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I’ve recently purchased a new home which by contract says I have a furnace which has the capacity to heat the finished part of the home plus heat 1/2 of the basement if I chose to finish it. A couple of construction related friends who have visited my home have been suprised by the small size of my furnace (Janitrol). I’d like to know if there are any simple calculations that I can do to get a quick ball park esitmate if my furnace is in the ball park, before I contact a independent heating contractor to do the professional evaluation. I have the same furnace as my neighbors even though my home is 500sqft larger.
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Replies
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John, why do you doubt the contractor, are you cold or uncomfortable in your house?
If the contractor has given you a written assurance that your system is properly sized and will do as promised based on the house as he built it, I fail to see the problem.
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But if it is undersized, it is my responsibility to verify that what I paid for is what I recieved. Since this is a builder who makes money by sub-contracting out all the work. The incentive is make money. I go by the rule of buyer beware.
John Edwards
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Yes John, it's buyer beware. But there has to be a reason to react. How do you know, that it wasn't your neighbor that was sold a bill of goods in the form of an oversized heating unit? His unit could be running at 50% capacity.
When I assume the role of GC, I get quotes from several Mechanical contractors. I give them the plans to the house and wait for their quotes and recomendations. On one house alone, (these were all licenced and ? reputable established contractors) the quotes varied from $6,000 to $16,000 dollars.
Some had quoted for 2 hot water units with a capacity of 150,000 BTUs each while others had 1 system capacity of between 125,000 to 200,000 BTUs.
Same house, same plans, same everthing.
Point is don't lose sleep over something that probably doesn't exist. The tendency is to sell oversized units as opposed to undersized units because of the percentage profit on the bigger unit is higher.
Let your house's comfort level be your guide and never mind what your neighbor has in his basement.
If your house is well heated with a smaller unit, then you got the better deal.
*Gabe,In my area I work with the heat subs mostly but have installed a few systems too...Here's my experience to date...No one has ever tried to sell me a bigger system for the added money...These guys all have learned one thing well though if you undersize a system, there will be all H*ll to pay for it...They try to oversize by 20% or so and though they all use heat anaylisis, the last thing they do is give it a reality check versas what is working in the field already. And the need to keep the bid price competitive especially in tract building means if anything the systems are kept as small as they dare while avoiding the midnight call from a cold homeowner.As for advice...most systems can put out extra heat for an addition. They just may have to run more often. Somebody in your yellow pages should be able to help you.J
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John, what size is your furnace and how many square feet is it heating?
*Even though the furnaces are physically the same size, that does not meant thier btu capacity is the same. The only way to be sure is to check the namplate.
*The size of my house if 3300sq ft. I will check the information package on my furnace again but I could find nothing about it's size or amount of house it is suppose to heat. What should I be looking for to determine size of the furnace? From the post from David I will look for something specifying BTU. Yes my house stays warm, but if I were to decide to finish half my basement, would it be enough? Yes this is a rather low chance I will do this (30%) but, I'd rather know now than later.I'm an engineer and if there is some sort of compulation such as measuring window square footing and home square footage to determine some heating need, I could do that as a first ballpark estimate. I thought there was some sort of calculation that a heating contractor does to estimate heating needs. That's all I want to do to get a simple balpark correlation. I would then contact the manufacturor (Janitrol) and find out if my unit would supply this heating need.If it is off then I would get an independent estimate.
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John,
I had to size the heating plant for my house a couple years ago. It is dependent on the design temperature for your area (the coldest temp that the system is designed to keep up with to heat the area to the temperature you want, ie: the design temp here is 10 below, though it often gets to 25 below) and to what temp you expect to be able to heat the house to, and what the heat loss in BTU's/hr is.
You have to figure out how many BTU's per hour the heated area loses at the maximum design temperature. I had it figured by three different people and came up with wildly different estimates, and chose the most conservative one.
How air-tight your house is has a lot to do with it, so basing it on rated R-values of the construction method chosen is often not very accurate. My heating supply house had three ranges of "tightness" for a house depending on age of the house.
New construction can be very loose indeed, if not done well, and a lot of it isn't done well. Anyway, there are heat loss calculation programs for your computer that'll do this, or you could run the plans by an independant heating contractor, though I would expect to pay him for the service.
Steve
*John,The builder's heat sub. knows your furnaces btu output and also should have the heat analysis that was done for your home...Oil fired furnaces put out different btus based on oriface size of the nozzle installed...More heat can be had by upping the size or letting the unit run longer which will happen automatically...You have adequate heat, so you are worried about nothing...Heat analysis is complicated and full of; guesses times guesses equals an answer that is far from exact...Cant't be done...Those that do the calculations know this and fudge fix all there answers...You can't make this "grey" zone "black and white"...I hope you are beginning to understand that it's a huge ballpark, and works about as well as batting averages do at predicting if this time at bat, the batter is gonna hit a sure fire homer...J
*John,If you finish a basement that was uninsulated before and insulated after, your house heat load will drop. If your furnace is big enough now (it should run at least 80% of the time but not 100% of the time on the coldest day of the winter), it will still be big enough after the basement conversion. The only thing to worry about would be blower CFM, but that is usually not a problem.
*John:A method called "manual J" should be used by HVAC contractors to size equipment. Search for it on the web. You will find lots of info. The process of size calculation is not trivial.
*John,Since you allready have the house and the furnace, if it is big enough now it should be big enough after the remodel if you insulate properly. Most homes have oversized components in their HVAC systems because most are designed by "seat of pants" engineering. Model J and others aren't used often but they should be.Ron
*Dear Fred,Your right. My statement about manual J is only that it is better than a seat of the pants guess. I remember seeing a house where three HVAC companys quoted an over 125,000 BTU furnace. The homeowner put in a 27,000 BTU unit and it never had to run 100% of the time even on the coldest days. I allways use modeling software.Ron
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I've recently purchased a new home which by contract says I have a furnace which has the capacity to heat the finished part of the home plus heat 1/2 of the basement if I chose to finish it. A couple of construction related friends who have visited my home have been suprised by the small size of my furnace (Janitrol). I'd like to know if there are any simple calculations that I can do to get a quick ball park esitmate if my furnace is in the ball park, before I contact a independent heating contractor to do the professional evaluation. I have the same furnace as my neighbors even though my home is 500sqft larger.