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Unico Hot Water Coil Piped Off Of Oversized Steam Boiler

Have two really unpleasant problems which resulted from adding a hydronic zone off a 400,000 BTU input Weil steam boiler, following OHJ article on this subject. Boiler is about 100,000 BTU more than we need. (1) The hot water zone, which is used to heat a kitchen and one room above it, feeds a Unico 1218 Hot water coil, but first runs through a Weil McClain Pool type heat exchanger. It turns out that this coil has about 7,000 BTU less output than my contractor thought, and the heating load is probably higher than he estimated, so I think I am about 10000 BTU short or so in the kitchen. On a 10 degree evening, the room is about 50F.
(2) The boiler is short cycling, and creating enormous gas bills. It turns on every 8-10 minutes for a minute or two to heat the hot water feeding the Unico coil which runs constantly because it can't make the thermostat temperature until it gets to 40F or so outside.
What should I do? Junk the 1218 and try to put in a bigger Unico unit? Use a supplementary heat source piped off the same zone-- radiator, toe kick heater, radiant floor? Don't pipe off the big boiler and buy a separate boiler? Shoot my contractors? Shoot Unico for advertising 4000 BTU an outlet when the 1218 really is delivering much less?

Comments

  • Steamhead
    Steamhead Member Posts: 16,719
    What

    does the pool-type heater do-isolate the boiler water from the hot-water zone? And what type of heat emitter is used in the kitchen and the room above?

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  • Noel Kelly
    Noel Kelly Member Posts: 43
    Water temp?

    What is the temperature of the water entering the coil?
  • kevin
    kevin Member Posts: 420
    I'd get a hold...

    of a mfg. rep. from Unico and find out why the discrepancy as to the output #'s.Maybe they will credit you. Sounds to me like you may be off more than 10,000 btu.Is the heat exchanger piped in a counter flow arrangement? to give you the best btu x-change? Get a total accurate heat loss so you can get all the #'s right.Estimates won't do. kpc
  • Ray M_2
    Ray M_2 Member Posts: 64
    unico

    There are many variables. The other questions in previous posts need to be answered.The 1218 coil is capable of 38k BTU @170* h2o 4gpm.As kevin wrote a true heat loss is the first step.

    LOL Ray M

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  • kf_2
    kf_2 Member Posts: 118
    My guess...

    The pool heat exchanger you are using is probably manufactured by Triangle Tube (Maxi-Flo) If so, even there smallest HX should be more than capable of transferring the need BTU's however, the flow rates on the heating (pool) side of these heat exchangers are very high due to the fact that they are usually installed with a high flow rate pump on a pool. My advice would be to check your pump sizing.

    kf
  • Chris Colman
    Chris Colman Member Posts: 2
    Undersized Unico HW Coil Off Of Oversized Steam Boiler

    Thanks for the posts. Absolutely agree that an accurate heat loss calculation is essential to get this right. However, here are answers to some of the other questions:
    Does the pool-type heater do-isolate the boiler water from the hot-water zone? And what type of heat emitter is used in the kitchen and the room above?

    Yes, the pool type heat exchanger does isolate the boiler water from the HW zone. The hot water created from the heat exchanger feeds a Unico coil, and high velocity air is blown over the coil to creat a high velocity forced hot air system. For what it is worth, the air temp at the outlet is about 115F. I do think that the heat exhanger does cause the water temp to the coil to be lower than it should be, say 155-165F. See below for some comments on inlet temp.

    What is the temperature of the water entering the coil?

    Oddly, I have a thermostat on the RETURN from the coil, and I have a thermostat on the boiler water, but NOT on the feed to the coil as it exits from the exchanger. What I see is a Minimum return temp of 149F/Boiler Water 178, Max return about 163F or so, Boiler water at 185. It takes about 10 minutes of the unico coil circulation to drop boiler and return water from highs to lows, which kicks the boiler back on. My guess is that the heat exchanger causes me to have, on average, 155 inlet water, clearly diminishing output.

    Is the heat exchanger piped in a counter flow arrangement?
    Not sure, but I would guess yes. Here is model and tech info for it:
    Model WMPH-95
    No. Output 95,000 BTU/hr
    Transfer Surface 2.0 Sq. Ft.
    I am curious whether I would be better creating a tank of HW to feed to the Unico. This reminds me of the types of problems instananeous water heaters have.

    The 1218 coil is capable of 38k BTU @170* h2o 4gpm.
    You are correct, as long as I put in 13 outlets and run the blower at 400CFM. My contractor put in 9, leaving my max of 28K BTU from bulletin 20-20. Interesting point, I could raise the ouput by raising CFM across coil, and add more lines. But I am pretty worried that the Unico is overrated on the coil, given my experience so far, though that could be due to feeding it water which doesn't maximize output by being too cold. Odd that 1218, as opposed to 2436 is not rated at 180F water, and is not even rated at 170F/4 gpm.

    Hope that adds some facts, I have a whole notebook full of notes. Thanks for the help!
    Chris Colman

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