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question on boiler size, operation and mixing valve

rrkelly Member Posts: 4
i have some questions concerning a radiant in floor system i am building. currently i have 4 200ft 1/2 pex loops stapled to 2 high density foam board
with a 4in pour of concrete in the basement 1 inch expansion/thermal break between the floor slab and the walls -- upstairs on the 1st floor there will
be 4 300 ft pex loops with a 1 & 3/4" pour of gyp-crete . the heat loss calc is somewhat of a educated guess -- the building is 150 years old with 22 inch laid up limestone walls that have been tuck pointed inside and out -- the basement windows and door have been replaced and sealed --there is a 7 x 8.5 foot door that has been boarded up insulated (R10) with a plastic vapor barrier stapled over it and the basement has a ceiling with 10" of dead air space -- used https://www.usboiler.net/heat-loss-calculator.html the calc say 23K for the basement which is what will be heated for now. (20 X 40 Ft)
when finished the upstairs should be pretty tight -- it currently has a new roof and windows and will be well insulated with a house wrap and new siding
total square footage is 2000, 800 in the basement 1200 upstairs.

this is the boiler i currently plan on using http://www.westinghousewaterheating.com/literature/WH-Combination-Heat-Hotwater-Wall-Brochure.pdf
the 140k btu floor model. one of the reasons i liked it was it looked like you could service it fairly easily and the sensor wires were accessible. i am not fond of 5lbs crap in a 1lb can

is the 140k model to big? i assume some of the 140K rating is going towards the DHW side as it is a combi boiler also i have not met a marketing man or brochure i totally trust.
my very rough guess is i will need about 60k plus to keep everything warm. the spec says a 5 to1 modulation on the burner but the tech says that has changed dropping the lowest
output to 14k. can i run it just for the basement? with just the basement pex runs the control piping and the boiler i am guessing total water load should be 10 to 13 gallons for the basement
is a mixing valve of any use at all? in the future when the upstairs is on line i think it will be <29 gallons will mixing valve be necessary? zoning with valves does not appear to be a good idea no clearly defined zones
i was planning on using a separate circ pump and thermostat for each of two zones 1 upstairs and 1 for the basement. I can set the CH water temp in the boiler over a range from 86 to 180 F
can i just set it to something in the lo 90's that works and leave it at that? this assumes i can get a delta change of 10 to 12 degrees between feed and return.
thank you


  • Zman
    Zman Member Posts: 7,542
    What do you mean by gallons?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • rrkelly
    rrkelly Member Posts: 4
    the volume of water in the pex tubing and the boiler in gallons
    based on the area of the tubing -- the numbers say 1.1 gal for every 100 ft of 1/2 pex 8 x1.1 = 8.8 gallons in the basement pex tubing plus what is in the copper and the boiler itself
    1 gal = 231 cubic inches with a weight of 8.34 lbs
  • Zman
    Zman Member Posts: 7,542
    The volume is relevant when you go to size your expansion tank or if you are trying to determine if the boiler will short cycle. It does not play much of a factor otherwise.

    If you have the space and the budget, I would recommend sizing the boiler to the heating load and go with an indirect for DHW.

    A combi boiler is almost always a compromise. It will barely keep up with the DHW spikes and will be significantly over sized to the heating load.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    Brewbeerdelta T
  • rrkelly
    rrkelly Member Posts: 4
    ok i kind of expected that -- space i have budget not so much
    dhw usage is minimal and far in the future i will add solar hot water with a small storage tank --gravity drain back i have most of the pieces
    currently girl friend has 2 4x8 german solar hot water panels that are amazing 151 F on a -11 F day 100-120 on the storage tank is typical and two very small zones of in floor heat. bedroom, closets and front entry and for a added treat she has 5kw of solar on the garage.

    so the boiler is to big? i could add an air exchanger to the lower floor of the addition -- there is a 16x16 addition that is why the upstairs is 1200 sq ft -- what would you suggest? i can instrument the hell out of it-- temps on all the feeds and returns a plus a ultra sonic hot water flow meter-- pieces collected over many moons and friends in strange places. when done the house will be very tight -- doing something right the first time is generally the fastest and the cheapest way to do anything.

    what is short looping ? and is there any major differences in having less than 3000' of tubing
  • Zman
    Zman Member Posts: 7,542
    If you are going with a combi, that is just the nature of it. The boiler will cycle when the output of the boiler is greater than the energy emitted from the floor. Short cycling is more than 6 cycles per hour.
    You should be able to minimize the issue by setting the boiler up with a wide on/off differential and/or enabling any anti cycling features it might have.
    It sounds like you have some fun instruments, if you do the math on the front side, the system will work great right from the get go. You can then prove how good the math was with the instruments :)

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Rich_49
    Rich_49 Member Posts: 2,766
    Do yourself a favor if you go with the combi and source the HTP branded units . They are identical but come with better support locally .
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
  • rrkelly
    rrkelly Member Posts: 4
    ok this westinghouse / htp thing is strange -- i have been told many stories about the 140F unit do you know any thing about who actually makes it -- i have not verified it but was told they paste a westinghouse sticker over the htp plate
    home depot sells it with a westinghouse sticker but tech support is htp other places list westinghouse for tech support
    westinghouse and htp both list new bedford ma as home.
    i can find parts online for both units you are right htp has more sources and better prices
    you have any opinion on the basic unit