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Question DetailsAsked on 9/30/2015

my house is 1154sqft, what size trane XT95 (or XR95)will fits house?60kBTU or 40k BTU?

In Michigan, house built at 1956.40KBTU for size 888sqft? and 60K BTU for 1333 sqft?the attic has insulation, crawl space has no insulation. wall has no insulation. 9 windows are double vinyl windows. i like small size 40kBTU, do you think I need to insulate crawl space in order to fit the size 40kBTU? i know there is load calculation, i am trying to convince the heating company to get load calcus. thanks

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4 Answers

0
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IF you have to "convince" him to do the ACCA Manual J, S, and D calculations to determine what size and peformance your new furnace needs, then use Manual E - as in EJECT the contractor, because otherwise he is just using a rule of thumb or a guess as to what size unit you need and is likely to give you a mis-sized unit.


It should not matter what you "like" - if you like small I can give you a 400W heat lamp that is even more efficient - of course, you will be limited to about a 10SF living space. You need to pick a unit that is sized right for your situation.


Depends a lot on what your air leakage situation is, but with decent weatherstripping and sealing of air passages I would imagine a 40K unit would likely work unless you are way in the backwoods on the Minnesota side where it gets WAYYY down there in the winter.


Cost difference between a 40 and 60K unit is pretty small and would probably not pay for itself in capital cost savings if you got an Energy Audit with blower door test to identify the thermal characteristics of your house - but that (if it has never been done) might very well pay off in spades in fuel savings, and as a side effect might (after taking the most significant energy saving measures recommended) allow use of the smaller unit, but with uninsulated crawlspace and walls, maybe not.

Answered 4 years ago by LCD

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Votes

Thank you for the answer LCD!


If i insulate the crawl space, the size 40k BTU may be ok for the house 1154 sqft?

Answered 4 years ago by Guest_9669352

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Sorry, but no - I did not mean that. While a 40K unit "may" work, it also may not - and you do not want to be counting on maybe's in choosing your furnace, or in spending a number of thousands of $. And imagine if you chose the 40K unit because it "may" be big enough, then a Polar Vortex dumps a week or two of 30 below blizzard on you and your pipes start freezing up or you can't get your house above 45 degrees ? Not a happy scene for the possible few hundred $ savings you may have achieved.


BTW - you are not stuck with a choice between a 40,000 Btu and a 60,000 Btu furnace (though the brand your HVAC guy is pushing may be limited to those) - there are 45, 50, 55K units made and readily available, so "right-sizing" your unit is not a matter of choosing between two that may be equally unsuitable.


The HVAC calculations have to be done, taking into account house construction factors and ambient climatic conditions (design heating and cooling termperatures and average wind velocity at extremes of the tempeature ranges). The Manual J (named for Joules, a measure of energy) determines the amount of heating (Btu's / hr) or cooling (tons of cooling capacity) needed by your house, and those numbers can vary depending on how well insulated or tight your house is - some more sophisticated computerized calculations take into account the results of energy audits for instance and specifics of house construction, some are quite crude and simplistic.


Manual S is for Sizing or Selection of the HVAC equipment - matching your heating/cooling load with specific capacity of equipment, and that determines the rated capacity of your unit.


Manual D is for sizing the Ducts for a central air / furnace system. There are related tables for determining air conditioner/heat pump refrigerant line size, but that is rarely needed in residential applications because the loads are relatively small and the runs short in most cases, though it can come into play in mini-split systems.


Without all three being computed (or the first 2 plus line and radiator sizing for A/C units and boilers), you cannot tell what size HVAC unit will work for you. Generally speaking, for furnaces by themselves (without combined central air A/C) you are better being slightly oversized than undersized, otherwise it will work too hard and run too long at each cycle, and may not even be able to maintain the desired temperature at the extreme cold end of your design ambient temperature range. However, too oversized and you can get uneven heating in the house due to inadequate run time to get good air mixing, and the short cycle times can also promote growth of mold in the air ducts due to inadequate run time to remove any accumulated moisture. Too large a unit also results in short run cycling which is hard on the equipment in the long run.


For stand-alone A/C's the same basic principles apply - undersizing is hard on the equipment due to long run times and risks not being able to achieve the desired cooling, oversizing causes short run cycles which promote mold growth in the system.


Where it gets more complex is with central air where the A/C evaporator or coil is installed in the furnace ducting and uses the furnace fan to drive the ventilation, because if you oversize the A/C unit it will run for only short times each cycle and not have a long enough run time for the airflow to remove condensation in the system, which can result in severe mold issues on the evaporator and even up into the ducts, but undersizing makes for ineffective cooling and makes the unit work too hard and wear out quickly, so it is best to get the right sized unit for your specific situation.


One thing on insulating the crawl space - if done incorrectly, it can result in an increase in the moisture in that area, causing mold even when that may not have been a problem before. In many cases, the heat loss from the house into the crawl space and rainsing the craswlspace temperature is what is keeping the humidity low enough that mold does not grow, so in probably a vast majority of cases for crawlspaces that are walled in with foundation walls and small vents rather than wide-open all around (visualize a porch or deck), if you insulate the crawlspace (either ground / foundation insulation or underside of floor) you also have to take measures to stop moisture from accumulating in there - generally by installing an effectvie vapor barrier over the ground, and sometimes with underfloor insulation adding additional outside venting or even artificial ventilation to keep the humidity level down. This can get very difficult to get right in areas with both hot humid summers and cold winters, because the winter cold protection can result in excess humidity in the summer if you increase outside ventilation, so sometimes temerpature and humidity controlled artificial ventilation is needed to control the moisture, so it has to be expertly done, and in extreme cases can end up with the entire crawlspace being insulated and vapro barrieried and ventilated to effectively turn the crawlspace into part of the conditioned space of the house to control the moisture.


Answered 4 years ago by LCD

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Thank you LCD for all the information.


a little more information. this is rental house.


a couple more questions:

1)the contractor did not mention i need to have energy audit with blower door test before "load calcu". I wonder if i need energy audit with blower door test before "Load Calcu"?


2)XR95 is $300 cheaper than XT95. XT95 can lower gas and electric bill a lot than XR95?

I am thinking choose XR95 since XR95 can give me $200 rebate from gas provider,XT95 can only get same amount rebate of $200(but XT 95 cost $300 more than XR95).


BTW, This is rental house.


Thank you again,

susan

Answered 4 years ago by Guest_9669352




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