Confused by Heat Loss Calc - Furnace Size

[Molo]

Thanks for the reply Dana.
Yes, the basement is usually warm.

The apartment is on the second floor. The ductwork runs in the walls of the first floor apartment and above the first floors drop ceiling.

The hot water heater for the apt. is in the basement (30 gal. nat.gas).


Does the 80k unit still seem too big?

I'll bet it keeps the basement warm too, with all of that uninsulated duct work.

I'm assuming that the apartment is 2 or 3 floors up from the basement? (In which case losses from the heating system for the apartment are heating the intervening space.)


Is the hot water heater for the apartment in the basement or in the unit, and is it gas-fired? For heating loads as low as this one likely has it's pretty cheap & easy to heat with baseboards or fan-coils running off the water heater, sealing off the supply & return ducts at the registers. With leaky uninsulated ducted systems like that the duct losses and air-handler-induced infiltration alone could be as high or higher as the actual heat load.

Alternatively, installing a right-sized mini-split heat pump would get you both air conditioning and high-efficiency heating, with comparable (sometimes lower) heating costs to natural gas. With a leaky uninsulated duct system outside of conditioned space it's probably cheaper to operate with a mini-split than the proposed condensing gas furnace. A 1.5-2 ton mini-split is on the order of $4-5K, installed, But if you spent a grand on tighting up and spot-insulating, adding storms to the single-panes, etc. you might get the heat load down to the 1-ton range for a similar installed cost, and even lower operating cost.

 

[Runs with Bison]

Is there a separate unit for the other apartment/level or is this a shared unit?

I'm with Dana on the heat loss to the basement. That is only helping the apartment below you. I would seal and insulate all of those lines and the trunk. Let heat losses through the floor and heat transfer from the ground will keep the basement above freezing.

There is likely no insulation in the ductwork in the walls leading to your apartment. So you are providing free heating to the apartment below...depending on how it is done there might be substantial leakage too (or a sly occupant might even tap in for some free HVAC although that probably isn't the case.)

 

[Sponsor]

 

[Dana]

Yes, the 80K unit is still WAY oversized, even with normal levels of duct leakage & conducted losses to the unconditioned space.

The burner on the existing gas HW tank is likely big enough to handle the design condition space heating load with a fan-coil or three in the apartment, but it requires some re-plumbing and a real design to both deliver the heat and avoid destructive condensation in the hot water heater. If one replaced the HW heater with a condensing Polaris or Vertex or similar it might even simplify the design, and you could use extra length cheap fin-tube baseboards or radiation, as long as it can deliver the heat with 120-130F water.) The 76KBTU/hr burner of the smallest Vertex would be oversized for space-heating alone, (but still OK due to the insulated thermal mass of the tank), with AMPLE margin for an apartment that has been working OK with a 30 gallon tank. (BadgerBoilerMN has plenty of experience with designing heating systems around condensing hot water heaters.)

The losses from plumbing would be a tiny fraction of that of the ducts, but putting a minimum of R4 (5/8" wall) closed cell pipe insulation on all of the distribution plumbing for space heating would cut that even further, and would be highly recommended for any new plumbing installed.