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Old 01-19-2022, 11:01 PM   #181
montcalm
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I wanted to have some point of compare between directly using NG for heat vs. electrical energy being generated by NG and using a heat pump to move that heat. Which is more efficient use of the NG?

It's pretty easy actually:

All you need to know is your NG furnace efficiency, the efficiency of a NG power plant, and the COP of your heat pump at any given temp.

NG furnace efficiency is pretty easy. If you have a gas furnace, just look, it will be on the yellow sticker. Mine is 93%, we'll use that for this example.

What's the efficiency of a NG power plant? Well maybe trickier. I thought about 40% - the old ones were around 42%. Multistage waste heat collecting plants are around 60% from what I can gather. OK, let's look at both.

Lastly we need COP. I was able to find some performance specs for a cold climate air-sourced heat pump:

COP = 1.8 @ 5F, 3 @ 48F

I believe that's about the range you want to run it.

On an unit of heat basis, just divide the efficiency of the furnace by the COP x efficiency of the plant. Numbers greater than one are the amount more energy you're getting from from directly consuming the NG in a furnace and numbers less than one are the fraction of NG energy you'd use compared to the furnace by electrifying.

So for a 42% plant:

.93/(1.8*.42) = 1.23 @ 5F

.93/(3*.42) = 0.74 @ 48F

For this, there's definitely a balance point. You're only using 3/4 of the gas volume you would for the same energy output at 48F but 23% more when you're at low temps.


When you look at the comp for a 60% plant, the numbers are way more in the HPs favor:

.93/(1.8*.6) = 0.86 @ 5F

.93/(3*.6) = 0.52 @ 48F

Here were in the black for both, and pretty significantly. Almost 50% less NG usage at moderate temps!

The issue is when you go to resistance heat (no NG backup for less than 5F):

Now your COP is around 1 (maybe a touch less), but good enough to get the idea here.

.93/(1*.6) = 1.55

Obviously we're 50+% better off just burning the natural gas in the furnace at low temps.

There's some transmission losses I'm neglecting here, but I think it puts into perspective why the trend is to move away from NG home consumption and to HPs.

Last edited by montcalm; 01-19-2022 at 11:11 PM..
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Old 01-19-2022, 11:22 PM   #182
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A couple notes there for anyone not familiar with some terms although they've been used extensively throughout this thread:

Efficiency - always useful output over price paid. In terms of natural gas, it's the amount heat energy with a furnace or electrical energy with a plant you extract from the energy stored in chemical bonds. Never can be greater or equal to one (although resistance heating elements are nearly 1).

COP (coefficient of performance) - ratio of energy output per unit of energy input for a heat pump, it's actually the same as efficiency but because heat pumps move heat and don't convert it, the number is greater than one. It's the amount of heat they can move per electrical power input.
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Old Yesterday, 07:04 AM   #183
John H Swanson
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I think this is the one area that lacks significant reference to this analysis.

I just assumed that regular leaky houses are about 1.0 air change per hour.
Tighter houses are 0.5 air change per hour as is without a HEX.

It would be great for someone to research actual values measured with different construction methods to understand the potential savings.

Then, if you can get say 0.2 per hour you can bump it up to 0.5 with a HEX for improved air quality without all the heat loss.

But if the best we can do is 0.5 then.....
This just read:
"25 February 2016 / Whole building air leakage testing is a quantitative test method that measures the air infiltration of a building after it has has been completely enclosed and finishes have been installed. It's commonly known as blower door testing, because the fans are often placed in an open door frame.Feb 25, 2016"

"The very best homes have an ACH score below 1 ACH, and ultra-efficient homes are in the range of 0.2-0.6 ACH. If you are looking into buying a home, and want one that is air-tight, then ask your builder about the ACH score and what their homes typically score.May 7, 2018"
Also this one Says 1-2 ACH.:
https://www.engineeringtoolbox.com/a...oom-d_867.html

So and ACH of 1 is not a leaky house but a tight efficient one and less than 1 is really tight. With this in mind, my calculations using 0.5 is a really tight house. And if extraordinary steps are not taken to make it this way, then the infiltration losses will be greater.
"ASHRAE (formerly called the American Society of Heating, Refrigerating and Air-Conditioning Engineers) recommends (in its Standard 62.2-2016, "Ventilation and Acceptable Indoor Air Quality in Residential Buildings") that homes receive 0.35 air changes per hour but not less than 15 cubic feet of air per minute (cfm)per person. as the minimum ventilation rates in residential buildings in order to provide IAQ that is acceptable to human occupants and that minimizes adverse health effects. ASHRAE also suggests intermittent exhaust capacities for kitchens and bathroom exhaust to help control pollutant levels and moisture in those rooms. ASHRAE also notes that "dwellings with tight enclosures may require supplemental ventilation supply for fuel-burning appliances, including fireplaces and mechanically exhausted appliances."

https://www.epa.gov/indoor-air-quali...or-air-quality
So I conclude that this confirms the need to include a HEX and take measures to have an ultra tight house or significantly more energy will be needed. Looking at my loss numbers for infiltration and imagine doubling it.
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Old Yesterday, 10:16 AM   #184
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That HEX seems pretty necessary. Even if it only recovers 70% of what exits, that's significantly better than just a leaky house. Some claim to be up 90% efficient. That seems optimistic to me for an air-to-air exchanger.

They cost about $1k from what I see + the necessary ducting and small electrical penalty of running the blower. I'm pretty sure if you did the math they'd pay for themselves fairly quickly. Especially if most homes are changing air volume well over 1X per hour (that's a lot of mass transfer, and thus heat loss via that transfer).
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Old Yesterday, 10:38 PM   #185
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Keep in mind the heat recovery ventilation does not offset air leakage. It's in addition to it. So if your house is doing 1x per hour, with a heat exchanger it may do 2 or 3x total, but the extra is at higher efficiency.

You can purposely set them up to make the inside to be positive/neutral/negative pressure which may have a slight impact comfort say be pushing warm air out versus letting cold in. But even that is imperfect with wind blowing on one side of house creating high pressure and eddy on other side of house creating relatively less pressure.

My personal take on HRV is they are a bit over rated in practice with the exception of replacing bathroom/kitchen fans. But then again, they move much less air than a traditional bathroom fan so they are less effective. I think codes/etc want them to run 24/7/365, which makes no sense to me. If we are not home for 8 hours, what is it venting? Similarly, when we are sleeping, why vent continuously?

When we bought our house it was on a light switch for on/off only. Then I had it hooked to thermostat that turned it on/off for 20 minutes an hour, then to a smart home switch that turned it on occasionally plus bathroom switches with 20 min delay.

I think it's a relative thing too. We are well sealed with foam on all outlets and holes, but not perfect to today's latest standard. Also larger house with two adults and two small children. If we had a very tight 800 sq ft of camp and no basement, an HRV would be more useful. I'd buy the smallest one possible. The new one I bought after I couldn't stand the noise on the 40 year old one is rated for a small house and has an ECM motor that allows me to run it at a low speed for 15 min/hr -- and it's silent -- and boosts to 100% for bathroom duty.
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Old Yesterday, 10:45 PM   #186
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Keep in mind the heat recovery ventilation does not offset air leakage. It's in addition to it. So if your house is doing 1x per hour, with a heat exchanger it may do 2 or 3x total, but the extra is at higher efficiency.
Right - I was assuming building tight and using the HEX vs. just letting her leak.

My house is nowhere near tight enough for one of these, but I bet my air quality would be a lot better with one.

I guess for the thermal analysis, it won't make a huge difference to just assume lower infiltration even if in practice you need this the heat exchanger system.
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Old Today, 11:29 AM   #187
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Our house is an 18 year old modular, quite tightly sealed. 1400 sqft, just two of us.

It came with a Lifebreath brand HRV, which has been very satisfactory. It's in the basement. It's very quiet, to the point where you cannot even tell if it's running unless you put an ear to the vent in the room you are in. It runs on a duty cycle similar to the one mooregm describes. And periodically cleaning the filters is pretty easy to do.

So this is a positive review for that particular brand.
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Old Today, 01:57 PM   #188
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TCD - care to share any details on your ~20yr old build? Foundation, heat sources, insulation, climate, costs?

I know you don't have good solar prospects, but the other stuff matters too.
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Old Today, 02:00 PM   #189
John H Swanson
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When the house is sealed more tightly, I understand that the HEX serves the two purposes of bringing in fresh air and also venting moisture to lower the indoor humidity to prevent mold.
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Old Today, 02:04 PM   #190
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When the house is sealed more tightly, I understand that the HEX serves the two purposes of bringing in fresh air and also venting moisture to lower the indoor humidity to prevent mold.
During the winter, I'd bet that's the case, but I was thinking about summer use and I bet it's the opposite. It may force you to run your AC more to dry the air more inside.
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Old Today, 02:41 PM   #191
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TCD - care to share any details on your ~20yr old build? Foundation, heat sources, insulation, climate, costs?

I know you don't have good solar prospects, but the other stuff matters too.
Sure.

The modular home was built by Guildcrest (an Ontario outfit). (Due to various regulations, Guildcrest no longer sells in the US.) It seems to be high quality. About the only issue I have is with some settling, which I think is inevitable. I have a few wall and ceiling cracks to repair, but they have not advanced any further in the last 5 years, so I think the settling is largely done, and one repair cycle will take care of it.

Foundation is poured concrete, pinned directly on bedrock (no topsoil here at this site; the deepest topsoil was about 18").

Not much moisture in the basement; it's pretty dry. One deep crack in the rock brings in a little trickle in the spring, but it's short lived and easily contained. The one thing I did do was add an "earthquake rider" to the homeowner's insurance (it's very cheap) because we have occasional minor quakes, and a house attached directly to bedrock is more subject to potential damage.

Heat is an oil burner, with hot water radiators. I supplement with small (one room) electric heaters, so I can keep the overall house a little cooler.

The wall insulation is factory installed foamboard, I think it's 6". Roof is 6" of closed cell spray foam. The house is pretty tight, as I mentioned, but I think there are a few small gaps. Each year I go around and seal a couple spots, but that's mostly to reduce mice getting in. I don't feel and drafts anywhere.

Don't know the exact costs; of course this year, everything is double...

In the shoulder seasons, I get pretty good insolation via large, south facing windows. But in the winter, the sun is below the trees on the hill across the street most of the day, and I get just about nothing.

I have minimal cooling installed. Even in the summer, it's not necessary 90% of the time. I'm at 1800', and there's almost always a cool breeze. For the 10 or so hot nights, I have a small portable AC in the bedroom, which does the job just fine.

When we selected this site, we were very aware that it was north facing, but other factors were a higher priority. This was in 1989. The realtor had shown us many properties, but this was the only one that was affordable, and which would likely eventually be on a town maintained road. There were some very attractive south facing properties, but they were either way out of our price range, or had major access problems (like only accessible in winter via snowmobile, or 1/4 mile hike from the last parking, etc.). We intended this to be a year round home, so it was required that we be able to drive to it, year round.
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Old Today, 05:04 PM   #192
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which would likely eventually be on a town maintained road. There were some very attractive south facing properties, but they were either way out of our price range, or had major access problems (like only accessible in winter via snowmobile, or 1/4 mile hike from the last parking, etc.). We intended this to be a year round home, so it was required that we be able to drive to it, year round.
That's one thing I'm always looking for even with a seasonal place. If I can't use it to go ski during the winter, I might as well just use a hotel and forget the whole deal. I also don't want to have to spend a fortune clearing a parking space. I'm willing to snowshoe up the driveway in the winter for weekend use, but I also don't really want that to be a 1/4 mile hike. I surely don't want to haul around a snow machine either.


ATM my wife is pushing me toward an all-season adventure vehicle. I'm not against it, per se, but I feel like vehicles are a terrible investment. Also I'm not sure about viability of use during the winter - I'm not aware of many legal, overnight parking spots where you can "camp" in one. During the rest of the year, no issue... but same deal as above. Hotels and tents work.
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