Lately the team has been struggling a bit with the HVAC budget and design. The original budget was around $10K which is doable until we starting putting things like an HRV and solar thermal into our wish list. One of the questions we have been asking ourselves from the beginning is whether or not we need a traditional Air Conditioning system in the home at all?
Philadelphia’s summer’s are fairly mild and do not have a high number of cooling load requirement. I have a number in my notes that says we have a mean total cooling degree days of about 1,172 degrees F (with a reference of 70 degrees F). The vast majority of these CDD’s come between June and August (861) which means there are only three months out of the year that we can really make a good case for needing A/C in an average home in Philadelphia (source data).
The real issue we have in our area is really not heat, but humidity. Since the air is so much stickier here than other areas of the country, it feels much hotter than it often is during the summer. It is not as easy to cool our homes with passive cooling due to the high humidity levels as we will be bringing humid air into our homes.
So this brings us back to our original question – can we make a home comfortable during the summer is we employ simple passive cooling techniques in conjunction with a whole house dehumidification system? I think the answer is yes, but we are not 100% sure yet.
Let’s break this down into two parts here. A traditional air conditioning system will both cool and dehumidify the air in a home. We are simply breaking these two functions apart and handling the cooling requirement my mostly passive means, while employing a whole house dehumidifier for the dehumdification requirements.
Let’s look at dehumidification first. The goal is to keep the humidity levels in the home below 50% ideally and 60% at the least. If we are successful in doing so, it has been proven that people are comfortable with their thermostats set between 78-80 degrees F during the summer. Since our baseline cooling degree days was calculated at 70 degrees, raising the target to ~79 degrees would drastically reduce and almost eliminate the total number of cooling degree days altogether. I don’t have the raw data to do this calculation officially, but there was a rough reduction of 600 cooling degree days for every 5 degrees added to the baseline from 55 to 70 degrees (source data).
The dehumidifier we are looking at is an Ultra-Aire 90H model that is capable of both dehumidifying the air as well as bringing in fresh air ventilation from outside (more on the ventilation benefits later). This model runs on a standard 110V outlet and only requires 6.3 Amps which means it should take much less power to run than a full A/C unit.
Passive (& some active) Cooling
Now for the cooling side. Here is a list of our cooling measures we are considering to employ in order to keep the inside temperature comfortable during the summer:
- Superior SIP Insulation – Keeps the house from getting hot and also remains a constant indoor temperature longer than a traditional home.
- Energy Star Windows – Low U values and proper glazing will help keep the home cooler.
- Cool Roof Coating – A white coating that will reflect more than 60% of the radiant heat from the sun.
- Landscape Shading – Trees in the back of the home will shade the south side and rear yard during the summer.
- Ivy Wall Covering – A wall of ivy over the entire south side of the home will shade and cool the home further from the summer sun.
- External Window Shading – Exterior shades above the south facing windows will block the summer sun from directly entering and heating the home.
- Low Wattage Lighting – Less heat from lighting indoors reduces cooling load.
- Ceiling Fans – Reduces the perceived temperature by three degrees (F) when running.
- Interior Thermal Shades – Increases the R-value of the window and can both help block heat from entering during the day or cool air from leaving during the night.
- Natural Ventilation – Natural ventilation from both the windows and the Ultra-Aire unit help to cool the home naturally day and night.
- High Thermal Mass – The exposed concrete slab floors on the ground floor (and possibly the bathroom) will absorb and store heat during the day and release it during the night if naturally ventilated. This is a cycle that can repeat each day.
Hey, that’s not a bad list. I have lived in a number of rowhomes and apartments in Philly now and many did not have any of these passive cooling features. We were still able to keep cool enough without A/C by simply opening the windows on both ends of the home. Many homes, we only had a window A/C unit in our bedroom to make it a bit more comfortable to sleep at night.
There are a couple of key features in this list to me. I think the first is the very basic ceiling fans. By lowering the perceived temperature by three degrees when blowing directly on occupants, we now can raise the thermostat to 81-83 degrees (F). With all of the other features helping us out, I don’t see how we couldn’t keep the home at that temperature simply by naturally cooling the house with outside air during the night when it is 17-19 degrees (F) cooler on average in Philadelphia. It could be 100 degrees during the day, but it will most likely drop to near 80 during some point in the night and this is a worst case scenario for Philly that would not last more than 1-2 days.
LEED & Budget Impact
So how will this effect our budget? With the model of dehumidifier we have chosen, we have the potential to eliminate both the HRV and the Ductless Split Air Conditioners from our budget. These two items are currently taking between $8K – $10K of our budget. I am assuming that installing the Ultra-Aire will be much cheaper than this figure and will free up some HVAC budget that we can shift towards a solar thermal system to handle all of our domestic hot water demands year round. I’ll take it if I can get it!
The last thing we need to verify is that we will not be violating any LEED prerequisite by not installing a traditional A/C unit to cover the calculated cooling load of the home. I would hope not and I don’t see anything at first glance, but we have MaGrann verifying this for sure now.
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