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Thread: Cast iron radiators and Takagi TH 2

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    DIY Junior Member Beekmansup's Avatar
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    Default Cast iron radiators and Takagi TH 2

    I have a house with very inefficient gas boiler, one 1” one way single loop pipe and 7 cast iron radiators. I wanted to replace this system with a radiant open system for the reason that is the most efficient system. I bought all the supply necessary to build that system including 2 Takagi TH2 high efficiency water heaters, but then I found that the city doesn’t aloud this kind of system for the legionnaire bacteria concern. Those cast iron radiators can work very nice if I have a decent efficient boiler. My question is: can I use this Takagi TH 2 which is 90% efficient with the existing one way T connectors for every ˝ “ risers and cast iron boilers and new mechanicals and controllers for single zone? I heard that instant water heater are not like heat boilers to accept hot water for intake but Takagi use them also for heating applications because it can go to 180F.Does anybody has any experience for cast iron radiators and TH 2 water heater?

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Tankless systems and boilers designed for heating are similar but not exactly the same. Personally, I think you'd get better reliability and easier installation using a boiler for your heating both the house and the hot water. Hooked to an indirect tank, a modern high-efficiency boiler makes great use of the energy for both purposes...a tankless may not. Some tankless systems void their warranty when used for space heating. Again, personal opinion, while some people think they work great, an open system has some pitfalls. One, especially when used with cast iron radiators is that you'd rust them out. An open system is constantly adding oxygen to the system in the incoming water supply - guess what, that will rust out your radiators. A closed system gets (ideally) filled once, and shortly, all the oxygen is used up, then, the water becomes relatively inert.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    In the trades Dana's Avatar
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    What Jim said about using potable water in cast-iron radiators- you'll ruin both the Takagi and the radiators going that route.

    It's possible to design a system around it using heat exchanger keeping the heating loop isolated from the potable, with a separate bronze-impeller pump in a short loop on the Takagi side and a standard iron hydronic pump on the isolated heating-system water side. You'll have to do the math to get it right, but it's possible to get decent efficiency out of it if you do.

    Do you have ANY idea what temperature water you would need to handle the heating load at +7F (the 99th percentile design temp for Wilkes Barre)?

    Do you have any idea what the whole house heat load is at design temp? (We can work backwards from fuel-use on the old system, if we have to.)

    With some idea as to the load and the amount of radiation you have out there we can probably thumbnail a system that would be close enough to work with some judicious tweaking.

    Some Takagi models auto-kill the flame when the return water gets up to about 130F- not sure if that's an issue with this model. No matter what, if you're running temps higher than that for return water you won't be getting anything like 90% efficiency out of it. Yes, you can put 180F out, but if you're constrained to 130F max return you have to design the flows very carefully. But if the TH2 is programmed for 180F output it's useless as a hot water heater, since you'd have to use a tempering valve at the output to mitigate scald risk, and the flow through the tankless at low DHW flows may not be enough to trigger ignition or regulate the output temp well Ideally you'd have sufficient radiator to run the thing with 130F water OUT, which would mean the return water temp would stay in the condensing range, and you won't have to fight to make it work as a hot water heater too.

    I used a KD-20 as the "boiler" on my single-temp ~125F combi system utilizing a "reverse indirect" as the heat exchanger for the potable side, running heating-system water through the tankless, but that's a pretty pricey heat exchanger if you don't need the thermal mass for buffering micro-zones (which is why I took that approach.) If you did something similar you'd be well advised to put a filter on the loop to keep iron crud out out of the tankless too- old iron heating systems can have a LOT of crud in them and work fine with a cast-iron beastie boiler, but not so much with a condensing tankless. But I did the math first rather than going with "hack & hope". If you have enough radiation to get your design day water temp requirements down to ~125F or so you might hit 90% with a TH2 and a plate type heat exchanger, but not if you need higher temps than that.

    Did you in fact install the radiant? If yes, please describe.

    You should also do a room-by-room heat loss calc based on a +7F outside temp, and use some reference like this to see if the radiators in each room can cut it on their own with 120-130F water, or if you would need more radiation.

    Condensing boilers have the advantage of being able to adjust the output temp automatically with changing outdoor temperature (called "outdoor reset" in the trade), but a tankless does not, so the only way you can get the efficiency out of a tankless is to have enough radiation to run it in condensing mode under all load conditions. You can then design/adjust the loop flows for long low-fire burns that match the design-condition load for output, and the thing won't short-cycle itself to death. But if it turns out you actually NEED 140F or higher water to meet the load at design condition, you'd be far better off with a boiler with outdoor-reset capabilities, and tweak the reset curve to the lowest temp that meets the load.

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    DIY Junior Member Beekmansup's Avatar
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    Thank you for your help guys. I decided to go with a Burnham ES2-4, 77,000 BTU and my cast iron radiators and an Outdoor Reset Module to make this 85% efficient boiler to save more gas and I will line the chimney with 5" ss liner and build a box around the boiler with a duct of filtered outside air. The TH2 I will use it for radiant floors in the joists and hot water with a mixing valve. I'm very impress about your knowledge in heating systems Dana and I appreciate any input from you regarding my project. Thank you.

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    In the trades Dana's Avatar
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    The Burnham ES2-4, is a bit more than 2x oversized for my whole-house heat load @ design condition (my outside design temp slightly cooler than Scranton/Wilkes-Barre, but comparable.) How did you choose that boiler?

    With part of the house heated by the Takagi the load on the cast-iron boiler will be less than the whole house load.

    A room-by-room heat load calculation is called for, but short of that, as a starting point, using the fuel use of the previous system it's possible to put some firm stakes in the ground as to the actual size of the whole house load. If you have a winter-season gas bill complete with the end dates of the billing period it's possible to look up the heating degree days (HDD) during the billing period. (Or if the billing has an mean-temperature for the billing period and a daily-use average it's even easier.) From therms/HDD it's simple arithmetic to come up with a source-fuel BTU/hour per degree F below the heating degree-day base (65F is an appropriate base for most homes). Then de-rate by the boilers name-plate efficiency fpr a net BTU/hr-degree, and multiply by the difference between design temp (+7F for Scranton/Wilkes-Barre) and 65F. Unless you leave multiple windows open all winter or have a great deal of single-pane window area (without storms) I'd be surprised if your heat load at +7F is anywhere near 77KBTU/hr.

    But the heating fuel bills will tell all. Unlike heat load calculation methodologies based on construction types and presumed U & R values, fuel-use against degree-day data is a heat load MEASUREMENT.

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    DIY Junior Member Beekmansup's Avatar
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    Thank you,Dana.This weekend I will go to that house in PA and measures all the SF of radiators to make a clean calculation of the heat loss. The problem is, I like to change the old boiler and mechanicals with a new more efficient cast iron boiler and mechanicals and keep the old radiators( 7pc.) connected in single one way iron pipe, but so far I didn’t find a satisfactory boiler, a blue print of mechanicals and electrical connections. There are some efficient cast iron boilers up to 90% but they have a confusing data of how to install them especially when it comes to an outdoor reset controller. If anyone have any idea where I can find such information will be much appreciated. I have to postpone the radiant system because I have to renovate the house. The radiant is useless where you have the cabinets and big furniture and king size beds so the old cast iron radiators will be the primary heating system and the radiant I will install later in the exposed no covered joists, mainly in the living and dining rooms. So right now I’m looking for an up to 90% efficient cast iron boiler and find a good diagram for mechanical and electrical connections which includes the outdoor reset device. The only one I found is the Burnham ES2-3 51000BTU and it has an outdoor reset module but the boiler controller doesn’t have an output for the outside circulator pump and have no idea how that circulator will be connected and controlled by the boiler controller board. Is there anyone who can help? Thank you.
    Last edited by Beekmansup; 04-19-2012 at 09:59 AM.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    If a boiler doesn't already have on-board multizone controllers, the most common way to deal with it is to add something like a Taco zone controller box. This OR's the theromstats for the zones into one on/off call for heat, and powers the circulators through on-board relays. The outdoor reset monitors the outside temp, and usually, the return water temp to the boiler, and uses that to decide what temp supply water needs to be which is different than turning the circulators on/off for the zones needing heat.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    In the trades Dana's Avatar
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    Quote Originally Posted by Beekmansup View Post
    Thank you,Dana.This weekend I will go to that house in PA and measures all the SF of radiators to make a clean calculation of the heat loss. The problem is, I like to change the old boiler and mechanicals with a new more efficient cast iron boiler and mechanicals and keep the old radiators( 7pc.) connected in single one way iron pipe, but so far I didn’t find a satisfactory boiler, a blue print of mechanicals and electrical connections. There are some efficient cast iron boilers up to 90% but they have a confusing data of how to install them especially when it comes to an outdoor reset controller. If anyone have any idea where I can find such information will be much appreciated. I have to postpone the radiant system because I have to renovate the house. The radiant is useless where you have the cabinets and big furniture and king size beds so the old cast iron radiators will be the primary heating system and the radiant I will install later in the exposed no covered joists, mainly in the living and dining rooms. So right now I’m looking for an up to 90% efficient cast iron boiler and find a good diagram for mechanical and electrical connections which includes the outdoor reset device. The only one I found is the Burnham ES2-3 51000BTU and it has an outdoor reset module but the boiler controller doesn’t have an output for the outside circulator pump and have no idea how that circulator will be connected and controlled by the boiler controller board. Is there anyone who can help? Thank you.
    The size/surface area of the radiation is of absolutely no use in determining the heat loss of the house (or room) at design temperature- NONE!

    Fuel use over a particular known period where the weather data can be correlated is a HUGE help in determining the maximum BTU output that would be required of the boiler, and the smallest boiler that meets that load will run the most efficiently, all else being equal.

    If fuel use data isn't available, a Manual-J or IBR type of heat loss calculation can get you there, and that is standard GOOD heating professionals would use. Those methods work by calculating the steady-state heat loss out of all exterior surfaces based on insulation & construction type, window type & are at a fixed difference in interior & outside temperatures. Typically 70F is used for the interior temp, and the local 99th percentile outside temp should be the lowest used for outside temp. Even though the 25 year peak lows might be 5-10F lower, it doesn't dwell there long enough to matter- the thermal mass of the house carries it through without a loss of comfort to the occupants.

    Doing the heat loss calc on a room-by-room basis, and correlating the size of the radiation in each room to that room's heat loss is what determines the water temp requirements for meeting the load. Most existing hydronic systems with a 180F nominal water temp are at least 2x over-designed for the true heat load, and will in fact meet the true heat load at design temp with 140F water. If that's true of your system you would be able to run a condensing boiler in condensing mode nearly 100% of the time if you set up the outdoor reset curves correctly.

    Outdoor reset is somewhat overrated for use with cast-iron boilers if the boiler is sized correctly for the load, and even more so when you have excess radiation that can deliver design-day heat at 140F or less. Since gas-fired cast iron boilers will need return water entering the boiler to be over 130F to avoid destructive condensation, simply setting it up with a thermostatic mixing valve on a system or boiler bypass plumbing to keep the return water to the boiler at 130-135F and letting it fire until the thermostat is satisfied it'll meet it's AFUE numbers, and the temp at the radiators may never exceed 140F in any cycle. Only if the radiation truly NEEDS 180F or something at design condition will outdoor reset deliver much in the way of enhanced efficiency or comfort. If the boiler is way oversized for the load there can be efficiency gains with outdoor reset, but in those cases a heat-purging smart controller would usually do slightly better.

    Outdoor reset mixing valves etc can make for a more even radiator & room temp though, which is more of a comfort issue than a system efficiency issue.

    If you're not up to doing your own Manual-J type heat loss calc, there are contractors out there capable of doing that, as well as specifying the hydronic boiler/system approach that makes sense for the amount & type of radiation you have. Spending some money up front for a real design can save you from buying unnecessary or inappropriate equipment that ends up costing you even more in reduced efficiency over it's lifespan. Without first calculating the heat loss of the house at design temp (which is NOT equal to the heat output of the radiators with 180F water, or at least not very often) it's all just a shot in the dark.

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    DIY Junior Member Beekmansup's Avatar
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    I did contact a friend plumber (he doesn't have college degree in plumbing) for a loss heat calculation and he told me that an accurate +/- 3% LHC will be $150 to find the correct size of a boiler but he appoint me to this BTU calculator which have a +/- 10% accuracy who give me 40,342 BTUs. The monster I have right now has a 90,000 BTUs output (calculated and installed by UGY gas Co. in 1988) and the open flue chimney efficiency. You don't have to be a boiler scientist to see that I was spending a lot of $ heating the air around the house. I know you have more experience (or education?) then more of us but please Dana let me know what do you think about this. Thank you.

    BTU Calculator
    1. Which best describes your heating system?

    High Temperature Heat (e.g. Baseboard, Fan Coils, Radiators)

    2. What is the square footage of your home (the area to be heated)?
    815 Sq.ft.

    3. Location: Please select the city closest to your home.
    • Pennsylvania


    • Scranton



    4. How old is your home?


    More than 50 years old

    5. Renovations: Have you made significant renovations to your home?


    No, my house has not had major renovations since the original construction.



    Your estimated BTU requirement is: 40,342 BTUs
    Last edited by Beekmansup; 04-20-2012 at 04:32 PM.

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