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trying to save a hydronic system

ross Member Posts: 37
I'm an engineer and have been given the assignment to convert an old residential hot water system to a forced air system, adding AC in the process. We are also building an addition to the home. The house is a 1400 SF ranch style home (the new room will add another 400 SF) with 1950's era cast-iron radiators. The boiler was replaced maybe 15 years ago (haven't seen it yet, I'm assuming it's a standard efficiency cast iron sectional; orignally there was a coal boiler). The occupants have complained about the big old radiators and that they can never get the system to work right. Once I've seen it and talked to them myself, I'll be able to tell more about how the system works (it works, they have lived there all winter with no other heat source, I just have to get the specific complaints). Right now I think they mainly don't like the big old hot radiators. I want to pitch a different idea:

1. I could switch out the radiators with panel radiators or baseboard so they are less obtrusive. If the boiler is cast iron, we'll be running fairly high temperatures, so would it be safer (from a burn perspective) to run baseboard rather than panels? If the boiler is OK, I won't want to switch it out or make changes that would lower the return water temps much.

2. We could make the new addition an in-floor radiant loop. Any suggestions regarding mixing in-floor with radiators? With a combined system, can I mix to low enough temperatures so the in-floor section won't overheat?

3. I can go ahead and add an AC system. The easiest way is to run duct through the attic space. What is the best way to keep cold air from dumping from the ducts during the winter? Is there an advantage to going to a high-velocity system?

Any advice is appreciated. If I can't convince them, it will be ripping out the old and in goes central air.


  • Mark Hunt
    Mark Hunt Member Posts: 4,909
    Old system

    First, I would not worry about a "burn" danger with the panel radiators, the old cast iron rads would have been more "dangerous".

    Incorporating non-electric, thermostatic radiator valves can achieve room to room comfort with fewer controls.

    You would be wise to incorporate variable speed injection technology to control the fluid temperatures to the in-floor zone.

    I have found that cold air dropping from ceiling diffusers is not nearly the issue that warm, moist air rising through them is. Use the highest R-value flex you can get, usually R-8, and plan on insulating over them. Depending on your location, insulating over the flex runs may not be necessary.

    Mark H

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  • ScottMP
    ScottMP Member Posts: 5,884
    How about

    Changing the relays to a TAco series and adding a outdoor reset control. Set a heating curve to adjust the temperature of the boiler water to the outdoor temperature. That cast iron basebaord loves reset temp.

    You'll need to lower the temperature of the floor heat also. As Mark said enjection works great. Three way mixing valve will also work.

    Lowering the operating temperature of the system saves money and improves comfort.


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  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    1) By the 1950s automatic boiler control and forced circulation were very common. Combined with more prevalent use of insulation this allowed rads to be sized smaller than in the decades before. Panel rads would likely be a closer "match" to standing iron--particularly if the customers are generally satisfied with comfort and balance is the major "problem". Constant circulation and thermostatic radiator valves will allow whatever device is connected to maintain space temp at the lowest possible surface temp. If they are complaining that the current rads are too hot during maintenance condition (i.e. NOT when turning up the t-stat after setback) you may want to measure the temp of the most offending rad and size panels (should you use them) to operate at a bit lower temperature. As long as the system is two-pipe it is relatively easy to add TRVs, constant circulation and possibly outdoor reset. Done this way their system will be more comfortable and versatile than ever before.

    2) If they choose radiant in the addition, you will need to set up the system to provide different supply temperatures. Particularly if constant circulation is desired, injection mixing is really the way to go. While not impossible to design a mixed radiant panel/radiator system to operate at the same supply temp, it takes VERY careful planning.

    3) Besides the relative ease of adding a high-velocity system to an existing structure there is an added benefit: they excel at humidity removal even with condensing units of the highest efficiency. Cost is certainly higher but "payback" in a humid climate seems to come with reasonable speed. As mentioned, a nice tight air system and good insulation in the attic [usually] prevents problems with air/condensation. HV systems won't even operate with the "standard" leakage in a typical FA system.

    Let the customer know that while it's a GOOD thing to deliver and return A/C from ceiling/high on walls, it is a BAD thing to do this with heat. Hi/Low returns might help but I've only seen this done when the delivery is low and the returns are "switched" with the seasons.
  • Mark A. Custis
    Mark A. Custis Member Posts: 247
    My Dear Mr. Ross Moore

    1. YES, www.runtalus.com
    2. Yes, see three way valve.com
    3. High velosity is the way a bunch of the wallis use, but I am an air head and know how to use variable speed motors to move the scorched air.
    3. Dampers.

    Where is this job?

    Good luck,


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