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Choosing a plan for my duplex

I recently purchased my first home, a purpose built up/down duplex I'm living in. It's a ~2500 square foot ~1920 structure in Upstate NY. It has completely separate utilities, and each apartment currently has very old gas boilers. My apt has a "newer" 1960ish cast iron gravity powered hot water boiler, and the other has the original coal burner with a gas kit and a pressure safety valve that looks like a physics class demo of a class 3 lever.

I'd like to replace these as a matter of updating infrastructure for reliability and efficiency, not to mention gaining some space and headroom in the basement.

My first floor apartment is ~900 square feet. Using loadcalc.net I come up with a design day heatloss of 24k BTU, and a design cooling load of 10k BTU. This squares well with my 5k window A/C that managed to keep the whole place quite comfortable all summer. The only assumption I'm very unsure of here is ceilings. Given that the ceiling is the uninsulated floor of another apartment presumably heated, I included zero heat loss through the ceiling. The installed boiler is 131k btu input. (huge?)

Using a couple different sets of tables, I've estimated the installed cast iron radiators at 25-28k BTU at the standard 180 degrees.

My goals are to upgrade to a modern system with that is considerably more efficient than the old, replace the failing DHW tank, while keeping the capital cost low. It'd be nice to add a warm floor to the bathroom, if I can do so without much expense.

My initial thought was something like this:

-AO Smith Vertex 76k BTU hot water heater with side loop

-Tempering valve for the DHW

-Taco XPB1 self contained heat exchanger/circulator/controller

-PEX manifold and home runs to radiators.

-If I'm feeling rich, a radiant loop in the bathroom?

Some concerns I have:

-Even though the place is OLD, and has all of the original radiators with none removed as far as I can tell, the heat loss seems similar to the radiation. And this is taking into account the insulation that has been retrofitted over the decades...foam under the vinyl siding and cellulose in the walls. Will I have enough heat on cold days with source water temps below 180?

-For efficiency, I'd want to operate the hot water tank at 140, not 170. I can't find any sort of automated reset control for the Vertex. Is there some other brand I should look at? Or is that just wasting effort and money for minimal gain?

-With efficient stuff installed and pex inside insulated joist bays instead of pipe knocking me in the head, how cold is this basement going to get?. The place came with the joist bays insulated...I've removed it all for now to solve a million unrelated old house problems, should I reinstall it or insulate the floors and walls of the basement?

Sorry for the wall of text, I just wanted to give all the background I could think of up front. Thank you for any advice you can provide.


  • GarrettM
    GarrettM Member Posts: 5
    Expansion tanks.

    I'd also like to get rid of these expansion tanks and change the floor plan for more useful closets.
  • Paul Pollets
    Paul Pollets Member Posts: 3,642
    Combi units

    I'd be considering other technology. Modulating boiler technology is the way to go. The new combi units from Viessmann offer a modulating boiler and SS 316Ti DHW tank in one box.

    They have an outdoor reset computer built-in that's very user friendly.


    If the existing radiator load is capable of satisfying design temps at 140 rather than 180, you'd see dramatic fuel cost savings. I'd also re-valve the existing radiator valves with TRV's (thermostatic radiator valves) to easily balance the system.
  • GarrettM
    GarrettM Member Posts: 5
    thank you

    That does seem to check all the boxes but cost. I don't dare ask how much...and Google didn't seem to know. Ideally I'd like to buy from a brand that's widely distributed and easy to shop around. Thanks for that...I'll try and track down a rough cost.
  • Paul Pollets
    Paul Pollets Member Posts: 3,642

    Viessmann equipment is not readily available to the DIY market. Purchasing the appliance over the internet will void warranty. Be cautious about taking on a project that has a requirement for both design and installation skillset. It's not like installing a faucet.
  • GarrettM
    GarrettM Member Posts: 5

    I don't mean that I intend on literally buying it myself and installing it.

    While I'm confident I could design and install it and do as good or better than the "average" job, I'd also make mistakes and spend far more time and money than necessary as well. I find it a fascinating subject and someday I'd like to design and install such a system myself, but I'd like to do that in new construction. :)

    What I mean is that I'm not at all interested in buying from a brand that ties me to a particular contractor, supply house, etc. There are 1000 brands out there and many of them are widely distributed. Some further research had me interested in Triangle as well, until I found out they won't accept support phone calls even from uncertified people.

    I deal with this kind of thing at my day job. When the heating system breaks it's going to cost me a lot of money in loss of use; I can't afford to be beholden to whoever the local certified Name a Brand guy is and his schedule. I need the ability to be on the phone with the parts house that's going to have the part in my hand the next day with no fuss. Its a huge hidden cost and hassle. And if Contractor A can't be here this afternoon, I need Contractor B or C. That's just good, common sense risk management.
  • GarrettM
    GarrettM Member Posts: 5
    reason for my interest in the details

    Given that I'll be hiring a contractor to do this, you may well wonder why I'm interested in the details. I guess I'd like to have a good understanding of the problem and possible, sensible solutions before I solicit or review any bids.
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