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designing for lower temperatures

weil_failweil_fail Member Posts: 69
what's the best way to design for lower temperatures in a boiler/radiator system, in your opinion?

three things I'm curious about:
1) what's the best way to increase radiator size? is it advisable to daisy-chain more than one hydronic panel in series, with one thermostatic valve? some other configuration?
2) what are your thoughts on fan-assisted radiators to increase output at a given temperature?
3) recommendations on a condensing boiler with a wide water temperature output range, to make the best use of low-temp operation while maintaining capability for very cold nights?


  • JUGHNEJUGHNE Member Posts: 6,985
    Is this an existing system with cast iron radiators?
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,099
    First thing is a heat loss
    Then you size the radiation based on the water temperature available.
    Lower water temperature in general decreases system pipe size but increases the amount of radiation that must be installed. Lower water temperature increases the efficiency of a condensing boiler.

    It's a balancing act, there is no one perfect design. There is no problem using 1 thermostatic valve to control multiple pieces of radiation installed in a room .............if it is designed correctly.
  • weil_failweil_fail Member Posts: 69
    Jughne, assume it's a from-scratch design.
  • hot_rodhot_rod Member Posts: 13,526
    My thoughts are that every hydronic system should be designed to cover design loads with SWT supply water temperature at 120F of less. That is usually doable with homes with loads under 25 or so BTU/ sq ft with radiant. Possibly with properly sized panel rads.

    Doing this allows the system to be heated with other heat sources like heat pumps, solar thermal, basically "future proofing" the home should fossil fuel ever become not available, or legislated away :0

    This journal describes the method to lower SWT on existing homes. the heat emitter math is the same for new or old applications.

    Load calc, floor coverings are two places to start.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • HomerJSmithHomerJSmith Member Posts: 929
    edited April 24
    No matter how you cut it, you're goina need to supply enough BTUs to overcome the building heat loss on the coldest day of the year at setpoint or have a supplemental heat source.

    Having a supplemental heat source give you more latitude in your low temp heating sys design. The demand of the coldest day of the year is at most 2%, which may save costs with a warmer low temp design.
    My house has 3 sources of heat. A redundancy that has paid off in the past.
  • BrewbeerBrewbeer Member Posts: 611
    I designed a low temp baseboard system for house, check the design thread linked in my sig. The big takeaway from my project was this: use big radiators and lots of them.
    Hydronics inspired homeowner with self-designed high efficiency low temperature baseboard system and professionally installed mod-con boiler with indirect DHW. My system design thread:
    System Photo:
  • weil_failweil_fail Member Posts: 69
    Brewbeer, what type of radiators do you prefer?
  • psb75psb75 Member Posts: 221
    I worked on the complete restoration of an 1855 farmhouse in southern MI. Kept all of the cast-iron, hot water radiators and added 100% more cast-iron and panel radiators. Using a Viessmann 222F boiler that runs on outdoor reset with no thermostats inside--other than TRVs on radiators. It works amazingly well--condensing 98% of the heating season. Planning to add an air to water heat-pump piped into the hydronic system for heating operation during the shoulder seasons and for cooling (to an air handler) in the summer.
  • weil_failweil_fail Member Posts: 69
    psb75, can you give me some tips on making such a system work as well as yours? which additional radiator panels did you use?

    without a thermostat, does it just use return water temperature to modulate?

    what kind of architecture are the radiators in? parallel 2-pipe?
  • flat_twinflat_twin Member Posts: 260
    Weil_fail if it helps at all I'll share the numbers for our old 1835 era home. Improvements to the envelope resulted in heat loss of 64k btu on design day (5 degrees) per the Slant Fin app. The hot water central heating system installed in the 1930's consists of 10 cast iron rads with an output of 102k btu @ 170 supply water temp. I have the modcon boiler with outdoor reset set to run from 87-140 supply water temp to cover outdoor temps from 65 to -20. All radiators are on one zone. The system is two pipe direct return. Boiler is a Weil McClain Eco110 which also provides our domestic hot water with a Bock 40 gallon indirect. We are extremely pleased with how well this works. It is amazing to feel a barely warm radiator and realize this low consistent heat is able to heat the entire house so well.
  • BrewbeerBrewbeer Member Posts: 611
    Weil fail, i am using Sterling baseboards in my system. Check out my design thread linked in my sig for details
    Hydronics inspired homeowner with self-designed high efficiency low temperature baseboard system and professionally installed mod-con boiler with indirect DHW. My system design thread:
    System Photo:
  • psb75psb75 Member Posts: 221
    weil_fail: the 1850 MI farmhouse heating system relies on 90% cast iron radiators (found a treasure trove of them in a warehouse in Detroit). 10% of the radiation is Ecostyle panel radiators and heated towel bars. Viessmann boiler with low-loss header. All radiators plumbed home run w/ 1/2" Uponor pex to Uponor manifolds. The boiler is running off of the outdoor temp. sensor and modulates according to that and its return water temp. and the general Viessmann control strategy. None of those radiators ever feel "hot". "Nicely warm" is a better description.
    Architectural details: All of the original lath and plaster was removed from inside the house. It was insulated well with spray-foam (for air sealing, minimal depth cavities.) I had 5/8 "blue-board" sheetrock installed everywhere (walls and ceilings) with 2-coat Diamond plaster finish. This was to retain internal thermal mass. Works great for cooling and heating.
    My great-great-great grandfather built the house. 7 generations.
  • hot_rodhot_rod Member Posts: 13,526
    Are you using the Synergy Series low temperature baseboard? The load calc for each room is needed to determine if you have adequate wall space for the amount of fin tube you need.

    These show output down to 120 SWT

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • HomerJSmithHomerJSmith Member Posts: 929
    edited April 28
    If you are going to use a low SWT, keep your runs short or boost the flow to the 4 gpm point. Long runs may run out of heat energy by the time you get to the last baseboard. If you plan for 4 gpm, I would use a balancing valve on each loop so you can be sure to lock the flow into the choice flow and not find yourself at 6' per/sec. In fact, I would use balancing valves anyway, as I always do.

    If you have uneven runs, the shortest run will get the flow. Caleffi has great balancing valves that can be set without a differential meter. You can balance your system yourself.
  • tim smithtim smith Member Posts: 2,317
    Hot rod, I think I would rather see a 12" tall Runtal UF series radiator which puts out 353 btu at 120 per ft. Only protrudes off wall about 2". Now probably not near the price point although but looks are better I feel??? jmho
  • HomerJSmithHomerJSmith Member Posts: 929
    edited April 30
  • PC7060PC7060 Member Posts: 40
    @flat_twin @psb75 - Thanks for the description of your systems, love the old cast iron units.

    @flat_twin With the old CI rads, did you install a dirt separator to to protect the Weil McClain? Or is this new piping?

    @psb75 - what is the flow rate of your Viessmann system? Although my current 90 yo systems has a large pump added about 40 years ago, it works very well with gravity feed alone. Currently updating the boiler and circulators looking to have minimal flow rate on the new system.
  • flat_twinflat_twin Member Posts: 260
    Our rads and piping are from 1930 best I can tell. 2.5 inch mains breaking down to 1.5 pipes to individual radiators. I found no sign of it ever being a gravity system. Appears to have always been pumped and electricity came to our town in 1927. House has been in our family for almost 60 years. Original coal boiler converted to fuel oil was here when my parents bought it.
    Contractors installed the mod con in 2016 when nat gas became available. They did not install a dirt separator but did flush the pipes. They were OK with our previous oil boiler but I felt compelled to educate myself about the mod con especially regarding outdoor reset settings. The tech left it with factory settings, 180 degree supply water. I have it adjusted now so it tops out at 140 when it gets down to minus 20 outside.

    A few months later I installed the indirect water heater. I do the maintenance by the book each year. Every other year seems to be often enough to open the HX and clean it. Cleaning the condensate trap, testing boiler water (Sentinel water treatment) and checking combustion is done every year.
  • PC7060PC7060 Member Posts: 40
    flat_twin said:

    I have it adjusted now so it tops out at 140 when it gets down to minus 20 outside


  • psb75psb75 Member Posts: 221
    I used Grunfos Alpha ECM circulators and they are set on Autoadapt. So the flow rate is variable. The Viessmann "knows" what temperature should be, and the Alpha "knows" what pumping rate should be. It works great. Some rads have temp. regulating valves.
  • flat_twinflat_twin Member Posts: 260
    Ours is piped direct, no primary secondary. Pumps are Grundfos 15-58 and are set to satisfy the boiler HX whether heating or dhw. System head with big pipes is low and very forgiving. System is one zone. The guy who designed the system knew his stuff. Rads are sized very well for each room. Very little temp variation room to room.
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