Sizing of replacement furnaces and boilers

[jessetayriver]

Hi everyone,

I have a background in carpentry and hvac. I've loosely followed FIRE and the community for some time and am very pro-diy. However, it appears that the great majority of replacement heating appliances in the FIRE community are grossly over-sized, perhaps even worse than the baseline professional replacement which is also quite horrible. For air-source heat pumps and condensing boilers, this substantially reduces efficiency (con boilers ~10%, ashp ~50%). For furnaces this manifests as noisy systems, poor room temperature control and the premature failure of blower motors.

Heating replacement doctrine is to have someone perform a Manual J. However, (as I describe herehttps://asteriskmag.com/issues/05/lies-damned-lies-and-manometer-readings) a high quality Manual J is often unattainable. The solution is simple. For houses with good fuel billing history and historic weather data, it's quite simple to calculate the capacity for a replacement heating system.

Here's a spreadsheet I made to help with sizing. This should be fairly accurate. It will generally specify a capacity that's 1/2-2/3 a Manual J built from menu items, which will in turn be often 1/2 the current capacity of existing equipment. The great majority of heating appliances are grossly over-sized!

https://docs.google.com/spreadsheets/d/1hD2pyfCrYx0j-bxaNftRuw7nIspJ3HdKcbxNk1AcTCY/edit?usp=sharing

Let me know if you have any questions!

[RWD]

Definitely. Technology Connections had an excellent video on this topic recently.
https://www.youtube.com/watch?v=_hAuKtoRxJI

[nereo]

I’m not sure I agree that a manual J calculation is unobtainable, but more that what’s offered is rarely that.

Thanks for sharing the spreadsheet. I’ll definitely take a closer look. Where we have struggled is that we don’t have reliable fuel billing history for two reasons : first / new owners. Second, every aspect of our innovation involved plugging and improving our building envelope. So we are quite a bit better than we were just six months ago, and will be much better in another six months. I dongg th have a great solution other than the manual J, which even thrr E n requires assumptions on what will soon be, not what currently is.

As most HVAC systems are put in with the expectation that they will be there for 2 decades or more, it seems wrong to base them on what was, instead of what will be.

[jessetayriver]

I’m not sure I agree that a manual J calculation is unobtainable, but more that what’s offered is rarely that.

Thanks for sharing the spreadsheet. I’ll definitely take a closer look. Where we have struggled is that we don’t have reliable fuel billing history for two reasons : first / new owners. Second, every aspect of our innovation involved plugging and improving our building envelope. So we are quite a bit better than we were just six months ago, and will be much better in another six months. I dongg th have a great solution other than the manual J, which even thrr E n requires assumptions on what will soon be, not what currently is.

As most HVAC systems are put in with the expectation that they will be there for 2 decades or more, it seems wrong to base them on what was, instead of what will be.

Edit: Replied to wrong post. I guess what I mean here is that from a typical layperson's perspective, it's extremely difficult to arrive at a realistic heat loss via the standard path of having contractors perform a manual j. The owner has to weed through 1) false claims that a manual j has been performed when it hasn't, 2) claims that the salesperson "knows the house" and therefore it isn't necessary to do one, 3) claims that the Manual J is broadly a representation of their house when it's generally not (in one case I documented for years a company kept files of manual j for each equipment size and just renamed them on every new job)

Wrt changes - I think this is fair, but for most light duty retrofits this might move the needle 10%-30%. I'm just trying to get people to <1.5-2x their heating demand & to be able to have conversations with HVAC people who make dubious claims about heating demand

[nereo]

I’m not sure I agree that a manual J calculation is unobtainable, but more that what’s offered is rarely that.

Thanks for sharing the spreadsheet. I’ll definitely take a closer look. Where we have struggled is that we don’t have reliable fuel billing history for two reasons : first / new owners. Second, every aspect of our innovation involved plugging and improving our building envelope. So we are quite a bit better than we were just six months ago, and will be much better in another six months. I dongg th have a great solution other than the manual J, which even thrr E n requires assumptions on what will soon be, not what currently is.

As most HVAC systems are put in with the expectation that they will be there for 2 decades or more, it seems wrong to base them on what was, instead of what will be.

I appreciate that guy's enthusiasm, but he probably could have used my spreadsheet! Definitely highlights the problems with oversizing. And things are even worse for multi-family where in many cases we don't have great access to fossil fuels appliances with low enough output.

What guy are you talking about? Me?

[jessetayriver]

Definitely. Technology Connections had an excellent video on this topic recently.
https://www.youtube.com/watch?v=_hAuKtoRxJI

I appreciate that guy's enthusiasm, but he probably could have used my spreadsheet! Definitely highlights the problems with oversizing. And things are even worse for multi-family where in many cases we don't have great access to fossil fuels appliances with low enough output.

[jessetayriver]

I’m not sure I agree that a manual J calculation is unobtainable, but more that what’s offered is rarely that.

Thanks for sharing the spreadsheet. I’ll definitely take a closer look. Where we have struggled is that we don’t have reliable fuel billing history for two reasons : first / new owners. Second, every aspect of our innovation involved plugging and improving our building envelope. So we are quite a bit better than we were just six months ago, and will be much better in another six months. I dongg th have a great solution other than the manual J, which even thrr E n requires assumptions on what will soon be, not what currently is.

As most HVAC systems are put in with the expectation that they will be there for 2 decades or more, it seems wrong to base them on what was, instead of what will be.

I appreciate that guy's enthusiasm, but he probably could have used my spreadsheet! Definitely highlights the problems with oversizing. And things are even worse for multi-family where in many cases we don't have great access to fossil fuels appliances with low enough output.

What guy are you talking about? Me?

Sorry! Not used to this forum and quoted the wrong post! Edited the original to reply to you!

[GilesMM]

Oversizing with furnaces may be inefficient but oversizing AC units in the humid south is a disaster. A too-large unit will not run enough to dehumidify the space and it will be uncomfortable.  At the same time, there doesn't seem to be a great deal of granularity in the sizing of AC heat pumps.  2-ton, 3-ton, 4-ton, 5-ton, etc.  If you think 3-tons might just barely be enough then look ahead to global warming and weeks of scorching temps each summer, you will probably leap to 4 tons without much convincing.