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Community and Q&A

All-electric Retrofit for Hot Water

kimbarkwilson | Posted in Mechanicals on

We’re working with an architect on a renovation of our 3-story 1880’s Victorian masonry home in Chicago (5a). This first phase of work includes significant improvements to the building envelope (air sealing, insulation, roofing, windows). It also includes update of electrical and plumbing (replacing lead pipe). One problem that we haven’t figure out: How to efficiently upgrade our existing hot water heating system (Cast-iron radiators) to electric from the 1950’s gas-fired boiler that is reliably heating out home. 

The GC’s we’ve reviewed our schematic designs so far haven’t been very keen to retrofit, and have instead recommended removing the entire system to replace with force-air heat pump systems.  GC’s have cited multiple concerns: 1) existing piping would need to be upgraded to work with new systems making cost prohibitive, 2) concern above mixing new and old radiators, and 3) lack of heat pumps available to retrofit to existing hot water systems like ours. 

I have reviewed the GBA archives for suggestions or case studies, but I can’t find what appears to be a solution for a comparable problem. We’d like to move to electric heating while keeping about 1/2of the existing cast-iron radiators, replacing a portion of existing with wall panels (e.g. Runtal), and supplementing selectively with in-floor radiant, possibly electric (e.g. war board), and/or electric wall panels (e.g. Runtal). 

My questions for this group: 1) In situations like this, are heat pumps an option for retrofit existing hot water cast-iron radiators? (I’ve seen the discussion on Air-to-Water systems, like Chilltrix. Seems like limited options). 2) Any recommendations for mechanical contractors in the region who might be able to advise on this system?

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  1. paul_wiedefeld | | #1

    1). yes, there is nothing technically preventing this. There are a handful of brands and solutions to make this work.
    Here's an example of a successful project:
    2). This is the problem - you’re seeking an extremely niche solution and need a specialist.

    Do you want AC in the home?

  2. walta100 | | #2

    I tend to agree with the contractor the low-cost option is to abandon the hydronic system.

    If you chouse to keep it given your humidity you will need to install a forced air AC system having two separate systems will more than double your HVAC budget.

    Note hydronic heat pumps are very uncommon and expensive equipment the suppliers tend to be startup companies likely disappear with zero parts availability.

    I am concerned that your to do lists said nothing about exterior improvements. If your plan is to have modern insulation and air tightness you are domed to failure without modern water proofing on the exterior. The old brick walls are not water tight because they did not need to be perfectly water tight without insulation there was lots of heat energy to evaporate any water that got into the walls and plenty of air moving thru the walls to carry away the moisture before things could get moldy and rotten.

    You may find these articles interesting.


  3. Expert Member
    DCcontrarian | | #3

    First, get a good Manual J done, taking into consideration all of the upgrades you've done. Sanity check it against the history of heating fuel usage.

    Figure out how big a system you'll need to meet your air conditioning needs. There's no way around getting that from heat pumps and air handlers. They could be ducted or minisplits.

    Figure out how much heating the heat pumps that you need anyway for cooling can deliver. Compare that to the heating load calculated in your Manual J. You need to make up the difference somehow. I see three options:

    1. More/bigger heat pumps and abandon the radiators
    2. Keep the gas boiler and wire it so it only comes on when the heat pumps are unable to keep up on the coldest days.
    3. Install an air-to-water heat pump and use the existing radiators. Wire it so that the air heat pumps only come on when the water is unable to keep up.

    Each of these solutions has drawbacks. Option 1 is the simplest in that pretty much any HVAC guy can do it, but it probably isn't the cheapest or the most comfortable. I would say option 2 probably is going to be the cheapest and is something all but the least competent HVAC guys can do. Options 3 will give marginally better comfort than option 2 but they will be close, and it will be a lot more expensive. The big downside of option 3 is you have to find somebody who can do it.

    It really boils down to how important it is to you to be all-electric.

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