Home Design

[SaskBob]

I recently built my new home and tried to incorporate all the ebergy efficient options. The house is built on an ICF foundation, I have WallTite Eco foam on the rim joists, walls and entire roof deck and my windows are triple pane, 2 compartment gas filled fixed or casement style. Heting/air conditioning is provided by 7 tons of GeoThermal system. When the initial plans where evaluated I was told I could heat the house with light bulbs and would require very little airconditioning. So far I am a bit disappointed and baffled by the performance of the house. We have recently experienced some fairly warm weather and the geoThermal system in cooling mode has kept the house at the set 73F. What baffles me is in the evenings and night time when the outside temperature drops as low as 64F the house temperature will still rise above 73 and call for air conditioning. it is as if the house is acting as a heat dam and releasing heat back into the house at night. This may seem like a good thing but causes the air conditionig to run excessively or else we must monitor the situation and turn off the AC and open the windows in the evening. This isn't such a bad thing but seems to defeat the idea of building an energy efficient home and relyying on the technology to keep us comfortable. I guess what I am wondering, is this behaviour to be expected. Is the foam in the walls absorbing heat during the day and releasing it back into the house at night? any explanation would be appreciated. I should note that we do have about 85 sq. ft. of glass on the West exposure. Thanks.

[54regcab]

What you are experiencing isn't uncommon in well sealed high mass homes. Most people just either pay for the electricity at night or open a window if the humidity isn't too high. If you run the A/C a little at night it won't add that much to your power bill. In commercial there are things call "economizers" that bring in cool air from outside during times when it's cooler outside than inside. For residential the payback time for economizers is too long to make sense.

[vstech]

how many light bulbs are you running at night?
how are your cooking appliances exhausting?
how many people living in the house?
what type of geothermal system? loop piping? vertical piping? water source from a well?
clothes dryer ventilation? water heating?

[energy_rater_La]

ashrae 62.2 ventilation

[SaskBob]

how many light bulbs are you running at night?
how are your cooking appliances exhausting?
how many people living in the house?
what type of geothermal system? loop piping? vertical piping? water source from a well?
clothes dryer ventilation? water heating?

I assume you realize heating by light bulbs was tongue-in-cheek
The kitchen exhaust is not yet installed but cooking generates minimal heat with just two of us old folks in the house; we seldom use the oven in summer and the cook top is magnetic induction.
geothermal is ground loop and the dryer vents directly to the outside. Thanks.

[skippedover]

There are two primary issues at play when you're trying to evaluate the efficiency of your home as built.

1. The 'fly-wheel effect' in the cooling industry is a term used to explain that it takes time for a home to both heat up and then to cool down when outdoor temperatures change. For example, in morning, the sun is low in the sky for the first part of the day but is adding Btu's to the outside shell of the home. As the day wears on and outdoor temperatures increase, the sun rises in the sky, the outdoor temps go higher and again, all that is being pushed into the exterior of your home. But you have insulation to slow the effect of those influences and so it's about 3:00 pm before they fully affect the interior. So your cooling system can 'loaf' along in the early morning, gradually increasing run minutes per hour until sometime in mid to late afternoon when the heat gain is highest. Now that your house exterior walls are fully heated, they'll continue to radiate that heat into the home until early evening. As the sun and outdoor temperature both recede, the walls and insulation of your home are still loaded with heat and will continue to raise the temperature for a few more hours, requiring operation of the cooling system well into the evening.

2. The second issue at play is the very insulation you put into the home. The assumption is that the heating and cooling equipment is smaller than would be needed had you not taken pains to make the house efficient. That means the cost to operate the equipment is also lower. But here's the important point. The insulation can only SLOW the transfer of heat from warmer to cooler. Heat always flows from warm to cold but the tighter the home, the slower that heat transfers back to the outdoors when the outdoor temperature is lower than the indoor temperature. In an ideal world, you've have high insulation values when the outdoor temperature is higher than indoors and low insulation values when the indoor temperature is higher but then only in the summer. In winter, you'd want the opposite. You want the insulation to keep the heat indoors. So in effect, the insulation works for you in one situation and against you in the other. Once the house is all warmed up, the insulation is just doing what you wanted it to do, that is, keeping the heat in!

[beenthere]

Check you electronics, depending how many you have plugged in, you are getting a fair a mount of heat from them. Also, lights can tend to put off a fair amount of heat, so that a well sealed and insulated home tends to increase in temp. What type of water heater do you have, and where is it located.

[SaskBob]

Skippedover,

I appreciate the good explanation. This is kinda what I thought was happening but not what i expected. I guess i need to evaluate the cost of simply letting the airconditioning go 24x7 in hot weather vs. switching it off and opening the windows when the temp dips in the evening and then switching back in the morning before the heat builds up. Again, thanks for the explanation.

[motoguy128]

How large is this hous and where are you located? 7 ton on a well sealed ICF home is a LOT, even if you lived in Houston or South Florida unless it's over 7000sqft. As others mentioned, the concerte provides thermal mass. Thsi means that you interior never "experiences" the leak tempratures of the day, so the equipment can be undersized against normal load calculations. Other than solar gain through windows and air leakage and ventilation, it will tend to behave as though the temperature stays equal to the average of hte outdoor temperature. Therefore I'd expect it to run at a similar duty cycle as late as 3-5AM before the structure equalizes to the outdoor conditions.

This is a good thing, because if the equipemnt was properly sized, it will have very long run times throughout the night keeping humidity more constant.

Opening windows at night in most climates is usually counterproductive. In many cases the dewpoints sharply rise after sunset and don't drop until early morning, so you'll bring in a lot of humidity. Opening windwos doesn;t pay off unitl your in "economizing" type of weather, usually in April and October in most areas where overnight temps are under 55F.

Actually in a large ICF home, using commerical equipment with econoimizers wouldn't be a terrible idea. You could also dehumidify using hot gas reheat as well. Even better, if your in a summer location, might have been to use a chilled water system.

[Shophound]

OP said house is built on an ICF foundation, which suggests the house has a basement. I'm also under the impression the walls are conventionally framed with external foam board sheathing, and perhaps the attic is also insulated at the roof deck.

What we did not receive is how large this house is. Seven tons may sound large even if my interpretation of the construction techniques is correct, but we have no disclosure from the OP how large the house is, or where it is located.

Skippedover nailed it, however. The house is performing exactly as designed...to hold heat in! Hence the need for a/c as exterior temperatures fall below room temperature setpoint. During the day, the driver of heat is very strong - the sun - so even great insulation will allow some amount of heat to enter the building, if slowly. At night, the energy forces between indoors and outdoors, at say 74 indoors and 65 outdoors, is not nearly as strong, so the transfer of heat outward into the cool night air is VERY slow. Hence the need for a/c or open windows.

OP, sounds like you live in a cold climate and your house was designed accordingly. Do you find the house easy to heat in winter?

[SaskBob]

To MotoGuy, Shophound & Beenthere

Thganks for your input You guys all seem to agree that the performance of the house is as should be expected and your explanations make sense.

For your info; the house is 2200 sq. ft. Bungalo with a full ICF basement. The house is stickbuilt (2x6) with 3 1/2" of medium density sprayfoam in the wall cavities. The entire roof deck is covered with 5" of spray foam on the underside and the Rim joist are spraued with about 5" of foam. Only 3 tons of the geothermal contributes to AC. The other 4 tons is water-to-water heat (should have made that clear).

Beenthere, you touched on a good point about the lights and electronics. That's what prompted my first statement that I could heat the house with lightbulbs. Incidently I have 2 connected 60 galllon water heaters. The first tank is heated by the Geothermal desupraheater and feeds the 2nd. tank that is electric.

The bottom line is, all of you Pros on the forumn offered a good explanation that confirms what I thought was happening and I feel a bit better knowing that this is normal behaviour. Again, thanks to everyone for your comments.

[Shophound]

Thanks for clearing up the tonnage distribution. That makes sense.

Even with your concern, I bet that house is a lot easier to keep cool and warm than an equivalent sized and built house that did not use the extensive insulation and air sealing efforts you undertook with your place.

[dan sw fl]

I doubt that the internal loads would be enough to create a significant runtime in the evening / night.

It seems like most OPs have a real reluctance to provide their location.

It seems that the cooling load would be less than 2 tons given normal internal heat gains.
I guess the internal heat gains must be > 3,000 wattts.
I don't see that a true conclusion has been arrived at here.

[motoguy128]

IF it had a lot of shade and depending on location, along with deep overhangs and a more thermal mass form somethign like real stucco (not that fake junk) or tile floors, it could probably get away with 2.5 ot 2 tons. But without knowing locations and total window area and solar gain, it's a guessing game. Interesting ot seperate heating and cooling system. Seems like it would have made sense to use a 5 ton water to water chiller with a heat pump mode instead. IF you already have a water to water heater, you have everything in place ot do both.

At some point, that's what i don't understand. These units have these desuperheaters but for some reason that industry is reluctant ot move towards chilled water. Yes, you'd need a storage tank and another pump, but equipment performance and effciency overall would improve and you gain so mcuh in what yo u can do for dehumidification and controlling systme capacity.

[54regcab]

With chilled water zoning is a cinch. Just put a separate air handler for each zone, similar to a minisplit. Storage of excess capacity during times of low demand to cover times of high demand could be useful too.

[Shophound]

With chilled water zoning is a cinch. Just put a separate air handler for each zone, similar to a minisplit. Storage of excess capacity during times of low demand to cover times of high demand could be useful too.

First cost is what kills chilled water system selection for most residential. Also does not seem a practical choice for smaller homes. For multi-story, larger residences, I see it as more viable than traditional zoning with ducts.

With a smaller house I would focus on the improving the enclosure and reducing the tonnage requirement of traditional DX equipment to notably below how an equivalent sized house with an inferior enclosure would require.

[SaskBob]

I was just reading through the whole thread and like I said before your explanations are good and make sense to me. Just to throw in a bit more info for those that are curious; we are located in Northern Saskatchewan, Canada. When we designed the system I wanted Hydronic heat but some areas of the main floor did not lend themselves to hydronic heat (hardwood floors) and besides we wanted some air conditioning. This is why we have a split system. This past winter was our first winter in the house. The water-to-water system was not yet in service and the 3 tons of heat we had worked fine up to about 0C after that it was a bit of a struggle but fortunately we have a big wood burning fireplace and it was a mild winter. Next winter will tell us how the heating portion works. The biggest disaointment has been the de-supraheater which only appears to work in heat mode and so we get no benefit from that for a good part of the year. Again, thanks to everyone.

[mark beiser]

The biggest disaointment has been the de-supraheater which only appears to work in heat mode and so we get no benefit from that for a good part of the year. Again, thanks to everyone.

The desuperheater only works when the system is running, and 7 tons for a 2200 sq. ft. super insulated house in your climate, probably in any climate, is massively over sized for cooling, but may be sized correctly for heating.
Desuperheaters don't perform very well when the equipment only has short run cycles.

[SaskBob]

The desuperheater only works when the system is running, and 7 tons for a 2200 sq. ft. super insulated house in your climate, probably in any climate, is massively over sized for cooling, but may be sized correctly for heating.
Desuperheaters don't perform very well when the equipment only has short run cycles.

Only the 3 ton unit has a de-supraheater to provide domestic hotwater. It was my understanding when we purchased it that it would utilize any capacity that was not required for the house i.e. when the unit was not busy heating or cooling the house it would work on bringing the domestic hotwater up to temp. That's not what seems to be happening. You're right, the only time it seems to work is when the pump is running in heating mode. This would seem to indicate that I am splitting the heat output of the pump between heating the house air coils and heating the domestic water tank.

[vstech]

Only the 3 ton unit has a de-supraheater to provide domestic hotwater. It was my understanding when we purchased it that it would utilize any capacity that was not required for the house i.e. when the unit was not busy heating or cooling the house it would work on bringing the domestic hotwater up to temp. That's not what seems to be happening. You're right, the only time it seems to work is when the pump is running in heating mode. This would seem to indicate that I am splitting the heat output of the pump between heating the house air coils and heating the domestic water tank.

the desuperheater should only be on the cooling system... there's no reason to desuperheat the heating side... it's needed to HEAT the loop...
I fail to understand why you have two systems... I'd think a heatpump style design could be incorporated, but I suppose having 3 tons dedicated to cool air, and a separate 4 ton system just to heat the water for your hydronic loop (I'm guessing radiant floor?) makes some sense.