You need to look at it as three requirements

Ventialtion Air - you need this for good air quality and in tight homes, you need this especially in the winter to keep your windows from condensating.

Combustion Air - this air that the furnace and water need for combustion

Make-up Air, in a tight home this is something needed when you have large exhaust appliances like a Jenn Air range or they have some imported Italian Stainless Steel Hood in the kitchen.

In an residential application HRVs are generally a neutral pressure ventilation scheme to give you your ventilation air. This means the exhauist and the fresh air rates are about the same. In commercial buildings you can run them out of balance to acheive the desired pressurization.

In a commercial building you could have the exhaust exceed the fresh air rate by about 10% to create a slight negative in the winter. Then in the summer you could run them with the fresh air rate 10% higher than the exhaust. These pressurizations are set to control which way air flows through the building envelope. In the winter it is better to encourage dry winter air to pass in through the walls as it will not condense on its way in. In the summer, you encourage dry room air to leak out through the building to prevent humid outdoor air from passing through the building envelope and causing condensation.

So with the basics out of the way, you need to make sure appliances have combustion air, and in my opinion directly connecting combustion air to a new condensing furnace is the way to go. Natural draft water heaters could be replaced by something direct vent, or power vented and then this takes care of these appliances from back drafting.

My opinion is when a house has to run a humidifier it means there is absolutely no shortage of fresh air moving through the building. A dry house is either a drafty house or an overventilated house. I usually have a saying that in the winter your windows will tell you if you have enough fresh air or not, as they will condensate if you do not have enough fresh air.

I usually recommend a ventialtion system that can provide about 0.3 air changes an hour and in the winter I would suggest running it off of a dehumidistat. As humidity builds to the point that windows will start sweating, the ventilation kicks on and gives a good blast of fresh air until RH drops. I usually recommend settting it for 35% RH in the winter.

Others seem to prefer a smaller amount of constant fresh air. I find in the winter that this can over dry a home to the point that people are then running humidifiers. To me the ventialtion is needed to get the humidity down in winter so running a humidifier on top of the ventialtion is like running heating and cooling at the same time.

For make up air, if you do have the large exhaust appliances you may be able to have a motorized damper interlocked with the applaince to open and spill air into the basement in an area where there are no water lines. With the larger exhaust appliances like the Italian hoods that remove 600 to 1200 CFM, you may need a make up air fan and perhaps an electric duct heater to temper air up to about 55F. With very tight homes with these large exhaust appliances and no make up air you may smell sewer gas when the large exhaust runs without make up air

 

I am wo

First priority appears to be: Accommodate the existing exhausting devices to avoid any sort of natural gas backdraft (?) - avoiding negative pressurization and achieving an optimal air pressure balance.

Next priority appears to be: Assure the proper number of fresh air changes are taking place per hour/day.

Finally, I want to assure that as much energy is being conserved as possible while keeping a safe and healthy house.

I ~think~ the best approach is to add a Heat Recovery Ventilator to the system and link it to the furnace blower. But given the two blowers in the HRV, I wonder if it will balance the pressure effectively. Do I need something more ... um ... passive?

Budget is not the highest priority but I want to make sure the selected HVAC contractor does not over engineer the system either. _Thanks for adding your advice and warnings._

Note: I am a newbie here @ HVAC-Talk and still learning to search this amazing knowledge base. I apologies in advance if my question has been throughly covered in an existing thread. I am still searching but for now, I am all ears!

Dealing with you 1st priority, make-up for the exhaust devices.
During cold weather, the stack effect pressure is comparable to the average 7 mph wind pressure. This means much more fresh air in the winter than during warm calm weather. Even air tight home may 70 cfm of fresh air infiltrating and exfiltrating. Calm, warm weather with temps inside the same as outside is near 0 pressure difference or no natural fresh air infiltration. I agree with the point that no moisture on the windows indicates enough fresh air to remove the moisture from the occupants. Two adults in a large home could have dry windows but not enough natural fresh air infiltration to purge indoor pollutants. But only supplent a small amount of fresh air, <50CFM when occupied. Ideally, comfort could also require a small amount humidification. It all depends on how crude you are willing make the design. If no or only a small amount of supplimental winter fresh air is needed, a small make-up air ventilation port on the furnace would suffice.

Dealing with your second priority- adequate air change when occupied. As the weather warms, the amount of fresh air infiltrating declines. You need more mechanical fresh air venitlation during the summer when the home is occupied and the wind is not blowing than during the winter. HRV provide no make-up air because they exhaust as much air as they introduce. Usually, 70-100 cfm of make-up fresh air when the home is occupied, meets the make-up air require ment and adequate air change to purge the indoor pollutants. If you have a green grass climate home that does not need fresh air during the cold winter months, providing minimal fresh make-up air when the home is occupied may be best provided by a ventilating dehumidifer. Mainly because a good dehumidifier is required in this type of home when an HRV/ERV is used. The a/c will keep the home <50%RH during hot weather. The dehumidifier part is to deal with moisture in the fresh air/occupants during moderate wet weather with low/no cooling loads. You could have both HRV/ERV and a good whole house dehu, high investment with minimal return on the HRV/ERV. An HRV/ERV <$150/year, Most can not afford both. Keep us posted on further question and results.
Regards TB

 

Thank you both for your thought-filled replies. Wow. I need to digest what you wrote and I will be back with a few questions and clarification requests. Thanks again!

 

A couple topic related questions...

1. Are HRV/ERV's typically suited for homes that stay closed up for long periods of time (weeks/months) without opening windows, etc?

2. If you are able to open windows and get cross breezes across a home regularly for an hour or so a day would that suffice?

3. When running high cfm exhaust fans (range vent systems, bathrooms, fireplaces, etc) if you're able to open/crack a window during that would that be sufficient to prevent a "negative" pressure scenario inside?

 

There is an ERV that does deal with pressurization. The UltimateAir RecoupAerator 200DX can be set to have a net bias to exhaust or intake air. It can be set up to 60cfm through switches. However, they have a pressure transducer that takes inside and outside pressure and will let the system automatically correct the bias up to 135cfm.

http://www.ultimateair.com/product.php

That may be enough to create the slight bias Carnak indicated is desirable. However, it has a lag on the automatic adjustment that prevents it from compensating immediately for bathroom fans. Kitchen hoods are moving far more air than the ERV.

 

I have yet to find pressure transducers that are reasonable in price, sensitive enough, and reliable to provide any consistant pressures levels inside a home. Through in a cornering 7 mph wind and forget it. The motors would be turning on/off continuuously. In most climate provide minimal fresh air when occupied or when CO2 levels are high enough to indicate that natural ventilation is not purging indoor pollutants. The cost to heat/cool/dehumidify are low cost enough that there is no payback unless you live in a very cold climate, use expensive fuel, and have a super tight home with a high ventilation requirement.
Regards TB

 

The have a transducer they say is reliable at 1-to-2 pascals. The reason they can't be used as a MUA device is the adjustment is gradual, 10 cfm every 5 minutes I believe, but I may have that backwards, until the pressure imbalance is corrected.

They also have an economizer mode that uses outdoor and indoor temperature and a 24v switch. If enabled it will turn off the ERV wheel to turn it into an economizer at 55-70F. Ideally the 24v switch may be connected to a device providing more inputs into the equation. They also have a CO2 sensor switch and the blower speed can be controlled on a 0-10v input.

Net: it can do a lot with standard or optional supplied switches and controls, and even more with a sophisticated control system.

The name is awfully close: UltimateAir vs. UltraAire.

 

If you're trying to advise without some schooling and certifications, it might not be in your best interest.

The first thing that needs to be checked is the Combustion Area Zone. All exhaust appliances running at top speed, do all combustion appliances properly vent out?

Once the blower door test is done, the certified tester can tell you how much, if any, additonal ventilation is required on an hourly basis. That can often times be accomplished with a simple bathroom vent fan set by a timer. 15-minutes of operation every hour will produce 6-hours of mechanical air exchange per day. Remember, the how is still going to exfiltrate and infiltrate, just not as many CFMs as before. So using a mechanical exfiltration device (the bathroom fan) will force fresh air into the home more quickly than it would enter naturally.

It's not all that complex. You just need to be trained to do it all correctly. Absent that training, you could leave yourself open to future claims.

 

I used to share your opinion about ventilation needs. I have been spending more time examining the way homes respond to typical weather conditions.
The earth shaking infor for me was that homes breathe much more during windy weather winter or summer. We all agree on cold weather stack effect making homes breathe through their imperfections. Blower door testing measures the leakage area of the home very accurately. The missing part is the the location of the leaks. This is important that the impefections be equal located on various planes of the home. If 80% of the air leaks are on the at the top of the basement wall or any single plane, not much air leaks from the stack effect. That is a small part of the problem of predicting the real air leakage of the home.
The blower door leakage is an average winter temperature/wind air leakage number. As the temperatures rise and wind declines, the natural infiltration declines. IN summer with a/c on, temperature inside/outside similar and calm wind, you are near zero fresh from infiltration. Yet we are trying to determine the need for fresh air based on the blower test. Another issue is the effect of all of equipment in the home with some dependent on gravity chimineies. This is a mess.
I started tracking a few homes with CO2 monitors. Interesting results. The CO2 meters are accurate enough to determine the amount of outside air that is infiltrating or ventilating into the home verses the number of occupants throughout the year.
Homes that BD test at .2 ach confirm an air change in 5 hours during typical winter weather These home indicated an air change in +24 hours in calm summer air. In addition, home that were not as tight indicated very low natural infiltration during calm summer weather.
Also, the mechanicals in the home, mainly the a/c can increase air leakage during the peak cooling hours. Especially with leaky ducts located in the attic of open crawlspace. These additional benefits of testing for a known quantity of CO2.
The Blower door is a great tool for tightening a building. Because a building is not tight enough to need winter ventilation, has little bearing on the need for fresh during the other times of the year.
Good CO2 meters are available for $200 that will indicate the everyday amounts of fresh air infiltration/ventilation that a really gets.
The safe bet is to provide minimal make-up fresh air ventilation when the home is routinely occupied. Ultimately, CO2 levels should be use to control make-up ventilation.
If a home gets enough fresh air during cold windy weather and other times of high winds and the home is unoccupied 12 hours/day, a time of day 75 cfm fresh make-up air system may be the most practical low cost ventilation system. Add a good dehumidifier for the green grass climates to keep the home dry during wet cool weather, I could not resist.
Regards TB

 

I think we are getting two separate issues intermingled.

The first issue is ventilation: providing fresh air to remove CO2 and contaminants. There is a study on hospitals floating around that the spread of infectious diseases has a high correlation with ACH. Lots of ways to handle it.

The second is make up air for an appliance or device that is depressurizing the structure: range hood, fireplace, bathroom fan. The last typically is under 100cfm and for relatively short periods. The first can be over 1000cfm.

There are kits that operate the bathroom fan through an HRV: http://www.iaqsource.com/article.php/bath-ventilation-with-hrv--smart-move/?id=88. As another thread here went around in circles for page after page, there are few residential make up air solutions for range hoods and most would be only partially effective as installed if the hood is actually used for an extended period at full capacity (1000cfm for an hour are nearly 2 ACH on my house which has an ACHnat probably around .5-.6, ACH50 of 10).