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By Kenneth in Lees Summit, MO on 12/18/2007


Now that I have over two years of data, I can actually look and make some comparisons and develop trends:

 

1)       I use Natural Gas to heat my house (no heat pump, 90+ furnace with variable speed air handler) coupled with passive solar. As expected, my natural gas usage is directly correlated to heating degree days over the past two years. In fact, if I plot the two together (after backing my baseline for clothes drying and domestic hot water (DHW) based on an average of my months with 0 heating degree days), and change the axes, the monthly usage in CCF and the monthly degree days correlate exactly, the curves are the exact same shape. This is how it should be, especially based on monthly data. My baseline (clothes dryer and DHW) stays within +/- 1 CCF, so fairly constant, although I anticipate that my DHW usage would increase as heating degree days go up because my intake water will be cooler, but I cannot capture that separately so I attribute all additional use to household heating. For reference, in 2006 I had a low temperature of 3 F with 4,271 heating degree days and in 2007 I have had (to date) a low temperature of 0 F and 4,373 heating degree days.

 

2)       I use standard forced-air cooling. Also as expected, my electric usage is directly correlated to cooling degree days over the past two years, with some discrepancy for estimated readings. It is interesting to note that as the A/C operates in its better efficiency range (less short-cycling) that this is reflected in the curve of electrical usage vs. cooling degree days. Less cooling degree day months clearly operate with less efficiency, indicating the importance of properly sized A/C equipment (as mine is oversized, this would clearly save me some money if/when it gets replaced). For reference, in 2006 I had a high temperature of 102 F with 1,536 cooling degree days and in 2007 I have had (to date, I don’t anticipate any more cooling degree days) a high temperature 99 F and 1,582 cooling degree days.

 

3)       And now, since I have this data I can tell you how much it costs to heat and cool my house. With 4,000 s.f., you can look up my heating and cooling degree days at www.wunderground.com based on my zip code and compare them to yours to have some normalization and comparison of the data, and over two years' data, my HVAC costs are right at $50/month. This keeps me at a toasty 71 F in the wintertime (I would stay lower, but my partner likes it warmer) and 73 F in the summertime. More importantly, my humidity levels are 50% in the wintertime and 40% in the summertime (I have separate dehumidification allowing me to control humidity levels separate from temperature, very nice and I recommend this to everyone).

 

4)       This also illustrates the importance of building the box right as your primary concern, and worrying about the efficiency of the units a secondary issue. Upgrading my air conditioner from SEER 14 to one of those really efficient SEER 19 units would clearly save me some money (or further upgrading it to a geothermal unit at even higher efficiency), but not much and certainly not enough to justify the increased capital or maintenance costs. I have a 90+ furnace, but my installer upgraded this for free because I didn’t need a flue for the DHW (direct vent, hooked up by the plumber, normally the HVAC tech exhausts them both together) and thus it was easier for him to direct vent the furnace as well, so my increased equipment costs were offset by reduced installation cost, for a net wash.

 

5)       A heat pump would save me money (I would still need backup heat source, so this is assuming a natural gas dual fuel heat pump) as the upgrade cost was only $250. However every HVAC tech I talked to recommended not installing a heat pump, so there must be something there (they cited increased maintenance costs). I would also qualify for separate metering and lower winter electric rates on that heat pump, saving me a bit more.

 

6)       Having natural gas definitely saves me money, but not as much as you might think. Over the course of two years, my administrative cost for natural gas (I have to pay a customer charge for the honor of having natural gas pipeline to my house) are almost 50% of my total natural gas bill. My customer charge is fixed and separate from my cost of gas factor. If I had less efficiency and used more natural gas, my customer charge as a percentage would go down. This is frustrating to me, as my last utility company provided both electric and natural gas, so I had no separate customer charge when I calculated my natural gas usage, and my decision to use natural gas as the most cost effective method to heat. Shame on me for not doing my homework. Anyway backing my natural gas usage and converting it to BTUs, also accounting for efficiency (86% for DHW, 90% for furnace, unknown for dryer so I assumed 86% which is probably too high) and converting the number of BTUs to KWH would indicate that natural gas is still more cost effective, including my customer charge (vs. all electric, including heat strips for backup heat). However if I consider that a heat pump allows me cheaper electric rates, it is much closer but still natural gas is more cost effective. Now if I consider that a heat pump offers me some gain in efficiency, this tips towards electricity, but not by much. Now again if I consider that all the HVAC techs can’t be wrong, I think my decision would remain the same, especially as close as the numbers are. Run your own numbers. A heat pump is likely more cost effective, but to gain more efficiency you need to focus on the box first and the appliance second. This also assumes the cost to run copper (wiring) and the cost to run CSST (natural gas) are the same; they should be very close.

 

7)       This also illustrates that energy recovery (HRV/ERV) is simply not as cost effective as reducing your energy usage in the first place. I certainly don’t advocate building with inadequate ventilation, but understand that an ERV/HRV is not a panacea or an ideal solution in most cases, even though they are purported as such. I am recovering no heat (or cool) from my ventilated air. Exactly how much would I be expected to save if I could recover 80% of that energy, as a percentage? Now then, how much money does that save me every month? And how much did that little unit cost in the first place? Understand that as it is, my AC isn’t operating at its peak efficiency, and operating less would actually increase my electrical costs, and suddenly that little unit looks even more like a waste of money.

 

8)       Given this, I would focus my investment on building the box, and once the box was built right, only then would I consider adding energy efficiency though efficient HVAC. And for each incremental increase of efficiency, I would determine where it ceases to be cost effective and where it becomes an exercise in spending money.


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By Jon in Ellicott City, MD on 12/19/2007


Great info, Ken!! Thanks for posting it. 

We're still evaluating these choices for our own project, so hard data is extremely helpful. Unfortunately, Nat Gas isn't an option for us. So, basically, it's Geothermal or Oil/Heat Pump.

I do have to say that it's a bit counter-intuitive to put such effort into sealing the thermal envelope against air infiltration and then purposely put 4" holes with blowers in that envelope. No arguing with the results though.

Jon


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By Kenneth in Lees Summit, MO on 12/19/2007


I don't think anyone would disagree that building your box tight without any ventilation would save you money. I do think they might agree that it isn't a very nice place to live. I cook (a lot, reflected on my baseline and those energy hogs in my kitchen eat some kWh), I take showers, I do laundry, and not venting any of that certainly would make for some unpleasant indoor air quality ;-(.  I think the key is controlled ventilation.

Energy recovery is not difficult, although cost effective energy recovery is something different entirely. A 90+ furnace recovers heat energy that would normally be exhausted out the roof, the ERV/HRV recovers energy that would normally be ventilated. If you use a geothermal you can also install a desuperheater to recover heat (for DHW) that would normally be transferred to the ground loop, you can buy devices to recover heat from your drain lines (I can't see how this would be cost effective, the only hot water going down the drains if from the shower, sinks, or dishwasher, and none of these are significant). Some are more cost effective than others.

My next house, I intend to have a mini-backhoe so geothermal becomes much more cost effective (assuming my time is free, which seems to be the way I calculate things). I will also use radiant (more efficient) and a smaller house (less heat to use anyway). My only early issue is that geothermaldiy.com (formerly arit.com) the smallest geothermal heat pump is three tons, too big for what I have now. Perhaps this can be addressed with a variable speed pump? I still have several years before I go down this road, I will sort it out later;-). I have seen one install where a guy used a solar panel to dump excess heat into the ground adjacent to the geothermal loops starting in October, further raising the energy efficiency of the geothermal - I would probably at least install the PEX to do this; solar panels can always be added later and PEX is cheap. And he used pre-owned solar panels, reducing his cost to almost nothing for this enhancement.


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By Amy in Richfield, UT on 12/19/2007


Ken,

Wow! Thank you for the update. We are building a 2,200 sq.ft. with same in the basement out of ECO-Block, 8 inches of concrete in the basement walls and 6 inches in the main walls. We put our basement ductwork under the floor with a 90+ furnace and the ductwork for the main in the attic with an 88+ furnace (to avoid any condensation problems) in the attic. We have an 18 SEER air conditioner for both levels.

We are insulating the inside walls and under the main floor, mostly for noise, and we are going overkill with R-50 blown in the attic. The attic upgrade from R-38 to R-50 was only $350, so we felt it was worth it to overkill up there. It would be a shame to spend $30,000 on ECO-Block and then let the heat out through the roof! We have been very excited to build this way, but have been wondering what to expect in energy savings. With 7 kids, I am sure we won't have $50-a-month bills, but I would be happy with $100! It's great to actually hear from someone who has had the savings!

Amy

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By Jon in Ellicott City, MD on 12/20/2007


There's no question that ventilation is mandatory. The question is whether or not it makes economic sense to temper the incoming air via an ERV/HRV. You say that it doesn't and you provide hard data showing that blowing untempered air into your house has a small effect on your energy costs. 

I presume your system provides something like 0.35-0.5 air changes per hour, or ACH. (Those are the recommended rates I commonly find, but I have no idea of the basis or validity of those numbers.) If that rate is correct, then every hour or so you're exhausting half the air you just paid to heat/cool and replacing it with air that needs heating/cooling. That SEEMS like it would be fighting the forced-air portion of your HVAC system pretty hard. 

That concern seems to be echoed in building science publications. (I still have to read the articles you referenced, but I will when I get the chance.) Every modern article I've read tells how air infiltration is a substantial problem. People go to great lengths to caulk/tape/foam every crack and seam in a tight-envelope home. That seems contradictory to the idea that it's okay to put a fan in your wall that regularly, or even constantly, blows volumes of outside air into your home. 

Again, you have real world data that supports your opinion. I'm just trying to figure out how all the pieces fit together.


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By Kenneth in Lees Summit, MO on 12/20/2007


When I see “rules of thumb,” I like to immediately go to extremes and see if those rules still apply. In this case, the extremes I considered were a camping tent and a gymnasium. Both of these examples serve to illustrate the fallacy of a fixed ACH that should be applied in every case (although your HVAC tech is quite likely to apply this rule of thumb to your house). You can see that 0.3 ACH in a small camping tent would be horrible and 0.3 ACH in a space the size of a gymnasium with only one or two occupants is also clearly poor design. So, I dismiss the “rule of thumb” and instead figure out what standard this is based on, and then use the standard directly to calculate how much ventilation is required for my system.

 

The American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) seemed to be just the starting point I was looking for. ASHRAE Standard 62.2, Ventilation for Acceptable Indoor Air Quality in Low-Rise Residential Buildings appeared to be just the standard I was looking for (although ASHRAE Standard 136-993, A Method of Determining Air Change Rates in Detached Dwellings would probably also work; I did not consult this standard). Now if your HVAC tech understands and applies these standards, you have found a rare tech. ASHRAE Standard 62.2 recommends 7.5 CFM/occupant and another 0.01 CFM/square foot of heated space. I have two people and three sizeable dogs (two at the time, and yes I counted the dogs, because they breathe and exhale probably as much as I do) so four occupants. The ASHRAE standard also recommends occupancy rates counting the number of bedrooms plus one. Either way I get four occupants, and based on my square footage the standard recommends 70 CFM. Now if you use the extremes identified above, you can see that this type of standard makes sense at these extremes.

 

Understand that a standard install in my area is based on at least one 250 CFM ERV operated continuously. One of my old houses, I had an old farmhouse of ~2,100 s.f., and this old house took two furnaces (100,000 BTU each) and 6.5 tons of A/C to get this thing comfortable – thankfully they don’t build them like they used to. With that much furnace, a “recommended” install would be two ERVs, even though you could feel the ventilation taking place through any of the 35+ closed windows; the recommended ERV sizing is based on the number of furnaces. And here I am building a very tight envelop, minimizing my HVAC requirements, and making ventilation actually much more critical.

 

Now then, 70 CFM ASHRAE recommendation is clearly not adequate for specific problems, such as humidity removal in a bathroom or cooking in a kitchen. In these areas, I wanted complete air changes on the order of minutes (IIRC I figured four minutes to completely exchange the air in a bathroom to get that excess humidity out), I want that problem out of the house – now. This is also why I cringe when I hear of people using their ERVs instead of bath vents, bath vents are to address very specific problem (and it is not general ventilation) and generally one ERV pulling from multiple bathrooms is insufficient ventilation for that (and in my area the code official agrees as these installs are not code compliant and will not get you a CO). The kitchen is another example of where you need better ventilation; I burned something on the stove just last night…

 

My makeup air comes through a ventilating dehumidifier to my furnace supply lines and is distributed through my furnace ductwork (all in the conditioned area), so there is tempering going on. In heating season (for most houses), this corresponds to when the furnace is operating, so tempering is taking place. In cooling season, you want your AC to operate constantly and again, any intake air is being tempered. In window-open season, who cares? In window-closed but not heating or cooling season (in the Midwest how long does that last?), I still need dehumidification so the ventilating dehumidifier is operating, and just exactly how much tempering for temperature needs to take place? Same thing for the ERV. The only time it is recovering energy is in heating or cooling season. Natural gas is cheap, so you really aren’t saving any money there. My 3-ton air conditioner is oversized, so recovering energy in the summertime would equate to less operation, and for me this equates to less efficiency and again nothing. Now the obvious question here is what if my AC were sized properly, wouldn’t an ERV work better then? Yes, but even then your AC is sized for one specific condition (unless you are using a two-stage compressor), and everything else is a compromise that reduces efficiency. So at one design condition everything works ideally, the rest of the time not so much. Why don’t I install a smaller AC compressor? Another good question, I found that anything smaller was built to a price point and not a quality standard. And the three-ton design size was based on Manual J calculations that I trusted (truthfully finding a HVAC tech that was willing to install a unit this small and base their reputation on it was difficult).

 

I make my own decisions independently and do not rely on arbitrary “rules of thumb” or vendor literature. I have met some great HVAC techs that will install, troubleshoot, and service these systems, and this does not take an engineering degree (and the corollary is also true, I don’t want an engineer doing actual installation or servicing; most of them can’t use tools).


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