Design Temps and extremes?

[WillieOH]

I live in an area where the design temps are 6 and 86 degrees (taken from table 1 of manual j. However, temps drop for maybe a week or so every winter into the -5 to -10 degree range or more and into the low to mid 90's each summer for a week.

Does this mean that if my heating and cooling system is correctly sized, I will not be able to maintain inside temp for these periods? For example, if temp drops to -15, my house will go from 72 degrees down to 51 degrees? Or are there safety factors built into manual j to compensate so I can maintain 72 degrees inside?

[seatonheating]

I live in an area where the design temps are 6 and 86 degrees (taken from table 1 of manual j. However, temps drop for maybe a week or so every winter into the -5 to -10 degree range or more and into the low to mid 90's each summer for a week.

Does this mean that if my heating and cooling system is correctly sized, I will not be able to maintain inside temp for these periods? For example, if temp drops to -15, my house will go from 72 degrees down to 51 degrees? Or are there safety factors built into manual j to compensate so I can maintain 72 degrees inside?

At design temps your system will run continuously while maintaining temps. At extreme temps it won't be able to keep up, but that is how it should be.

Why have your system oversized and running inefficiently for these extreme conditions?? During extreme temps throw on an extra blanket in the winter and tough out those couple of days in the summer.

[WillieOH]

51 degrees inside would equal more than an extra blanket. Would it be that extreme? Because we get that low outside almost every year here. Thanks. This is a real sticking point for me. If I spent an arm and a leg for a new hvac system and then had to go out and buy space heaters to keep warm in the winters, I'd have some 'splainin' to do here

[seatonheating]

51 degrees inside would equal more than an extra blanket. Would it be that extreme? Because we get that low outside almost every year here. Thanks. This is a real sticking point for me. If I spent an arm and a leg for a new hvac system and then had to go out and buy space heaters to keep warm in the winters, I'd have some 'splainin' to do here

No no no, it doesn't work that way. If your system is designed to keep your home at, say, 72 degrees at 6 deg. oudoor temp, then it will more than likely keep you close at -15...probably 68 or so at worst.

Your home will not drop a degree for every degree it is below design temp if that is what you were thinking.

[WillieOH]

Thanks. That's good to know. I did a full manual j (not abridged) on my house and came up with 49600 btuh heating and 20600 cooling. I currently have a 64000 output furnace (80000 input x 80%) and 2-1/2 ton a/c. I'm thinking of downsizing to a 56000 (60000 x 93%) and 2 ton 13 seer unit, since my old ones are 20 years old now.

My existing hvac is seemingly oversized, but is still very comfortable and good control of humidity in the summer (33 grains diff outside for the area). The proposed system is 13-16% oversized according to manual j results. Do you think this will hold a steady temp inside all winter and summer through the extremes I mentioned earlier considering they're oversized?

[seatonheating]

Thanks. That's good to know. I did a full manual j (not abridged) on my house and came up with 49600 btuh heating and 20600 cooling. I currently have a 64000 output furnace (80000 input x 80%) and 2-1/2 ton a/c. I'm thinking of downsizing to a 56000 (60000 x 93%) and 2 ton 13 seer unit, since my old ones are 20 years old now.

My existing hvac is seemingly oversized, but is still very comfortable and good control of humidity in the summer (33 grains diff outside for the area). The proposed system is 13-16% oversized according to manual j results. Do you think this will hold a steady temp inside all winter and summer through the extremes I mentioned earlier considering they're oversized?

Should work out great, couple of things I would do though being that it is oversized. Make sure the gas furnace is modulating and make sure your AC/heat pump is 2 stage. That way it can adjust for varying loads.

[WillieOH]

I'm looking at a Trane XV90i two stage, variable blower and XR13 single stage (since it only runs maybe 6 weeks a year total).

[seatonheating]

I'm looking at a Trane XV90i two stage, variable blower and XR13 single stage (since it only runs maybe 6 weeks a year total).

I wouldn't limit myself to Trane and I would definitely look into a modulating gas furnace.


You do realize that Trane is no better/worse than anything else don't you?

[WillieOH]

A modulating furnace is better than a 2 stage right? Would that be the trane xv95?

[jpb2]

Please check the actual not nominal capacities of the system you think you need.

[WillieOH]

I purchased and read the manual S. They discuss output bthu and sensible and latent heat capacities. Is that what you mean?

[seatonheating]

I purchased and read the manual S. They discuss output bthu and sensible and latent heat capacities. Is that what you mean?

For a true modulating furnace, go with the Rheem or York/Coleman furnace. Trane hasn't quite mastered it yet.

[jpb2]

I purchased and read the manual S. They discuss output bthu and sensible and latent heat capacities. Is that what you mean?

Yes. Not to split hairs but a lot of higher seer units have compressor capacities far less than their nominal capacity.Good luck

[Joker]

At extreme temps it won't be able to keep up, but that is how it should be.

Says a Yank! LOL

I would have been run out of business if I did not account for extreme temperatures.

Yes with heating you can add more blankets and what not but down south if we use 100 as specified by the manual We would have months extreme temperatures!

Use a bit of commons sense is the best advice I can give you. If we are talking a week then maybe its not such a big deal but weeks of extreme then its time to add it in.

Thank God for 2 stage systems!

[seatonheating]

Says a Yank! LOL

I would have been run out of business if I did not account for extreme temperatures.

Yes with heating you can add more blankets and what not but down south if we use 100 as specified by the manual We would have months extreme temperatures!

Use a bit of commons sense is the best advice I can give you. If we are talking a week then maybe its not such a big deal but weeks of extreme then its time to add it in.

Thank God for 2 stage systems!

Before I read your last sentence I was going to mention 2 stage equipment.

Why not just keep a couple of portables for extreme temps?? That's what I would do.

[Joker]

Before I read your last sentence I was going to mention 2 stage equipment.

Why not just keep a couple of portables for extreme temps?? That's what I would do.

I like the 2 stage because Texas is either Hot, damn hot or freaking hot! LOL Oh we do get real cold for a 2 week period.

Some people are very picky and those that don't care already have a window unit in the master bedroom.

[Joker]

Before I read your last sentence I was going to mention 2 stage equipment.

Why not just keep a couple of portables for extreme temps?? That's what I would do.

BTW I have seen 100 days of 100+ degree days here.

[seatonheating]

BTW I have seen 100 days of 100+ degree days here.

Maybe Manual J needs to update its weather data?

[Joker]

Maybe Manual J needs to update its weather data?

I have lived in Texas all my life and there are summers we don't hit a 100 and there are summers that we hit a 100 100's.

2 stage system is perfect for Texas. It can get humid on top of being just damn hot! LOL

We have about a 9 month air condition cycle here. 2 months of cold but nice and about 1 month of biter cold.

Its just real hard to account for all needs. Between the old ladies and the coke heads you just have to account for the extremes.

[jimj]

Manual J does have a safety factor built into it, or so says the co-author. If memory serves me rite he says 15%.