Anonymous wrote:

Anonymous wrote:

Anonymous wrote:Here's a really good article (written by a PhD in building science!) comparing the results of a thorough Manual J with using rules of thumb:

https://www.greenbuildingadvisor.com/article/manual-j-load-calculations-vs-rules-of-thumb

This guy's firm does HVAC design for people who are building and remodeling their homes. If you are just talking about an HVAC replacement for an older home with old duct work, it is more nuanced than the results of a load calculation. Our contractor said he could do Manual J but at the end of the day he had to rely on how much air our ductwork could move. Basically it's be a fair amount of labor and then it wouldn't make a difference. So far we are super happy with our system in terms of both comfort and efficiency.

Duct work is only a constraint if you want to increase the size. As he points out, most existing systems are oversized, often by a factor of 2 or 3. An oversized system will cycle on and off, whereas a properly sized system will run more continuously. This results in better comfort as it does a better job of dehumidifying, lower electrical bills and longer equipment life.

I get why most systems are oversized. From the installer's perspective an undersized system is a disaster, the customer will know right away that it's not keeping up on hot days. The problems with an oversized system are more subtle, the customer may not realize they're even happening. So if you're just guessing anyway the incentive is to guess too big rather than too small.

The Manual J doesn't eliminate guesswork, there are still some assumptions the installer has to make, but it constrains guesswork.

Anonymous wrote:

Anonymous wrote:

Anonymous wrote:

Anonymous wrote:Here's a really good article (written by a PhD in building science!) comparing the results of a thorough Manual J with using rules of thumb:

https://www.greenbuildingadvisor.com/article/manual-j-load-calculations-vs-rules-of-thumb

This guy's firm does HVAC design for people who are building and remodeling their homes. If you are just talking about an HVAC replacement for an older home with old duct work, it is more nuanced than the results of a load calculation. Our contractor said he could do Manual J but at the end of the day he had to rely on how much air our ductwork could move. Basically it's be a fair amount of labor and then it wouldn't make a difference. So far we are super happy with our system in terms of both comfort and efficiency.

While well intentioned; a majority of the building science people I've met and worked with do not live in the real world. Can't tell you how many firms that have approached me to do the HVAC side of their projects that are dissolved or merged within a year.

Allison Bailes, the author of that piece, has been doing building science consulting for over 20 years and been with the same firm for 15 years. His bio is at:
https://www.energyvanguard.com/about/allison-bailes-iii-phd/

Anonymous wrote:

Anonymous wrote:

Anonymous wrote:Here's a really good article (written by a PhD in building science!) comparing the results of a thorough Manual J with using rules of thumb:

https://www.greenbuildingadvisor.com/article/manual-j-load-calculations-vs-rules-of-thumb

This guy's firm does HVAC design for people who are building and remodeling their homes. If you are just talking about an HVAC replacement for an older home with old duct work, it is more nuanced than the results of a load calculation. Our contractor said he could do Manual J but at the end of the day he had to rely on how much air our ductwork could move. Basically it's be a fair amount of labor and then it wouldn't make a difference. So far we are super happy with our system in terms of both comfort and efficiency.

While well intentioned; a majority of the building science people I've met and worked with do not live in the real world. Can't tell you how many firms that have approached me to do the HVAC side of their projects that are dissolved or merged within a year.

Anonymous wrote:

Anonymous wrote:Here's a really good article (written by a PhD in building science!) comparing the results of a thorough Manual J with using rules of thumb:

https://www.greenbuildingadvisor.com/article/manual-j-load-calculations-vs-rules-of-thumb

This guy's firm does HVAC design for people who are building and remodeling their homes. If you are just talking about an HVAC replacement for an older home with old duct work, it is more nuanced than the results of a load calculation. Our contractor said he could do Manual J but at the end of the day he had to rely on how much air our ductwork could move. Basically it's be a fair amount of labor and then it wouldn't make a difference. So far we are super happy with our system in terms of both comfort and efficiency.

Anonymous wrote:

Anonymous wrote:Here's a really good article (written by a PhD in building science!) comparing the results of a thorough Manual J with using rules of thumb:

https://www.greenbuildingadvisor.com/article/manual-j-load-calculations-vs-rules-of-thumb

This guy's firm does HVAC design for people who are building and remodeling their homes. If you are just talking about an HVAC replacement for an older home with old duct work, it is more nuanced than the results of a load calculation. Our contractor said he could do Manual J but at the end of the day he had to rely on how much air our ductwork could move. Basically it's be a fair amount of labor and then it wouldn't make a difference. So far we are super happy with our system in terms of both comfort and efficiency.

Anonymous wrote:Here's a really good article (written by a PhD in building science!) comparing the results of a thorough Manual J with using rules of thumb:

https://www.greenbuildingadvisor.com/article/manual-j-load-calculations-vs-rules-of-thumb

Anonymous wrote:

Anonymous wrote:

Anonymous wrote:Something you have to realize in these discussions: every house is unique. Every house is hand built. There is no such thing as a "normal" or "average" house.

As to the questions on sizing and heating vs. cooling, there are no rules of thumb. In our climate some houses are heating dominant, some are cooling dominant. Proper sizing of equipment involves doing what's called a "Manual J," which is a computer model of the energy use of your house. Code now requires it in most places. Doing a proper Manual J means going into every room in the house, measuring the dimensions of every room and all the windows and exterior doors. Also going into the attic and basement and looking at the construction. If someone wants to quote you a system without doing a Manual J, they're just guessing. Even if they just want to replace existing equipment with a similar size. In fact, especially if that's what they want to do.

The Manual J uses what's called the "design temperature," which is basically the hottest and coldest temperature we regularly see. In Washington, DC, the design temperature for heating and cooling are 21F and 92F.

Since the cooling temperature is only 20F from room temperature and the heating temperature is 50F, you'd think we'd need much more heating than cooling. But this is where the uniqueness of houses comes in. As part of the Manual J you have to figure out the solar gain and account for it. That means measuring the dimensions and orientation of every window in the house, and factoring in the solar gain factor for the type of glass in it. These aren't hard calculations, but it is work. You also have to figure the removal of humidity in the summer, which takes cooling capacity.

I had a Manual J done for my house. The results were that the heating load is 27,600 BTU/hr, and the cooling load is 29,800 BTU/hr. This is Washington, DC. The solar gain is almost half of the cooling load, 14,000 BTU/hr. I have lots of windows and my house faces south.

As part of the process I had the air infiltration of my house measured, using what's called a blower door test, a fan is mounted in the front door and pressure and flow are measured. My house is much tighter than most. This means that air infiltration is low, which means that I need less dehumidification in the summer than typical.

In my case, since the loads are so close, a heat pump sized for my winter heating load keeps the house comfortable year-round.

But every house is unique.

OP's house is 70 years old. It's true that new builds may have very similar heating and cooling loads. Unlikely that's the case for OP.

You're completely missing my point. The reason to do a full Manual J is that every house is unique. It's not true at all that new builds have similar loads, two similar-sized houses built last year could differ by a factor of ten. Yeah, with a 70-year-old house there's some guesswork, but it's educated guessing.

Anonymous wrote:

Anonymous wrote:Something you have to realize in these discussions: every house is unique. Every house is hand built. There is no such thing as a "normal" or "average" house.

As to the questions on sizing and heating vs. cooling, there are no rules of thumb. In our climate some houses are heating dominant, some are cooling dominant. Proper sizing of equipment involves doing what's called a "Manual J," which is a computer model of the energy use of your house. Code now requires it in most places. Doing a proper Manual J means going into every room in the house, measuring the dimensions of every room and all the windows and exterior doors. Also going into the attic and basement and looking at the construction. If someone wants to quote you a system without doing a Manual J, they're just guessing. Even if they just want to replace existing equipment with a similar size. In fact, especially if that's what they want to do.

The Manual J uses what's called the "design temperature," which is basically the hottest and coldest temperature we regularly see. In Washington, DC, the design temperature for heating and cooling are 21F and 92F.

Since the cooling temperature is only 20F from room temperature and the heating temperature is 50F, you'd think we'd need much more heating than cooling. But this is where the uniqueness of houses comes in. As part of the Manual J you have to figure out the solar gain and account for it. That means measuring the dimensions and orientation of every window in the house, and factoring in the solar gain factor for the type of glass in it. These aren't hard calculations, but it is work. You also have to figure the removal of humidity in the summer, which takes cooling capacity.

I had a Manual J done for my house. The results were that the heating load is 27,600 BTU/hr, and the cooling load is 29,800 BTU/hr. This is Washington, DC. The solar gain is almost half of the cooling load, 14,000 BTU/hr. I have lots of windows and my house faces south.

As part of the process I had the air infiltration of my house measured, using what's called a blower door test, a fan is mounted in the front door and pressure and flow are measured. My house is much tighter than most. This means that air infiltration is low, which means that I need less dehumidification in the summer than typical.

In my case, since the loads are so close, a heat pump sized for my winter heating load keeps the house comfortable year-round.

But every house is unique.

OP's house is 70 years old. It's true that new builds may have very similar heating and cooling loads. Unlikely that's the case for OP.

U.S. households installed 4 million new heat pumps last year, about half of new sales of residential heating systems, eclipsing gas furnaces for the first time.

Since several regions of the country have been installing them for years, 16 percent of U.S. homes already use electric heat pumps for space heating.

Anonymous wrote:

Anonymous wrote: we do them (Manual J) for all our installs because having to rip out and reinstall incorrectly sized equipment is a fools errand.

You want a contractor who says things like this.

First, they're up on the latest science. Second, they intend to stand by their work.

Question for PP: How do you integrate the Manual J into the bid process? Because I'd imagine a proper Manual J takes about $500 worth of labor. Do you bid just the labor of the install and say equipment is contingent on the results of the J? Or do you charge up front for the J?