4 ms·

You can energy model it if you have the time.

by bumby 2y ago

tguvot 2y ago
if you do fresh manual J calculation, with updated design temperature (which in reference manual is still incorrect) and blower door test, than maybe. And it still very theoretical but realistically, very few people do it when they update hvac system. in fact, most (all?) of hvac installers simply refuse to do it.
- tguvot 2y ago
  ps. manual j actually won't give you runtime i think. only needed capacity.
- bumby 2y ago
  Like I think you are alluding to in your own response, manual J is just a design load calculation. It won't give you the annual energy usage so it probably isn't suitable for estimating annual energy costs. You want something that gives you (ideally) hourly load calculations. Like with all models, it depends on how accurate you need to get. Very few energy models (mainly TRNSYS or eQUEST) that I've been involved with go to the level of blower door testing. They generally take design information (materials, orientation, etc.)
  - tguvot 2y ago
    you can't get hvac installers to do even manual j. i had to retain third party to do it when I wanted to replace hvac. if there are more progressive models, they are most definitely out of question for home owners :)
    - bumby 2y ago
      100% agree that HVAC techs are not going to do energy modeling. That's not really their role and I'd expect you to have to contract it to someone else. It's like asking a transport pilot to sign FAA certification paperwork. Design load calculations are more a part of the A/E design process than an installer task. >they are most definitely out of question for home owners Maybe the general home owner, but my comment was geared towards the HN crowd. I think running an energy model is within the grasp of most tech-savvy people if they have the inclination. Again, it depends on the accuracy you want. Even Excel can be used for a bin energy model approach (and is even advocated for in some commercial energy auditing texts as a suitable approach for of ASHRAE Level II audits). For example, eQUEST is free from the DoE and there are plenty of tutorials out there. Edit: for anyone interested, this link gives an example of spreadsheet load calculations for residential applications (scroll to "ResLoad22") https://geokiss.com/free-design-software/ https://geokiss.com/free-design-software/
      - tguvot 2y ago
        looked at spreadsheet. strong vibes of manual j :) curious, does it take into consideration heat gain/loss by walls and insulation through the day ? or is there any model that does it ?
        bumby 2y ago
        >strong vibes of manual j In the simplest terms, they're all derived from the heat transfer equation of Q=U*A*deltaT so all load calculation models will seem similar to an extent. It ultimately comes down to how nuanced you want the model to be. >does it take into consideration heat gain/loss by walls and insulation through the day Yes, it has factors like heat gain coefficient that are used in conjunction with U-factors, weather, and the orientation of the structure. Most of the more widely used models include those aspects.

You can energy model it if you have the time.

by bumby 2y ago

You can energy model it if you have the time.

if you do fresh manual J calculation, with updated design temperature (which in reference manual is still incorrect) and blower door test, than maybe. And it still very theoretical but realistically, very few people do it when they update hvac system. in fact, most (all?) of hvac installers simply refuse to do it.

tguvot 2y ago
ps. manual j actually won't give you runtime i think. only needed capacity.
bumby 2y ago
Like I think you are alluding to in your own response, manual J is just a design load calculation. It won't give you the annual energy usage so it probably isn't suitable for estimating annual energy costs. You want something that gives you (ideally) hourly load calculations. Like with all models, it depends on how accurate you need to get. Very few energy models (mainly TRNSYS or eQUEST) that I've been involved with go to the level of blower door testing. They generally take design information (materials, orientation, etc.)
- tguvot 2y ago
  you can't get hvac installers to do even manual j. i had to retain third party to do it when I wanted to replace hvac. if there are more progressive models, they are most definitely out of question for home owners :)
  - bumby 2y ago
    100% agree that HVAC techs are not going to do energy modeling. That's not really their role and I'd expect you to have to contract it to someone else. It's like asking a transport pilot to sign FAA certification paperwork. Design load calculations are more a part of the A/E design process than an installer task. >they are most definitely out of question for home owners Maybe the general home owner, but my comment was geared towards the HN crowd. I think running an energy model is within the grasp of most tech-savvy people if they have the inclination. Again, it depends on the accuracy you want. Even Excel can be used for a bin energy model approach (and is even advocated for in some commercial energy auditing texts as a suitable approach for of ASHRAE Level II audits). For example, eQUEST is free from the DoE and there are plenty of tutorials out there. Edit: for anyone interested, this link gives an example of spreadsheet load calculations for residential applications (scroll to "ResLoad22") https://geokiss.com/free-design-software/ https://geokiss.com/free-design-software/
    - tguvot 2y ago
      looked at spreadsheet. strong vibes of manual j :) curious, does it take into consideration heat gain/loss by walls and insulation through the day ? or is there any model that does it ?
      - bumby 2y ago
        >strong vibes of manual j In the simplest terms, they're all derived from the heat transfer equation of Q=U*A*deltaT so all load calculation models will seem similar to an extent. It ultimately comes down to how nuanced you want the model to be. >does it take into consideration heat gain/loss by walls and insulation through the day Yes, it has factors like heat gain coefficient that are used in conjunction with U-factors, weather, and the orientation of the structure. Most of the more widely used models include those aspects.