I am in the midst of working to figure this part out, but I still want to figure out how to do it based on the current parameters. The building is 145 years old, so "making sure the plumbing is correct for all tenants" is not possible in the foreseeable future...

By current plumbing, I meant that he gas piping for respective units is hooked up to the right gas meter if separate meters are installed.

If you are really ambitious you could do a heat loss calculation based on the indoor temperatures you stated. Once you have that loss then you can figure the ratio of how much gas is being used between the two apartments. What will further complicate this is how the distribute the hot water heating and if one apartment has a gas stove and/or dryer.

If it were up to me I would just come up with a reasonable split (60/40?) and be done with it. Then have the gas company install two meters if it is feasible. Two hot water heaters would also be be fair but all this split plumbing is going to be difficult in a 145 year old house.

There are two ways of doing this. One way is to have gas usage meters installed and the other way is to get an estimate, yes an estimate only by performing manual J load calcs on each apartment then plugging those numbers into the usage estimate software that looks at weather history and estimates cost on current utility pricing.

Heat loss calculations look at the building envelope, not the load between floors. You could apply basic heat transfer principles and calculate each space on its' own and estimate heat loss and usage, but this would be a theoretical exercise and have very little legal standing. No one would or should pay utility allocations based on a unsubstantiated calculation. Where you really need to start is with the landlord/tenant laws for your state. Depending on what is allowed your options, best to worst, would be:

1) Split up utility services (gas, water, electric) by apartment and require tenants to have their own account.

2) Submeter utilities with additional gas/electric/water meters and chargeback tenant. That mean's one main feed to the building with separate in-line meters that you log and allocate a portion of the monthly utility bills based on the reading.

3) Remove the upstairs thermostat. You can replace the thermostat with a space sensor, and then remotely set the thermostat to an agreed temperature above the legal requirement.

The most important part of all this is the state laws. Some states don't allow submetering. Regardless of what you choose you need to get the tenant to accept it in writing in their lease.

What do the leases say about the utilities, and what has been the practice in the past?

Thanks for all the input, I appreciate it.

The lease says each tenant pays "half", so this exercise is just for myself, really, since I cannot legally hold the tenant to a higher cost. My immediate plan is to invest in a thermostat that has a temperature control limit, but I am also going to be revising the leases for next year soon, and this is going to inform my revisions.

At the rate you are going you'll be lucky to still have tenants next year. In tenant's eyes there are many things they rather you do than changing their lease aggrement. If and when you have dealt with renters a few years you come to understand what is meant by "let sleeping dogs lie". After you have upset them and run your tenants off,try this. In less than freezing weather,turn upstairs heat off and set downstairs to whatever you think renters should. Return any time from a few hours to a few days and you will find upstairs is about as warm as downstairs,maybe even warmer in an old place. Run that through some manual Js,heat loss calculations and remote t-stat theories and see if they make any sense. One of your expert hvac advisors should have mentioned that instead of sending you off to spend money on a worthless pursuit.

My general take-away so far has been that this is "too complicated" to actually come up with a way to calculate it for now, and that I need to just figure out how to not be stuck with the higher bills next year.

Fortunately, the apartments are unique enough that I don't actually anticipate having trouble keeping them rented.

Also, both of my tenants already know that there will be changes, and are aware that this is currently in a state of flux precisely because I have just inherited the building, and am consulting with an attorney to make sure that I am doing everything correctly and not quite as "under the table" as my dad was doing it.

If each tenant is willing to pay half of the gas bill, then you should not have a problem.

If this is a legal two family house then there should be a meter for each tenant. You should discuss this with an attorney who knows the allowed zoning before making any changes.

Good luck.

"Consulting with an attorney to make sure that I am doing everything correctly",on a 125 year old building should bring to light various things that need testing and updating. Like non-compliant locks,inadiquate egress, lead paint, galvanized pipes,lead solder, asbestos material,mold and the list go's on. Government bureaucrats agore sheep volunteerly coming in for shearing. While you have the attorney's ear,ask what is an adiquate amount of liability insurance in case you neglect to correct something you have been made aware of. Also ask about difference between oversight and gross negligence when it come's to building code compliance.

Ha. I already have a long list of these things. My dad owned it my whole life, and I lived there for various periods of time (not just the 7 years in the apartment as an adult), so it will probably be more of a "have I covered everything?" kind of list.

Margretta, if you are just looking for a way to do the calculation for informational purposes then look up lumped capacitance heat transfer and ignore radiation. You will have to make a lot of assumptions and simplifications because heat transfer is very complex, but it may get you an idea of how much heat is needed to maintain the 2nd floor vs the first floor.