Reliabilityman, i don't know. It sounds a little off to me, too. (I posted a similar, if less extreme, question when we were being sized for our heat pump a couple of years ago.)

A two stage heat pump being oversized is not the same as a single stage gas furnace being oversized, as it simply means the heat pump will rarely, if ever, be called on to deliver full output. A older style gas furnace delivered full output whenever it was called on, but a two stage unit will work at a more relaxed pace much of the time. If your area is subject to rare but severe cold or hot spells there may be a good reason for the extra capacity, and your contractor might it consider good insurance against a callback some January or August day. It'd be worth asking, IMHO, whether the cost differential is justified. Heck, maybe he makes an extra buck or one hundred on the bigger unit, or has a couple lying around!

I think you'll get more informed opinions -just thought I'd throw in my two bits worth.

Good luck,

Mark

Thanks for your reply, marknmt.
Yes we are building on a cost plus basis, so the more I spend the more he makes. Not that I don't trust my builder. I am just watch everything. I hope some other guys can chime in on this sizing question. Sometimes we do have bitter cold spells, but not often, however we see a few weeks in the summer up in triple digets.

Thanks

I would ask the contractor 2 questions. What outdoor design temperatures he used for the summer and winter. Also what indoor temperatures he used for the summer and winter. If you agree with his numbers and 3.6 tons then I would go with 4 tons. The extra ton will probably cost you about $3k with the extra ground loop and equipment. For the bonus room I don't think you will be able to find a unit under 1 ton.
The bigger question is will you be happy with only 1 unit for your house. You didn't mention the sq ft but it must be fairly large. I would consider a 1st floor & 2nd floor unit or something similar.

The manual J does not show what outdoor design temperatures he used for summer and winter? Total heating required for zone 1 (3,927ft) is 55.721 MBH. He did not give a cooling load? For the bonus room it called for 483 MBH. I just recieved a quote from another company. They call for a 4 ton system for the whole house without the split system for the bonus room. Decisions Decisions?

Re: relabilityman

Funnycide is absolutely right regarding winter/summer outdoor & indoor design temperatures & relative humidities. This omission puts into question the value of the entire calculation.

IMO

SR

Steve- am I right in my assumptions about a two-stage unit? Can you sraighten me out on that?

Thanks,

Mark

Hi Mark,

Yes you are right about 2-stage equipment with variable speed blowers. 1st stage is very efficient and with 2-stage, variable speed you donÂt have to be as worried about being oversized. However, even an oversized 2-stage gas furnace will be more efficient than a single speed (oversized ) gas furnace in that it will likely run on lower stage for longer run times.

Reliabilityman may benefit from the larger 5-ton 2-stage variable speed unit with zoning (and a remote temp sensor) so as to avoid the additional cost of installing 2-separate systems.

There are always trade offs. You could install a smaller HP, as long as it would be adequate for cooling, and rely on more aux-backup if you feel itÂs not worth the price of installing the larger system. It may all depend on how deep your pockets are DonÂt forget, itÂs not just the extra cost of the HP, itÂs the drilling, labor, if you go larger than 3-ton youÂre looking at a load center with 2-pumps etc. - more $$$!

Hope that answers your question. (I think you know more than you give yourself credit for. Maybe youÂre ready for the IGSHPA installers course?)

Best regards,

Steve

Thanks Steve, for the info and for giving me a little credit!

But nah, I ain't no tech. Usually I can avoid doing really stupid things, but I'm constantly amazed at how many facets there are to all these "simple" items.

So I lurk on these boards, learn a lot of (other people's) stuff, and once in a while get to add a voice to the choir. It's always fun to see if you know what you're talking about, and if it can help somebody else get a handle on a question, so much the better. It's a pleasure to deal with pro's who don't mind sharing with rank amateurs like myself.

Back atcha,

Mark

ps- at age 58 I'm just hoping my current job as a baker lasts long enough for me to reach full retirement age! But being an installer would be fun. I know a couple of well and pump people who would probably work for less, they like their work so much- Same kind of thing with problem solving and doing real, visible work in HVAC and such.