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Old 11-27-2009, 08:51 PM   #1
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More Accurate Load Calculation


There are all kinds of Manual J load calculation tools, software, books, etc.

They all rely on measuring rooms and guessing at insulation of walls/windows/cracks, etc.

Do any contractors actually install a data logger and just take a measurement of building load?

Basically if you installed a little box that hooked up to the furnace red and white wires (single stage furnace), you could know when the furnace is being told to turn on. From this you can gauge run time and duty cycle. If you had an outdoor and indoor thermostat also hooked up, you would know the temperature differential and the nature of the settings, etc. Based on how long the furnace runs and the BTU rating of the furnace, you would know how much heat is going in and how much is going out. If you measure the inside and outside temps, you can calculate the actual load of the building (assuming the BTU rating of the furnace is accurate) If you then cross checked these numbers with gas meter readings before and after you install the load tester data logger module, you can validate the readings.

You could install this for a month and give a home owner a really good idea of the houses thermal load.

Does anyone make anything like this?
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Old 11-27-2009, 08:58 PM   #2
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If the house is already balanced
And if the existing HVAC is already sized correctly
Which negates the need for the Load calc

You want a contractor to monitor for a month ?
So they would only be able to install 1 system a month ?
Spring...summer...fall...winter, which to measure ?
Seems like that would be more work then the Load Calc
I did my Load calc in a few hours
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Old 11-27-2009, 09:23 PM   #3
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More Accurate Load Calculation


Quote:
Originally Posted by zootjeff View Post
There are all kinds of Manual J load calculation tools, software, books, etc.

They all rely on measuring rooms and guessing at insulation of walls/windows/cracks, etc.

Do any contractors actually install a data logger and just take a measurement of building load?

Basically if you installed a little box that hooked up to the furnace red and white wires (single stage furnace), you could know when the furnace is being told to turn on. From this you can gauge run time and duty cycle. If you had an outdoor and indoor thermostat also hooked up, you would know the temperature differential and the nature of the settings, etc. Based on how long the furnace runs and the BTU rating of the furnace, you would know how much heat is going in and how much is going out. If you measure the inside and outside temps, you can calculate the actual load of the building (assuming the BTU rating of the furnace is accurate) If you then cross checked these numbers with gas meter readings before and after you install the load tester data logger module, you can validate the readings.

You could install this for a month and give a home owner a really good idea of the houses thermal load.

Does anyone make anything like this?
The avg. is 5.9 BTU per sq. ft. per heating degree day, from my own samples and a magazine article. Here's my few samples for North American residences, single family I think

1.7 was the least heat loss
3.8
4.0
4.4
5.7
8.0
8.8
11 was the most

My leaky 1964 house comes in at 8.0.

The HVAC guys will be all over us, in about a minute, for a variety of reasons.

http://www.focusonenergy.com/files/D...udy_report.pdf

http://utwired.engr.utexas.edu/siege...ngloads_II.pdf

Last edited by Yoyizit; 11-27-2009 at 09:50 PM.
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Old 11-27-2009, 09:31 PM   #4
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Originally Posted by Scuba_Dave View Post
If the house is already balanced
And if the existing HVAC is already sized correctly
Which negates the need for the Load calc

You want a contractor to monitor for a month ?
So they would only be able to install 1 system a month ?
Spring...summer...fall...winter, which to measure ?
Seems like that would be more work then the Load Calc
I did my Load calc in a few hours

No I want to know the actual load of my house with my existing furnace. It would help in the sizing of an AC install later on. More importantly, I want to know where the crossover point is for my heat pump.

To get usable numbers, you would only do the test during the Nov-Feb months if you are measuring heating duty cycle. Obviously it would do no good to do it in August for a furnace.

I had three contractors out doing a Manual J load for my AC install and they came up with 2.5 tons, 3 tons and 3.5 tons respectively. That begs the question, what is the actual load of my house? Guessing at insulation of a 30 year old house is retarded when you could just measure the duty cycle of the furnace and cross reference it with the outdoor vs indoor temperature.

That would give me a correct sizing, it would help me determine the actual cutoff point of my heat pump, and it seems like something that customers would want to have an accurate energy audit of their house for an extra cost.

If I knew my house was X BTU-Hours per degrees-F of temperature differential that could do the following:

1. Let me know at what temp my heat pump could no longer heat the house if on 100% of the time
2. Provide something to look up against comparable sq-footage homes to see if I should install insulation, new windows, etc.
3. Provide both furnace and AC sizing references.
4. Give me an idea of Green factor for this "Green Energy Fad" that seems to be rocking the nation. Maybe I use more or less energy as a result of the test.


-Jeff
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Old 11-27-2009, 09:38 PM   #5
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Are you also going to track the days when its windier ?
Thus pushing more cold air into the house ?

You are asking about heat on the one hand.....then switch to the fact you are trying to have an AC sized

Heat rises.....cold air sinks
Summer you the need to worry about a hot roof

They have thermal sensors that can show heat loss
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Old 11-27-2009, 11:32 PM   #6
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Quote:
Originally Posted by Scuba_Dave View Post
Are you also going to track the days when its windier ?
Thus pushing more cold air into the house ?

You are asking about heat on the one hand.....then switch to the fact you are trying to have an AC sized

Heat rises.....cold air sinks
Summer you the need to worry about a hot roof

They have thermal sensors that can show heat loss


I'm just surprised that none of the actual load calculations take actual measurements. Sure, track the house for a year, don't just track it for a month.. Install a few bluetooth temp sensors, hook it to a computer via USB and then run it for year. Seems like a month or even a few heating days would put you right in the ball park. For example, for three heating days in November it looks like it was on average 50 degrees outside and 65 inside so with an average of 15 degrees over a few days your 60,000 BTU furnace was running a duty cycle of 20 % so that was 12,000 BTUs for 15 degrees delta. So the load of your house is 800 BTUs per degree f.

That means if it's 25 degrees outside you'd need about 32000 BTUs to heat. If your heat pump only has 16000 BTUs output at 25f, it's not going to take up the load, hence the reason they have a cutout in most cases at 45. But maybe it's 50? or 40?



Seems like something basic to measure. I'm surprised none in this industry generally take real measurements, or if you wanted one, you can't really order it done.

Last edited by zootjeff; 11-27-2009 at 11:43 PM.
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Old 11-28-2009, 10:01 AM   #7
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Played around like that on my own house here in Nebraska. It's a small,1000 sq ft, built in 1955 with new windows and insulated siding but need more in the attic. Two ton heat pump could maintain 72 degree indoor temp untill the outdoor temp fell below 9 degrees. Of course it ran non stop that low. I have the heat pump set up to run now untill the outside temp gets to 18 which cycles the thermostat 3-4 times per hour.
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Old 12-13-2009, 10:56 AM   #8
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Quote:
Originally Posted by zootjeff View Post

Does anyone make anything like this?
No one makes a plug-n-play system like what you are asking. You could develop your own though.

I deployed my own.
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Old 12-13-2009, 11:42 AM   #9
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You would need to also have a way to determine how much gas was used for hot water. And how much for cooking.

You will find load calcs from companies off because of the salesman's fudging.

If you want an accurate one. Use ASHRAE's long form. Manual J is a compressed format of it.

Or you can measure the R value of your walls yourself.


Would you be willing to pay 1500 to $2,000.00 for a company to use data loggers on your house for 3 months?

Remember, they would have to have several ses, so they can do this kind of data logging on 10 to 20 houses at the same time. So they can do load calcs on all of their sales presentations.
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Old 12-14-2009, 12:06 AM   #10
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Quote:
Originally Posted by AndrewF View Post
No one makes a plug-n-play system like what you are asking. You could develop your own though.

I deployed my own.


What kind of hardware are you using to do this logging? Did you write software or just use something off the shelf?
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