Heat Loss Calculator Software

[Exequiel Mondragon]

Youcan use this heat loss calculator to estimate the power of a heater needed to keep your room at a comfortable temperature. From the text, you will learn how to compute heat loss and what is the BTU heating calculator.

All materials conduct heat. You can warm up your place to a comfortable temperature, but as long as the temperature outside is lower, your home will get colder. The flow of the heat from warmer to colder places is practically unstoppable, no matter how high the quality of insulating materials you choose to use. To compensate for the loss, we need to supply energy at a constant rate. This power is the power of a heater that this calculator will help you to compute.

The first point is simple, the larger the surface, the more heat can be transferred at the same time. The second point touches upon the characteristics of the materials. Materials used in the construction should comply with specific standards. Among other things, this means that they should have particular properties when it comes to heat transfer (see thermal conductivity calculator).

A common characteristic is a heat transfer coefficient, also called U-value. It specifies the transfer of heat through one square meter of a material divided by the difference in temperature. For example, an 11" brick wall might have a U of order 1 W/(m K), whereas a standard window might have a U-value five times larger. The last factor is the difference in the temperatures. Heat flows only between regions of different temperatures, so if the temperature is the same, there is no flow of heat. In general, the heat transfer is proportional to the temperature difference.

To compute the heat loss, we need to sum up heat loss through all the surfaces of the room and take into account the different characteristics of the materials used in the construction. The total heat loss is a sum of losses through walls, floor, and ceiling. We compute the loss through a single surface from the equation:

To use the heat loss calculator and to find the power of the heater, you need to give the dimensions of your room, specify on which floor it is, and what type of insulation the walls have. If you are unsure which insulation type to choose, go for the worse insulation. It's safer to be pessimistic. Finally, you should also specify how many external walls there are. In the section labeled additional information, you can also include the number of windows and doors.

The last bit of the information needed is the difference in temperature between the inside (internal temperature) and outside (ambient temperature). The internal temperature depends on your comfort. The ambient temperature should be the minimum temperature occurring in your region.

In some places around the world, it is more common to use the BTU (British Thermal Unit) per hour instead of watts to specify the power of a heating system. If you wonder how many BTUs I need, you can easily change from watts to BTUs per hour in our calculator.

Building materials directly impact heat loss. Materials with good insulation properties like fiberglass or mineral wool help maintain indoor temperature, whereas conductive materials such as metal and glass facilitate heat escape.

Air changes, through ventilation or leakage, can cause significant loss of heat. For example, buildings that are well-sealed and have controlled ventilation systems tend to lose less heat compared to those with air leakages.

The building's surface area exposed to the outside environment correlates with heat loss. Buildings with a larger external surface area tend to lose heat more rapidly due to increased opportunities for heat transfer.

The difference between the indoor and outdoor temperature plays a significant role in heat loss. Higher temperature differentials cause heat to flow more quickly from inside to outside, increasing loss of heat.

There are two quantities you want to calculate. The first is how big a heater you need to maintain temperature on the coldest days. That is given in BTU/hr, that looks to be the number you already have. The second quantity is the annual heating cost, which is given in BTU. To calculate that, the assumption is that every degree-hour takes the same amount of energy. If your tub needs 6,392 BTU/hr when it's 102F inside the tub and -16F -- 118 degrees -- that's 54 BTU per degree-hour.

My opinion is that treating it as a basement is underestimating your heating needs. With only about 22 inches below ground you're not getting much insulation from the ground. I would re-run it assuming the whole thing is above-ground.

Second:

I've recreated the spreadsheet as if the entire thing is above ground, but it seems to actually have made the heat loss go down. For that reason and for verification, would you know how to do a basic calculation to confirm or deny what this calculator is spitting out? I just want to make sure this is roughly accurate and I am not way off.

- Did I use the right numbers for interior/exterior? What temps would I use for the slab since it is below the surface?

- Then I tried to follow your previous math trail to divide over degree days and estimate annual usage. Does it look like I did that right?

Most pools/hot tubs lose their heat through evaporation. Insulating helps for sure. A sealed lid helps tremendously more. I think it's between 80-90% of heat in pools/hot tubs is lost through evaporation.

The heat loss to the ground is going to be different from the loss through the walls. The ground is roughly at the same temperature year-round so the loss is constant year-round and doesn't depend on heating degree-days. So it's a different calculation.

I have a similar hot tub in Central Texas. The OD are 8' x7'. The walls are all masonry and 10" thick. The base is 12" thick to allow for plumbing and extra rebar support (unit has over 600 feet of No. 5 rebar) You are wanting to use a 5 kva heater that I think in reality will be way too small. I run a 11 kva heater and need every kva to keep it hot in a texas winter. When i had an element go out it took almost 8 hours of heating a day to keep the tub at 102 degrees. With the 11 kva, 2 - 3 hours a day of heating will keep the tub at desired temp. I also have a very high end 5" cover, so it pretty well insulated for a hot tub. Also, once the masonry heats up, there is a pretty good thermal mass that doesn't cool very fast.

The short answer to your question is, "Using Passive House Planning Package (PHPP) software." Understandably, you aren't interested in buying $800 software; how does $225 grab you? You can purchase PHPP from the Passive House Institute US; for more information, click here.

Building scientists are still arguing this question; it's a complicated one, because isotherms in the soil complicate the heat loss calculations. Suffice it to say that many North American building scientists recommend 4 to 6 inches of rigid foam under a slab, with comparable or slightly higher amounts of vertical insulation at slab perimeters; slabs that are deep underground need less insulation, while slabs with hydronic tubing might need more. (Gary Proskiw's calculations, reported in this month's Solplan Review, call for even less insulation, even in northern Canada.)

Martin: Thanks. I have been assuming that I was looking for an illusive dude w/ this one, as there just does not seem to be much info on it. I'm glad to hear from your end that there is a reason for the lack. $225 does not sound bad at all, esp since, I assume, it pretty well looks at the whole house. I will check into it. Upon looking at my calcs for the 30th time while grappling w/ this, I realized that although you don't have a huge delta T under the slab, its (potential) duration somewhat compensates for the small dT, and I had not previously respected the impact of that. The search continues. john

John Klingel,

I don't have any answers for you but I always thought these thermal cartoons were interesting.

Also Robert's Comments from the "Murky discussion" about heat storage.

The area below the house is not as cold as the "ground temperature" and the further you get from the edge..the warmer

Brooks: What? You don't have the silver bullet? And I was counting on you... Yes, I've read Robert's take on this before, and agree w/ that generalized look at things. It would still be nice to have more than a gut feel, but that may just be asking too much. I was planning on 6", but now think 8" is more likely, and I could not argue one second about 10". But, that is all "gut". I looked at the PH software Martin linked above, and will have to inquire about their assumptions, etc, before I plunk down the cash for it. (BTW: For you Mac users, Maclinks opened their free xl spread sheet easily.) Thanks for the reply. john

Viel Danke, Herr Chlupp. Ich studieren werden. (Bitte, ich erinnnern mich nicht so viel von 1969.) When you get your data ready, we are waiting with bated breath. I am about convinced that 8" is a minimum, but going the full 12" like you do is rather tough on a ranch-style house. Thanks very much for the input. john

I'd be interested to know how you have "seen" these gains in performance. Did you build identical homes but with varying levels of slab insulation and then closely monitor them for a year? Maybe you built 50 homes with 2 inches of slab insulation and another 50 with 6 inches and another 50 with 10 inches and then compared their utility bills? I'm just curious how you have seen the gains in performance.

I'm also curious about any data to support your claim that foundation heat loss is underestimated. I would guess that it's over-estimated, but that's just conjecture based on comparing modeled and measured whole house energy use for many relatively inefficient homes (with no insulation anywhere in the basement -- slab, walls or ceiling) and finding that whole house heating energy use is typically dramatically over-estimated.

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