Radiant electric heat in floors in one building and gas furnaces in another.

A sonar scan will tell you more.We've done the repair for both water, heat, and steam.

(vapor barrier and foam insulation)* Ridge vent is totally clogged with dust, dirt or something that is preventing flow* There is a heat cord on the roof below where the furnace is located so I am sure the roof is hot in the winter and melts the snow which causes ice damn in the section.* vents and returns are in ceiling..so won't the heat come down and go right back up the return?

It needs foundation work, heat, electric updating.    

The units share a wall, heat/ac source, and address. 

If the house is 100 years old with wood heat and split shaker shingles, you need higher capital reserves. 

Also, tell your tenant they can pay for the clean up from that know it all dad who pulled the heat register up and disturbed the flooring if it's required. 

Also did it for a fridge: $220 and it lasted another 5 years before the heating element in the freezer door went out (I had warranty at that point).  

What temporary heat source should I use while rehabbing an apt building in the winter?  

Built in 1973, in great condition, although does not have current upgrades.Since I am not yet a PRO user, I based on my rental calculations using great calculators from ListenMoneyMatters.com.SUMMARY:ESTIMATED VALUE (Zillow): $214,714.00 (In line with averaged real estate agent's Comps but slightly more conservative, as I think he threw in a couple outlier properties to bump the average a bit to entice my folks.)PURCHASE PRICE: $195,000.00 (price based solely on the amount left on my parents 4.25% HARP non-assumable mortgage)UPFRONT COST: $50,587.42 RENT ESTIMATE: $1750.00 ANNUAL NOI: $11,633.00GROSS YIELD: 10.79%CAP RATE: 5.98%CASH ON CASH: 5.56%CASH FLOW:ANNUAL: $2760.32MONTHLY: $230.031ST YEAR EQUITY: $2363.49ACQUISITION COSTS:DOWN PAYMENT: $48,641.75 (25% ) CLOSING COSTS: $1,945.67 (1%) *anticipate only using real estate attorney (no real estate agents) for more cost effective deal and save parents 6% commission; do not know actual attorney fee costs for closing so this number is a WAG (Wild *** Guess).MORTGAGE RATE: 4.5% *anticipated MORTGAGE TERM: 30 years ONGOING COSTS:PROPERTY TAX: $1,927.00/yrPROPERTY INSURANCE: $300.00/yr PROPERTY MANAGER: $175.00/mo (10%) VACANCY RATE: $350.00/mo (20% vacancy rate)MAJOR/MINOR CAPEX: $175.00/mo (10%)HEAT/AC: $92.00/mo (Averaged)HOA: $420.00/mo (has remained approx. $420.00 for decades; although the HOA just hired a property manager for the first time last year, but so far no indications of changing HOA fees)Notes:Rent estimate: $1,750.00 (local property manager assessed realistic rents between $1600-$2000+ per month on long-term lease or as vacation rental $140-200 a night/minimum 1 week stays).My parents are open to any type of creative financing deal, that would free them from paying mortgage (their current monthly mortgage payment is $1,000.00 (+ $420 HOA).