Heatloss- theoretical vs experimental.....

[kuribo]

As I am finalizing all of my hydronic system component purchases, I looked back at a whole house heat loss calculation I did some time ago. Calculating the heat loss through the roof, walls, windows, doors, and basement walls, I arrived at 41,000 BTU/hr. It seemed low based on the size of the house, so I was tempted to do what engineers like to do, and use a x2 "factor of safety" to size my equipment.

We just got through 4 days of extremely cold weather here in SW Wisconsin. I have a large masonry heater in the house as the only current source of heat while the construction is completed. I burned a fire in the masonry heater every 12 hours and was able to keep the inside temp of the house right around 40F. Doing the math (150lbs of wood per 24 hours, 75% efficiency. 6000 btu/lb) and a (40 - (-20)) 60F temp difference, I calculated a whole house UA factor of 500. Using this UA factor with a 90F temp difference (70F inside, -20F outside), I calculate a heat load of 45,000 BTU/hr. I was shocked at how closely the two figures were! Of course my theoretical calculation above didn't include infiltration so taking that into account, the two figures are really quite close! Coincidence????

 

[TCaldwell]

Not familiar with a ua figure but going from a 60 deg dif to a 90 deg dif and only increasing the load 5000 btu/hr seems worth doing the calcs through another method to check.?

 

[kuribo]

Q= U * Area * delta T

U= 1/R

U (1/R) and the area are fixed, so Q, heat loss, just varies with the inside and outside temp difference.

The heat loss went from 30,000 BTU/hr calculated from the wood burned to keep the house at a constant 40F to 45,000 BTU/hr calculated at 70F, so not 5000 BTU/hr for a 30F difference, but 15,000BTU/hr.....

 

[TCaldwell]

Must be very tight construction, solar gain to have those numbers in your climate

 

[kuribo]

There is a good deal of south facing glass and it has spray foam insulation, so that surely helps...

 

[TCaldwell]

Nice house, we’ve just finished up a 4 day cold snap, they usually don’t last that long. Last night was cold but but no wind either, loaded up the garn about7:30 pm fired 2000 gal to 194, woke up to the oil boiler cycling at 6:30 am? Honestly thought would have woken up to a boiler at 163 or better as the night before was colder, first thought was a open window or door but nothing. Went to exercise class saw local burner buddy there, he experienced the same thing last night. We guessed the extended cold time period allowed the cold to creep a little farther into the reaches of the house through foundation, windows ect.
For me with outdoor reset, at 0degf the curve calls for 155 deg water, so when the garn reached 152 deg the Oiler supliments.
I guess I need to think about insulating the poured concrete foundation walls among other areas.

 

[peakbagger]

Passive solar gain and night time radiant loss is not something that can be modeled very easily. I know that if its 10 degrees and blue sky I can open open all the cellular blinds on the sunny side house and it will stay warm , it it clouds up it will cool down rapidly. A standard calculation is going to just assume worse case, no sun. On the other hand if I leave the blinds open at night on clear night it will suck the heat out quick.

 

[maple1]

Nice house, we’ve just finished up a 4 day cold snap, they usually don’t last that long. Last night was cold but but no wind either, loaded up the garn about7:30 pm fired 2000 gal to 194, woke up to the oil boiler cycling at 6:30 am? Honestly thought would have woken up to a boiler at 163 or better as the night before was colder, first thought was a open window or door but nothing. Went to exercise class saw local burner buddy there, he experienced the same thing last night. We guessed the extended cold time period allowed the cold to creep a little farther into the reaches of the house through foundation, windows ect.
For me with outdoor reset, at 0degf the curve calls for 155 deg water, so when the garn reached 152 deg the Oiler supliments.
I guess I need to think about insulating the poured concrete foundation walls among other areas.

Same thing here this morning. Just as cold as last few days but no wind. Thought storage would have some ooomph left when I got up this morning (also nice & sunny out), but it's pretty well tapped out.

 

[JRHAWK9]

woke up to the oil boiler cycling at 6:30 am? Honestly thought would have woken up to a boiler at 163 or better as the night before was colder, first thought was a open window or door but nothing. Went to exercise class saw local burner buddy there, he experienced the same thing last night. We guessed the extended cold time period allowed the cold to creep a little farther into the reaches of the house through foundation, windows ect.

yep, day after day after day of extreme cold can really make it an uphill battle for heating. A night here and there of extreme cold is easy to manage. For me, being a wood furnace guy, I need to keep my basement warm, once my basement gets a bit cooler, it makes things very difficult. Our cold snap consisted of 6 of the 7 days being 70+ HDD's (75, 78, 71, 58, 71, 84 and 84). Towards the end of the last of the two 84 HDD's is when my basement temp dropped to a point where it was tough. The whole house was now feeling the effects of the extreme cold. The LP only ran for a total of 86 minutes during the whole 7 day stretch, and most of it happened at the end of the second 84 HDD.

 

[kuribo]

No doubt I am having a large loss at night through all the glass. Once we move in I intend to put in some sort of blinds we can lower at night. In Japan they have sliding doors on the outside that they close at night to cover the doors. I might do that down the road but it has already been 13 years I have been working on this house...we will finally move in this Spring.

Something mentioned above is something I have wondered about....I have radiant throughout the basement and am wondering how much of the basement heat will migrate up through the floor into the 1st floor....Anyone with radiant in the basement that can comment?

Thanks to all who have responded....