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OT: Heat loss calculations for house

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  • OT: Heat loss calculations for house

    I have been trying, on and off, for several years to determine the heat loss of my home. I tried to simply “measure” the heat loss characteristics by timing the cycling of my natural gas furnace (when the temperature was at 5 degrees F outside) and timing the cycling of the heat pump (when the temperature was at 35 degrees F). Using the efficiency of the furnace and the documentation for the heat pump, I was able to calculate that the heat loss per hour when the outside temperature was 5 degrees at about 950Btu per degree of temperature differential as compared with the inside temperature; however, when I used the data from the heat pump, I came up with a heat loss of 460Btu per hour for each degree of temperature difference. The inside temperature for the 2 measurements was close (69 degrees for the 5 degree outside temp and 70 degrees when the outside was 35 degrees).

    Is temperature loss a linear function, or does the rate increase as the temperature difference increases? Do my numbers of 950Btu per degree @ 5 degrees and 460Btu per degree @ 35 degrees make sense?

    I paid around $50 for a limited-time access to a software program to try and calculate the thermal properties of the house – I was unsuccessful in utilizing the software due to the complexity of my house. Our house is a “tri-level”, with ½ of the "footprint" about 4 feet below ground. You basically enter the front half of the house on ground level and can go either up or down to access the 2-story back half. The ground level portion has a cathedral ceiling that limits the airspace between the outside roof and interior ceiling (think very little room for insulation here) and would make it difficult to provide for proper venting of the roof and allow any type of insulation. I could probably blow it full, but then the roof would get too hot in the summer and trying to shove those foam vent sections down the long roof and into the overhang would be extremely difficult. Several years ago, I removed all the siding and installed a layer of insulated foam board with foil backing before installing new vinyl siding. While the house was stripped, I installed new, high-efficiency windows and doors; since the front is brick veneer, I left it alone. All this detail is necessary to explain that there are all kinds of different wall materials with differing insulation properties. Total square footage of the house is about 1700 with about 575 sq ft below ground, 550 sq ft @ ground level, and 575 sq ft above ground. The shadow cast in the second picture, does a pretty good job of depicting the portion of the house that is actually 2-stories (partially submerged in the ground).

    Last edited by Jim Caudill; 02-07-2010, 04:34 PM.

  • #2
    You have more variables......

    Wind speed affects rate of heat loss, just as with a "heatsink" in power electronics.

    And air infiltration leaks hot air out, and brings in cold air, adding another loss path.

    Then also, the running of uninsulated ducts (or hot water pipes) through cold basements loses heat that may or may not be still effective in heating.

    A decent formula should have inputs for those..... Somewhere I have a couple HVAC books, I will see what they say, since I am supposed to do some similar calculations.

    Keep eye on ball.
    Hashim Khan


    • #3
      If you don't care about units of measure one good way to understand heat transfer is to calculate your annual bill for gas and electricity. That will give you your energy consumption rate in dollars/day. Once you know what your dollars/day average is you can break it down seasonally, monthly, and even get a rough estimate on day vs night.

      All energy that enters your home leaves as heat. That includes burners, light bulbs, motors, sunlight, etc. It's all heating your house or removing heat from your house. Even the occupants contribute to heat. Your house is 100% inefficient which means all heat that enters your house will leave your house. Using nothing but your power bill you can come up with a rough idea of what the rate of heat loss is in dollars/hour and that converts to watts/hour. This also has the advantage of calculating energy consumption rate for cooling as well as heating.


      • #4
        Get a "pro" in


        I think you are nearest the mark as regards simplicity as I guess the OP needs a reliable "ball park" figure to start with.

        There are some excellent tables, guides etc. that various manufacturers put out for use by Contractors and HVAC engineers etc. to estimate/"size" HVAC components for optimum capital and recurrent costs. This includes multiple zones. They have taken most or all variables for a particular locality, size/s, insulation, construction materials, etc. etc, into account.

        Hiring in a specialist may well be a very good proposition as the longer it takes for a non-skilled person to work it out the more it costs and so the "break even" point for paying to have the assessment done can be very short.

        A "DIY" method assumes that the occupant gets the capital costs (equipment plus installation etc.) and recurrent costs (power, gas etc.) right. If he gets it wrong and it is inadequate - it may be either or both very unsatisfactory or very expensive or both. And if the funds are borrowed it makes it even worse if it is wrong.

        While a good professional may seem expensive in raw $ terms, it should be very good value at the end of the job as it is more likely than not that he will save the client a lot more that his fees were - and while the fees are are a "one off" cost the benefits are on-going.


        • #5
          Not all energy that enters the house leaves as lost heat through the walls and roof, a goodly proportion leaves as hot water down the drains too.


          • #6
            Some power companies will come out and do a heat loss study. I don't know if they charge for it or not. You might call your power company and see what they can do.
            It's only ink and paper


            • #7
              I would also think if a motot is turning, like washing clothes, or making a mudslide in a blender then that mechanical effort would not leave as heat as some work was accomplished. a drill press might do some effort as well. FWIW.



              • #8
                I actually had a full house efficiency survey done this past December. I frankly was disappointed.

                I hired the best rated firm in the area.

                They were competent at operating their equipment, but quantitative data was lacking, and the report was not what I would call a complete report of the type I would give to a client. I could not use teh data they gave to prioritize actions, and I am not at all sure that they actually GOT data that would allow it.

                Perhaps that disappointment is the effect of being in the consulting business, but I don't see it as a great and helpful thing to have done.

                As for ways heat leaves, or inputs of heat, they tend to mostly pale in comparison to the furnace and ordinary heat loss thru walls and windows, unless the house is very tight and well insulated. Few are.

                Keep eye on ball.
                Hashim Khan


                • #9
                  I think you are talking about HVAC calc.
                  It is to my understanding a very good piece of software. With some work you will be able to get your data. BTW the person who wrote it is very knowledgeable and very helpful. Or at least that was my impression when I asked him a few questions. Another associate also have been very pleased with the software.



                  • #10
                    The power company here does an infrared study to determine where the heat loss is and somehow they convert that into how much is lost at those spots. It's mostly to show where you need to insulate from what I understand and it's not exact.
                    It's only ink and paper


                    • #11
                      I found the information required to be too daunting to collect. I have different roof structures with varying insulation, flooring that is below grade, walls that are partially below grade and partially above (but insulated on the outside), a front wall with very different properties from the side walls, etc.

                      It is not the software's fault, it is my ability to asess, characterize and quantify all the elements that are needed.

                      I would gladly pay to have an "expert" figure all this out. I don't know where to find one. All the HVAC folks I know, size units on some "rules of thumb" and recommendations from the distributors. Just like the settings for the heat pump, the distributor will tell them where to set the "crossover" switch (or whatever you call it). Replacement units are sized based on whatever was installed and the anticipated improvement in efficiency - no calculations whatsoever.

                      Years ago I received a free energy audit from my power company - I don't think they do that anymore, since the gas and electric were "split up". Since I have a "dual fuel" system with heat pump and natural gas, some of my heat comes from the gas folks and some of it comes from the electric folks - neither get all my money. Neither can tell me how much energy I use to heat my home. Since I have resistance heat for the shop, and use a fair amount of electricty for heat, lights, and machinery - even the amount of electricity used for the heat pump is hard to determine.

                      The energy audit didn't tell me very much, other than the payback for new windows would be something like 17 years. They said the single best thing I could do, was to dig down and insulate the concrete block walls that are below grade. But even that wouldn't make sense unless I was already going to dig them out for some other reason. It seems like if that were the case, they would recommend adding insulation to the interior walls that are simply wood panelling over firring strips.


                      • #12

                        Your idea to compute the loss by energy balance was a great idea but comparing the furnace and the heat pump is apples and oranges. If you measure the gas or electricity usage vs. the temperature drop, you have account for the fact that the efficiencies of the heat pump and the furnace are different.

                        Given your circumstance, the numbers for the furnace are much closer to the actual heat loss of your house than those from the heat pump. I say this because the furnace efficiency is likely between 70% and 90%. The heat pump efficiency is likely between 100% and 200%. The efficiency of the heat pump varies dramatically with temperature. (I'll leave the explanation of heat pump physics to the pedants.) As a result of the efficiency differences, the heat pump used significantly less energy than the furnace to deliver the same amount of heat.

                        Simplistically, the heat exchange in a system is proportional to the temperature difference between the inside and the outside times a constant. See for an explanation of how its computed in practice.

                        If you are really interested in a number that professionals in the industry would believe, the correct way to do it is an ASHRAE Manual J calculation or the software automation of it. I've never actually seen manual J or done the calculation so I can't help beyond that. All I can say is the book is bloody expensive.

                        I think in principal it might be possible to calculate the loss coefficient with newton's law of cooling but it is difficult to recover the loss coefficient without knowing the specific heat of the house and the area.

                        The code word to ask for what you want is "What would it cost for you to do a manual J calculation on my house?"



                        • #13
                          Manual J heat loss methods/calculations are what the quality heat loss programs are based on. Still, I heat loss calculation has the potential for a fair amount of error due to the many variables that cannot be accurately measured and inputted. For example, the color of the roof, shading on the house etc. Also, calculations are based on a "worst case outdoor temperature" for the region during the heating season.

                          The desired result in obtaining a accurate heat loss is so that heating equipment can be sized for the house and be equal (actually a little larger) than the calculated loss. When loss equals the furnace output we have equalibrium.

                          With equalibrium in mind........ Your method of timing the furnace was not a bad idea all together. First you need to know the real BTU input to the furnace, you get that by reading the gas meter for a timed peroid when the furnace is running. Exact details of how this common procedure is done is available on the net. A adjustment for the quality of natural gas can also be made (therms).

                          Next, you need to apply a efficiency factor to account for the efficiency of the furnace. Furnaces 90% and over use PVC for their venting. A 80% furnace is the most common by far. Depending on the age, there might be a sticker on the doors with the efficiency.

                          Next you can do your measurments/calculations. Don't forget that the only furnace time to be used is when the burner is on, not just the fan running! A neat method is to put a 24V AC hobbs (time) meter across the gas valve wires. This then gives operating hours for the burner only.

                          ALTERNATELY, you could simply turn the furnace off and measure the time it takes for the house to drop say 10 degrees. That may be more accurate with less potential errors. If you think about it, that would be a TRUE heat loss number. (varies according to outside temp, sunshine etc. of course) Don't open a door either! LOL Personally, I believe this to be the best method of all.

                          Remember though, manual J calculations and good software programs are based on the worst case temperature for your region. Your measurements will be for the given day.
                          Last edited by Sparky_NY; 02-07-2010, 08:25 PM.


                          • #14
                            Oh yea........ I forgot to ask........
                            Why do you want a heat loss? What do you plan to do with the "number" once you have it?

                            Let say the number is 55,000 btu/hr (a reasonable guess for your house).
                            Now what? I suppose that provided you get a detailed output you could look at how that loss is divided up among various line items. You could then change a line item parameters and see how much differerence in makes in btu's for that item. Having someone do a one time loss calculation for your house wouldn't provide that information however.


                            • #15
                              Something i cant work out

                              wouldn't it be more efficient to have a boiler that turned the flame down lower and not out ..infinatly variable flame ..than have some thermostat turning it on and off at full blast every ten Min's or so.

                              i mean its like using full aceleration in your car to reach the top speed every time .we all know that uses more fuel

                              why is this sort of system not made ..a system that cruises

                              i can also see such a system lasting a lot longer, if it were done this way ..because you're not getting all the expansion contraction and cooling down of the boiler.

                              the flames would be mostly quite small most of the time this will not eat the metal like huge flames

                              such a system would not heat the crap out of the water and lead to calcium deposits either

                              you could put a smaller more electrically efficient pump in the system ..that runs twenty four seven

                              so all someones got to invent is a robust infinitely controllable valve. and a 1500000 hour pump

                              I'm sure the above would improve efficiency a lot over the conventional systems.

                              all the best.markj
                              Last edited by aboard_epsilon; 02-07-2010, 08:46 PM.