new boiler piping

Discussion in 'HVAC Heating & Cooling' started by ffp20, Mar 2, 2014.

  1. ffp20

    ffp20 New Member

    Mar 2, 2014
    New York
    Hi folks. i will be replacing a utica starfire oil fired boiler that heats cast iron radiators with a new gas fired boiler (non condensing). The house is 1250 sq ft with a heat loss of just shy of 62000 btu. I will be splitting two bedrooms off into its own zone and adding a zone for an indirect water heater for a total of three zones. The mains are 1-1/2" for a short run and reduced to 1-1/4". The runs to the radiators are 1". There is no boiler bypass now and the return temps are pretty cold and i know what that does to a boiler. So, i plan on installing a boiler bypass or use primary/secondary pumping with circulators, not zone valves. My question is: What size pipe do i plumb the boiler with?
  2. nhmaster3015

    nhmaster3015 Master Plumber

    Jul 30, 2008
    The granite state
    The primary loop should maintain the boilers feed and supply pipe size. You can use a simple ball valve to balance delta t or a three way mixing valve. You really don't need primary secondary piping there either. Pipe it exactly as your old boiler was piped although, if your circulator was pumping on the return side you are going to want to swap that around so that it pumps away from the boiler / expansion tank (point of no pressure change). Owing to the recent shortages of gas and the rising price of gas, you might also want to reconsider changing from oil.
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  4. ffp20

    ffp20 New Member

    Mar 2, 2014
    New York
    thanks. with multiple zones ( one being for the indirect water heater, the other two being for cast iron radiators), how do i plumb the bypass? is primary/secondary pumping overkill for this?
  5. Soapm

    Soapm Member

    Dec 18, 2013
    Aurora, CO
    I hope the OP don't mind but I wanted to ask a question for my own learning, in a closed loop, pressurized system, why would it matter where you put the pump?
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    Activating the pump creates a momentary low-pressure works best if the expansion tank is behind it to provide a little 'boost' and prevent cavitation. That may be a simplistic answer, but I think it will hold up.
  7. Dana

    Dana In the trades

    Jan 14, 2009
    Not that it matters regarding the question asked, if you have glass in the windows and doors that shut I'm a bit skeptical that a 1250' house anywhere in NY has a true heat load anywhere near 62,000 BTU/hr. That's about 50 BTU per square foot of living space, a ratio rarely seen even in Fairbanks AK. Maybe an uninsulated double-wythe brick with single pane windows could come in that high. Maybe you used unrealistically high air infiltration/ventilate rate numbers or something? Most 2x4 insulated houses with storm windows and even 20 in the attic will come in at half that number or less.

    For 2-zone system with 1" distribution plumbing and radiators rather than long runs of skinny-pipe baseboard plumbing it primary/secondary would very likely be overkill unless it's a high-head/low flow condensing gas boiler or something (in which case you'd want to be pumping toward the boiler, with the expansion tank at the hydraulic separator.)

    I disagree heartily with nhmaster3015's comment/suggestion " Owing to the recent shortages of gas and the rising price of gas, you might also want to reconsider changing from oil." From a source-fuel BTU/$ point of view oil has not been cheaper than natural gas for over a decade. At recent years' $4/gallon New England pricing you pay about 30-35,000 BTU/$ out of oil, and 70-100,000 BTU/$ for natural gas, or 2-3x the amount of heat per dollar. (Propane would be a whole other story, and not a happy one- don't go there!)

    Spot market pricing of natural gas supplied to electric utilities has been very spiky recently due to thin reserves from space heating use, but even if the residential retail price were exposed to that price volatility (which it isn't, in a regulated and contracted price market), there are no local gas mains in danger of running out. The longer term prospects for both gas and oil pricing is upward, since the cost of shale gas & oil is much higher than the old-school formations that are currently being depleted. But whereas the price of oil is determined by the world market for transportation fuel, natural gas is still pretty much a local & regional market, since the cost of liquifying and shipping that gas to a world market comes with a huge cost-adder, whereas oil is shippable in un-pressurized un-refrigerated tankers, with minimal processing required.

    It's hard to come up with a scenario for the next 20 years puts heating oil at a lower price average than natural gas. As the rest of the world takes up the habit of driving, it's unlikely that the price pressure on oil will go away, even if (as isn't too likely) the US weans it's privately owned car fleet completely off of diesel and gasoline over the next 20 years. The US may have already passed it's peak oil consumption rate, but the world hasn't.

    The price of heating oil would have to literally be cut in half, or the price of natural gas to more than double from current pricing to be in the same ball park as a heating fuel. And if the price of natural gas were to double, heating with ductless mini-split heat pumps would be significantly cheaper than gas, since they're roughly comparable at current regional electricity and natural gas prices. The price of electricity in the ISO/NE region rises and falls a bit with the price of natural gas, but it becoming more heavily moderated by the low marginal price of renewables (including imports from Quebec.) Even at current electricity prices the levelized cost of even grid-tied photovoltaic solar is at parity with the residential retail prices, and it's expected to drop considerably over the next 10 years rather than rising, becoming the absolute cheapest source feeding the grid. If the cost of electricity pricing jumps, so will the rate of grid tied PV installation- there's real market feedback there, and few technology or infrastructure limitations (including storage), so the long term outlook for electricity pricing is pretty good, as long as you can leverage it with heat pumps in heating applications. In a 1250' house it's highly likely that at least one large zone could be heated with a mini-split if it comes to that.
  8. BadgerBoilerMN

    BadgerBoilerMN Hydronic Heating Designer

    Mar 4, 2011
    hydronic heating designer/contractor
    Ya, what he said.

    The boiler contemplated is too "big". Likewise the piping and the pumps, pumps, pumps!

    You don't have to be Dana to do this math. I love how Dana "guesses" better than most of the "professionals" that show up here. We will go with this 50mbuth (low-efficiency, waste of time and fuel) boiler. With the paltry efficiency of cast iron (DOE, since IBR output has been obsolete for decades) we end up with 41mbtuh. Let's be extra conservative and use 40.

    At 40,0000 / 500 / 20°F delta T = 4gpm. That's it. You have to move 4 gallon per minute to transfer the usable heat from this boiler to the radiation.

    Even if you were installing a condensing boiler (recommended), you would not need more than one pump with three zone valves, one dedicated to the water heater. And no, you don't have to up-size the boiler to add and indirect water heater. There is a legitimate concern for minimum return water temperature, even more so when the boiler in not grossly over-sized since return water temperatures below 130°F will lower the stack below 350° and like reduce to dew point producing sustain flue gas condensate.

    As suggested (if you must install a low-efficiency atmospheric cast iron boiler) you can use a bi-pass to maintain minimum boiler temperature and since the pump you have on the truck will run 8-10 gallons per minute you should have no problem with flow. I would use an Alpha or Eco if I wanted to dress up the pig and use 25% of the 88watts the old fixed speed will burn for the next 30 years. But that's just me.

    This boiler serves a 5000+ sf house in Minneapolis with a mix of cast iron radiators, baseboard with a radiant ceiling, radiant floor and indirect water heater. Boiler output 142mbtuh. Note the number of circulators.

    The boiler system at the bottom of the page is an earlier installation done before I knew better. Both work fine. The latter burns more electricity than the latter, significant in a smaller home using this 50mbtuh boiler pictured.


    PS. I also agree with Dana on the availability of NG.

    I believe the price of electricity will naturally be tied to energy source used to produce; increasingly natural gas, the abundance of which seems to be assured by our cousins in Canada for many decades to come. The price can only come down once the pipeline is approved in the next US administration.

    PSS. The OP asked about pipe size. My B&G "wheel" put 40mbtuh at the magical 20° delta T at 4gpm and 3/4" pipe. Believe it or Not!
    Last edited: Mar 10, 2014
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