Hi have a fully gutted house row house. It’s 2 story, 2 bedroom 840 sq ft. House already has a newer furnace for heat and there’s a chimney so looks like no high efficiency.
The heat load of a row house is pretty small since it has much less exterior surface area to lose heat from than a standalone single family house, making all but the very smallest gas fired (and literally all oil fired) furnaces WAY oversized.
If this is a middle unit rather than an end unit and in the full gut remodel the insulation & windows are being brought up to current code minimums you are probably looking at a design heat load of less than 10,000 BTU/hr @ +70F indoors/+17F outdoors (Philly's
99th percentile temperature bin). ASHRAE recommends a 1.4x oversize factor for the load at the 99% design temperature as the best compromise on having sufficient capacity to cover Polar Vortex event cold snap coolth, yet still have a high enough duty cycle during normal cold weather to have reasonable comfort. At 1.4x oversizing the furnace would be running (1/1.4 =) ~70% of the time when it's +17F outside, for very long "warm summer breeze" on-cycles, and with relatively short off cycles.
When the furnace is 3x oversized for the load it only runs 33% of the time even when it's +17F outside, which is OK for efficiency, but terrible for comfort, delivering the hot-flash with a temperature overshoot, followed by the extended chill & draft. Look at the ratings on your furnace. If it's likely to be crazy oversized, it may be worth running an aggressive Manual-J load calculation using
CoolCalc or Man-J-ish calculation using
LoadCalc. Either of those tools will oversize from reality, especially in the hands of a newbie. To avoid crazy oversizing with those tools be very aggressive on air tightness & ventilation assumptions and duct losses (zero infiltration, zero ventilation, zero duct loss), and DON'T upsize from there.
More on how oversizing detracts from comfort see
Nate Adams' freebie
book chapters &
videos.
A 1-ton or 1.5 ton hydro air handler(s) running off a condensing water heater is one way to right-size (or even zone) low load houses/apartments. Ducted modulating right-sized mini-split heat pumps is another (usually more expensive, but more comfortable too.) Yet another is to calculate the heating & cooling loads and use a reasonbly-sized
Dettson Chinook, which uses
tiny plastic ducts small enough to run horizontally through 2x4 studs (on non-structural studwalls.)
Are you intending to heat and air condition with the same ducts?
My contractor who isn’t that familiar with HVAC wants to try to do it because the house is straight through on 1st floor, 2nd floor has 2 bedrooms and a bathroom. Basement is just a cellar for now(down the road we are going to dig it out to make it a livable space.
Contractor says we might be able to use flex duct since house is small. Trunk line from basement will be hard pipe and some hard pipe in between.
Can anyone suggest a basic HVAC layout for this? Like location of registers etc? He thinks we can make soffits in the first floor for the heat and central air. That means heat and cooling will come from about 8-9 feet high on first floor.
Why he’s trying to figure this out I figured I would do some research myself.
Thanks!
With the right types of diffusers it's possible to heat and cool just fine with soffited ducts 9 feet (or even higher) from the floor. On a project I was involved with a handful of years ago in a split level "mid-century modern" type house the diffusers were ceiling mounted more than 15' off the floor of the lower level of the house. With a right sized 2-stage gas furnace the duty cycle was high enough to keep the air mixing with relatively high-throw diffusers (even at modest cfm), with very little temperature stratification effects.
That house was about 4000' of conditioned space (2000' of which is walk-out basement) with lots of window area, and is served with 60,000 BTU/hr 2-stage furnace and a 3 ton single-stage AC, at an outdoor design temp of +5F, and the house does just fine when it's -10F outside. It replaced 200,000 BTU/hr of gas heat and 9 tons of AC that had been making it a wretchedly
miserable place to live (even though it's a very pretty house.)