Some heat pump enlightenment
I am getting ready to have a heat pump installed and need some guidance. I live in a rural area and I am all electric. I had a couple of contractors come out and only 1 did a manual J. He came up with a 2.5 ton heat pump with 10kw of electric backup. He didn't give me the results of the manual J but I did one on my own using loadcalc.net. My results were 20647 btu of cooling, 19499 sensible load, 1148 latent load and 48331 btu of heat. Now I'm sure the contractors calculation was more accurate than mine but if I convert the cooling BTU to tons it comes to about 2 tons. The heat portion seems like it is going to come up short. This is where I get a bit confused. In our area the electric rates are pretty high so I would like to let the heat pump do as much of the heating as possible. It also gets very cold (Iowa) for 3 months a year. So my question is will the heat pump still operated when the temps dip into the teens? I hoping that the heat pump can still run and the electric elements will kick in to supplement it if necessary.
A 2 ton would not meet your sensible needs. A heat pump will still be providing heat when its in the teens, or even in the single digits outside.
Why would a 2 ton not meet the sensible load? If 24,000 BTUs of cooling are available and the latent capacity were no being utilized, wouldn't the unused latent capacity provide sensible cooling?
Originally Posted by beenthere
You have to look at the expanded data on the units. Even though they are called a 2 Ton, they never put out 24000. Some of the latent can be "used" for sensible-but not all of it. Manual S allows 1/2 of what is left after the latent load is taken into account.
Originally Posted by air1
I wish I had a $1.00 for every response I deleted.....
"Decidedly Superior in a twisted pathetic way".....
Ok I think I'm going to pull the trigger with this contractor. A couple more questions. They are a lennox dealer but they install a variety of other brands. The equipment I was quoted is Lennox 14hpx-030-230 heat pump and a lennox CBX27UH 036 air handler. There is a note that the equipment will provide 15 seer, 13 eer and 9.00 HSPF. There is also a loss of charge kit, low ambient kit and crankcase heater. Now I'm not trying to start a brand war or anything but I am considering asking him about another brand. I have been told a few times that lennox stuff is expensive to repair when it does break. This was told to me by people that don't push lennox stuff so I take that for what its worth. Also, I just went over my current electric rates. We are on a tier system, so the first 600kw is really expensive but after that it drops to about 8 cents. I use well over the 600 kw so the unit will be operating on 8 cents per kw. Would it make sense to go with a higher efficiency unit? Finally, can anyone tell me if the air handler is variable speed or not? Thanks for all the input so far.
Heat pumps need to be sized for cooling load for your home. With that per your numbers I agree with Beenthere a (2) ton will not meet sensible load requirements for your home.
While heat pumps can be sized to heat your home only but in most all cases you will oversize the a/c system by sizing for heating only.
Per your load cal. I would install a (2.5) ton heat pump with 15kw aux. heat pack. The heat pack when wired with t-stat and heat pump correctly will bring on heat pack as needed to provide supplement heat to keep your home warm when the heat pump can't keep up or goes into defrost.
As for the heat pumps depending on selection, how it installed, how is wired etc... Should work well into the teens and provide you heat. What temp do you like for your home to be in the winter vs summer and or both.
As far as the contractor goes that did the load cal. If he feels that (2.5) heat pump with 10 kw aux. heat pack will heat and cool your home properly then I would go with that suggestion. As he has ran a load not me. He will be the one responsible for the sizing. I also would not leave my load cal. With a customer unless they signed a contract with my company for the work. If they did sign I would give them a copy of my work. To many times customers take load cal and use them with other companies to get lower prices. Sure another company can do it cheaper if customer gives them the work that I have already done that's roughly 1-2 hrs worth of work they don't have to do.
I am not saying you would do that but I am sure most people on this site will agree it happens more often then not. As far as you the customer once you have decided which company to use make sure they do provide manual j load cal. To you for peace of mind plus just to make sure they have done it.
Latent sounds a little low. It still gets fairly humid even in northern Iowa and that number doesn't seem to include much for occupants breathing, cooking or taking showers.
Given it's a heat dominated climate, a 2.5 ton or even 3 ton 2 stage might be the best option with dehumidify on demand to slow the blower for dehumidification.
If you had a gas furnace, I'd go 2 tons and not use any setbacks.
It does get quite humid here. A little more info on the house: It is a single story earth home with approx 1100 sq ft of conditioned area. The house currently has no AC and heating is handled with base board heat in each of the south rooms and a wood stove in the central room. I added on a 900 sq ft garage with 900 sq ft of living space above it. At present time with the baseboards off and no fire going the house only dips to the low 60's on even the coldest days. The addition on the other hand is going to be the opposite, it will be more like the second story of a normal house.
Originally Posted by motoguy128
As for the calc that I did, I was just playing around to see what it would come up with. I went pretty conservative on the insulation ratings I think. The addition is going to be very well insulated. R21 in the walls R 50 in the attic and R 69 for the floor. I also input 4 occupants in the house (there are currently only 2) and listed 4 appliances.
The contractor that did the manual J thought 2.5 tons may be a touch undersized but thought it would help with dehumidification due to slightly longer run times.
When looking at sensible & latent capacities of a system, remember they are rated at some pretty sticky, warm conditions. Under standard conditions, a system might have 70% sensible. But if you put that unit in the desert with no humidity, it could really be doing much better on latent. Or put the same system in a steamy house that's been without A/C for a week and could be more like 50% latent.
Being a native Iowan, it does look like latent figure was a bit low. Lots depends upon how tight the joint is. Not sure I'd go 3 tons and in IA, a high SEER unit will never pay you back, but if you like it cold, might consider 2.5 ton. I took a gander at that site and they size to 88 for Indy. That's too low for reality. 15kw is what you'd need for backup and don't set back if below say 40.
Earth home? Underground 1100 sq ft? Is the 900 garage living area going to be on this system and insulated like a fortress? If so, gonna need more than a 2 stage. Remember, low isn't that much less than high.
Yes this system is going to feed both areas. One 10x 14 trunk line will feed the underground portion and another 10 x 14 will feed the upstairs. The upstairs will be insulated as much as possible. I will be spray foaming the walls for an R value of 21, ceiling will be R 50 blow in cellulose and floor will be 2 inches of spray foam and 14inches of cellulose for an R value of 69. The existing earth home is covered on all sides by dirt except the south wall. All earth covered walls and roof have 2 inch foam for insullation and atleast 2 ft of dirt over them. The south wall is R13 with 1.5 inch foam board over that. It takes very little effort to heat and cool it. The contractor wants to install a damper in each truck line to prevent over conditioning the underground part of the house. What do you mean by "low isn't that much less than high"?
Originally Posted by BaldLoonie
all that and you need 2.5 tons of ac and 48000 btu of heat? have you considered a duel fuel system
Originally Posted by burhead
I am all electric, I would have to have natural gas brought in or go with a propane tank. From my understanding propane is pretty high but so are my electric rates. Those numbers are from the load calc.net site which I did. I have not seen the actual manual J that was done by the contractor but he is recommending 10k heat strip and 2.5 ton heat pump. My gut feeling is that the earth covered part of the home, which is basically a well insulated basement, isn't going to require much heat or AC but it does get quite humid. During the summer months we don't have an AC but we run 3 dehumidifiers.
Originally Posted by catmanacman