Posted By RMD on 26 Aug 2017 12:29 AM
One more quick question. How did you determine the 12-15 BTUs/sqft for my age home and location?

Thanks again!

That's basically it. In most of the '90s 2x6/R19 and U-0.5 windows were code-min. The calculated or measured heat load 2x6/R19 houses with clear-glass double panes typically comes in around 12- 13BTU/square foot at an outdoor temperature of 0F (which is probably a degree or three below your actual design temperature).

But there are many (and sometimes wide) exceptions to the rule, which is why it's always a bad idea to size the equipment with a dumb rule of thumb. Most HVAC hack in our area using a rule of thumb use 25 BTU per square foot for newer stuff, 35 for old, but that usually oversizes the equipment by 2x.

Oversizing a gas or oil hot air furnace by 2x is typically less than a $500 cost adder. Oversizing a ground source heat pump by 2x is a few tens of thousands of doolars of additional cost...

...which is why there is NO SUBSTITUTE FOR A PROPERLY AGGRESSIVE MANUAL-J !!! Even if it costs a grand to have a competent person run the numbers it's dirt cheap compared to even a half-ton of additional geo.

I totally agree with all of the above. Pi started the GSHP quote process before I realized how badly the house was performing. I guess it's partly because I've always lived in 150+ year old homes and am used to terrible perf. Huge eye opener to hear that I should have half the load i do.


Energy audit happening next month (MassSave).


One question. I was planning on doing aeroseal duct sealing, but it sounds like some of you think my entire duct system might be garbage. No point in spending $3300 to seal ducts I'm going to year out, right?

One grand for a man J sounds pretty steep to me. I paid less than half that for both my man J and radiant floor design. That was a couple years ago but still. Also the did the actual heat loss drill and it was maybe 2% higher than the man J. But yeah you need to do man J before sizing system...

Your duct system might be garbage or not. You need to sort out why you using so much oil. If it is all crappy duct that's good cause it's headed to garbage. If it is infiltration you know you have bigger problem and can decide if throwing more $$$ into hole makes sense.

Another interesting tidbit.. I have the oil consumption data for 2015. Now, I didn't own the home then, and I know the previous owners weren't living here and instead had tenants. (One in finished basement and one in a suite above the garage.) So a large portion of the living area could have, theoretically, been turned way down. But I doubt they did. They weren't exactly cost-conscious people afaict. The results? Their oil consumption, normalized via HDD, was about 30% less than mine! They did about 5.76 BTUs/HDD/SqFt. I'm averaging 8.07.

A grand for a Manual-J is indeed pretty steep, unless it also includes blower-door testing. Some of the communities 20 miles from Boston are pretty gold-plated, which also drives the price of services. (On gold-plated Martha's Vineyard I saw asking fees north of $1K for an absolute basic Manual-J! ) It's possible to shop around and find a P.E. or RESNET rater willing to do a basic (no blower door) calculation for $500-700, but it's hard to get a qualified person to do it for less than that anywhere in eastern MA.

Aerosealing the ducts would only be in order if the design was verified and deemed close enough to Manual-D to work, and only if the leakage can't be dealt with by other means. ($3300 buys a LOT of duct mastic & $50/hour labor.)

Are you using programmable setback thermostats, or are you keeping it at the same temp 24/7?

Alas, I do not have programmable thermostats. I tried to upgrade to an Ecobee 3 or Nest, but discovered they're not compatible with my system. I've got two "primary" thermostats and 3 zone thermostats. The fact that the zone thermostats both control dampers and can call for heating/cooling make this setup not compatible both Nest and Ecobee.

You might review your zoning controls - from what I've seen, they are almost never done correctly and will pressurize/depressurize portions of the house, increasing air ex/in-filtration.

What jonr said. Zoned ducted systems with zone dampers that can close (nearly) completely are notorious for having gross imbalance issues creating air-handler driven infiltration. The duct design is still a prime suspect.

Are all of the ducts fully inside the insulation and pressure boundary of the house? (No ducts in the attic, above the insulation, no ducts in exterior walls?)

I have ducts in my attic, and the attic is... well.... confused. It has insulation on both the floor AND the ceiling joists, yet it also has soffit vents. Yeah. I'm going to be taking care of that asap as all that joist insulation is doing nothing.

Ducts in a ventilated attic are nearly always an efficiency hit.

If they're not well sealed (=tested with a duct-blaster with the registers taped over, with measured leakage @ 25 pascals pressure coming in less than 5% of air handler's rated flow), and the register boots & other penetrations are not well sealed to the ceiling gypsum it's usually an efficiency DISASTER.

Took a trip to the attic today. Discovered two things. First, I do not have soffits up there. It's completely insulated space. R30 between the rafters and a combination of spray foam and r19 bats on the floop. Gotta be close to R60 between the floor, air space, and roof. But I also discovered at LEAST two flex duct branches that are hanging on by a thread - barely connected to the main trunk. I bet there are more too. Taking a trip to the hardware store to get some mastic foil tape pronto.

Yeah, like I said before first fix your crappy ducts. Then sort out if you also have outdoor air infiltration problem too. Only then can you size best replacement if any is even needed.

Oh, it gets better. Went up to see if i could re-attach a few of these, and I discovered that what appears is a main branch (non flex duct) mostly detached as well. Not only that, but I did, indeed, find about 4 feet of soffit vents. The entire rest of the attic is seal up, but these soffits empty right into the insulated space, rendering a lot of this insulation moot.



On the bright side, this is some serious low hanging fruit in terms of fixing my oil consumption issues.

Oh, it gets better. Went up to see if i could re-attach a few of these, and I discovered that what appears is a main branch (non flex duct) mostly detached as well. Not only that, but I did, indeed, find about 4 feet of soffit vents. The entire rest of the attic is seal up, but these soffits empty right into the insulated space, rendering a lot of this insulation moot.



On the bright side, this is some serious low hanging fruit in terms of fixing my oil consumption issues.

Yeah you are on the right path now. You had a poorly insulated/sealed heated attic and you didn't even know it... If you had just done the GSHP thing blind you would be bragging how amazingly great it performed compared to the oil system and well worth the $$$.

It's "worth it" to verify/adjust to meet Manual-D with the ducts, and use hard pipe rather than flex to avoid these kinds of crazy faults. If staying with flex, they have to be stretched straight & tight to come anywhere near meeting flow specifications. More food for thought on that topic: https://www.energyvanguard.com/blog/joy-flex

From a code-R compliance point of view only the insulation at the roof deck counts, and unless the rafter-R is at least 40% spray foam on the underside of the roof deck or rigied foam above the roof deck you're at risk of moisture issues in the attic if you seal up venting to the Great Outdoors. The exception would be if the only venting is soffit-to-ridge venting on the exterior side of the rafter insulation, with at least an inch of air between the insulation and roof deck. Without either the roof deck ventilation or the R20 out of R49 at the roof deck it's technically a code violation, and it's worth inspecting the roof deck in a few areas to make sure it's still OK. The roof pitches at highest risk are those that get little to no sun in winter, such as those on the north side of the house.