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    Scott's radiant heating project (45 Posts)

  • Scott_Mountain_View_CA Scott_Mountain_View_CA @ 12:29 PM
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    Scott's radiant heating project


    I’m designing and installing a wall radiator hydronic system in my home and am hoping to get some advice along the way from the pros on this forum. I have an extensive background in kitchen/bath remodeling and have adequate plumbing skills. I’ve done a considerable amount of reading on the subject of hydronics, and recently purchased “Modern Hydronic Heating” by Siegenthaler as a reference. A good friend with 20 years experience installing hydronic systems is helping with the project. The area where I need the most help is system design and pipe sizing.

    My home is 1,550 sq feet, single story SOG with a T&G built up roof. I live in the San Francisco Bay Area (warm summers, mild winters) and my house is very well insulated including dual glaze windows. In short, my heating requirements are minimal. I’ve done a preliminary heat los calc using Slant Fin’s Heat Loss Explorer software. I plan to attempt a more thorough heat loss calc using the MHH book as a reference.

    I have a 1960’s AO Smith Copper boiler that was installed when the house was built and is still running strong. Here’s the rub: the boiler has 80,000 BTU output @180 degrees, far in excess of my needs. I'm fully aware that using this oversized and inefficient boiler will result in frequent short-cycling. I also understand that frequent short-cycling will increase my fuel consumption. I’m selling this house in 18 months and am not interested in spending $5K + to upgrade the boiler at this time unless presented with evidence that keeping the over-sized boiler will cost more than the extra few hundred dollars/year in natural gas expense. It has been suggested that I install a buffer tank to minimize the short-cycling. I would be open to any suggestions on how to improve efficiency using this boiler.

    The preliminary plan is a simple no-manifold 2 loop system (one loop for the front rooms, another loop for rear bedrooms). I’m installing wall mount Runtal rads with TRV’s. I’m not interested in series looping, and am deciding between parallel direct return or reverse return piping. Since I have no crawl space, all piping will have visible exposure, either over the roof or under the eaves (with pipe insulation of course). I'm leaning toward direct return to minimize the copper.

    I’ll be posting more after I've had time to review the MHH book, which will no doubt raise more questions.

    Thank you.
    This post was edited by an admin on February 20, 2014 12:31 PM.
  • RobG RobG @ 1:52 PM
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    What type of heating system is in there now? What was your heat loss? Exposed piping? This must not be a luxury home?

  • kcopp kcopp @ 2:35 PM
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    A simple wall...

    mount combi would work. Don't have to go 95%... Biasi makes one version.
    This post was edited by an admin on February 20, 2014 2:35 PM.
  • Scott_Mountain_View_CA Scott_Mountain_View_CA @ 4:32 PM
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    Update: Biasi Riva NOT available outside New England (I'm in Calif)

    "A simple wall mount combi would work. Don't have to go 95%... Biasi makes one version."

    You got my attention....the Riva is under $2.5K, and modulates as low as 37K BTUH. This sounds like a more affordable option to the higher priced, high efficiency boilers I had been looking at.

    Where is a good place to purchase one if i go that route? I have a contractor's license, would want wholesale pricing so retail web stores may not be my best choice.

    Thank you.

    Edit: I just called the importer, QHT, and the Biasi is available in New England only; I'm in California so not an option.
    This post was edited by an admin on February 20, 2014 4:49 PM.
  • Scott_Mountain_View_CA Scott_Mountain_View_CA @ 3:30 PM
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    Response to Rob

    "What type of heating system is in there now?"

    Right now there is no heating system. When originally built in the early 60's, the house was heated with floor radiant galvanized pipe embedded in the slab. Some years later, before I purchased the house, the floor system failed and was replaced with a fin-tube baseboard system, using the original AO Smith boiler. I recently remodeled the house and removed the old, rusty baseboard units and am installing the Runtal wall system.

    "What was your heat loss?"

    Using the SlantFin Heat Loss Explorer software, I came up with 20 BTUH/sq foot, or roughly 30,000 for the entire house.

    "Exposed piping? This must not be a luxury home?"

    I gather you've never worked on a SOG T&G home with neither a crawl space or attic? When I say "exposed piping" I'm referring to the need to either run the pipe across the roof and into the individual rooms, or along the outer wall under the eaves and into/out of each room. The roof piping would of course be insulated, as would the under-eave piping (contained within an insulated soffit). There are really no other choices to get the heat from the boiler room to the radiators. It has nothing to do with whether or not this is a 'luxury home". It has everything to do with the type of construction.
    This post was edited by an admin on February 20, 2014 3:31 PM.
  • Zman Zman @ 3:23 PM
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    Good call on the book.
    A home run or reverse return system is going to be much easier to balance.
    I would not even consider copper especially on a remodel. You can run hepex or pex-al-pex cheaper and easier.In a homerun system you would likely use 1/2"  or even 3/8" lines. Maybe you could hide the behind a baseboard.
    You should replace the boiler especially if you are selling the home. That 50 year old R2D2 unit is going to cost you much more than $5k once the home inspector makes his report. A new high efficient heat system is a great selling point.

  • Scott_Mountain_View_CA Scott_Mountain_View_CA @ 3:46 PM
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    "A home run or reverse return system is going to be much easier to balance."

    True, but wouldn't TRV's do the same job using a direct return?

    "I would not even consider copper especially on a remodel. You can run hepex or pex-al-pex cheaper and easier.In a homerun system you would likely use 1/2" or even 3/8" lines."

    I'd love to avoid copper...I'll research the pex solutions. One question is the 1/2" size....will this deliver the volume of water needed for my system? Seems too small. I was looking at a primary pipe 3/4" or even 1".

    "Maybe you could hide the behind a baseboard."

    Not sure what that means?

    "You should replace the boiler especially if you are selling the home. That 50 year old R2D2 unit is going to cost you much more than $5k once the home inspector makes his report. A new high efficient heat system is a great selling point."

    LOL...I knew I was going to get flack on the boiler. But here's the deal: That boiler is in good working condition; it's the predominant boiler in homes that sell in my neighborhood. I've lived here 28 years and know many people that have sold, not a single mention of any inspection issues. I do plan to pay a boiler expert in the area, a guy who has been working on AO Smiths for 40 years, to clean up and replace any components as needed. Yeah, I'm sure some buyers would appreciate a new high efficiency boiler but they can do the upgrade after I sell....I'm not in a position to commit $5,000+ cash to replace a working boiler, albeit a highly inefficient one. I would consider adding any components that improves efficiency, such as a buffer tank.
    This post was edited by an admin on February 20, 2014 3:47 PM.
  • Zman Zman @ 4:49 PM
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    You will understand the pipe sizing thing once you get the book.
    Trv's will not balance your flow, they will make the rooms heat more evenly. Again the book will explain.
    You could hide the pex tubing inside an oversized wood baseboard or crown molding. I hate to see it outside the heating envelope.
    R2D2 is cute. You would save about 40% with properly sized panel radiator and a condensing boiler. Take that cute little guy and plant him in the front yard!
    Be sure to stock up on Tylenol, Modern Hydronic Heating may make your head hurt.
  • Scott_Mountain_View_CA Scott_Mountain_View_CA @ 4:57 PM
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    Zman question

    "You would save about 40% with properly sized panel radiator and a condensing boiler."

    Could you elaborate? 40% savings on what? Monthly Nat Gas bill?

    Also, wouldn't the "properly sized panel radiator" be a function of the heat loss + water temp? I've been advised that I should size my panels based on 150 degree water so that when the boiler is eventually replaced the rads will be properly sized. I'm sure my thinking is misguided and this will serve as yet another learning opportunity, one of many, many more to come. :)
  • Scott_Mountain_View_CA Scott_Mountain_View_CA @ 5:12 PM
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    Any suggestions on inexpensive wall mount gas fired boiler for 30K BTUH requirement?

    Looking for nat gas powered alternative to "R2D2". Yuk, yuk.
  • Rich Rich @ 9:23 PM
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    30 K

    Cadet or HTP MC Series . Low bottom end modulation , both are inexpensive as far as wall hung mod cons go .
    You didn't get what you didn't pay for and it will never be what you thought it would
  • Zman Zman @ 5:35 PM
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    Yuk Yuk

    I would be sizing the panel radiators for 130 to 140 degrees on your design day . I would also use outdoor reset. This will allow the condensing boiler to condense all the time. You generally need a return water temp around 135 to get the boiler in condensing mode.

    Your existing boiler is likely running a net efficiency of about 50%. Partly because it is an oversized single stage and partly because of the massive amount of heat loss out the vent.The new boiler should get you in the 90% range.

    It is hard to find a boiler that small. I really like the triangle tube and lochivar with the firetube design, even though they don't get down that small, they will modulate 4or 5 to 1. You may end up needing a buffer tank on a system that small.

  • Scott_Mountain_View_CA Scott_Mountain_View_CA @ 6:02 PM
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    Thanks've given me some terms and concepts to research when my MHH book arrives tomorrow. I look forward to the challenge of designing my system, and am grateful for this forum and it's members for the time they devote to helping folks like me.
  • Scott_Mountain_View_CA Scott_Mountain_View_CA @ 11:11 PM
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    Rich's boiler suggestions

    "Cadet or HTP MC Series . Low bottom end modulation , both are inexpensive as far as wall hung mod cons go ."

    Found the HTC, but couldn't find anything about a "Cadet" boiler when I googled it.
  • SWEI SWEI @ 1:03 AM
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    Lochinvar Cadet  Giannoni heat exchanger, far simpler controls than the current Knight line, but attractive prices and (in the CD*040) the smallest mod/con with the lowest minimum modulation rate available in North America.
  • Scott_Mountain_View_CA Scott_Mountain_View_CA @ 7:40 PM
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    Lochinvar Cadet 40K quote

    I'm being quoted $2,350 for the Cadet 40K. Does this seem like a reasonable price? I'm tempted to bite the bullet and upgrade the boiler now....but will need guidance on the boiler installation. I'm skilled at plumbing, hopefully someone on this forum with experience with this boiler can offer some tips.
  • Rich Rich @ 8:21 PM
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    is taboo discussion here .  Dos it sound like too much to you ? The right price is whatever you are willing to pay or whatever you are willing to perform some task for . That said , it's not a bad number in my market (NYC Metro) .  HTP is a bit less expensive .
    You didn't get what you didn't pay for and it will never be what you thought it would
  • Bob Gagnon plumbing and heating Bob Gagnon plumbing and heating @ 10:47 PM
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    Don't Replace The Boiler

    The warrantee goes to the original purchaser, so in effect the new owners would be buying a new boiler with no guarantee, give them some money off the selling price, they will appreciate your honesty.

    THanks, Bob Gagnon
  • Scott_Mountain_View_CA Scott_Mountain_View_CA @ 11:00 PM
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    Bob Gagnon advice

    Bob, you make a good point. I didn't realize that the warranty applies to the original owner only. A $2.5K credit in escrow would be a better choice for the buyer. Thanks for your suggestion.

    From the Lochinvar Cadet warranty:
    "Warranty extends to the first owner as long as the
    boiler remains installed at its original place of installation."
    This post was edited by an admin on February 22, 2014 12:03 AM.
  • Zman Zman @ 12:02 AM
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    Respectfully disagree

    The new buyer is not likely to consider the warranty issue.
    The new buyer would have to pay significantly more to have a boiler installed than your cost for parts.
    I would replace the boiler and give potential buyers nothing to balk about.
  • Scott_Mountain_View_CA Scott_Mountain_View_CA @ 12:05 AM
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    ....and then there's the other side of the coin.

    I'll ask my wife, she always knows the right thing to do....she married me, right? :)
  • Zman Zman @ 12:10 AM
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    That's the smartest thing you have posted so far... and you have made some good posts.
  • Zman

    Most people won't consider it because they don't know the warrantee only extends to the original purchaser. I have gained a lot of customer appreciation when they call and are selling their house, and I forgo the boiler installation because the warrantee will end with the sale, being honest will get you customers for life. Give the buyers all the information and let them make the decision, some may not want to deal with a boiler installation, others will want a full guarantee.

    Thanks, Bob Gagnon
  • RobG RobG @ 5:01 PM
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    I'm with Z-man, purchasers don't want to worry about replacing their boiler when they buy, they want move in and decorate ready. High efficiency is also a big selling point. 
    Bob, If you were to replace the boiler and put a sticker on it with whom and when to call for service you might just gain a customer. Versus losing the one who is moving away.
    Scott, Instead of running pipes outdoors and insulating, why don't you save yourself some money and time, just remove the baseboard (trim that is). , cut 12'' of drywall up from the floor, run your pex and re-drywall the 12" all the way around and reinstall / replace the baseboard. Only one seam to drywall and you will use less pipe, not have to insulate and you will make the home ascetically more pleasing. I have always re-painted the homes I have sold in a neutral color anyway. You are going through so much effort as it is, why not do it the correct and easy way? You could probably run the piping in a day.

  • Scott_Mountain_View_CA Scott_Mountain_View_CA @ 8:43 PM
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    Water temperature question

    Runtal provides BTU/H ratings based on 180 degree water temp. They also display a table of factors to adjust BTU/H for different water temps.

    I checked both the Lochinvar Cadet and HTP MC series specs and see nothing about water temp.

    What am I missing?
    This post was edited by an admin on March 13, 2014 8:56 PM.
  • Zman Zman @ 9:26 PM
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    You can run a  condensing boiler  as low a temp as you like.
    130 or lower is good target on your design day. You will want to use outdoor reset as well.‎
  • Rich Rich @ 9:27 PM
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    will make any temp water you want , they do not mention temp too much . The lower the temps the more it condenses , the more efficient it is , Pretty simple .
      Now panel rads on the other hand give more BTUs with higher water temps , What to do ?   Buying the larger rads to deliver the BTUs you need at a lower temp is the ticket . The price difference on these rads is not that great as the amount of money you will waste buying a Mod Con that doesn't Con . Depending on the rads you use sizing can get challenging and very cerebral . I use Buderus for the most part , easy sizing , no confusion , 2 sizes (depth) . Most of the time because of size limitations by customers , interior designers and the like I end up sizing for 158* with an 18 degree Delta T for design days and utilizing outdoor reset so that the boiler will condense for 80-90% of the heating season  
     Only my opinion , I am sure others will differ , but not by much
    You didn't get what you didn't pay for and it will never be what you thought it would
  • Scott_Mountain_View_CA Scott_Mountain_View_CA @ 9:40 PM
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    ModCons question

    You are implying that one can select an operational water temp....a thermostatic control on the modcon? Why would the mfg not spec that? Couldn't find anything on water temp for those models.

    Should my Runtal BTU/H be based on 180? Or 150? I'm still looking for a thermal spec. on the modcons.
  • Zman Zman @ 1:47 AM
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    You can set any mod con boiler on the market today to whatever temperature you want. In order to achieve the efficiency they are designed for you need a low return water temp and an outdoor reset curve. I would size the radiators to 130 degrees max on the design day.
    What happened to the modern hydronic heating book?
    This is all in there.
  • J Van Lund J Van Lund @ 2:31 PM
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    Water Heater for Radiant System

    Hi Scott: We used a 40 gallon electric water heater with two 5,500 watt elements wired in series for a 24,000 btu/hr heating load in the Pacific Northwest.  Main floor area is 1,500 SF with a 14+ ft great room which has a heating load of approximately 11,000 btu/hr.  The system is simple and is very comfortable.  We used a 1.5" thick Youcker "real" concrete slab heavy on the pea gravel topped with tile for a thermal mass.  Pex tubing was 1/2" in diameter on 12" centers.  Being a structural engineer, I designed the floor supports for a maximum deflection (DL + LL) of 1/16" to eliminate cracking. It has worked for over 12 years, is very confortable, and requires no maintenence.  We got the idea from an article by Bill Clinton of Bay Hydronics.  See: and
  • Scott_Mountain_View_CA Scott_Mountain_View_CA @ 2:59 PM
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    J Van Lund

    Thanks for the post. For my home, in-floor radiant is not an option. We're using Runtal wall mount radiators fitted with TRV's, and parallel return piping. Our total house heat loss is around 18-19K so am looking for a low BTU/H tankless wall mount boiler.
  • SWEI SWEI @ 10:23 PM
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    Dealing with the facts

    We encounter design day heat losses under 20k on a regular basis here.

    The smallest gas-fired boilers currently available in North America have output ratings ranging from around 32k to 50k.  This means you'll be short-cycling on a design day and have a truly unhappy boiler the other 95% of the heating season.  A mod/con with a sufficiently low minimum firing rate will fare far better here (see Cadet suggestion above.)

    If you have sufficient solar gain and a well-insulated house, an electric resistance boiler can actually make sense.  They're compact, inexpensive, and don't need any venting. has the smallest models I have found with onboard ODR.
    This post was edited by an admin on March 15, 2014 12:32 PM.
  • Scott_Mountain_View_CA Scott_Mountain_View_CA @ 11:47 AM
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    Looks like your last post isn't complete?
  • SWEI SWEI @ 12:27 PM
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    Third time's the charm

    Aggggggravating!  Turns out using a less than symbol will truncate everything you write afterwards.  Starting over -- again.
  • Scott_Mountain_View_CA Scott_Mountain_View_CA @ 5:04 PM
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    Thermolec TMB

    I checked it out....looks great except I pay more than 20 cents/kWh where I live. Nowhere near as cost effecting as gas, but I like the easy gas line or vent. Something else to consider.
  • SWEI SWEI @ 12:55 AM
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    20 cents per kWH

    is definitely on the high side, but total annual use must be taken into account when making decisions like this.  If your boiler only runs a few hundred hours per year, the math is entirely different than it is for someone living in the northeast (especially this winter!)

    My current rule of thumb is that the minimum firing rate of a boiler should be no higher than 1/3rd of the design day heat loss.  I might be wrong, but I doubt it will be by a whole lot.
  • Scott_Mountain_View_CA Scott_Mountain_View_CA @ 1:24 PM
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    SWEI rule of thumb

    "My current rule of thumb is that the minimum firing rate of a boiler should be no higher than 1/3rd of the design day heat loss."

    Thermolec spec's their B-6TMB at 20K BTUH but they don't provide the low range. Why is this? Seems that anyone offering a mod-con would spec the mod part. What am I missing?

    I've had to rethink elect cost. PG&E, my provider, has tiered pricing. I typically end up in their 3rd tier, which means that INCREMENTAL elect. is costing me in the mid-30 cent range...Yikes! California's population growth has exceeded it's in-state power generation, forcing outside purchase. and it's going to get worse not better. Elect rates in Calif have doubled in the last decade. Nat gas in increasingly more economical, a trend that is expected to continue.

    One problem with a gas fired mod-con is that they don't seem to be available under 40K BTUH, which means even a small mod-con won't run at max efficiency. I'm at a loss how to calculate annual gas cost under this condition.
  • SWEI SWEI @ 10:52 PM
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    Electric resistance boilers

    Can PWM themselves down to nearly zero output.

    How do you manage to hit the third tier with only 1550 square feet?  Time to step back and look at the big picture.  Lighting and appliance upgrades could be a smart move.  Are you heating your DHW with an electric tank?  High flow shower heads or a huge soaker tub?
  • Scott_Mountain_View_CA Scott_Mountain_View_CA @ 10:38 AM
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    elect usage

    Home is occupied all day everyday. 2 Delonghi elect heaters (temporary until hydronic system installed). Attached 400 sq foot cabinet shop loaded with frequent use high amp power tools. Getting the picture? Yes, losing the 2 elect space heaters will help.

    Any suggestions on how to project operating cost of a Cadet mod-con running at less than peak efficiency?
  • SWEI SWEI @ 11:31 AM
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    mod/con cost

    Assuming it is properly installed will be lower than anything else burning NG.  With some history, you could derive a BTUs per square foot per degree-day number and estimate from that.  I think you'll be just fine with the Cadet.

    You sound like a good candidate for PV.
  • Scott_Mountain_View_CA Scott_Mountain_View_CA @ 12:25 PM
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    We're selling the house and leaving the area in about 1 year. My primary objective is to install a credible heating system with as little cost as possible. PV is not on my short list. My original plan was to install the Runtal rads on a new parallel piping system connected to my existing 50 year old AO Smith 180K BTU cast iron boiler and live with the short cycling. But after listening to advice here, I'm coming around to the idea of a mod-con...I believe it will make the home more attractive for resale.

    I know talking component cost here is frowned upon, but I need to have a sense of the total cost of boiler room valves/pumps/tank, etc that I'll need to put between the Cadet and the pipes (I've already received a quote on the Cadet). I'm hoping someone here could throw out a ball park number as well as offer suggestions on internet hydronic parts sellers.
  • SWEI SWEI @ 12:37 AM
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    Internet hydronic parts sellers

    are fine for pipe and fittings and hydronic accessories.  Boilers are a different matter -- you have to carefully read the fine print from the manufacturers.  Many of them will not warranty (or not warranty fully) if mail-ordered.  Find a factory-trained installer for whatever brand you select and see if they're willing to work with you.  Be sure to mention that you're planning to sell the house soon and see if you can transfer the warranty.  Sometimes the factory trained installer can negotiate things that you as a homeowner can not.  If you can get yourself classified as a 'builder,' your buyer becomes the 'homeowner' for warranty purposes.  If you're lucky.
  • Scott_Mountain_View_CA Scott_Mountain_View_CA @ 10:10 AM
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    I was asking for suppliers for the valves etc. not the boiler.

    I've used our contractor's license to get a Cadet boiler quote from the local Lochinvar distributor. I'll discuss the warranty with the distributor and manufacturer.

    The Cadet installation instructions and schematic appear straight forward. I believe I can do the install. You seem to be suggesting that I hire an installer. Are there special skills required that I won't be able to learn through study? I don't trust either of the hydronics "pros" that I've talked to, neither seemed technically competent.
  • SWEI SWEI @ 5:41 PM
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    Radiant education

    quite a bit is available gratis online.  You should start by checking these out: (start with #12)
  • Scott_Mountain_View_CA Scott_Mountain_View_CA @ 5:57 PM
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    Thanks for the links...I'll check them out. I'm also reading Modern Hydronic Heating. It has a wealth of info. I'm less concerned about the actual piping and valve assembly than I am with setting the boiler controls properly. I figure that if I use the manufacturer's schematic showing the positioning of the various valves, pump, tank, etc. I can't go too wrong. I'm accomplished at sweat soldering. This is going to be either a 1 or 2 zone system....very basic.
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