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Air intake and HRV

Post in 'The Green Room' started by Beno, Apr 24, 2007.

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  1. Beno

    Beno New Member

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    Hi there,

    Still, about design. The 1800 sq.ft. basement will have an HRV (heat recovery ventilation) to ventilate a well insulated house.
    Do I still need to provide my VC Defiant wood stove with an air intake?
    The wood stove will be located in the basement. Shall I design a pipe for air intake of the stove in the concrete slab, to be there when they pour the concrete?
    Even more, I plan to have a wood stove also at the main level. Not much air flow between the 2 levels. I thought I'll put an HRV also at the main level. Same question there, do I need an air intake for the second stove, at the main level?
    Thanks,
    Beno

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  2. titan

    titan Minister of Fire

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    Hi Beno,you should certainly be able to install one properly- sized HRV to serve the entire house.IMO you should install combustion air lines for any wood stove in a newly constructed tight home because a properly-installed HRV will have its' supply and exhaust air balanced to each other with regard to the homes' minimum ventilation requirements.This is done to try maintain the home at a neutral pressure:too much supply air=positive pressure, too much exhaust=negative pressure.A home under severe negative pressure could pull combustion gasses from any fuel-burning appliance back down the chimney and into the living space.The HRV is also not designed to allow for the extra supply air required by any fuel burning appliances within the home.
  3. Beno

    Beno New Member

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    Thanks Titan. I wonder if the wood stove can have its air intake installed in parallel with the prefab stainless steel chimney? If not, what's the best practice to design the air intake?
  4. stoveguy2esw

    stoveguy2esw Minister of Fire

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    air intakes should be installed in as short and straight a manner as possible, long runs or elbows reduce effectiveness due to resistance also a vertical installation similar to a chimney could be problematic as it may attempt to "draw" as well, though not nearly as much as the flue will this stack effect can reduce the ability of a chimney to pull the correct volume of air to operate the unit at its peak.

    hope this helps

    p.s. i agree whole heartedly with titan about the effect on the hrv, bottom line, if you are building now and can install it as the house is being built, i'd definately do it
  5. titan

    titan Minister of Fire

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    Beno, when installing your combustion air pipes,try keep them as short and straight as possible-within reason of course.As for the HRV-Stoveguy is correct-install it during new construction if possible.I've put them in fully finished homes before and it can be a real PITA with all the required cutting and patching.
  6. Beno

    Beno New Member

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    In the basement I can locate the air intake pipe straight, but at the main level I have no choice, it will have to come under the floor, between the wooden joists. If I'll put it on the floor people will have to jump over it.
  7. sbedelman

    sbedelman New Member

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    Well I am a newbie here but this is my area of expertise so I'll put my two cents in.

    Here is the story and I am assuming here that you have a new fairly tight house. You have to do a whole house audit. Anything less and you can quickly get into trouble.

    First you need to gather a bunch of numbers. How much air your house leaks, and how much the stove or stoves and all other appliances will exhaust from inside the house.

    Since the goal is not to depressurize the house to an extent that draft is adversely affected you start by checking the specification for your stove for maximum decompression. It is almost always expressed in negative Pascals. It probably won't be published so may need to call the vendor. Even then some vendors don't make this information generally available to their support departments. Worst case you won't get it all and have to make an assumption (we can talk about this later).

    Next you add up all the other devices in the house that exhaust air, dryer, bath fans, kitchen fan, whatever. The safest course is to assume that all of these could be in use at once, along with the stove. Don't forget to include appliances like water heaters etc unless these are sealed appliances that have their own air supply. This is the total amount of air you need to balance against makeup.

    The next step is to figure out how much of this makeup air will come naturally via leaks in the house. To do this you do a blower door test. This will tell you how much air you house will admit and different levels of decompression. The test is cheap and I recommend you do a number of different setting but for sure do it at the depressurization maximum for your stove. Keep in mind this number may be quite low, perhaps only a few Pascals.

    So once you have the total leakage at the maximum decompression you compare that to the air requirement. If you come up short you need to either reduce the requirement or supply makeup air.

    If the house is older you almost certainly will be ok unless you have one of those fancy restaurant ranges with their 1000-1800 cfm fans. But if the house is newer you need to be very careful particularly if your appliances are not sealed because backdrafting these isn't a matter of getting a poor draw but in fact could kill you.

    So assuming you come up short and all your appliances are sealed, what then? The easy solution is to make a few assumptions. Assume you are smart enough not to turn on all the bath fans and the dryer and the kitchen exhaust without opening a window. Assume only the bath fan and the dryer or the fan and the kitchen exhaust will all be on at one time. Revise your makeup air requirements and see if you can make it work.

    However if you can't cover the gap you are going to have to supply makeup air. There are a number of way to do this including active and passive. Each has advantages and disadvantages so I will only cover some very basic techiques.

    First of all what most people (including installers) and what some codes even require just doesn't work. A 6" or even a 14" pipe is generally not sufficient to supply enough air to keep a house from depressurizing a couple of Pascals. If you don't believe me run the calculations yourself. The methodology is on the web.

    The problem is that the amount of air moving from outside to inside with that small a pressure differential extremely small. If the amount required is that low then eliminating the pipe altogether and just living with the slight increase in negative pressure will be essentially the same. No you need either a big hole (like a window) or a fan blowing air through that pipe to get enough air into the house to make a meaningful difference.

    But the funny thing is you already have exactly that. It is called your HRV. The other poster is absolutely correct that the HRV really needs to stay balanced. But imagine this. Your kitchen fan is on and blowing 400 cfm our of the house. The makeup air is coming from leakage. But what if you put a damper in the exhaust leg of the HRV so that when the kitchen exhaust came on the air was diverted from going outside to a vent right behind the stove. Rather than going directly outside, the dirty air goes to the stove and, right out the kitchen exhaust to the outside.

    Instead of using up your precious leakage air to supply the exhaust you have a free makeup air. That leaves the leakage air for the other appliance and your stove. All you need is either an interlock from the fan that kicks the HRV into high (most HRVs come with such an override) or a pressure sensor that does the same thing.

    You can remove the number one user of air from the debit side of the chart. This alone is generally enough to bring the house into balance. The HRV is better balanced than it otherwise would be (stove or no stove) and there is more makeup air to go around.

    That little HRV with its small pipe is going to give you maybe 150-600 cfm of free makeup air (depending on the model). You might even upsize if you need a bit more. Ifpossible get one with an ECM motor since it will then run nicely at lower speed and use a fraction of the electricity when doing so.

    Bottom line. You have to do an audit. You need to know how much air the house admits at the stoves rated depressurization and you need to know how much everything else in the house is using from that budget. With that you have the information to do the job right.

    Anything else is guesswork. And if you have unsealed appliance, potentially deadly.

    I hope this helps.

    PS If you house is leaky, then you can ignore all of this. But then you don't need the fresh air pipe either. The house is leaky. That is why most of the time things work out even without and audit.
  8. titan

    titan Minister of Fire

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    Sbedelman, what would really be accomplished by diverting exhaust air from the kitchen fan to the rear of the woodstove?ans.=condensation on nearby windows.If a person is running the range hood for exhaust,odds are good it's because they are boiling something on the stove;why divert this saturated air and keep it in the house?
  9. Beno

    Beno New Member

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    All good points.
    On every level I design the use of: Venmar HRV 2500.
    For bathrooms I'll use also HRV: Venmar THH 1.0.
    The range hood will filter the air and recycle it in the kitchen, not outside.
    The Kerr oil boiler (Comet) will be also provided with air intake.
    I need to find out if a modern dryer can also have its own pipe for air intake.

    I got the idea, in an air tight house every consumer of air must have its own air supply and the floor HRV is for refreshing the air we breath.

    Thanks,
    Beno
  10. sbedelman

    sbedelman New Member

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    I think you misunderstood. I was probably less than clear.

    You divert the exhaust air from the HRV (which normally goes out a vent) to behind the kitchen stove. Then the kitchen hood exhausts this stale air right out of the house.

    This rather cute trick (not my idea by the way) uses the HRV as a free way to provide active makeup air. The diverted air is exiting the house anyway, just via the kitchen stove exhaust hood rather than its own vent whenever the fan is on means that the kitchen hood has essentially its own dedicated makeup air. This is good.
  11. sbedelman

    sbedelman New Member

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    I also realized I was less that clear on one point.

    The problems associated with putting a wood stove in tight house are NOT associated with the additional air the wood stove requires. This number is quite small. The problem is that wood stoves don't take kindly to depressurization. Its the kitchen hood that is causing the problem.

    So imagine a house without a wood stove. It's pretty tight. It has a big honking range and 1100 cfm hood. The GC knows nothing about makeup air. The appliances are sealed (which is pretty much required in a tight house) with their own air supplies.

    The owner turns on the fan. Let me tell you that house is going to depressurize. If there is an HRV it goes completely out of balance. But no big deal. People don't cook all day. The only affect is that air is drawn in via cracks, windows etc and under some circumstances excessive moisture is drawn into the walls. Generally it isn't bad enough to cause mold so all we have is a case of bad design practice.

    Now the owner puts in a wood stove. Suddenly depressurization isn't just a theoretical problem, it is a very real one. Every time the kitchen stove gets used, or perhaps a couple of bath fans are on and the dryer is going the wood stove has problem. Don't blame the stove. The problem was always there.

    Putting a vent in the wall to supply air to the stove isn't going to do much of anything. The amount of air coming in is near zip when the kitchen hood is off and the amount that gets sucked in when the hood is on won't be enough to fix the depressurization.

    That said a passive vent can be used to fix the problem, but the vent needs to be behind the kitchen stove where all the air is going out not near the wood stove. And it has to be big enough to keep the depressurization below the required specification. Do the calculations and you will find this is a big hole.

    Well if you need a big hole why not just open a window in the kitchen? Its not quite a swank as a hidden vent, but a lot cheaper than a passive.

    Alternately if one doesn't have an HRV the next cheapest way is to use back to back fans. Rather than a large passive vent put a vent in that matches the size of the kitchen hood exhaust vent and a fan that blows air into the house (right behind the stove) using a blower motor roughly matched to the exhaust blower cfm. This is cheap reliable and clean design. When the exhaust hood goes on so does the supply blower.

    In summary, know how tight your house it. Know how much depressurization your stove allows. Make sure the house is not depressurized more than that. Either keep other exhaust sources low so that there is plenty of leakage available to source the air for the stove without excessive depressurization or supply air to those places that are exhausting all the air.

    This problem could be easily solved if residential hoods came with integrated make up air the way commericial ones do. With tight houses becoming more common that may yet occur, but until it does I recommend people design and HRV into all new construction with the exhaust from it routed as I have described or use back to back fans.

    I hope this helps.
  12. sbedelman

    sbedelman New Member

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    I have two comments. I think hoods that recycle air are a poor idea. They don't remove smells all that well and are just not as good as conventional exhausts. Dump the stale air coming out of the HRVs behind the stove and use a conventional hood for much higher customer satisfaction.

    Don't sweat the dryer. If I remember these run 50-100 cfm so it just isn't that much and even in a tight house leakage should cover it.

    If you aren't already doing so eliminate bath fans altogether. Hook the vent into the HRVs. Run the HRV all the time at lowest setting with a push button that kicks the HRV to high for a set time period where you would have put the fan switch. HRV manufacturers supply this and it is plug and play. One HRV can support multiple baths.

    If you really want to get fancy put a pollutant monitor in the house that does the same thing. If the level gets high (or CO2) then the HRVs kick to high until it is reduced.

    This kind of design is not only the best it leverages your investment in the HRVs to the maximum as fans and separate make up air equipment and costs are eliminated.

    Finally if you really want to get fancy put an air intake where the garbage goes. It can be quite small and shared with the bath. This little bit of depressurization will keep the smell from leaking out. Works especially well with trash compactors where the garbage hangs around longer.

    I'll save some more tricks just in case anyone is still interested.
  13. Beno

    Beno New Member

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    We don't cook too much, and also the range is not facing an outdoor wall.
    The HRV is not located near the bathrooms, how can the HRV connect to the bathrooms fan?
    Thanks,
    Beno
  14. sbedelman

    sbedelman New Member

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    Well if the layout is such that you can't get there then the answer is it can't. I was thinking about new construction. Since an HRV is pretty small one can pretty much always find a way to put it somewhere that allows access to both the bathroom in lieu of a fan and over to the kitchen to supply make up air.

    Depending on the size and layout of the home it is not uncommon to use 2 HRVs. Then its even easier to have one of them supply the kitchen range make up air.
  15. titan

    titan Minister of Fire

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    I like the trick for the make-up air at the rear of the kitchen stove;never heard of it 'til now but it makes good common sense.I could just interlock a 6" motorized damper to the range hood exhaust fan-I like it!I must bring this up with my local municipal inspectors to see if they'll allow this on my next installation.
    Beno-I put a new direct vented Comet boiler in my last house with radiant heat.Get the Riello burner ent package;it's a nice setup and quiet too.As for bathroom exhaust fans,I would put them in if I were building a new home;yes the HRV can handle the bath exhaust duties but not nearly as quickly as a good dedicated stand-alone exhaust fan.Why bother letting the moisture build up while showering when you could be rid of it as quick as it's made?
  16. Beno

    Beno New Member

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    How much did it cost the radiant heating and for what surface, not including the Kerr boiler?
  17. titan

    titan Minister of Fire

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    Beno, I'm scratching my brain here as I sold that house 3 years ago,install was6 years ago.I think the list price of .5" PEX tubing runs at roughly $1.00 t. The house was a 1.5 story chalet roughly 30'x30' cw a full basement.The complete heating system cost me about $4000.00 but I buy materials at contractor rates and installed myself.The average homeowner would pay around $9000.00 to $11000.00 for such a heating system locally.I am a fan of radiant heating if it's done properly-it will be more efficient than rads and certainly more comfortable throughout the home.
  18. sbedelman

    sbedelman New Member

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    Glad you liked it. Note you can also use a pressure switch as an alternative to the interlock. I am of two minds on this however. On the one hand it means that if any depressurization in excess of the setpoint occurs makeup air is supplied (good) but if that occurs without the fan being on then the exhaust air from the HRV is not going out the kitchen hood (bad).

    I think you should go with the interlock particularly if you have no other fans or appliances exhaust sizeable amounts of air.

    Let me know what the inspector says though I can't imagine he will have a problem once he understands it (ah yes). Trace the air for him. It comes in, goes to the house, goes back to the HRV and then out the hood. No different than dedicated make up air except for the trip from the fresh air supplies and back to the HRV.

    Oh one other thing. I recommend against installing an HRV with the fresh air supplied to the plenum of the forced air heating/cooling system. Much better supply the fresh air to a small duct next to one of the HVAC systems returns.

    Here are a couple of hints as to why.

    The pressure in the plenum swings wildly as the HVAC cycles on and off. What does this do to the HRV balance?

    Most HVAC systems are not set to run 24/7 at a low speed (they should be but that is another story) but cycle generally 20 min on/40 off.

    If you hook the HRV fresh air into the HVAC what is the true rating for your say 250 cfm HRV when it is only running one third of the time? Kind of like buying a car with 300 hp and then instantly derating it to 100 hp. Duh.

    If you feed the fresh air to the house directly the HRV can run 24/7 as it was intended then run the main HVAC cycled. The fresh air still comes into the house even when the HVAC is off and builds up a plume of fresh air in the area near the HVAC return then when the HVAC cycles on it gets circulated around the house.

    Much better than only running the HRV a third the time.
  19. keyman512us

    keyman512us Member

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    First off: "Welcome to all the 'newbie's..." here in this thread...

    Secondly...I'm enjoying following this thread...very informative.

    This is the "first mention" of HRV's I can think of here on the forum.
  20. sbedelman

    sbedelman New Member

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    Tight houses are still more the exception that the rule, but they are becoming much more common given concerns about mold and the desire to save energy.

    As houses become tighter (less air leakage) they provide less fresh air for occupants and it becomes necessary to introduce fresh air mechanically to the structure. HRV's do this while recovering a significant amount of the heat from the exhausted stale air. ERV's do the same thing but also recover moisture however they are limited to operation either in climates or during times of the years that there is no risk of freezing (one can swap an ERV core for and HRV during the winter for areas where there is freezing).

    The problem is that very few builders, engineers or stove installers have ever worked extensively with tight homes and virtually none I have come across fully understand how to do a make up air audit let alone design a well thought out system. It's not that it is complex, but rather no one skilled in the art has every published an accurate point by point how to manual.

    My own interest in the subject started when I was told it wasn't possible to have a traditional Rumford open hearth fireplace in a tight home. Two years and a fair amount of research later I came to the conclusion this was nonsense. It certainly wasn't possible if one failed to provide for adequate make up air (which seems to have been the source of this belief) but was quite straight forward if one did.

    Having calculated the amount of air the house I was working on would require to keep depressurization below a level that would materially impact either the wood stove or the fireplace I then began to research various method to provide it.

    Active make up air is the default practice in commercial buildings but I decided against them for reason of cost and long term reliability. It just wasn't clear to me someone would understand the necessity to have the fan in 20 years and if having eliminated it would fail to comprehend the reasons why the fireplace was smoking.

    I therefore went with a passive system. Since I wanted whole house fan anyway for evening cooling on those days AC wasn't required I calculated what size opening would be enough to supply the air. 16 square feet was about the answer which was pretty close to the size of a standard whole house fan. That was convenient so I figured out a way that the same opening could be used to supply the make up air. If there is sufficient interest I'll describe it as it is a pretty cute design.

    But as I continued to work on the HVAC system the idea of using the HRV came into focus. That was just too sweet to ignore so that is what I went with in the end. I still have the honking hole if for some reason the HRVs aren't able to supply the required flow, but I am fairly certain they will for all but the most extreme conditions.

    Of course I won't know for sure until the sheet rock and all the window and doors are in at which time I will be able to do the blower door test. If the leakage is less than I project (which is good actually) and we go with a grill on the kitchen range (which requires gobs of air) then I might need to make use of the passive air when the kitchen hood is going full blast.

    If nothing else what I have learned from this experience is to be wary of experts who can't adequately explain why what they say is true, because it often is a sign they are operating from long held beliefs that may or may not be supportable once examined carefully.

    I also came to believe that extremely tight houses are a very, very good idea, as are unvented crawl spaces.

    And finally that these types of structures work fine with an open hearth fireplace. You just have to plan carefully and know what you are doing.

    Just don't forget the air tight chimney cap.
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