BTU Sizing

boobird

I live in NJ in a 1880s 3 floor renovated victorian with new windows and a new kitchen extension. It's not a drafty old house, but it's also not new construction with foam insulation and r-38 everywhere. There's an oil steam boiler for heat. There's also 4 ton and 3 ton ducted AC system that was installed in 2005.

- 3 ton A/C serves the first floor (1600 sqft) and basement (1000sq finished but rarely used). = 1600 sqft

- 4 ton A/C serves the 2nd floor (1200sq) and 3rd floor master (1000sq) = 2200 sqft

For various reasons (expensive, lack of radiators), I am looking to decommission the oil steam by installing (2) heat pump using the existing ductwork.

Q: As a rule of thumb for sizing, does the 3 ton (36K btu) and 4 ton (48K btu) seem reasonable or massively oversized for NJ heat pumps? I will ultimately request a full manual J from the contractor that I hire, but it hasn't gotten to this point yet. I'm just wondering if 3T/4T is wildly oversized..

Q: Would you recommend a dual-stage heat pump + furnace for my NJ heating needs? I understand the Mitsu and Daikin Fit have 100% efficiency down to 5F. I'm getting mixed opinions on whether a backup furnace would be needed.

Thank you for your feedback.

Hot_water_fan

https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

With the oil receipts, you’ll be able get the heat loss. 

There are fine reasons to use a dual fuel system - you won’t necessarily need it but it could be helpful if you avoid a electric service upgrade. It also provides a backup system and could let you sign up for some very economical electric rate plans. 

boobird

Hmm, the oil boiler serves "most" but not all of the house...The new extension for example uses electric radiators. And the 3rd floor lacks any radiators (gas fireplace ).

Hot_water_fan

That’s fine, include gas receipts too.

boobird

There's also a pellet stove in the living room

Hot_water_fan

A different path: is the cooling well sized? It runs mostly constant on a hot afternoon?

boobird

I'm reviewing the Google nest a/c usage from August.

First FL - Only turned this on AM til dinner. It looks like there was many hours when it ran the full 60 minutes. There's a few times where it ran 15 minutes every hour.

2nd FL - Only turned this on after 8pm (bedrooms). It would run 30-60 minutes every hour.

boobird

I'm also hearing that my ducts are undersized vs sufficient.

1st FL ducts (3 Ton A/C):
return: 930 CFM (with a segment squeezing down to 700 cfm!)
supply: 930 CFM + 300 CFM round ducts off the initial 2 ft supply trunk

2nd/3rd FL ducts:
return: not sure
supply: not sure

Hot_water_fan

I’d start with the oil and gas receipts. 

If the AC is not too oversized, which it doesn’t seem to be, you could replace like for like. Then provide supplemental heat with a furnace, resistance strips, the existing oil boiler, etc. You should be able to lower your bills substantially with the heat pump vs oil/pellets. 

boobird

Thanks.

I was reading that Mitsu hyper heat is 100% BTU delivery at 5F. Is this actual in real life scenarios?

I was also trying to find the efficiency of a Daikin FIT heat pump at 5F. But am unable to find the comparable efficiency data.

Jamie Hall

It is quite possible that the heat pumps -- with some revisions to the ductwork to suit a heating application rather than an air conditioning applicaation, since the requirements are quite different0 will provide adequate heat. Whether you will be as comfortable as you are with steam... doubtful. Whether it will be less expensive to run -- also doubtful.

If it were mine, I'd keep the steam, upgrading it (better venting, careful boiler tuneup) and use the heat pumps as supplements or in the warmer fall and spring months. But that's me.

Hot_water_fan

I  was reading that Mitsu hyper heat is 100% BTU delivery at 5F. Is this actual in real life scenarios?

Yes that’s legit. But you’ll have other constraints: ductwork being the main one. You might not be able to get the full capacity through the ductwork. But a good installer will be able to answer that and design around it. 

Since you’re replacing an AC system that’s 18 years old, you’re in the prime situation to add the heat pumps. You could live with both the heat pumps and the boiler for a year before deciding on the boiler’s fate. 

For cost analysis: 

oil: $/gallon x 1000/138 x 1/COP. 

So $3.5/gallon oil costs you $31.7/MMbtu if the boiler is 80% efficient. 

For heat pumps: 
$/kwh x 293 x 1/COP, so $.15/kWh (my price!) and a cop of 3 gets you $14.65/MMBtu. 

boobird

I read something about mitsubishis sometimes running more expensive than oil. Is there a possibility that heat pumps are most costly than oil??

Hot_water_fan

I read something about mitsubishis sometimes running more expensive than oil. Is there a possibility that heat pumps are most costly than oil??

Absolutely. In which case you could just turn on the boiler. But it depends on energy prices and climate. We know your climate, now what are your energy prices? The calculations are above. Plug in your values. 

In my locality, oil is severely behind and is basically extinct. In Jamie’s, it’s close. It’ll vary. 

boobird

Looks like I'm around .20/kwh for electric $19-20/mmbtu