Posted: 2/16/2010 8:14:02 PM EDT
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The long and short of it is that after divorce and putting one kid through private school I found my self sorta homeless, well not really, just had to sell the house I was living in and give half of it to the ex. Fast forward a few years and I find my self the owner of my fathers property 10 acres with a single wide with a 13x24 room added on. Total SF is around 950 sf. Instead of building a new house on the property, I decided to fix up the 42 year old mobile home that I grew up in, so I put a roof on it, have been gutting it and re-insulating it with R-13 outside walls and ceiling (not enough room between the steel roof and ceiling, may blow in insulation later on in the attic and below the floor), been dry walling it (it will never move, unless it is in a dumpster), new windows, doors, plumbing, electrical, etc. Basically, I have built a new home from the inside out. Good decision on not building, as I was laid off in September 09. I figure that I can rent it out to help pay for new house in a few years. Anyway, I have pretty much learned everything as I go using the internet and ton and tons of forums. The only thing that I am still a little fuzzy on is the HVAC system. So anyone with HVAC knowledge, please comment on my system. I am in Zone 3 and after reading a ton of info, it looks like I am somewhere around a 2 ton to 2.5 ton unit (I know, do a manual J and it will give me the number). Which I am pretty sure it will spit out that I am at the upper end of a 2 ton unit. So I was leaning toward a 2.5 ton, since we do get some pretty hot summers here, but honestly, I cooled the place with two small 115vac window units (did I mention the walls had R-7 insulation), so I think that the 2 ton will probably be sufficient 95% of the time, it is the other 5% ball sweating time that I am worried about. Due to the space limitations, I will be using a packaged heat pump (all electric, although I am considering using propane and running a line inside so that I can have emergency heat if the power fails). Anyway, I am looking at running an 8x22 rectangular up the outside wall and 45ing into an area above the water heater which will lead into the attic. From there, I will go to a 14" flex duct (less than 23 feet and teeing into a 9 inch line (that will serve a bedroom and bathroom (about 300 sf) and a 12 inch line that will serve rest of the house (around 650 sf). I figure the 9 inch should give me around 300 cfm and the 12" should give me around 600-700 cfm. I will tee off of the 9 and 12 inch and reduce my flex duct down after each tee as necessary to keep the cfm sufficient for the next rooms. Due to space limitations (again), the return duct will be at one end of the house and I plan on using a 20 x 20 grill and using a 14" or 8x22 rectangular in the crawl space to get back to the unit. So a couple of questions... 1) am I close or will I be sweating my ass off in the summer and peeing Ice cubes in the winter 2) given the choice should I go all elec. or use propane for heat 3) is my return sufficient, should I oversize the return with a 16 inch. 4) if I go with a 2 ton unit, should I keep my supply line at a 14" or go ahead and reduce it to 12"? Same with return. 5) and lastly, do I need a plenum in the attic where the 8x22 enters or can I just go to a round pipe and then tee off of it Sorry for the long post and thanks in advance. BTW, looking a Goodman Seer 13 or 14. Oh, all of this is on the cheap, did I mention I was laid off in September  .Reisco |
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1) am I close or will I be sweating my ass off in the summer and peeing Ice cubes in the winter
2) given the choice should I go all elec. or use propane for heat 3) is my return sufficient, should I oversize the return with a 16 inch. 4) if I go with a 2 ton unit, should I keep my supply line at a 14" or go ahead and reduce it to 12"? Same with return. 5) and lastly, do I need a plenum in the attic where the 8x22 enters or can I just go to a round pipe and then tee off of it 1. You need to get a Manual J done. You can compare 2 window a/c's all day long but you are missing factors like humidity and average moisture levels for your region by guessing. Square footage, cumulative btu output of the window a/c's and whatever else is just guessing...I can't emphasize this enough. A Manual J will take into account everything about your home. You said you've searched around on the internet a bunch; if that's true then you should have already found appropriate software to do a Manual J yourself. 2. You mentioned propane for emergency heat? I assume a propane stove of some kind? If you're thinking propane will still work when the electricity is out for your furnace, you are incorrect. You do know something has to power that blower motor right? But to answer your question I personally would choose propane over electricity but since you live in Georgia purchasing the heat pump without the strips might be a good move. There's a somewhat complex formula for figuring up which would be cheaper to run at a certain efficiency based on the costs of electricity and propane. I'll have to look for it. 3. You need to have a Manual D done for this stuff. Undersizing your ducts will kill your system; oversizing a little won't hurt anything but if it's too oversized you won't have good airflow. 4. Again, Manual J and D. You may find out that 2 tons is too big for your home. 5. It's hard to say without seeing the setup but usually any hard 90 degree angles are bad for airflow. |
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Thanks for you input, I figured I would get a lot of people telling me to do a manual J calc, but was hoping I would also get "your on track". 1. You need to get a Manual J done. You can compare 2 window a/c's all day long but you are missing factors like humidity and average moisture levels for your region by guessing. Square footage, cumulative btu output of the window a/c's and whatever else is just guessing...I can't emphasize this enough. A Manual J will take into account everything about your home. You said you've searched around on the internet a bunch; if that's true then you should have already found appropriate software to do a Manual J yourself. I guess I will purchase a copy of the Manual J. A lot of systems I looked at with similar characteristics (single floor, crawl space, older home, southern region), were using 2-2.5 ton units, and using the "rule of thumb" that is how I came up with the size. But if a 1.5 ton will suffice, then I agree, that over sizing the unit will be as bad as under sizing it. Also, I did happen across an online Manual J calc and it showed a two ton unit, but at the high end, this was before I started getting into the HVAC stuff more in depth, so I don't think I completed it correctly. 2. You mentioned propane for emergency heat? I assume a propane stove of some kind? If you're thinking propane will still work when the electricity is out for your furnace, you are incorrect. You do know something has to power that blower motor right? But to answer your question I personally would choose propane over electricity but since you live in Georgia purchasing the heat pump without the strips might be a good move. There's a somewhat complex formula for figuring up which would be cheaper to run at a certain efficiency based on the costs of electricity and propane. I'll have to look for it. Regarding the propane, yea the propane would be for a small wall mounted heater (non electric) that I am currently using, I would keep it in storage and use it if needed. I am planning on getting the heat strips for when it is too cold so I would not have to deal with the wall mounted heater. 3. You need to have a Manual D done for this stuff. Undersizing your ducts will kill your system; oversizing a little won't hurt anything but if it's too oversized you won't have good airflow. One thing I thought of last night is that I probably should oversize all my duct since flex duct is not as efficient. 2.5 ton flex duct is 16" to be safe, 2 ton will need 14", so I will need to go up in size on all my ducts (or use the lower end of the round pipe cfm since it ranges, for example a 9" round duct ranges from 250 to 300 cfm.) 4. Again, Manual J and D. You may find out that 2 tons is too big for your home. 5. It's hard to say without seeing the setup but usually any hard 90 degree angles are bad for airflow. Issue is I don't have much room for a plenum, otherwise I would just build one (or have it built for me). So if I can do without it, it will make life a lot easier. |
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Thanks for you input, I figured I would get a lot of people telling me to do a manual J calc, but was hoping I would also get "your on track". 1. You need to get a Manual J done. You can compare 2 window a/c's all day long but you are missing factors like humidity and average moisture levels for your region by guessing. Square footage, cumulative btu output of the window a/c's and whatever else is just guessing...I can't emphasize this enough. A Manual J will take into account everything about your home. You said you've searched around on the internet a bunch; if that's true then you should have already found appropriate software to do a Manual J yourself. I guess I will purchase a copy of the Manual J. A lot of systems I looked at with similar characteristics (single floor, crawl space, older home, southern region), were using 2-2.5 ton units, and using the "rule of thumb" that is how I came up with the size. But if a 1.5 ton will suffice, then I agree, that over sizing the unit will be as bad as under sizing it. Also, I did happen across an online Manual J calc and it showed a two ton unit, but at the high end, this was before I started getting into the HVAC stuff more in depth, so I don't think I completed it correctly. 2. You mentioned propane for emergency heat? I assume a propane stove of some kind? If you're thinking propane will still work when the electricity is out for your furnace, you are incorrect. You do know something has to power that blower motor right? But to answer your question I personally would choose propane over electricity but since you live in Georgia purchasing the heat pump without the strips might be a good move. There's a somewhat complex formula for figuring up which would be cheaper to run at a certain efficiency based on the costs of electricity and propane. I'll have to look for it. Regarding the propane, yea the propane would be for a small wall mounted heater (non electric) that I am currently using, I would keep it in storage and use it if needed. I am planning on getting the heat strips for when it is too cold so I would not have to deal with the wall mounted heater. 3. You need to have a Manual D done for this stuff. Undersizing your ducts will kill your system; oversizing a little won't hurt anything but if it's too oversized you won't have good airflow. One thing I thought of last night is that I probably should oversize all my duct since flex duct is not as efficient. 2.5 ton flex duct is 16" to be safe, 2 ton will need 14", so I will need to go up in size on all my ducts (or use the lower end of the round pipe cfm since it ranges, for example a 9" round duct ranges from 250 to 300 cfm.) 4. Again, Manual J and D. You may find out that 2 tons is too big for your home. 5. It's hard to say without seeing the setup but usually any hard 90 degree angles are bad for airflow. Issue is I don't have much room for a plenum, otherwise I would just build one (or have it built for me). So if I can do without it, it will make life a lot easier. My advice is to purchase the Manual J software and do your own calculation...this one costs $50 and works quite well. http://www.hvac-calc.com/ What I would do once you purchase it (since it's good for 2 months) is to work up a calculation as your home exists now; then work up a couple of additional calculations based on making your home more energy efficient, such as more insulation or different windows, some trees, stuff like that. You can save all of these calculations as reports in .pdf format and keep them even after the software expires. I don't know if I would agree with you on the heat strips part. Generally in terms of providing heat, natural gas/propane/oil are usually more efficient. But I don't like making estimates either so here's the formula I mentioned earlier to help you choose which would be cheaper: running a heat pump (without strips) or turning on the furnace. For natural gas 1 therm = 100,000 btu = 100 cubic feet; 1000 btu per cubic foot 100 cubic feet = 29.307 kwh A therm is equivalent to 29.307kwh. Divide your delivered cost per therm for NG to get the equivalent energy cost of NG in kwh. $1.21427(example price) / 29.30722 = $0.041432 per kwh by natural gas. Divide by the AFUE of your furnace to allow for efficiency losses. $0.041432 / 0.95 = $0.04603 (the 0.95 is assuming a 95% efficient furnace) $0.041432 is 54% of of what you pay for your first 600 kwh of electricity at a delivered rate of $0.084772. (Your electric rate schedule will obviously be different; this is just an example from a previous workup) Take the reciprocal of 54% to determine the minimum COP at which a heat pump will be cost effective. 1 / 0.54 = 1.841 A heat pump will only be cost effective during the interval of the winter that it has a COP greater than 1.841 and it is able to generate enough heat to meet your house's heating needs. You'd need to look at heating degree days and typical winter conditions for your area to determine for how much of a typical winter a heat pump would be cheaper to operate than a gas furnace. Now let's compare the cost of 1 million btus of heat. Heat pump w/electricity at $.10 per kw-hr delivered, C.O.P. = 3.25 at 35F ambient (1,000,000 / 3414) x .10 / 3.25 = $9 Gas furnace w/gas at $1.21 per therm delivered, 95% efficient furnace (1,000,000 / 100,000) x 1.21 / .95 = $12.73 The heat pump is 30% cheaper for the same 1 million btus of heat. For propane 1 gallon propane = 91,000-93,000 btu (figure 91 to be safe) Electricity = about 3400 btu per kwh So 1 gallon propane = about 27 kwh Use the same formulas as above for propane with the values listed here. |