Multiple HVAC Systems in one Home

Replies to This Discussion

Permalink Reply by Allison A. Bailes III on November 6, 2010 at 8:14am

You can't do that with REM, John. You'd have to enter everything at least on a zone by zone basis, and REM gives you no way to do that. You have to use Manual J software for that info, like Right J by Wrightsoft or RHVAC by Elite.

Permalink Reply by John Nicholas on November 6, 2010 at 9:13am

Allison,

Can I copy the REM file into three files. Delete the other two zones. and then be half way close. I have a client with 6575 sf. 3 zones. His actual heating capacity is 235K. The whole house number from REM is 141k. I'd like to be able to provide some simple guidance as to Right Sizing when he gets to replacement time. The newest furnace is 3 years old. It is 80 AFUE 110K input for 859 sf 2nd floor. His house is good for the age. I can't work on infiltration until he installs mechanical ventilation. With the blower door numbers he should do that anyway. I can recommend additional insulation for attic and foundation walls.

Permalink Reply by Allison A. Bailes III on November 7, 2010 at 4:47am

Yes, I think you could split it up into three different files, John, but I don't trust REM for sizing equipment. I've seen some pretty big discrepancies (> 50%) between the results from REM and a true Manual J program. I don't completely trust Manual J programs either because more often than not, they oversize equipment even when you specify everything correctly.

The bigger issue here, though, is the question of providing guidance on HVAC. You open yourself up to more liability by venturing into HVAC consulting, so I recommend really understanding as much about HVAC as possible before you go down that road. I got myself into a couple of sticky situations early on by providing sizing guidance based on REM results.

The equipment size needed depends not only on the loads of the house but also a properly designed and installed duct system with the correct air flow. In most houses, the equipment is oversized, and it needs to be because the ducts are undersized. Can you reduce the size of equipment without changing the ducts? Probably, but you have to be careful when doing so because you might reduce the air flow too much to properly cool and heat the house.

I'm not trying to discourage you from giving HVAC advice. I'm just cautioning you that it's a tricky business and requires you to know a lot more than you get from your HERS and BPI classes.

Permalink Reply by John Nicholas on November 8, 2010 at 8:50pm

Allison,

Thanks for the words of wisdom. I finished modeling a home tonight. The home has been great with a geothermal unit, sized at 52.2K REM says 105K.

Is that the kind of discrepancy you are referring to?


What do you do when you can't trust your tools? REM has discrepancies, so get WrightSoft. These have discrepancies so what then?

Sounds like what you are telling me is - practice, practice, practice! Get experience. Read.

In the meantime, I'm doing audits and specing equipment replacements. What kind of assurance do I have that the advice is right? I am already using this: These whole house calculations are completed in Manual J. if your Manual J calculations require different sizing, please attach your complete calculations, including design inputs for review by Energy Auditor and Efficiency Kansas,

Ideas???

Permalink Reply by Allison A. Bailes III on November 16, 2010 at 6:48am

John, who sized the system at 52.2 kBtuh, and how did they come up with that number? The difference I was talking about is not between equipment capacity and load but between load from REM and load from a Manual J program.

What to do when you can't trust your tools is a key question in this field. The answer is that with enough experience, you start to get a feel for the precision of your results and what kinds of things you need to adjust for. A few years ago, for example, Right J underestimated the cooling load from ducts in attics. They seem to have fixed it now, but if you weren't aware of that problem when it existed, you could end up with a system that didn't cool enough in some cases.

Another thing you need to be careful of is specing equipment size without doing a full duct design. I almost created a serious problem for myself by doing that 6 years ago. The HVAC contractor installed ducts that didn't move enough air, and the house wouldn't cool off.

The right way to do this is to do the full design process: Manuals J, S, T, & D. Here's an article I wrote about this a while back: HVAC Design Done Right - Manual J, S, T, & D http://hub.am/dAXJmk.

If you're serious about doing this kind of consulting, I recommend getting either Wrigtsoft's or Elite's HVAC design software packages and take some classes on the subject, too. ACCA and Wrightsoft offer classes, and I'm sure Elite does, too.