Insulation heat-loss improvement, system sizing

[American Locomotive]

Family member is looking to replace their AC system with a heatpump to augment their oil heat. While I was up in the uninsulated attic looking at the unit, I noted that there was only about 3" of sad fiberglass insulation on the ceiling joists, which is roughly what, R12? Looks like the recommended insulation area for this area is now R60. So we're going to pump that insulation up to R60 equivalent.

The current system is a ~25 year old R22 3-Ton unit, and they tell me it just about keeps up on the hottest days. It's a single story home, about 1600 sq-ft. New metal roof.

So with the insulation improvement, would keep the existing sizing of 3 ton, or would you be comfortable downsizing to 2.5 ton?

 

[The Cobbler]

with similar insulation My original a/c would run all day long & well into the night . I upgraded the insulation to ( probably r60) and based on recommendation from installer( friends Nephew inlaw) replaced the A/C with the same size, ( I think 1-1/2 ton)
it cycles frequently even on the hottest days . I don't have any humidity issues that would be seen from too large a unit . the house id very comfortable
I know that's probably not answering your question, but hope to give you some insight to savings
the summer I ran the new unit, my electric bill was half of the previous year , as well as the gas bill for heat in winter
I insulated the basement walls at the same time

 

[American Locomotive]

I figure it's probably safest to go with the 3-Ton unit again.

 

[fitter30]

Contact their electric company see if they offer a energy audit with a blower door test. Might also include some rebates on new equipment and insulation upgrades.

 

[PoorUB]

Family member is looking to replace their AC system with a heatpump to augment their oil heat. While I was up in the uninsulated attic looking at the unit, I noted that there was only about 3" of sad fiberglass insulation on the ceiling joists, which is roughly what, R12? Looks like the recommended insulation area for this area is now R60. So we're going to pump that insulation up to R60 equivalent.

The current system is a ~25 year old R22 3-Ton unit, and they tell me it just about keeps up on the hottest days. It's a single story home, about 1600 sq-ft. New metal roof.

So with the insulation improvement, would keep the existing sizing of 3 ton, or would you be comfortable downsizing to 2.5 ton?

IMO 2.5 ton is a bit large if properly insulated. I think 2 ton would do it.

I assume attic air? The AHU and duct work in the attic? I also would be very concerned about the quality of the duct, and how well insulated it is. When I did HVAC service, pretty much all the attics I was climbing through the duct was in poor condition.

 

[danski0224]

So with the insulation improvement, would keep the existing sizing of 3 ton, or would you be comfortable downsizing to 2.5 ton?

It would actually be best to do the one thing that almost no one in these forums ever does, and that would be doing a load calculation. Then you will know the answer.

A load calculation will also provide crucial data that's needed to set the balance point. The economic balance point will tell you if the whole thing is worthwhile... or not. Or, you can guess.

IF (and this is a big if) the ductwork is properly sized, and also IF an inverter condenser is used, it's possible to put in an oversized condenser for greater heating capacity- but the ductwork needs to be sized for the higher airflow capacity.

 

[pcmeiners]

Agree with PoorUb , properly insulated you could get away with an efficient 18k unit but 24k gives an added margin for open doors/hot cars. Mind you Rhode Island is low temp unit country. For higher efficiency you could consider (2) 12k units, only using the second unit for really hot/cold days, resulting in redundancy should one unit fail.

 

[BurtEggley]

3 ton with R60 insulation. Make sure you do the math well on the replacement unit. Electric prices have not caught up yet to oil, but they will. For example, much of the SW uses power from hydro electric on the Colorado River. It has been so overused that it is at historic lows, and that power is no longer reliable. That means power from natural gas, oil, or coal is likely to replace it.