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geothermal with cast iron radiation

Heatman_2 Member Posts: 65
Has anyone successfully re-used existing cast iron radiation with a geothermal water to water, somehow boosting the water temp with aux. heat?


  • yeldarb
    yeldarb Member Posts: 8
    re: geothermal with cast iron radiators

    Wow no response in 3 years. I guess that says something?

    I have to think that at some point, oil/gas will become so expensive that it's cost effective to do Geothermal + (temp booster of some sort) for use with cast iron radiators.

  • Mark Eatherton
    Mark Eatherton Member Posts: 5,839
    Hybrid system only...

    The problem with using older heat emitters with the newer technology is that when design conditions hit, the GSHP will not be able to generate temperatures high (160 to 180) enough to satisfy the call for heat. During the shoulder seasons, the 120 to 130 degree F fluid temperatures from the GSHP can handle the load, but you will have to have another source (modcon) to satisfy the need for heat and pick up where the GSHP leaves off.

    THe economics of doing this are also dictated by the use of the system. If it is doing heating and cooling, then the economics look decent. If it is doing heating only, then it stretches the economics beyond most peoples acceptable range.

    So, as with any hydronic question, the only correct answer is, it depends~

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Brad Barbeau
    Brad Barbeau Member Posts: 48
    Water temps (from a homeowner)

    I just put a cast iron system into my old house in February.  We're using outdoor reset and the rads were purposely oversized by about 15%.  Outdoor design temp here is -10 and the corresponding water temp is 135.  Most of the time it's around 100.  That being said, we don't use setback at all which I think would throw a wrench in things. 

    Assuming your radiation is oversized, I think those temps would be fine for you.    Most old houses I've seen around here have more radiation than I do. 
  • yeldarb
    yeldarb Member Posts: 8
    but will radiators feel "hot" ?

    (for reference, details of my current system are in other thread:

    http://www.heatinghelp.com/forum-thread/136804/geothermal-with-cast-iron-radiators )

    So, I understand some may suggest that 130F might be ok in my radiators. While that's certainly one option, it wasn't exactly my question. My concern would be that 130F cast iron radiators won't feel the same as they do now at 180F. In other words, part of the appeal of the heating quality, the "warmth", of the large cast iron may be lost at lower temperatures. Thus the question about boosting it.
  • hot_rod
    hot_rod Member Posts: 16,097
    less output

    with lower temperatures is the result. What temperature can you expect to supply with the GEO you are considering? Has the building shell been upgraded to lower the heating load?

    With panel rads we have developed a "cheat sheet" It takes about 4 times as much rad surface area when you drop from 180F to 120F supply, all things being equal.

    You can could add some forced convection with a small fan to force convection. Jaga radiators do this with a small group of quiet "muffin" fans attached to the radiator.

    ideally you could perform a room by room heat load calc, see what is required in the rooms and then calculate how the current rads match that load. it's tough to answer your question without putting some numbers to the question.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Ironman
    Ironman Member Posts: 6,857
    A Misconception?

    If you have to "feel" heat coming from the rads, then the heat is not being evenly distributed throughout the room or structure. A well designed system distributes heat evenly. By lowering the water temp, you will actually have a more comfortable system. Assuming the temp is not lowered below the heating demand.

    Think about it: The most comfortable heating system is radiant floor that is properly designed. The surface temp of the floor is never more than 85* and the floor feels neutral. Yet you have the most comfort from a heat that you don't "feel". :~)
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Brad Barbeau
    Brad Barbeau Member Posts: 48
    Mine feel hot

    135 feels hot to me.  Even at 100, they are nice and warm.  There are no swings in temperature either because we are using constant circulation too. 
  • yeldarb
    yeldarb Member Posts: 8
    A matter of preference

    I think there are many people who would debate the question of whether it's desirable to "feel" the heat coming from a cast iron radiator. It probably has more to do with what kind of heat one grew up with as a child, etc. (i.e. psychology) than objective facts.

    In any case, my goal would be to replace ONLY the oil-fired boiler with geothermal system of some sort, and leave everything else the same and have it deliver the same heat. I stated at the beginning the premise that if I had to rip out everything or generally change my historic structure too much I just wouldn't bother - easier to keep using my 1944 oil burner and leave things as they are. Everything works acceptably now, it's just getting expensive. At some point I thought perhaps it would be economically sensible to do geothermal plus (something) to keep my radiators at their current higher temperatures. I like it, you can't argue with what the customer likes, even if it's not technically optimal.

    So far, most people have just told me "no", rather than the answer I'm looking for, which is something more like "yes, but it will cost $X+$Y up front and $A+$B to operate, compared with a standard geothermal system costing $X and $A. Looking for some creative thinking, versus cookie cutter installation.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,839
    edited June 2011
    Lots of misunderstandings here...

    First off, let me tell you MY interpretation of "comfort". Comfort is NOT being aware of your environmental surroundings. You are not hot, nor are you cold, and you don't hear things running in the back ground. Simply stated, you ARE comfortable, or not. If you are NOT comfortable, you will naturally seek out the warmest spot in an effort to become comfortable. Your definition may vary, but human physiology dictates otherwise.

    If you perform a heat loss calculation (required first step for ANY heating system replacement) and perform as MUCH energy conservation as is reasonably possible, then perform an EDR study of the existing radiators, then you will be able to determine how hot the water needs to be in order to satisfy the load. It is entirely possible that you can satisfy the load with 120 degree F water, but until you crunch the numbers, it is all speculation.

    As for getting the installed cost, MOST contractors do NOT understand how to interface this equipment to allow it to work in unison, and therefore will tell you it can't be done. It can and is done by competent designers/installers every day. As I previously stated, the economics are dictated by the end use. In many cases, you'd be money ahead to hire a competent designer to design a system for your application, and then you will have something to give to the local contractors to give you an apples to apples bid.

    Otherwise, your numbers will be ALL over the place, and you will be just as confused as you are now.

    As it pertains to delivering good human comfort, I would strongly recommend the use of non electric thermostatic controls valves on all radiators. This will eliminate the hot radiator syndrome, but will significantly increase the comfort conditions within the house, and if you are comfortable, you won't be looking for a warm radiators to snuggle your butt up to.

    One possible design candidate to offer up to you would be Rob with NRT. Hopefully he will see this and chime in. If not, I can find his contact info some place.

    EDIT: I just realized that you are in Dave Yates back yard, and if there ever was a competent designer/contractor with GSHP and high efficiency experience, it would be Dave. He owns/runs F.W. Behler Company. If he can't help you, he may know of someone who can.


    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • scott markle_2
    scott markle_2 Member Posts: 611
    integration problems

    I'm not aware of a design that can heat effectively with a supply of 160-180, and still provide the 100-110 return we need for Geo.

    We can SWITCH from geo to an alternate heat source, but supplementing (as in running simultaneously) is impossible because ultimately we need return water that is only 20-30 degrees hotter than room temp., 50-80 deg. deltas are not realistic, and would require massive radiation and very controlled flow rates.

    Solar integration has similar hurdles, once we start supplementing we loose the super cool return water that drives the solar efficiency. A recent Sigenthaler article on low temp heating troubled me a bit because it showed a reverse indirect solar drain-back tank with boiler back up. The problem I see is that the boiler is committed to a 130 plus operating temperature (to provide DHW) even though the heating can be done at much lower temperatures for much of the season.

    There is no clever piping way around this. If a heat source requires low return temperatures then the radiation and structure must be designed with this in mind.

    Mark, if you can show me a schematic that shows integration (simultaneous operation) and provides the 50-80 degree deltas required, I'd be very interested to study it.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,839
    Can't be done Scott...

    As you pointed out, it can't be done. Theoretically speaking, the GSHP's COP will drop to near 1:1 when it is at its upper operating limit.

    When I set a GSHP system up with a mod con boiler, I use a "soft" interface, meaning that the boiler is doing the primary control, and if the GSHP is able to satisfy the call, then it is allowed to. But if not, the modcon picks up and takes over. There may be a brief period of time that both are running simultaneously, but as soon as the return water temperature comes back adequately warm, the GSHP will automatically shut down on its high limit set point.

    With some higher escehelon control logics (ENV for example) when the cut over point happens, the GSHP can be disabled. I just like keeping things as simple as possible,whenever possible.

    Got a link to the troubling Siggy article that I can look at?


    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • NRT_Rob
    NRT_Rob Member Posts: 1,013

    you send 180 out to a radiator to make it feel hot, the return water would have to be something like 100 degrees to really allow the geo to do anything, and then you're still heating with 3/4 whatever your booster is.

    that would be an incredibly low flow system, that's fun, but you'd be wasting your money on geo completely.

    If you want geo efficiency you need geo water temps which means cool radiators and invisible comfort. If you want hot blasts of heat, you need high temps.

    with modern low mass panel radiators, you could do both... have a timer switch to switch to your high temp heat source for, say, a half hour. the rads would heat up in a few minutes, you could bask, and then it could go back to the serious business of efficiency. but that's probably a total radiator replacement... standing cast iron has a lot of mass and would take a while to heat up.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
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