Tight House + Reg WH = CO problems?
I was hoping to avoid a power vented water heater in my new house. It is my understanding that they are not more efficient (than regular tank style) and more prone to failure.
I just learned from an energy star inspector that energy star certification requires a power vent on the waterheater (tank or tankless). He says that a tight house and a regular venting waterheater can present a back-drafting problem; especially when things like bath or range vents are turned on.
We are not going for energy star, but his point got me thinking; energy star or not, I don’t want CO gases in my house. I made provisions for a regular vent unit from basement to attic in regular B-vent (in fact, the vent is already in).
I briefly considered Tankless, but opted against it becuase the Cat III SS flue would set me back another $600 to $1000 in addition to the cost of the unit. Due to the design of the house, there is no easy way to make that run shorter by going out a side wall. I suppose I could go tankless and mount it in the attic or other area nearer a roof . . . (Hmmm . . .see what happens when you write things out – new idea some to mind.)
We will have an HRV/ERV.
However, what are my options here with the regular WH? Should I “wait and see” if there is a problem by mounting a CO meter near the unit? I might be able to enclose the mechanicals in the basement and provide an outdoor air source? Can I later retro to power vent and keep the B-vent (I don’t think that’s an option). Should I run the PVC vent now from basement to attic so it is there if I need it later?
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An online journal covering the preparation and construction of our new home.
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You're wise to be alert to the possibility.
A lot more info is needed: will the furnace or returns be near the water heater? How many contiguous cubic feet are there in the space the water heater will be in?> Will the furnace be in that space? The clothes dryer? Have/use a whole house fan?
After installation, draft tests can be performed testing performance of the water heater draft (And technical and procedural knowledge is needed to do it right)
Note: the new style water heaters with the anti-flame roll out devices tend to burn much hotter than the earlier style (I've been consistently measuring 500 - 500 on them, compared to 400-450+/-) That means better draft, less chance of backdrafting.
Contact the National Comfort Insatiate at the 800 # on their website http://www.NationalComfortInstit0ute.com for a heating contractor or home inspector near you who has been trained in CO Safety and draft analysis.
>>Should I "wait and see" if there is a problem by mounting a CO meter near the unit?
Not a bad idea, but not definitive
After you move in, a good quality CO detector on each living level is recommended, in my opinion the most important area to protect is the bedrooms - you're most vulnerable when you sleep
Use one of the digital readout types, get the battery operated and mount it slightly above eye-level in the bedroom of the most vulnerable member of the family
If there is a child under 18; elderly folks; someone with a heart condition; someone with anemia or immune system issues, get a low level detector - described at the NCI site.
>>I might be able to enclose the mechanicals in the basement and provide an outdoor air source?
Usually a good approach
>>Can I later retro to power vent and keep the B-vent (I don't think that's an option).
No, the poewer vents have low flue gas temps and corroisve vapors way condense in the flue and damage it.
>>Should I run the PVC vent now from basement to attic so it is there if I need it later?
Usually, the power vent units are vented a couple of feet above grade through a wall
“Experience doesn’t bring wisdom, experience evaluation does. When you fail, stay down there for a while and figure out what went wrong. Don’t run away from your failures, evaluate them and learn from them.”
Dr. John C. Maxwell
I'm far from an expert on the subject - Just have a subjective opinion based on limited experience.
We have the power vents in the house we live in as well as the rental house. Both of them have performed well. We did have an ignitor thingy fail on one unit, but it was replaced under warranty.
I really like the things, and would never go back to conventional water heaters again.
The only downside I can think of is the noise of the fan. But it's not a big deal.
First of all I don't think that the "energy inspector" is correct.
Is he saying that it mandatory to get the power vent to meet energy star. If understand it correctly you get points for doing different things and for passing different tests. Enough points and you get the star. I don't think that a single item will keep you from getting it.
Or does he mean that because of the high sealing of an energy star house that it makes it hard to use other than a power vented WH?
If so there are other solutions.
One of which is the Direct Vented WH. It has a coaxial vent that brings in outside combustion air and exhaust there the center. But I think that they are limited in the distance that you can vent and they are designed for venting through a side wall.
You need to check the catalogs, but I think that there are also power vented direct vent WH's.
Also there are direct vent tankless so that amount of expensive flue pipe is minimal.
(But I see that sidewall venting would be a problem in your case).
"He says that a tight house and a regular venting waterheater can present a back-drafting problem; especially when things like bath or range vents are turned on."
You need to check on this, but I think that you can set them up to "over boost" when an other fan is running to eliminate backdrafting problems.
" It is my understanding that they are not more efficient (than regular tank style)"
Have you checked the EF ratings for them?
>>"He says that a tight house and a regular venting waterheater can present a back-drafting problem; especially when things like bath or range vents are turned on.">>You need to check on this, but I think that you can set them up to "over boost" when an other fan is running to eliminate backdrafting problems.He's talking atmospheric/gravity draft, I think you're talking power vent. Haven't hear of "over-boost" though>> >>" It is my understanding that they are not more efficient (than regular tank style)"
>>Have you checked the EF ratings for them?I spent an idle day last winter looking at as many energy/efficiency ratings as I could find on manufacturer's web sites: I found very little difference among models as well among design types.My belief is that we see furnaces with draft inducers have higher efficiencies, so we tend to assume water heaters do, as well.But the draft fans do different things on furnaces and water heaters.In an 80+, it regulates the flow of air and gas mixture through the heat exchanger, allowing the unit to be more "tightly" designed/set up: a natural draft has to be designed to operate in a wide range of conditions and rates of movement of air and combustion gases into the burner and heat exchanger, the 80+ is much more controlled. I believe, but have not been able to confirm this, that the tighter control on operating conditions allows design for higher efficiency.Similarly with the 90+, although note the 80+ just "pulls" air/gas through the heat exchanger; the 90+ also pressurizes the flue, "pushing" the lower temp gases out.On the draft assisted water heaters, the draft fan is pulling in a lot of air from the immediate area and diluting the flue gasses significantly, reducing CO levels, which is the basis for allowing them to be vented out the side wall.The burners and combustion chambers on natural draft and fan assisted are pretty much the same, though, so the fan doesn't affect efficiency or allow any finer tuning" of the design.
“Experience doesn’t bring wisdom, experience evaluation does. When you fail, stay down there for a while and figure out what went wrong. Don’t run away from your failures, evaluate them and learn from them.”
Dr. John C. Maxwell
">>You need to check on this, but I think that you can set them up to "over boost" when an other fan is running to eliminate backdrafting problems."My copy and paste, the paste got lost.That was suppose to refer to the ERV system.Set it up to put in more air than it takes out when you already have another vent fan taking air out.And I think that comes are integrated some way or another with the bath vent fans.
A power vent solution on the WH was one of the "must have" qualifiers. This is not related to the attainment of the star, but rather an necessity learned from experience. As mentioned, we are not going for the rating, but are interested in staying alive to enjoy the new house.
Related to "over boost" - that assumes that there is a power vent unit installed. My hope was a regular tank style WH that uses a B-vent - no fan.
The more I think about this, the more I may try and do tankless, but not install it in the basement. We have a bonus room over the garage. I could probably figure out a way to put it in there and then it would be a short run out the roof with the vent. Running a cold supply to it and from it should not be a problem.
It's either that or use the regular one and CO meters and "see what happens"
Power-vent, tank-style is not an option I want to pursue.
http://jhausch.blogspot.comAdventures in Home BuildingAn online journal covering the preparation and construction of our new home.
My quote got lost.That over boosting was refering to the HRV which I see that Tim give more info on.
I understand now. I will look into that, too.
http://jhausch.blogspot.comAdventures in Home BuildingAn online journal covering the preparation and construction of our new home.
A natural draft appliance should be provided with enough combustion air and excess air to meet the mechanical code(s) and should never be subjuect to negative pressure.
An ERV is an air exchanger, which is usually a net zero air provider. Fresh air is exchanged for stale air, no make-up provided.
"...a tight house and a regular venting waterheater can present a back-drafting problem..." Yes, they can, amongst other "issues".
A house that does not account for all the exhaust required and where that air volume will come from, whether "tight" or not, is going to be under a negative pressure from time to time, like most of the winter and some of the summer. No mention of any other gas appliances in this house, like a furnace or boiler. In WI, you have to have some heat, right. All gas burning appliances should be accomodated properly as well. If the water heater details were overlooked, the other appliances might be of concern as well.
However, even if you don't backdraft the GWH and/or other appliances, you could potentially be causing undue inleakage and could have problems with mold, summer humidity, dry air in the winter, and at the weakest point in the envelope (and the air will find that point), structural damage due to moisture instrusion and rot.
Some ways to address the limited/no makeup aire after the fact:
1 - buy sealed combustion appliances, 2 - have the HRV setup as imbalanced as possible, 3 - duct combustion air directly into a closed and sealed room that contains the gas burning appliances, 4 - provide enough outside air to keep a very slight positive pressure (which would require matching the exhausts, measuring the leakage (i.e. blower door test) and differential pressure control.
For me, I have sealed combustion appliances, in a closed mechanical room and I provide a surplus of outside air via the return side of my FA system.
Perhaps you should consider a non-powered direct vent model. AOSmith has one and I would guess that all major manufacturers do as well. That way you use outside air for combustion but without a fan. There are installation limitations but they might work for you.
Bill
I had never heard of such a thing. I went searching and found this:
http://www.hotwater.com/products/residential/rg-ssl-dv.html
http://www.hotwater.com/lit/im/media/res_gas/184961-001.pdf
It is an interesting option, but may still not work as desired due to front and back decks on the house and where the utilites will be in the basement. I have to think about this one for a bit . . .
Thanks again.
What I need is a ducted fresh air supply in from below the deck and the B-vent up through the roof. . . . I may seal off the utilities room from the house and then open up some supplies into the room from outside. The Furnace in that room will be std high eff type - which reminds me - I need to talk to them about that pvc flue.
http://jhausch.blogspot.comAdventures in Home BuildingAn online journal covering the preparation and construction of our new home.
This gives you some idea of ways to handle combustion air.http://www.codecheck.com/pg21_22mechanical.html#combustionairBTW, I did that when I built my house in 79. But the room was never sealed so it is really not as eff as it should be.When I replace my furnaces I am hoping to go with seal combustion units and a direct vent WH and then seal up the outside vents.
A tight house containing any natural draft combustion appliances needs to have makeup air provided somehow. There are various ways to handle this, but a commonly used one is a simple air duct from the attic, ending in the utility area.
Without such a makeup air provision, the appliance may not draft sufficiently, causing combustion products to spill into the house. These combustion products are not initially high in CO, but over time, as the oxygen in the house is used up, combustion becomes less efficieent and CO will begin to be produced.
The combination of lower oxygen and high CO can obviously be quite toxic.
I assume that this need for makeup air is what the inspector is talking about. Because of the CO hazard, it is a code requirement to have the makeup air provision, but I've been given to believe that there are ways to provide it that shouldn't themselves cause a significant loss of energy efficiency.
It's a bad idea to use the attic for combustion air. Venting the attic can and does create a negative pressure in the mechanical room. Bring the combustion air directly from outside.
The general rule is that for a natural draft appliance, for every cubic foot of gas burnt, you need 30 cubic feet of air.
An appliance burning 100,000 btu's per hour needs 3000 cubic feet of air per hour.
Yeah, where the combustion air can reasonably be drawn from is dependent on a number of factors. If the attic is used then a rooftop attic fan could prevent and actually reverse the flow of air in the combustion air vent, while a "whole house" fan could produce more airflow than is needed (though generally the house is open when a whole-house fan is used, reducing the problem). Under extreme circumstances (poorly ventillated attic, "unfortunate" arrangement of vents, etc) simply the wind and "chimney" effects could create a vacuum and result in poor/reverse airflow.The far end of the combustion air vent must be placed to avoid effects due to wind, fans, etc.For a water heater, the best approach is probably an enclosure for just the heater, with combustion air brought in through a sidewall. But for certain check local codes and heater manufacturer recommendations.And, of course, if there are ANY combustion appliances in the house, or the house has an attached garage, there should be a CO detector (or several) installed.
If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison