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Delta T / Flow Question

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by JaxGarage, Nov 2, 2012.

  1. JaxGarage

    JaxGarage New Member

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    I have a new garage with a radiant system that I fired up late last winter. Garage is 900sqft and is well insulated and has 2" under floor and on walls of foundation. I have four loops all around 270', 6" centers on exterior and 9" everywhere else. System works well but I have a higher delta T than what most suggest is optimal. I am using a tankless heater set at 110F. Temp in garage is set at 50F and slab temp is 55F. When systems fires up return temp is initially 55F and rises to approx 67F after about 20 minutes. I have approx 0.55gpm per loop for a total of 2.2gpm. Any ideas as to why the delta T is so high?

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  2. BravoWhiskey

    BravoWhiskey New Member

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    Flow of 0.55 gpm per loop seems low for a normal 75 watt pump and four 270 ft loops in parallel. The lower the flow, the higher the delta t. Is the flow limited on purpose? Also is the heater output actually 110F? 2.2 gpm with delta T of 40F would be 44000 btu, does the heater put out that much? At any rate optimal is delta T low enough to make your floor temperatures even enough. If your floor temperatures are even enough then nothing to worry about.
  3. DaveBP

    DaveBP Minister of Fire

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    High flow X Low delta T ... or... Low flow X High delta T. You end up with the same BTUs.

    Remembering that most instantaneous water heaters are designed to operate most efficiently at very high delta T you may be running at optimum condition just as you are. You'll know better when the real cold comes.
  4. Bob Rohr

    Bob Rohr Minister of Fire

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    Something is not adding up. 900 square feet at 8" on center tube spacing would be 1400 feet of tubing. Your 4 loops of 270= 1080 feet of tube. So an average of about 10" on center.

    Regardless with a well insulated space your loads at design should be under 20 BTU/ foot, depending on design conditions in your location.

    Tankless heaters have a large pressure drop, not ideally suited to be a hydronic boiler. Is one pump flowing the boiler and loops? 75W is not a very large pump if you have a restrictive hx.

    You will always have a large delta T on a cold slab, does the delta t close as the slab warms up? the delta T is ever changing based on the load conditions. If the space heats, boiler cycles off, then it working.
  5. JaxGarage

    JaxGarage New Member

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    Thanks for all the replies. I will try to answer all questions together.


    1. pump is a grundfos 15-58FC which claims 87W at high speed which is what it is running at.
    2. heater is capable of 140k btu, however it will modulate lower if delta T is lower. That is why i was hoping to narrow the delta T to get better efficiency.
    3. 6" on exterior and 9" everywhere else.It adds up.
    4. I didn't realize tankless heaters had large pressure drop and yes one pump for whole system. Should i use larger pump?
    5. delta T does close the longer system runs but it normally only runs approx 20min every 2-3 hrs.
  6. Bob Rohr

    Bob Rohr Minister of Fire

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    What brand and model tankless? The installation manual should show the pressure drop at various flow rates. It sounds like a lot of boiler for 900 square feet of heating load. Even with a 20 btu/ square foot load that is 18,000- BTU at design condition. If the building is well insulated the load could be 12- 15 BTU/ sq. ft. That would explain the short cycles also if the boiler is oversized and cannot modulate down to the small load. What is the lowest modulation output?

    Generally when a tankless water heater is used as a boiler with a small pump you will not get full output from the unit. It's really designed to operate at a large delta T 55- 60°F incoming temperature 120° out, 60 degree delta T. Tankless are attractive for small radiant boilers due to cost, small footprint, ease of venting, efficiency, etc but the HXer is designed for a different type of load, compared to a radiant heating load.

    Now there are some tankless manufacturers that offer boilers or combi boilers that uses a small plate HX for the domestic HW. those are controlled and modulated differently and have a better HX design for hydronic loads. I'm not sure exactly what you have for a tankless and what your heat load is.
  7. heaterman

    heaterman Minister of Fire

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    A couple questions. ...... How are you measuring the flow in each loop? A little inaccuracy there could really skew the resulting math. I have found the flow meters on a lot of the less expensive manifolds now on the market to be worthless for anything other than determining that water is moving somewhere.

    I may have missed it but I did not see any reference to the actual supply temperature in your post. Only that the tankless was set to 110*. There could be a huge difference between the two. What is your measured supply temp?

    When we use a tankless as a heat source we always pipe them primary secondary and use a high head pump like a 009 as the "boiler" circ. They need a lot of flow to ramp up to full fire and as Hot Rod said, they are a different beast than a real boiler in terms of how the burner functions.

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