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How Tight is Too Tight?

Crissie
Crissie Member Posts: 122
Bought a tri-level home 9 years ago. Home needed work and we did a complete rehab. Insulated walls with R15 Rockwool and Poly vapor barrier (manufacturer's spec). Attic had R45-50 Rockwool. Just had a blower door test done. The house is very tight, 25% tighter than code requires. It take 5 hours to do a full air exchange. Have a fresh air intake in the return of furnace, pulls 90 CFM but only when furnace or ac runs. How much fresh air should I bring in through an ERV? I feel like the house is too tight and it is definitely making me ill.

Tore out ceiling to expose HVAC ductwork. Someone had disconnected a large section of the return ducts and left huge gaps. Every time the system runs, it was pulling in ceiling cavity air with rockwool insulation and 40 yrs of construction dust / dirt. Major portions of the ductwork needs to be completely replaced. Feel like I need as much fresh air as possible. Could I bring in 250 CFM and only exhaust 200 CFM?

Comments

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 3,314
    If your house is 100% tight, and you bring 250 cfm and only exhaust 200 cfm, you will eventually over-pressurize your house and it will explode and then you will experience rapid decompression. If you have oxygen masks in the overhead compartment, they will probably deploy at that moment. be sure to put your mask on before helping anyone else with their mask.

    But seriously, if you bring air in, if has to leave somewhere else. Once you get to a certain pressure difference between inside and outside, the fans will stop moving air. Think of it like this... if you place a piece of plywood in the path of a fan blowing 1000 cfm, how much air will the fan move with the duct blocked 100%? No air will move, and you will most likely burn up the fan motor in the process!

    You are experiencing off gassing of the fabric, glues, paint, cleaning chemicals, and other items in your newly renovated home. You need to get some fresh air in that home. Open a window for Cripes Sake! (that would be Jesus Cripes, of the church of Holy Moly!)
    Edward Young
    Retired HVAC Contractor from So. Jersey.
    Services first oil burner at age 16
    P/T trainer for EH-CC.org
    Crissie
  • Crissie
    Crissie Member Posts: 122

    If your house is 100% tight, and you bring 250 cfm and only exhaust 200 cfm, you will eventually over-pressurize your house and it will explode and then you will experience rapid decompression. If you have oxygen masks in the overhead compartment, they will probably deploy at that moment. be sure to put your mask on before helping anyone else with their mask.

    But seriously, if you bring air in, if has to leave somewhere else. Once you get to a certain pressure difference between inside and outside, the fans will stop moving air. Think of it like this... if you place a piece of plywood in the path of a fan blowing 1000 cfm, how much air will the fan move with the duct blocked 100%? No air will move, and you will most likely burn up the fan motor in the process!

    You are experiencing off gassing of the fabric, glues, paint, cleaning chemicals, and other items in your newly renovated home. You need to get some fresh air in that home. Open a window for Cripes Sake! (that would be Jesus Cripes, of the church of Holy Moly!)

    LOL. I have a window open as much as possible, can't do that in zero or 90 degree weather. The HVAC is making me ill because of the breaches in ductwork and i think there is mildew / mold in it. When that doesn't run, it is very stuffy and the lack of fresh air is clear.
    I thought it was good to pressurize a home slightly.
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 12,126
    @Crissie

    Yes you want to bring in slightly more than you exhaust. The reason is that the make up air is usually heated or cooled before entering the space. By slightly pressurizing you keeping untreated air outside the home. Slight exfiltration versus infiltration.

    If you use an eRV same thing it is recovering the BTUs from the conditioned air air that is being exhausted
  • ratio
    ratio Member Posts: 3,216
    Set the fan on the furnace to "on" & it will bring in fresh air the whole time. Might not be a fix, but it should help, & you can always turn it back to "auto".
    Crissie
  • HVACNUT
    HVACNUT Member Posts: 4,622
    edited October 2021
    If its moldy in the ductwork, what's it look like in the blower compartment? You might want to replace it unless a duct cleaning company can get rid of it.
    Look into UV light, ionization generator, whole house steam humidifier, whole house dehumidifier, a pleated air filter, etc. When you move into the "tight house" area, it's now a controlled environment. 
    Does anything control the intake air or is there something like the Aprilaire kit shown above?
    The same company that did the blower door test should have been able to tell (sell) you everything you need concerning Manual J, duct sizing, air sealing and how much air exchange is needed. They can even do a Duct Blaster test to show how much duct leakage there is. 

    CrissieCanuckerSTEVEusaPA
  • Jamie Hall
    Jamie Hall Member Posts: 19,116
    There is, so far as I recall, no hard and fast requirement for air changes per hour in a residence (commercil nd industrial uses have requirements). The general guideline is between 2 and 4. So for a 1200 square foot house with normal ceilings you'd be wanting 320 to 640 cfm. If it were mine I'd go on the high side. Your house is probably bigger than 1200 square feet total floor area -- so you will need a good bit more than that.

    Note that reading off the lines in the Aprilaire chart above does not produce the ASHRAE required cfm from the formula; the formula is correct.

    The requirement is for both chemical and biological hazards.

    The ventilation should, ideally, be through a heat recovery ventilator (sensible heat only -- do NOT recover latent heat, as that will recycle most chemical and biohazards back into the space).

    You mention that you feel the house is too tight and may be making you ill -- and I'd say you are quite correct. Do something about it... don't skimp!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Crissie
  • Crissie
    Crissie Member Posts: 122
    HVACNUT said:

    If its moldy in the ductwork, what's it look like in the blower compartment? You might want to replace it unless a duct cleaning company can get rid of it.
    Look into UV light, ionization generator, whole house steam humidifier, whole house dehumidifier, a pleated air filter, etc. When you move into the "tight house" area, it's now a controlled environment. 
    Does anything control the intake air or is there something like the Aprilaire kit shown above?
    The same company that did the blower door test should have been able to tell (sell) you everything you need concerning Manual J, duct sizing, air sealing and how much air exchange is needed. They can even do a Duct Blaster test to show how much duct leakage there is. 

    The return ductwork is panned through ceiling and wall cavities. Very tight area as it is a split house and all ducts are in ceiling of lower living area. Someone tampered with the return ducts and disconnected the returns to the upper level, left holes. I need to replace all ductwork and was planned to use boiler heat for a large part of the house. The company that did the blower door test doesn't do any J manual cal's or anything else. I wasn't impressed, it was sort of a waste of time - other than learning how tight my home was.
    I have three levels in the house and feel like I need ventilation in all three.
  • Crissie
    Crissie Member Posts: 122

    There is, so far as I recall, no hard and fast requirement for air changes per hour in a residence (commercil nd industrial uses have requirements). The general guideline is between 2 and 4. So for a 1200 square foot house with normal ceilings you'd be wanting 320 to 640 cfm. If it were mine I'd go on the high side. Your house is probably bigger than 1200 square feet total floor area -- so you will need a good bit more than that.

    Note that reading off the lines in the Aprilaire chart above does not produce the ASHRAE required cfm from the formula; the formula is correct.

    The requirement is for both chemical and biological hazards.

    The ventilation should, ideally, be through a heat recovery ventilator (sensible heat only -- do NOT recover latent heat, as that will recycle most chemical and biohazards back into the space).

    You mention that you feel the house is too tight and may be making you ill -- and I'd say you are quite correct. Do something about it... don't skimp!

    House is 2800 sq ft. What is difference between sensible and latent? Is sensible supply duct air? Well, ductwork is contaminated. I am trying to replace the entire HVAC system. Was hoping to use boiler heat and then reinstall ductwork for ventilation and AC. I can't seem to find a contractor who can help me. Have had a dozen guys come through, and 25% of the time I don't hear back, or weeks later they come back with something stupid, like just replace your furnace.
    Started back in April with the thought of using ductless mini-splits. Was talked out of that (Chicago, would need secondary heat), then moved onto a boiler heat system. In the middle of boiler quotes and after calling in 6 companies, still don't have one I feel confident with. A few couple have said just replace all ductwork with a single furnace. But, they don't follow with a proposal to do that. There are three levels to house, they all have different temp's and air quality. Duct work is inadequate in lower level. One company looked at ducts and said they can't replace them, not enough room. Another promised me a proposal 2 weeks ago, haven't heard back. Another can replace all ducts, but is booked until March. So I am not skimping, I seriously cannot find a company to take this on. Getting an education!
  • Jamie Hall
    Jamie Hall Member Posts: 19,116
    You do, @Crissie . That 2 to 4 complete air changes per hour applies to each room individually, as well as to the structure as a whole. The return ductwork should be complete -- it's actually more important than the supply ductwork.

    It may be, technically, inadequate in size, even if it is complete. That's not unusual, but can be compensated for (at the expense of some noise sometimes) with adequate fan power.

    It is not at all uncommon for houses to be built much too tight. Saves energy, you know, and we're all for that. But the air changes per house which I mentioned above have been found out to be really important for health This was not a problem in much older structures -- they usually have adequate air changes per hour as is to begin with and they don't usually have the assorted contents and finishes which produce chemical problems in more modern structures, though they still have people!.

    As I said in my previous post, use the formula given in the Aptrilaire page above. Don't use the graph -- it doesn't compute, and I have no idea how they came up with it.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Crissie
  • kcopp
    kcopp Member Posts: 3,996
    I dont think your house is too tight....hard to actually do that.
    There are Passive house assemblies out there that are less than 1 ach50
    I think your Scorched air heating system was installed poorly.
    You could install a HRV (not ERV) to deal w/ moisture issues.
    What was the blower door test result? how many air changes per hour at 50 pascals?
  • Crissie
    Crissie Member Posts: 122
    edited October 2021
    kcopp said:

    I dont think your house is too tight....hard to actually do that.
    There are Passive house assemblies out there that are less than 1 ach50
    I think your Scorched air heating system was installed poorly.
    You could install a HRV (not ERV) to deal w/ moisture issues.
    What was the blower door test result? how many air changes per hour at 50 pascals?

    Result of Blower Door: "The home is rather tight at ~2.9 air changes at 50 Pa (ACH50) -
    about 75% the leakiness of a basic code compliant home (<4
    ACH50). Mechanical fresh air exchange is necessary to maintain air
    quality and the fresh air intake should continue to run as
    necessary for this purpose (at least 90 cfm, 24/365)." "The home is relatively tight at about 1 air change every 5.13 hours."

  • kcopp
    kcopp Member Posts: 3,996
    Crissie said:

    kcopp said:

    I dont think your house is too tight....hard to actually do that.
    There are Passive house assemblies out there that are less than 1 ach50
    I think your Scorched air heating system was installed poorly.
    You could install a HRV (not ERV) to deal w/ moisture issues.
    What was the blower door test result? how many air changes per hour at 50 pascals?

    Result of Blower Door: "The home is rather tight at ~2.9 air changes at 50 Pa (ACH50) -
    about 75% the leakiness of a basic code compliant home (
    That isnt bad.
    That I believe is just under the 2015 Energy code minimum for climate zones 5 and lower... 3.0 ach50.
    Running ductwork in an attic has always been a poor idea....but there again I'm not a fan of FHA system
    period for heating.
    Crissie
  • STEVEusaPA
    STEVEusaPA Member Posts: 5,720
    edited October 2021
    There's a super knowledge person on another forum who is an IAQ expert. Some of his recommendations:

    "Keep your home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter."</i>

    The usual recommendation is a whole house dehumidifier, bring in fresh air. Keeps humidity down on warm rainy days when it's not cool enough to run the AC and shoulder seasons.
    steve
    Crissie
  • Jamie Hall
    Jamie Hall Member Posts: 19,116
    Some 50 years ago, with Pres. Carter's oil embargo, everyone jumped on the energy efficiency bandwagon, and many have never fallen off. Indoor air quality, as a result, became somewhere between bad and horrible, and as a result in many houses built or tightened up since then the indoor air quality is actually considerably worse than outdoor air -- even in unban areas where the outdoor air quality is not too good.

    The problem isn't moisture. The problem is airborne pathogens (Covid is only the most recent in a string of them) and outgassed chemicals from finishes, carpeting, glues in furniture (and sometimes structure), insulating materials... the list goes on. Yes, if you are in a tight building, moisture can be a problem -- but other contaminants are a much worse problem and, unfortunately not visible or apparent to smell.

    We ran into this problem with some of the early passive solar houses we built, and very quickly -- because we were paying attention-- found that there was no reasonable substitute for adequate outside air changes. While very high level filtration (HEPA or MERV 20 or higher) combined with UV disinfection can control pathogens, oxygen depletion and contaminant build up can only be handled with adsorption and regeneration. This is practical of, say, the International Space Station -- but hardly for the average house.

    We experimented initially with energy recovery units which recovered latent as well as sensible heat, but very quickly realised that they simply recycled the pathogens and contaminants. So we went with sensible heat recovery units.

    Put very simply, a structure which is too tight will save you energy -- and make you sick or kill you.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • neilc
    neilc Member Posts: 1,805
    so I'm reading both your threads,
    and questioning the boiler sizes proposed, the existing 115k furnace, and that it replaced a 80k,
    that the 115 "cycles" tremendously,
    were you present with the 80k? and do you remember that one cycling on it's worse design day ?

    because my thinking is that even the old 80 was too large(cycling), and if you're as tight as you say, 60 or even 40 is more likely your number, do that heatloss calc as suggested by others, and get your ball park number,
    another option is to "Math" your annual gas usage history, and degree days for your zip, subtracting the summer domestic cooking and hot water average, from the winter months, dividing the degree days, and there's your input BTUs you used for heating, math again for the efficiency rating for the furnace , , , output BTUs.

    and I write all this because,
    find and fix all that return duct work, inspect and fix any supplies in need,
    (yeah, it may mean opening ceilings and walls)
    duct work is probably sized large enough for a proper sized furnace, maybe not the 115, but for, say, the 60.
    the guys that said your ductwork was too small, for what size furnace were they proposing ?

    and get the ERV running, tied into furnace ductwork
    known to beat dead horses
    Larry WeingartenCrissie
  • kcopp
    kcopp Member Posts: 3,996

    Some 50 years ago, with Pres. Carter's oil embargo, everyone jumped on the energy efficiency bandwagon, and many have never fallen off. Indoor air quality, as a result, became somewhere between bad and horrible, and as a result in many houses built or tightened up since then the indoor air quality is actually considerably worse than outdoor air -- even in unban areas where the outdoor air quality is not too good.

    The problem isn't moisture. The problem is airborne pathogens (Covid is only the most recent in a string of them) and outgassed chemicals from finishes, carpeting, glues in furniture (and sometimes structure), insulating materials... the list goes on. Yes, if you are in a tight building, moisture can be a problem -- but other contaminants are a much worse problem and, unfortunately not visible or apparent to smell.

    We ran into this problem with some of the early passive solar houses we built, and very quickly -- because we were paying attention-- found that there was no reasonable substitute for adequate outside air changes. While very high level filtration (HEPA or MERV 20 or higher) combined with UV disinfection can control pathogens, oxygen depletion and contaminant build up can only be handled with adsorption and regeneration. This is practical of, say, the International Space Station -- but hardly for the average house.

    We experimented initially with energy recovery units which recovered latent as well as sensible heat, but very quickly realised that they simply recycled the pathogens and contaminants. So we went with sensible heat recovery units.

    Put very simply, a structure which is too tight will save you energy -- and make you sick or kill you.

    Jamie,
    I respectfully disagree with you.
    Outside air is seldom great Especially where pollen and pollutants are always around. Is is not better to control enters the building and then filter what's inside? There are 4 filters in an HRV (heating climate) vs an ERV (cooling climate)
    Canucker
  • ratio
    ratio Member Posts: 3,216
    If the duct size is small, it's in your interest to see how small you get get away with WRT any furnace/AC.

    Take a look at https://www.slantfin.com/slantfin-heat-loss-calculator/, you can make a reasonable stab at the heat loss yourself, as a double-check of someone elses' heat loss or sizing.

    Crissie
  • Larry Weingarten
    Larry Weingarten Member Posts: 2,583
    Hi @Crissie , You are correct that you want a slight over pressure in the house. This keeps the not so good air in crawlspace and attic out of the living space. But, that can’t work with such damaged ductwork. I particularly like the holistic approach that @neilc suggested. The ductwork needs to be almost tight enough to hold water. All joints sealed with mastic, even the lengthwise seams. And, indoor pollutants, like particleboard or gas stove need to be dealt with. These things will help make your home livable again.

    Yours, Larry
    Crissie
  • Crissie
    Crissie Member Posts: 122
    neilc said:

    so I'm reading both your threads,
    and questioning the boiler sizes proposed, the existing 115k furnace, and that it replaced a 80k,
    that the 115 "cycles" tremendously,
    were you present with the 80k? and do you remember that one cycling on it's worse design day ?

    because my thinking is that even the old 80 was too large(cycling), and if you're as tight as you say, 60 or even 40 is more likely your number, do that heatloss calc as suggested by others, and get your ball park number,
    another option is to "Math" your annual gas usage history, and degree days for your zip, subtracting the summer domestic cooking and hot water average, from the winter months, dividing the degree days, and there's your input BTUs you used for heating, math again for the efficiency rating for the furnace , , , output BTUs.

    and I write all this because,
    find and fix all that return duct work, inspect and fix any supplies in need,
    (yeah, it may mean opening ceilings and walls)
    duct work is probably sized large enough for a proper sized furnace, maybe not the 115, but for, say, the 60.
    the guys that said your ductwork was too small, for what size furnace were they proposing ?

    and get the ERV running, tied into furnace ductwork

    The 90 BTU furnace was operating when I bought the house, I wasn't living in it, was doing rehab work, so had the temp set at 60 degrees. It was not cycling, but I wasn't living here so do not know how adequate it was. however, I did upgrade all insulation, the attic maybe had 5 inches. So the house is much tighter.
    I did tear out about 450 sq ft of ceiling in lower level to expose the main duct runs. They were a mess. Returns panned in ceiling cavity, second main return disconnected. Holes pulling air from ceiling cavities, and the duct stink, even after a cleaning several ozone treatments. The issue is there is no headroom to run the ducts, very tight area. There are four 90 degree turns of the main duct runs. The guy said there was not enough room to replace the ducts, because of the tight area, number of turns, and the return ducts are panned, which I would not want to do. I cannot aero seal if any of the ductwork is panned, they will not touch it.
    I was considering a boiler system because of all of this and love the idea of that type of heat. If anything, I could replace the ductwork and just use it for one or two levels. Still would need to find a contractor to do that - it is challenging. They say they can, but then I don't hear back from them. Interesting how this trade works.
    Yes, I am doing the load calculation and selecting an ERV. Is it unusual for a homeowner to have a mechanical engineer design a boiler or HVAC for a contractor to work off of? Thanks for your help.
  • Crissie
    Crissie Member Posts: 122

    Hi @Crissie , You are correct that you want a slight over pressure in the house. This keeps the not so good air in crawlspace and attic out of the living space. But, that can’t work with such damaged ductwork. I particularly like the holistic approach that @neilc suggested. The ductwork needs to be almost tight enough to hold water. All joints sealed with mastic, even the lengthwise seams. And, indoor pollutants, like particleboard or gas stove need to be dealt with. These things will help make your home livable again.

    Yours, Larry

    Larry, how much positive pressure, like 50 CFM? thanks
  • pecmsg
    pecmsg Member Posts: 2,948
    kcopp said:
    Some 50 years ago, with Pres. Carter's oil embargo, everyone jumped on the energy efficiency bandwagon, and many have never fallen off. Indoor air quality, as a result, became somewhere between bad and horrible, and as a result in many houses built or tightened up since then the indoor air quality is actually considerably worse than outdoor air -- even in unban areas where the outdoor air quality is not too good. The problem isn't moisture. The problem is airborne pathogens (Covid is only the most recent in a string of them) and outgassed chemicals from finishes, carpeting, glues in furniture (and sometimes structure), insulating materials... the list goes on. Yes, if you are in a tight building, moisture can be a problem -- but other contaminants are a much worse problem and, unfortunately not visible or apparent to smell. We ran into this problem with some of the early passive solar houses we built, and very quickly -- because we were paying attention-- found that there was no reasonable substitute for adequate outside air changes. While very high level filtration (HEPA or MERV 20 or higher) combined with UV disinfection can control pathogens, oxygen depletion and contaminant build up can only be handled with adsorption and regeneration. This is practical of, say, the International Space Station -- but hardly for the average house. We experimented initially with energy recovery units which recovered latent as well as sensible heat, but very quickly realised that they simply recycled the pathogens and contaminants. So we went with sensible heat recovery units. Put very simply, a structure which is too tight will save you energy -- and make you sick or kill you.
    Jamie, I respectfully disagree with you. Outside air is seldom great Especially where pollen and pollutants are always around. Is is not better to control enters the building and then filter what's inside? There are 4 filters in an HRV (heating climate) vs an ERV (cooling climate)
    What happens if you live in a plastic bubble or a walk in box? Eventually all the indoor pollutants build and become Deadly. Yes a complete change of air is needed every 3-5 hours. Your better off controlling how much air and conditioning it as it enters.  A simple CO 2 monitor in a tight home will show how quickly the levels rise. 
    Crissie
  • Crissie
    Crissie Member Posts: 122
    ratio said:

    If the duct size is small, it's in your interest to see how small you get get away with WRT any furnace/AC.

    Take a look at https://www.slantfin.com/slantfin-heat-loss-calculator/, you can make a reasonable stab at the heat loss yourself, as a double-check of someone elses' heat loss or sizing.

    That Slant Fin web app freezes when I enter a job name, not sure it is working.
  • ratio
    ratio Member Posts: 3,216
    Crissie said:

    That Slant Fin web app freezes when I enter a job name, not sure it is working

    Huh, IDK. I use the Android app.
  • Jamie Hall
    Jamie Hall Member Posts: 19,116
    Well. Three threads with the same message. Download the program to your PC and run it on your PC. If it's a PC. It may or may not run on an Apple product.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England