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Need to determine HVAC for 1930’s home with addition in construction

Nausten | Posted in General Questions on

Hello everyone,

Nick here coming to you from sunny Buffalo, NY with a rather loaded question. 

The wife and I bought a nice 1932 two story “craftsman” style house a few years ago that currently has a gigantic boiler from 1973 and cast iron radiators. Square footage of the house is a full first floor with second floor knee walls that comes in at around 1000 square feet all told. With the first baby on the way we decided it’s time for an addition for several reasons, and the main question I have is about which HVAC system would be best. As is typical in this era of houses the walls are plaster over plaster gyp-board in the existing house, obviously the new addition is to be built but will have open walls to adapt to whichever system is chosen. I mention the existing walls because of the added cost/mess of opening the walls to run ductwork and/or mini split piping which is not something we’re opposed to but just an added fun step. The existing house has NO insulation aside from open cell spray foam applied to the attic space. Removing the exterior cladding and insulating and installing a proper breathable water barrier(zip) and new siding is in the plans but not this year. Having had experience with the minisplit type systems in a commercial setting I see their appeal on many levels however I feel myself leaning towards a geothermal type system. I love the idea of the system as a whole but just want to make sure I cover all my bases on this topic. There’s obviously a multitude of different options to choose from and we’ve toyed with radiant floor heat with minisplit just for the AC, keeping the radiators and replacing the boiler again minisplit AC, conventional furnace system etc. I just want to make sure we’re making the best decision possible based on everything that’s available right now. Given that the house with the addition will total about 2300 square feet with two very different building envelopes it presents several challenges. Any and all help on this topic is greatly appreciated as my brain has become overwhelmed. One small caveat is that my family is in the plumbing/heating wholesale business so most of the equipment I have available to me at cost which saves a substantial amount and I pay time and material to the installer. I mention this to keep options open not to “brag” so hopefully it doesn’t come off as such. Thanks again in advance. 


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  1. GBA Editor
    Brian Pontolilo | | #1

    Hey Nick,

    I suspect that you'll get a lot of responses to this. There are a lot of GBA members with deep understanding of heating and cooling equipment. I'll start by recommending that you find a professional to perform room by room load calculations for you. This will be particularly important in your house that will include some old, uninsulated and leaky areas, and a new addition that I hope will be much more efficient. Given your family business, it shouldn't be too hard to find someone who can do this for you. There's a lot of info on this site about this process. Here are two articles I suggest to get you started:

    1. Nausten | | #3

      This is why I love this website. The knowledge base available to tap into is fantastic. Thanks guys for both the starting points. I may have forgot to mention that dust is a pretty huge issue in the house and the forced air type system provides an opportunity to filter and clean that air so in that regard it gains a pretty big leg up at least in the new addition, in the older part perhaps some high velocity ductwork to keep things small in the walls? I’m just spitballing here.

      1. Expert Member
        Dana Dorsett | | #4

        >" the older part perhaps some high velocity ductwork to keep things small in the walls?"

        High velocity ductwork trends toward power hoggery, and probably isn't needed.

        Run the load numbers first- you're getting a bit ahead of the spitball...

        Fujitsu has a pretty good line of mid-static mini-split cassettes suitable for somewhat higher velocity ductwork if needed, but prescribing any solution before the load numbers are sketched out is really premature.

  2. Expert Member
    Dana Dorsett | | #2

    Getting to the right solution starts with a careful Manual-J or similar load calculation using aggressive assumptions about R-values and air tightness (as per the instructions in the Manual), otherwise any solution is just a guess. For a first rough cut, use an online Manual-J tool such as, using extremely conservative air tightness assumptions and aggressive R-value assumptions or it will overestimate reality. Run the numbers only on the "after planned insulation, windows & air sealing upgrades" version of the house.

    If the rads are 1-pipe steam rather than hot water it's probably not worth keeping them. If it's hot water, assess the equivalent direct radiation (EDR) per room relative to the calculated heat load of those rooms, from which one could derive the water temperature requirements. See the attached guide.

    Most homes in the 1920s & 1930s were over-radiated even for the heat load of the original air-leaky version of the house. In most cases the insulated tighter version of the house can be heated with 120-140F water even at design condition using just the pre-existing rads. If supplemented with mini-split you can go even lower.

    If the rads are painted a metallic brass/bronze or silver, that was commonly done to REDUCE the heat output to minimize the common temperature overshoots followed by the chill. Painting them any non-metallic cover restores it to the original EDR.

    It's not inconceivable that the radiators alone could have a low enough temperature requirement to be able to use a ground source heat pump solution, but from a cost point of view reversible hydronic chillers such as a 2 ton Chiltrix might just do it on a house that size. But a tiny modulating condensing gas boiler (or a condensing water heater) can cover a wider range of temperature requirements more cost-effectively in the short term, assuming you're on the gas grid.

    If the new system isn't going in before this coming heating season it's possible to measure the heat load of the house in it's current condition using weather data and fuel use via these methods:

    With a condensing boiler right-sized for the load it's probably not going to be an issue even if you break the system up into a few zones, but for a sanity check on boiler vs. radiation sizing, run these numbers too:

    1. Nausten | | #7


      I posted the reply to your original question for calculations below. However, I’m curious as to your thoughts based on the old house vs the new. Given that the new addition will have a much greater R-value with 2x6 walls vs zero insulation in the existing house I’m wondering if I should go with two forced air systems or one system with multiple zones. After thinking about this and given the proximity of the new baby(December) I felt like getting into the wall(s) to install ductwork is something I’d rather get done sooner than later. Is a standard high efficiency furnace with AC equally as efficient as a heat pump sourced system? I’m a complete novice when it comes to any of this information. Also important is that we’ve decided that ideally we’d like to remove the radiators to gain the space in the existing house which therefore means the option left is forced air as the wife is not fond of minisplit systems. Thanks again. I can go on forever detailing things so feel free to tell me when enough is enough.

  3. MattJF | | #5

    Dana covered all the HVAC.

    If dust is an issue, hvac is likely not a solution. You likely have lots infiltration or crumbling plaster and paint or a dog.

  4. Nausten | | #6

    Ok, so first off sorry for the delay.

    Second I have done the calculations you mentioned above.

    According to my math all of my radiators have an output of 11883 btu/hr according to the EDR calculation link.

    Next I did the calculations based on the “out with the old in with the new” article to arrive at 19894.5 btu/hr implied heat load for the existing house. Bringing the ASHRAE number to 27852.3 btu/hr.

    Not that this matters but currently my boiler (from 1978) is sized for 175,000 btu and 133,000 output. It’s incredible how oversized the unit is, just ridiculous.

    Now that’s all done I’m curious as to any recommendations you all may have. Thanks again!


  5. Expert Member
    Dana Dorsett | | #8

    >"I’m wondering if I should go with two forced air systems or one system with multiple zones."

    Zoning hot air or AC with zone dampers has all sorts of design issues and design/efficiency compromises. Every time a zone damper closes the static pressures are higher, and the flow lower. It takes the collaboration of an Einstein and Michaelango grade duct designers to really find comfort & efficiency when it's done that way. If separate systems can be found that are SMALL enough to be right-sized multiple units is probably better.

    With a design load of 20K and an ASHRAE sizing of 28K you're probably better off using right-sized (for their zone loads) hydro-air handlers running off a condensing tank type water heater with a ~75KBTU burner (HTP Phoenix "Light Duty", or AO.Smith Vertex). The smallest versions of the FirstCo HBXBX series air handlers might work, but run the real Manual-J:

    They have other series that may be even more appropriate- it's an extensive catalog of products.

    The 3/4 ton or 1 ton mini-ducted Fujitsus may be a better bet (but might be more money.)!/product/25310!/product/25312

    They make bigger versions in their 'mid-static" -xxRGLX series if you want to run it with one heat pump as a single zone.!/product/25351

    With a right-sized modulating heat pump & ducts the new-construction vs. old-construction issues go away- since it's running a 90% duty cycle the room to room balance can be tweaked by air flow with balancing vans, and it can be comfortable operated as a single zone. But the upstairs/downstairs temperature balance changes seasonally, and is potentially a sticking point for the upstairs in an under-insulated kneewall attic craftsman bungalow.

    >" It’s incredible how oversized the unit is, just ridiculous."

    C'mon, it's only 5-6x oversized for the design load, and you COULD have cold snap that gets down to -250F or something, y'know? :-)

    Ridiculous oversizing like that is common when the domestic hot water was provided by an embedded tankless coil in the boiler. Right sizing the boiler for the heat load wouldn't have provided anywhere near satisfactory hot water service, so efficiency took a back seat to utility. Times have changed.

  6. Nausten | | #9

    Awesome stuff. I’m definitely going to look into this much more heavily because I realized after that I do know what a heat pump system is and have a lot of experience with it’s VRF counterpart made by Samsung. I thought I’d pass along some info based on my own experiences in case you were interested. I own a 750k sq foot commercial building here in buffalo and we’ve begun to switch over our hvac systems to the Samsung branded system. It’s relatively new here in the states but like most things has been used extensively in the Asian market for years. Cost wise it’s cheaper than Mitsubishi and most others and is fantastic(when installed correctly). They introduced a 360 style cassette that has markedly improved both flow and comfort. As with most heat pump style systems it loses efficiency as the temperature Drops below 5 degrees Fahrenheit and will at times freeze up given extreme cold and wind. From a commercial standpoint it’s been a game changer as we have a food manufacturing facility with retail that is about 25,000 sq feet and their heating and cooling bills are incredibly low due to the VRF system and Samsung’s quality. With energy rebates and the like it’s become a draw to our building for tenants like that. Anything that comes with a 10 year warranty(After registered) on the entire system raises a few eyebrows. I don’t mean to sound like a Samsung salesman here as it has come with several learning curves and a very large hole in their support system from a consumer side, but overall I’d recommend it to anyone. I’d do it in my own house but my wife doesn’t like the look of the wall hung units and flush mount isn’t really an option just yet. As they say happy wife happy life :-). Returning to the real topic here I’m glad I had my light bulb moment with your help to turn myself on to the heat pump systems. It’s definitely going in the addition! Sized right I do believe I can make it work in the existing house as well. Given it will eventually be insulated it can only get better. Thanks so much once again for the time to help on this topic.


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