Posted By ICFHybrid on 26 Aug 2014 11:03 AM

i guess he took down his data, and then tore the houses down, so that he could run another experiment.

You'd have to show us what part of the articles indicate the houses were torn down upon completion.

you said it was an experiment.

Posted By eugenep on 25 Aug 2014 07:08 PM

even your bad case is only $4000/yr, so how can someone save $6000/yr when you believe these bad homes are only paying $4000/yr.

I also know people who are spending well over $6K / year just on heating oil, but they aren't living in 1500' houses.

The cases I cited are more "typical" than "bad" cases, but intended to illustrate just how much more expensive it is to heat with oil in a small but at least somewhat-insulated house compared to heating with natural gas in uninsulated leaky homes 2x that size that I consider energy-use disasters.

Electricity cost in MA are on the high side- most families in the National Grid or NStar service territories are paying ~$1500/year, except for those who have taken extensive conservation measures, live in the dark (or those heating with electricity. :-) ) I'm sure if your sister in Lexington were not on the gas-grid (or heating with electricity, given NStar's higher than average electricity rates) she would be paying more than $500/month in heat & utilities during the winter months, and a large fraction thereof during the shoulder seasons.

 The average household in MA may only spend ~$2500/year on utilities, but that average is heavily skewed by the fact that about half the homes in MA are on gas-grid and heating with gas.  (Gas + wood accounts for fully half.):



The $500/month savings cited by the Net Zero homeowner may have been rounding up or an exaggeration (or not), or may have been only referring to monthly costs during the heating season. But it's not even a 2x exaggeration over typical homes in MA off the gas-grid.

you said it was an experiment.

That's correct; I did. But, there is nothing about an experiment that requires the homes to be torn down upon completion. Nothing.

Thanks for the pointers to different products.



I played a little with the new Borst steady state room temperature calculator, and if I'm understanding this correctly, it would seem that the air transfer between rooms doesn't help a whole lot. On a test 12'x12'x9' room I entered 1296 for the volume of room and then for the indoor air infiltration rate of the room I entered 0.15 (considering using a 200 CFM fan to transfer air from the hallway to the room -> 200/1296 = 0.15). Doing this boosted the steady state temperature by only 0.3 degrees, so I'm not sure if this is correct as it seems like it doesn't do much? I'm also not sure how to estimate how much air transfer would occur just from an open air space like the standard door undercutting.



One other product I came upon in my search so far is the Mitsubishi Zuba Central. It is a heat pump that uses the same Hyper Heat technology that Mitsubishi uses in their mini splits but ties into a central forced air ducting system instead of being just a point product. 40,000 BTU/hr which would be enough for my heat load, and operates down to -22F. The main problem I've seen so far is that it is only sold in Canada and I contacted them and they have no plans so far to sell it in the US, so I'm not sure if I would be able to import it or not.

DarkNova, 0.15 ACH (i.e., 0.15 room air changes per HOUR) for a 1296 cubic feet room is equal to 194.4 CFH (i.e., cubic feet per HOUR) or only 3.24 CFM (i.e., cubic feet per MINUTE). This why you are not seeing much of a change. If you want to enter the infiltration as 200 CFM (i.e., cubic feet per MINUTE), you must enter 0 for the room volume and then enter 200 in the infiltration field. If you want to enter the infiltration as ACH (i.e., room air changes per HOUR), you must enter the actual room volume and then enter the ACH value in the infiltration field and 9.26 ACH is equal to 200 CFM for this 1296 cubic feet room. This is all explained in the calculator instructions. I don’t know if you noticed, but we also optimized the iteration convergence algorithm so the calculator now provides an even more accurate solution and much quicker too.

I'm also not sure how to estimate how much air transfer would occur just from an open air space like the standard door undercutting.

Would also be interesting to model the open door situation. I have a corner bathroom that will drop > 5F, even with an open door. But it's not super insulated.

Posted By sailawayrb on 26 Aug 2014 10:42 PM
DarkNova, 0.15 ACH (i.e., 0.15 room air changes per HOUR) for a 1296 cubic feet room is equal to 194.4 CFH (i.e., cubic feet per HOUR) or only 3.24 CFM (i.e., cubic feet per MINUTE). This why you are not seeing much of a change. If you want to enter the infiltration as 200 CFM (i.e., cubic feet per MINUTE), you must enter 0 for the room volume and then enter 200 in the infiltration field. If you want to enter the infiltration as ACH (i.e., room air changes per HOUR), you must enter the actual room volume and then enter the ACH value in the infiltration field and 9.26 ACH is equal to 200 CFM for this 1296 cubic feet room. This is all explained in the calculator instructions. I don’t know if you noticed, but we also optimized the iteration convergence algorithm so the calculator now provides an even more accurate solution and much quicker too.

Gotcha, thanks a lot. I changed it to 200 like you suggested and that really makes a big difference. That value brings the steady-state temperature up to over 65F which seems quite good. I'll have to play with it to see what kind of CFM rate I would need to get acceptable temperatures. I picked a value of 200 CFM kind of randomly as I saw some inter-room fans that could go that high, but that kind of volume might be kind of loud.

And I'm also curious about the open door situation, or if there is a vent a certain size (say 1 square foot), but no forced ventilation, what kind of heat transfer would take place.

Posted By DarkNova on 27 Aug 2014 10:59 AM
And I'm also curious about the open door situation, or if there is a vent a certain size (say 1 square foot), but no forced ventilation, what kind of heat transfer would take place.

A single square foot hole will do pretty much nothing.


Splitting that square foot into two vents, one at the top the other at the bottom to promote convection would do something, but it's still not enough cross sectional area to come anywhere near the convection through 20 square feet of a 7' tall open doorway. 

Glad to hear that we successfully resolved that confusion DarkNova :-)

If you are willing to give up temp zone isolation (i.e., being able to maintain significantly different temps in your different zones) or willing to accept some additional design complexity to maintain it, you can take advantage of passive thermal differential ventilation.

Typical ventilation is accomplished via active (e.g., electro-mechanical blower/fan) pressure differential ventilation by creating a pressure differential which results from air moving from the higher pressure area to the lower pressure area.

Thermal differential ventilation is accomplished by creating a temp differential which results from lower density warmer air moving above the higher density colder air area, then subsequently cooling and moving below the lower density warm air area.

Thermal differential ventilation can be accomplished by having an opening in the interior wall at the ceiling location and having an opening in the interior wall at the floor location. These openings can be in different interior walls of the room needing the ventilation. These openings can be simple “always open” openings or be more complicated closable openings (manual or powered/automated). Thermal differential ventilation is commonly used in passive solar house design or in buildings heated with a centrally located heat source such as a masonry heater. As Dana indicated while I was typing this, there is some analysis involved in getting this to work properly.

Posted By sailawayrb on 27 Aug 2014 12:25 PM
Glad to hear that we successfully resolved that confusion DarkNova :-)

If you are willing to give up temp zone isolation (i.e., being able to maintain significantly different temps in your different zones) or willing to accept some additional design complexity to maintain it, you can take advantage of passive thermal differential ventilation.

Typical ventilation is accomplished via active (e.g., electro-mechanical blower/fan) pressure differential ventilation by creating a pressure differential which results from air moving from the higher pressure area to the lower pressure area.

Thermal differential ventilation is accomplished by creating a temp differential which results from lower density warmer air moving above the higher density colder air area, then subsequently cooling and moving below the lower density warm air area.

Thermal differential ventilation can be accomplished by having an opening in the interior wall at the ceiling location and having an opening in the interior wall at the floor location. These openings can be in different interior walls of the room needing the ventilation. These openings can be simple “always open” openings or be more complicated closable openings (manual or powered/automated). Thermal differential ventilation is commonly used in passive solar house design or in buildings heated with a centrally located heat source such as a masonry heater. As Dana indicated while I was typing this, there is some analysis involved in getting this to work properly.

Interesting, thanks for that information. I'll have to look into how that could work to see if I can figure out how to estimate how big of vents I would need. I understand how I would need 2 different vents and oversimplified things when I indicated 1 sqft vent previously. I do not care about zoning individual rooms and am still in the planning stages of my design so I'm just trying to figure out the best way of doing everything.

For passive, you need the large opening(s) because there is so little pressure involved. This might be approximately correct (try 16"/.4m ducts). On the other hand, at that size, why not just leave the door open (most of the time)?

Yes, those are indeed the appropriate differential equations for the physics we are discussing. However, for our bedroom ventilation problem, they need to be solved for a different boundary value condition (i.e., solved for two unconnected orifices in lieu of a single duct).

Jonr, I think the answer to your question is likely the same as the reason that we put doors in some of our interior walls to begin with…to provide personal privacy in some rooms (e.g., in bathroom/bedrooms). While we often use passive convection ventilation in bedrooms, you certainly wouldn’t want to use that in bathrooms!

DarkNova, in many of our passive solar designs we create an opening that is 18” tall and between 6’ to 8’ long just below a flat ceiling that is 9' to 10’ in height. This is often designed as a shelf where the homeowner can place and grow some hanging style plants. This is especially attractive when done on a masonry thermal mass interior wall which is common in passive solar designs.

I've been thinking about this more, and I appreciate all the advice I've been getting. It turns out the electric cost is actually $0.08/kWh, so a little more than I was originally thinking, but still very cheap. I've been doing more heat modeling, and I'm thinking about the option of putting one Mitsubishi Hyper Heat MSZFE18NA 18,000 BTU/hr Mini Split in the common area of the house, and putting in some sort of electric resistive heaters (cove heaters?) in each of the bedrooms. The floor plan we're working on is quite open, all on one floor, with only the bedrooms and bathrooms having doors to close things off from the common area.

My thoughts about this:

- Based on hourly heating simulations, this heat pump should be able to provide the vast majority of the heat annually. It looks like about 5% of the year we would need additional heat added from resistive heat. Based on COP data I could find and outside temperatures in the simulation it would appear the annual COP would be a little over 2.1.

- I'm not sure if I would install extra ventilation fans between the bedrooms like previously discussed, or just rely on the resistive heat in the bedrooms. I would have to think about that more. It kind of depends on how often it would be needed.

- Our climate is very much heating dominated. Air conditioning is nice to have for just a few weeks of the year when it gets really humid. My model shows this 18,000 BTU/hr unit being able to provide what we would need for cooling. The steady state analysis shows a much friendlier result for cooling since the temperature difference between outside and in is much less, so it would seem like the closed bedrooms would only get 1-2F more than the common area in the most extreme case, which wouldn't happen at night anyway, so it doesn't seem like that would be a problem.

Question: Can I assume that the heat from the mini split will work its way through the common area (not closed off by walls/doors) pretty well? If it is 69 in one end of the house, can I assume it is close to that on the other end of the house that isn't blocked by anything?

Any comments on if this make sense to do? I haven't received quotes for systems yet, but the standard around here would be a propane forced air system. If I assume a COP of 2 with the mini split and resistive heat combined at $0.08/kWh, propane would need to cost $1/gallon in order to be competitive. And it would seem like installing the mini split and cove heaters around the house would be cheaper than doing ducts with a furnace and central air, wouldn't it? Or am I wrong about that?

All things being equal, I'd rather do electric than propane as I'd like to do PV in the future. Thanks.

The primary factors for choosing between open doors, passive ducts (high and low), active ducts and supplemental electric heat are visual privacy, noise, cost and comfort. Only you can value them.

If the bedrooms get used in the day, consider solar gain when calculating closed door temperatures.

Our entire house stays at a very even temperature running one mini head, and the doors open, but I have noticed that the humidity creeps up in the more remote rooms. I would have thought that if the temperature was good, the humidity would be as well, but that hasn't proven to be the case. Here in SC, it is very humid in the summer, so it is a key factor. At night, we shut the 12K main unit off, and run the 9K in the master bedroom, which keeps the room very cool and dry, which is what I prefer for sleeping. We keep our house at 78 degrees, which I consider to be the high limit for A/C comfort. I'm sure if we kept it lower, the humidity would be less of a problem. We haven't been through a winter with this system yet; I'll be reporting on my observations when it gets cold out.

I'm wondering why you shut off the other unit - wouldn't it help with dehumidification if you left it on? If you don't want it in cooling mode, just change the setting to "drying" mode.

I have noticed that the humidity creeps up in the more remote rooms.

It would be interesting to investigate this. In a well sealed (to the exterior) room and with only a couple people as humidity sources, an open door and 1 degree temperature difference should create more than 10x the air flow needed to control humidity.

Even the very low CFM rates required for ventilation can be sufficient for humidity control (this is how DOAS/radiant cooling works, but it requires attention to the details).

I just run one unit at a time to save a little electricity. I wonder how much they draw in dehumidfying mode?

I've wondered about the humidity source, too. It's not like we have a bunch of houseplants or some such. I haven't actually measured the humidity- I need to do that. One issue is that the basement is not yet sealed well from the upstairs, and we definitely have humidity in the basement.