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replacing cast iron with veha panel rad


Thank you SO much for all of your time in thoroughly answering my questions and schooling me on my heating system. You don't happen to know of someone in the Pittsburgh Pa area that is as knowledgeable and patient for when I start to price out boiler installs, do you? Based on the heating bills from the last owner of this house, I suspect that the boiler is already blowing much of its work up the flue.

I am dead set on replacing the kitchen radiator, but the bedroom (and yes its the master... are you psychic as well?) I will leave. I thought that the panel would be a "nice" upgrade, but I have no problem with the space that one takes up. A few last questions if you don't mind donating a little more of your time.

Should I install TRVs on all radiators if I am replacing the kitchen rad, or just if I was doing the bedroom too? Should I still install a TRV on the panel in the kitchen? Any good suggestions for model or make of TRVs?

If I ever did want to do panel rads throughout the whole house, would it require a complete re-pipe, or would I then be ok because all of the rads are the same?

Any suggestions on a boiler that would be priced middle of the road but still a good manufacturer and efficiency?

Again, thank you so much for your time in dealing with my questions! I have been amazed at the willingness of people to help on these forums and you have certainly done more than your share for me.


  • replacing cast iron with veha panel rad

    I am planning on replacing 2 of the cast iron rads in my house with Veha panel rads. The current radiators are not connected in series, although they all branch off of one main loop. All of the piping is done in iron, from the boiler to the rads.
    Three questions:
    1. I am new to radiant heat and was looking for information on the most efficient use of a boiler system. I have heard that adjustable thermostats actually make the system work harder and waste energy, but I haven't been able to confirm this. Does anyone know how to most efficiently utilize this system?

    2. It is my understanding that since cast iron and the new panels are made of different materials that they will retain heat differently and placing them on the same zone may create uneven heating. Is there any way, short of installing another zone, which I am unwilling to do at this point (may eventually do when I get to replacing the boiler and the rest of the rads, but right now am renovating these two specific areas), to maintain as even or even-ish heat. My thought was if I kept the water temp low and the pump working more often than not that I would have as even heat as possible. Any thoughts? Will this mean an decrease in efficiency?

    3. One radiator currently has 3/4" (second floor bedroom) running to it and the other 1" (first floor kitchen). The guy at Veha told me that since the flow required by a panel (1-2 gpm) is much less than cast iron I would be fine running 1/2" pex to them both. Is there a benefit to running 3/4" to both even though the tappings and nipples will be 1/2"? Would it be ok to run 3/4" to both or should I maintain the sizes proportionally?

    Any help would be greatly appreciated. Thanks!
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    From your description of the system it's almost certainly a gravity conversion system. The real clincher is smaller piping to higher, farther radiators.

    These systems have extraordinarily low head loss (resistance to flow). As a consequence, you have to be very careful about making piping changes. If you significantly increase head loss to any radiator (as could easily happen if 1/2" piping were used) flow to the radiator will drop.

    Be forewarned that heating disasters often begin by removing a kitchen radiator that's in the way...

    Gravity systems are almost always two-pipe systems. You should see at least one pair of large pipes running parallel to one another that get gradually smaller the further they get from the boiler. The two connections on each radiator will lead to separate main pipes. Is this what you see?

  • Yes, this essentially what I see. In the basement there is a set of parallel pipes that run around the perimeter of the house. Each radiator has two pipes running from it, one to each of the pipes in the basement. The main pipes in the basement are the same size throughout the basement, but the pipes to the radiators are larger for the radiators that are on the first floor and smaller for the radiators on the second floor.

    I expect that flow will drop with smaller piping, but Veha tells me they need far less flow than the cast iron, so is this really an issue?

    "heating disasters often begin by removing a kitchen radiator that's in the way..."
    Could you please elaborate on this? I was planning to replace it with a radiator that is of similar BTU output, just a lower profile. Any suggestions on how to move forward?

  • one clarification

    One clarification, however...
    I have an expansion tank in the basement and a circulation pump, so I am assuming it has been modified.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    I presumed that it had already been converted to forced flow.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    You definitely have a converted gravity system.

    Unfortunately, few people actually understand how gravity systems function either originally or when converted to forced flow.

    I won't quibble with Veha's statement that their panels need far less flow than cast iron (even if my gut tells me that it's dead wrong), but I will tell you that original gravity systems maintain a reasonable balance after converson ONLY because the system flow is MANY, MANY times greater than under gravity.

    Let me give you a VERY important term with regards to hydronics (hot water heating): Delta-T

    Delta-T is a combination of the Greek (I hope) word for change--delta--and an abbreviation for temperature. In other words, it is "change in temperature".

    Delta-t and elevation are the engines that drive gravity circulation. Under gravity circulation you could VERY easily feel the delta-t across a radiator by putting one hand on the supply pipe and the other on the return. Under forced circulation not only will you not feel the difference, but you'd have a hard time measuring the difference. Flow through the radiator is so many times greater than required that delta-t (like resistance to flow) becomes effectively immaterial.

    Change the piping such that your new panels receive the "proper" flow for the "proper" delta-t and despite being similar in output to the original, their output will drop relative to the iron rads because their average temperature is significantly lower.

    If you pipe such that far higher than required flow is allowed through the panels in an attempt to duplicate the delta-t in the rest of the radiators, you'll be bitten in the **** because the panels have less mass and hold less water and consequently have less heat to deliver after the thermostat is satisfied and circulation stops.

    I know of one and only one way to "zone" an original gravity system in a typical residence serving multiple floors without a COMPLETE re-pipe: Thermostatic Radiator Valves (TRVs). Those things someone told you can be wasteful... Again, I won't quibble but only say that they are ignorant of gravity systems both before and after conversion. This is ESPECIALLY true if you're trying to mix fairly compatible (say standing iron and panel rads) in the same system.

    TRVs are the highest value control device I have ever seen, but to get the greatest value in a converted gravity system you should also use a condensing/modulating boiler--preferrably using only a SINGLE circulator, just as you have now. Combined, fuel savings in the order of 50% can be expected in a typical gravity conversion system while comfort level increases!

    If you cannot handle the added expense NOW, my best advice remains, "Keep those iron radiators!"

  • bob young
    bob young Member Posts: 2,177
    the gospel on gravity

    Great treatise ,Mike. you covered it all
  • replacing cast iron with veha panel rad

    Thank you for all of the information... I hope you don't mind me continuing to pick your brain with a few follow up questions.

    Having been educated in physics and chemistry, I was smart enough to try to do research in advance of this conversion, and my research led me to believe that I should have some concerns about it. I wasn't happy with the answers I was getting before. My main concern originally was the once the "thermostat is satisfied and the circulation stops" issue. Not only is there less water, but also the specific heat of the material is less, both factors translating to less energy (heat) transfer to the room. This is how I came up with the theory that a lower water temperature would lead to a more frequent circulation, and possibly minimize the heat transfer issues occurring once circulation stopped. But I had no idea if that was practical or efficient use of the system.

    I was planning on outfitting the new panels with TRVs but I wasn't sure how they would help once circulation ceased, as it is my understanding that they simply restrict flow once the temp in that specific room is reached. If the temp in the specific room dropped, but circulation is not triggered by the main thermostat, there is nothing that the panel rad can do to heat back up, right?

    "Change the piping such that your new panels receive the "proper" flow for the "proper" delta-t and despite being similar in output to the original, their output will drop relative to the iron rads because their average temperature is significantly lower." I am not sure I understand this... isn't the temp of the radiator (at least while the system is circulating) mostly dictated by the temp of the water circulating (the rad wants to come to thermal equilibrium with the water)?

    When you say handle the added expense, are you referring to the added expense of TRVs or the added expense of re-pipe? If you mean re-pipe, would it require replacing every pipe in the house, or just re-piping the basement to separate it into two zones, each with its own pump, or including some sort of manifold? Do you have any idea what that would cost? If I was already thinking of replacing the ancient boiler that I have, is that something that can be a simple add on while I am at it?

    The "adjustable" thermostat that I referred to in my original...I was attempting to say (apparently I had a total mental block) programmable, as in the main thermostat with the timer to go off during the day. Random acquaintances that have had radiant heat have suggested that it takes so long for the house to heat after its been off for a few hours that the system is actually working harder than it would have just having stayed on through the day.

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


    You have a very good basic understanding, but again gravity systems (converted or not) are a very different beast than modern systems designed for forced circulation. They have some unusual characteristics, that if ignored, can result in unintended operation and poor comfort.

    I work almost exclusively with gravity conversion and have one in my own home. Mine has been updated with TRVs, a condensing/modulating boiler, true constant circulation, reset, warm weather shutdown and even a bit of additional reset compensation for solar gain. Comfort and versatility are extraordinary with fuel savings that I can only describe as phenomenal.

    You are VERY correct in that all other things equal, a lower (but still adequate) supply temperature will result in longer and/or more frequent periods of circulation when such runs on a call for heat from a traditional wall thermostat. You are also VERY correct that a TRV can do nothing when water is not circulating. You would NOT however want to add TRVs only to the new panel radiators. Why? Because I can nearly guarantee you that unless you use panels with significantly more output potential than the existing iron radiators the panels will wind up relatively undersized compared to the standing iron. When TRVs are installed selectively (not on all radiators) they are used ONLY on those that tend to overheat. In multi-floor structures (and especially 1 1/2 floor homes that have been insulated) it is almost always the upper floor rads that tend to overheat the space. TRVs can ONLY reduce output by restricting flow through a rad--since they are self-contained and non-electric they have no way to let the system know that they actually need more heat*.

    You've probably discovered in your research that radiators are very commonly sized to deliver the design load at about 180F supply temperature with a 20F delta-t. This was true in the days of gravity systems, just as it is mainly true for systems designed for forced circulation. Such conditions will probably occur (on occasion) in a well-designed forced circulation system as well as in a gravity system STILL operating under gravity, but I can guarantee that a converted gravity system (without TRVs) will NEVER see such conditions! Why? First, the system flow rate after conversion is FAR higher than under gravity--this reduces delta-t greatly. Second, the radiators are so generously sized that they don't need to be anywhere near 180F to heat the space. Even if you start the system in a very cold house (where higher rad temp will raise space temp faster), even the typical greatly oversized boiler can't produce enough heat to get the entire supply system up to 180F as the rads (in a cold space) will be liberating their energy as fast as the boiler can supply and unless you want a virtual sauna the thermostat will shut down the system well before the supply system reaches 180F. (I once tried to get my system--before TRVs--up to 180F--in mild weather yet. The more than 2x oversized boiler fired continually for more than 10 hours. The house was a sweatbox even with every window open. I gave up when supply temp reached about 165F.)

    You may now wonder why the flow rate in gravity systems is so much higher after conversion (but without TRVs). Remember how I previously said that elevation is one of the engines that drives gravity circulation? The higher a radiator above the boiler, the more the water wants to flow through it via gravity. Remember also how I said that head loss (resistance to flow) in gravity systems is effectively zero? On an absolute scale, this is true, but on a relative scale the differences are extreme. The piping serving a second floor radiator will have about twice the head loss of those on the first floor. This was a CRITICAL part of the design with the purpose being to force water to essentially force the water to flow through the lower radiators. This is why pipes serving upper floor rads are typically smaller than those serving lower floor rads. This is the exact opposite of how you design a system for forced flow. If during conversion to forced flow you could actually find a circulator that could deliver near the original gravity design flow (at effectively zero head loss), the water would now favor the LOWER radiators! The solution is to move far more water than under gravity. This adds a touch of "real" head loss to the system (more flow means higher velocity which means more restriction). This overwhelms the absolutely tiny but relatively great difference in head loss among the floors and you get reasonably balanced circulation.

    The greatly increased flow does more than add some head loss--it greatly decreases delta-t across the radiators. How? Because there is one certain way (all other things equal) to increase the output of a radiator--increase its' temperature. BUT, you need not increase the supply temperature to increase the temperature of the radiator! You can also increase its' temperature by increasing the flow rate. Increasing the flow rate reduces delta-t (the difference in temperature between supply and return of a radiator). Reduce delta-t and the average radiator temp (simple average between supply/return) increases and the now warmer rad gives off more heat! There are definite limits to this way of increasing the rads' output, but again original gravity systems are a VERY different beast. With essentially zero head loss you have a lot longer way to go before you hit limits compared to a system designed for forced flow. This is why I said that in a typical gravity conversion not only is head loss effectively zero, but delta-t itself is extremely difficult to find across any given radiator. The ONLY place you're likely to find much delta-t is across the boiler itself! I have spent countless hours measuring temps in my system (and even use datalogging of numerous temps) and can only say that these things are statement of fact.

    Once you have a decent grasp of what I've said above, you'll understand what follows. Without that basic understanding it will seem hideously complicated, counter-intuitive and rather ridiculous.

    While (ideally) some fairly minor near boiler piping changes are made when a gravity system is converted to forced flow essentially all that has occurred is that a circulator (almost always a B&G 100 or Taco 007) is thrown into the system. The wall thermostat does two things: it tells the boiler to fire and the circulator to run. The boiler's temperature control (the aquastat) is typically set to about 140F and everything works fine--as long as you don't monkey with the piping or radiators!

    Guess what though? The boiler rarely--if ever--reaches the aquastat setting even though it's much lower than the norm (180F). The thermostat is satisfied well before the boiler comes up to temp and circulation stops. This results in a rather interesting form of reset--you'll notice that the radiators automatically seem to be warmer as the outside temperature drops. While this seems like a VERY neat and virtually free way to add reset, it is FAR from efficient!!!! In fact, unless system bypass (preferrably thermostatic) was added as one of those relatively minor near-boiler piping changes, I'd be willing to wager that it actually reduces efficiency of a given boiler! (I have yet to see a converted gravity system with bypass-before I got hold of it at least...)

    By modern (forced flow) standards, this should result in an early death for the boiler from condensation damage. Why no problem? Things get VERY complicated, but just accept that converted gravity systems driven by a conventional cast iron boiler are virtually immune to condensation damage The simplest thing I can say as to why they are immune is that they are protected by adding inefficiency! Do not ever though install a copper tube boiler into a gravity system without low temperature protection--without they will try to maintain their efficiency and destroy themselves in the process--the root cause of this is, I believe, because compared to traditional cast iron, copper tube boilers have very high head loss.

    Now on to how radiators of any form deliver their heat to a space. Three forms of heat transfer: conduction, convection and radiation. While conduction (heat transfer between solids) is [supposedly] the most efficient, there is so little in any common form of radiator that you can consider it zero. Heat transfer via radiation has been and still is the most poorly understood method of heat transfer--consequently I say that it is currently impossible to reasonable determine the proportion of heat delivery among these methods in all but the most controlled of conditions. However, radiator manufacturers LONG AGO learned that they could play with the balance via the general design of the radiator. In general, the more massive the emitter the better it is at radiating, but that's FAR from the whole story. What can be easily proven is that, all else equal, the proportion of radiant to convective output increases as the temperature of the emitter decreases. Don't forget that the total output is dropping as the temp of the emitter falls--just remember that as this happens, a greater portion of its output is via radiation.

    When your thermostat is satisfied and the circulator stops, the mass of the iron radiators as well as their high water content ensure that not only is abundant energy available to liberate, but that as the rad cools, more and more is delivered via radiation. This is EXACTLY why hot water systems using standing iron radiators are so widely recognized for delivering steady, cozy, even heat.

    Aesthetics aside, the ONLY complaint I have ever heard about cast iron rads in a well-functioning system is their physical presence!

    There are two ways around this problem--make the radiator "invisible"--say by using the floors, walls or ceilings, or reduce its size by increasing its proportion of convection. Fin-tube baseboard is the current ultimate in mass and size reduction. If you consider that the tube running through it is the very same size as the tubes supplying and returning, it has zero water content and especially with aluminum fins it has extraordinarily little mass! Once the boiler stops firing and regardless of whether or not the circulator is running, fin tube rads deliver all of the available heat extremely quickly. This is why fin-tube radiators are UTTERLY incompatible with standing iron in the same zone and poorly compatible even in the same system.

    Modern steel panel radiators fall somewhere in between (mass and water volume wise) standing iron rads and fin tube. There are however different forms of panel rads. While all are more compact than standing iron rads of similar rating (AT THE SAME TEMPERATURE) some are designed to be even more compact by increasing convection by essentially sandwiching fins between panels. In your case and no matter what you do, you want to avoid mixing "convection enhanced" panels into your system. They more they are designed for enhanced convection, the less compatible they are with your iron rads. But guess what? If you're trying to save space by removing that big iron rad you've likely chosen a convection enhanced model by virtue of its relative compactness!

    By now, I hope that you have thought, "I'll just keep the circulator running almost all the time. When the boiler stops firing the circulator will keep moving the heat through the system. It won't have time to sit and slowly deliver itself via the iron rads. Instead it will keep it available to the new panels." With properly sized (and such is FAR from straight-forward) panels, this will work. Unfortunately though there is a SERIOUS problem in a gravity conversion system. Doing this will turn your boiler into the most efficient radiator in the house with the VAST majority of its ouput going straight up the flue! If you think, I'll just add an automatic draft damper to the boiler, think again. They are short lived, trouble prone and not particularly effective.

    Simply running a circulator constantly is NOT true constant circulation. TRUE constant circulation ONLY occurs when you are also, and as constantly as possible, deliving energy (via the boiler) at a level slightly more than adequate to meet the demand on a real-time basis. Believe it or not, the ORIGINAL, solid fueled boiler in your original gravity system did just that as long as someone did a decent job of tending the fire and adjusting the draft. The fanciest gravity systems even included automatic draft control! Gravity systems are INHERENTLY modulating. A "modern" boiler, "modern" on-off wall thermostat, and "modern" circulator DESTROYED this ability to the detriment of efficiency! Sometimes new really is old--it just took decades and extraordinary engineering to replicate the best of the old solid fueled boilers without the inconvenience of tending the fire! Better yet, such boilers are FAR more efficient than those old iron beasts and with the possible exception of high mass radiant floors/walls/ceilings, gravity conversions are the best possible place to use a TRULY modern modulating AND condensing boiler--ESPECIALLY if you restore the original MODULATING FLOW via TRVs on ALL emitters. In "ideal" conditions (which at least in my location occurs FAR more often than I ever believed possible) overall system efficiency comes so kissingly close to 100% that inefficiency--just like head loss in a gravity system--becomes inconsequential. Even better, TRUE SEASONAL EFFICIENCY IN THE REAL WORLD is at LEAST that of the idealized, laboratory-based AFUE. I will GUARANTEE you that a conventional cast iron boiler driving a converted gravity system of just your sort can appear to operate at it's AFUE rating until you try to look at SYSTEM efficiency. The difference between the two is HIDEOUS waste going out the flue with the ONLY positive being that such boilers can last seemingly forever when driving that old system.

    Now, finally, I can get to your specific situation. With a panel rad of appropriate design and size, I'd say there's a good chance you wouldn't have any problem in the kitchen as long as you follow monkey-see, monkey-do piping. Why? Because kitchens have lots of internal heat gain. The refrigerator after all only cools by moving heat into the room...

    The upstairs bedroom however (and why do I suspect it's the master bedroom) is another case. Even if you--like many--prefer to "sleep cool" I can nearly guarantee that balance problems will result in every other room in the house being relatively warm.

    Now, specific suggestions:

    1) If in any way possible, keep your iron radiators! This holds true no matter what!

    2) Don't even consider a complete re-pipe. By the way, you CANNOT do such by merely replacing the main pipes and creating zones. When I say complete, I mean COMPLETE!!!! Your branch piping will STILL be sized for gravity! Unless your water is unusually aggressive or your system is leaking, the life of your piping is probably best measured in centuries.

    3) If you must replace both standing iron rads with panels, install TRVs on ALL radiators and replace your boiler with a condensing/modulating version of the finest quality you can possibly afford. At present, and provided that your Manual-J heat loss is at least 80,000 btu/hr, I would suggest that the Viessmann Vitodens 200 is the best choice for a fully TRVd gravity conversion. While also the most expensive, it is virtually "plug and play" into such a system and by its very philosophy of design will deliver the utmost of efficiency.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928
    The Asterick

    I said, "TRVs can ONLY reduce output by restricting flow through a rad--since they are self-contained and non-electric they have no way to let the system know that they actually need more heat*."

    Why the asterick?

    Because in some conditions TRVs in a FULLY TRV'd system CAN increase the output of their connected radiator and with a directly connected modulating boiler they can actually increase boiler output almost instantly even though they have no electrical connection.

    To understand how this is possible, you'll not only need an extraordinary imagination, but you'll have to understand everything in my previous message. I like to believe that I have such an imagination, but it has taken me years, countless hours of observations/measurement and thousands of dollars of "senseless" equipment.

    This ability occurs when a FULLY TRV'd, TRULY CONSTANTLY CIRCULATING MODULATING BOILER system has achieved SYSTEM-WIDE balance with each and every space being maintained at the desired room temperature. Such takes at least two full solar cycles with ZERO changes to TRV settings and I suspect can only be truly realized when a single circulator is used for the entire system.

    In such condition the system is in a dynamic state of steady balance. (How's that for seeming contradiction?) Even the movement of the sun will have little affect on the balance as long as your emitters aren't excessively massive relative to the solar gain in the space. (Usually not a problem with cast iron rads--they seem to be just massive enough...)

    Once such state of balance is achieved, if you "crank" a TRV its resistance to flow will drop markedly and can temporarily rob flow from other radiators because its Delta-p (change in PRESSURE) has relatively dropped. As a conseqence, Delta-t across that rad will decrease and its output will increase while delta-t in the remaining rads increases as their output decreases. Why? Because the change in delta-p in the remaining rads will lag behind the change in delta-t required to maintain the desired setting. (Again another concept VERY hard to imagine until you've measured the effect.)

    Now I have to add the term that I consider most important to efficiency with a modulating boiler driving a TRVd (proportional flow) system: Heat Authority

    Heat Authority is nothing but the ADDITIONAL energy available to a system in excess of its' real-time requirement once it has achieved balance.

    The heat authority will be available almost exclusively to "cranked" TRV(s) with such authority limited only by the boiler's ability to provide additional heat at the level of authority allowed by the current reset curve.

    If your reset curve is optimized for ultimate efficiency which I will define as allowing heat authority as close to zero as possible for your general climate and emitters only the general construction and occupant desires remain as variables. The more massive the shell (say solid masonry compared to stick built) the less heat authority you need to meet occupant desires efficiently.

    To finally answer you last question regarding "people with 'radiant' heating tell me it's more efficient to keep the inside temp fairly steady" I can only say that they, like me, have learned by experience.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Yes, I am abnormally psychic. Why? Because I believe that I understand the essence of radiation. At moments I can actually see it.

    I cannot offer any suggestion regarding a contractor other than to use "Find a Contractor" at this site.
  • Always an educational experience here

    So the Swampeast is not a place with swamp gas causing these visions of water flow and radiation ;-}
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    It might be the summer weather. I've searched and searched for comparables, but out of my own ignorance I forgot that the Eastern hemisphere of this big ball existed!

    Beijing, China IS the comparable and unfortunately it would best be compared to St. Louis about 1880. Yes, there is a natural haze, but when the crap that's being spewed into the atmosphere can't make its way out by virtue of being trapped by a dome of high pressure* what you see and breath is FAR from natural.

    *Yet another seeming contradiction. When high pressure naturally moves to low how can high pressure trap the crap?
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Before I answer you questions, let me explain a bit about what TRVs do to a gravity conversion system.

    Provided you use true constant circulation and a varying supply temperature--the colder the weather, the warmer the supply, TRVs on ALL radiators will restore the original flow rates in the system. Not only will be flow rate be similar, but the system will effectively act just as it did with a solid fueled boiler. With a properly tended fire and draft, gravity systems worked just like the most modern hydronic system with both supply temperature reset AND variable flow. The colder the weather, the bigger the fire and the higher the circulation rate.

    TRVs are the ONLY way to restore this type of operation because the system again, when converted to forced circulation and with no restriction to flow in the system
    the flow rate is greatly increased at all times.

    Anyone who has ever tried to balance a gravity conversion system with the original hand valves knows that such
    is worthless cause. Even the tiny little hole that helped prevent freeze-ups if the valve was shut off in cold weather often provides enough flow to heat the radiator!
    The valve adjustment is hideously finicky and even if you do manage to get everything adjusted nicely, moving a single valve in the system will throw of the others!

    TRVs handle the adjustment of all those hand valves continuously and automatically. Even better (for an original gravity system) they add a "base" level of flow
    restriction to each and every radiator. With zero restriction in the piping and the radiators, the TRVs are themselves providing all of the restriction. While the
    "base" restriction is usually quite slight (it's partly reflected in the cV value of TRVs--the lower the number the less the base restriction) it is significantly
    higher than the existing, intentional imbalance in piping that finds higher, farther radiators using smaller pipe than lower, closer radiators. (Unless smaller than most, the lowest, closest radiator in your system will probably have the largest hand valve and branch piping!)

    With the TRVs themselves providing the only and effectively idential restriction in the piping and emitter system, they operate under IDEAL conditions. They respond rapidly and accurately and will prevent any radiator or group of radiators from "hogging" flow should operators be cranked and irregardless of the wide open flow potential of the circulator!. It doesn't matter if your circulator can deliver 6 or 26 gallons per minute with all TRVs wide open!

    To maintain this ability you must follow "monkey see, monkey do piping practices". Don't even think of touching the mains. When modifying branch piping, use the existing size throughout (or as close as possible to say a modern panel radiator whose connections are smaller). If making new runs, use similar to what you see. Use black pipe and brass connections to emitters. Not only does this eliminate the possibility of galvanic corrosion, but it looks far more professional, will have an indefinite service life and with copper tube sky high it is cost competitive when done by an experienced pro and a relative bargain if you DIY.


    1) With a properly sized panel rad of appropriate design in the kitchen, I would say you could get by without making any other changes to the system. DO NOT use a TRV on the new panel unless you are positive that the kitchen radiator will be the MOST oversized in the entire system! (Nearly impossible. Old houses with gravity systems usually didn't have a radiator in the kitchen! Why? Because the big, iron, solid fueled cookstove gave PLENTY of heat! Bigger kitchens had bigger cookstoves... When the cookstove went and a radiator came in, it was very likely already somewhat undersized relative to the rest in the system. When considering a replacement and unless you have first-hand experience with the system to the contrary, I would assume that the existing rads are never more than 140F in any reasonable weather or user desire condition. Calculate heat loss room-by-room, normalize output to heat requirement at 140F for each room, determine a reasonable average degree of existing radiator oversizing deducting about 15% because you're in a kitchen and choose a panel rated for the required output at 140F. I would GREATLY prefer "plain" panels as opposed to those designed to enchance convection. You may very well have just as hard a time finding a place to fit the replacement as the original! If you existing rad is rather short, consider finding a standing iron replacement (used or new) of 38" height and similar EDR.

    2) No matter what you do, there is no need to re-pipe the system. Even if you replace ALL of the rads with panels at once--keep the piping and use TRVs everywhere. If for any reason you feel compelled to replace the main piping, replace the branch piping as well--even if you have TRVs everywhere!

    3) The Triange Prestige boiler would be a well-suited for the sort of system I would propose you choose WHENEVER you replace the boiler in your (or any unmolested) gravity system. SINGLE circulator directly connected to the FULLY TRVd emitters via the boiler. The control system is not as well suited as that in the Vitodens 200, but it should prove quite acceptable. The Presitige is known for low head loss in the heat exchanger. This is a VERY relative thing--most (all?) other mod-cons have quite high restriction through their HX. (Effective infinite high when comparing to gravity systems with old cast iron beasts...) The Presitige would be just high in restriction and you will be able to find a number of well-suited modern, wet rotor circulators that cannot normally be used in a gravity conversion system. You can even use one of the most modern, extraordinarily efficient DC circulators but you CANNOT at present take advantage of the built-in variable speed control in most--such would have to be disabled. The Vitodens 200 is the only condensing/modulating boiler I am aware of that can directly drive a system via a variable speed circulator. Why? Because the circulator is built-in and under control of the boiler itself.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928
    italic fix

    sorry Can't edit message.
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