't rule the world.

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