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  1. #1
    Join Date
    Jul 2008
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    Newfoundland, Canada
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    4

    Lightbulb Hydronics Tech Question: Temp vs Pressure

    This post really belongs under the hydronics forum but I don't seem to have posting priviledges in that area so here goes:

    Our company is not yet in the hydronics business but is working on developing solutions for the residential oil fired hot water radition retrofit business.

    The main obstacle is that existing radiator installations were designed for higher water temperatures (160-180F) and than we are trying to use more economical water temps commonly used in geothermal installations (100-115F). Luckily we've found that most past installations were greatly oversized and this works in our favour. However, we are still short of meeting the design heat loss once you degrade raditator output at lower temperatures.

    We considered using a 2nd stage heating source to boost water temp when called for by 2nd stage thermostat, zone controller etc, however, doing so is futile in our case since the return water temperature is higher than our original incomming water temperature and this would effectively negate and reverse our heat exchanger.

    Therefore I am wondering whether we could increase the pump speed as a means of incresing flow and therefore boosting radiator output. On paper this seems like it should work but our lack of experience in this field makes me wonder whether there are reasons this should not be considered with an older installation desgined for lower speed, higher temp installations.

    Anyone out there with practical and theoretical experience on this matter? I believe most installations will have current pump speeds of 3-5 G/M so we would anticipate rasing this by a factor capable of maybe doubling the heat output of the rads without blowing gaskets.

    Your feedback would be appreciated.

  2. #2
    Join Date
    Jul 2008
    Posts
    12
    Like fan blades, Pump impellers have a pressure curve and the pumping capacity is not directly proportional to speed.

  3. #3
    Join Date
    Jul 2000
    Location
    Northern Wisconsin
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    1,934
    Like everything else in the universe there are laws that govern what can be accomplished with the energy available.

    If your equipment is capable of putting out the required btu's/hr that the structure needs at design temperatures and you're getting a proper temperature drop across your radiation there's nothing else you can do with what you started with. In other words a temp. drop of say 10F is about the best you shoot for and that assumes that the original system was designed and installed correctly.

    Try B&G's website for information, contact numbers etc. to help you design good systems that will not be constant callbacks. http://www.bellgossett.com/

    Most forced air heat pump systems (no matter the style or make) have some form of auxillary heat producing component to take over when design conditions are beyond what the main system can deliver.

    If all you're doing is heating then size the HP to deliver to the maximum the radiation system can produce and add your aux. heat source in other forms if return water temps are an issue. Another avenue to try is figuring out if improvements to the building envelope in these cases would bring your system's capabilities and the heat loss to the same number.
    Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.

  4. #4
    Join Date
    May 2008
    Location
    North Texas
    Posts
    105
    Flow velocities below 2 feet per second will not move air bubbles in a vertical pipe. Flow velocities above 4 feet per second will increase noise. Faster velocities increase pump power needs.

  5. #5
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    66,824
    After your GPM gets so high, you actually lose capacity. Not enough contact time, for efficient heat transfer.

    You would need to establish if you can get more heat from the rads by increasing flow.
    It may help the second floor, but not the first floor. Often, the first floor has a higher GPM then the second floor to begin with.
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  6. #6
    Join Date
    Jul 2008
    Location
    Newfoundland, Canada
    Posts
    4

    Update on Original Post

    Thanks to all those who offered their advice so far. I will now fill in some of the details on one case we're working on:

    The home's design heat loss is 37MBTU at 7F. However, our real climate's average temperature is 32F. We've calculated the actual MBTUs required during the entire heating season and we can more than accommodate it. However, we could use a 2nd stage for 20% of the time when temperature will drop and the home's heat loss will be greater than our heat pump's output at those same temperatures.

    The existing radiators are 3 times longer (in total) than they need to be when we determine their output at 170F. The radiators will perform at 30% at our colder 110F outgoing water temperature. This leaves us with a shortfall of 10% (on average) to the home's design heat loss.

    This is why I am wondering whether increasing the pump speed might be used as a means of increasing radiator output by 10% + sensible margin. We are planning on using an indirect hot water storage tank between the heat pump and the existing distribution system. Using an in-line electric spa heater as a means of boosting water temp if/when it drops might be considered again if the result of the increased pump speed is a greater delta T which the heating source & buffer tank combination cannot absorb. I am thinking of a spa heater outside the tank because I don't want the tank temp to go up or it will negate our heat exchanger's effectiveness.

    Therefore, do any of you think this strategy will work and if so, what equipment, controls, logic or other advice would be of use to us.

    Thanks in advance for your time considering this matter.

  7. #7
    Join Date
    Jan 2004
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    Lancaster PA
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    Cast Iron Radiation is based on the average temp of the rad.
    If your current GPM water flow is going to be able to maintain that average temp in the rad, then increasing GPM won't help.
    And the only thing you can do is increase the water temp by some other means.

    What is the highest return water temp you HP can have.
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  8. #8
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    Jul 2008
    Location
    Newfoundland, Canada
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    Reply

    The heat pump produces up to 117F. It would then be circulated through a heat exchanger in a buffer tank. The radiators are not cast iron but fin type made by Haydon.

  9. #9
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    Jan 2004
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    Then you would need to know if your getting 4GPM, through teh baseboard, or less.
    But increasing to more then 4 GPM won't get you any noticable BTU output. Probably 2 to 3%, but thats about it.

    Under 120°F, you'll be hard pressed to get 200 BTU per foot of element.

    Any chance you can add floor boxes.
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  10. #10
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    Jul 2008
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    Newfoundland, Canada
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    Reply

    Of course we could replace some or all the rads with low lemp rads made in Europe which are designed for 120F water temps, or we could try to fit more rads. We could also upgrade the customer to underfloor on the main level.

    However, the point of this excersise is to learn if there are any ways of boosting existing baseboard heater output by means of tweaking pump speed etc.

    You noted that raising the speed up to 4 GPM would only increase output by a negligable amount. However, is there a chance that boostng water temp and increasing pump speed when 2nd stage is called for would increase output and lower return water temp enough that it won't be higher than 117F?

    Ironically, even if this were possible, the boosted return water temp would effectively knock-out the heat pump and run solely on 2nd stage heat. In that case I might be better off simply setting up a dua-fuel type of configuration where the 2nd stage heating is separate from the 1st stage altogether. Remember that we've already determined that we will only need 2nd stage heating for 20% of the total annual heat loss. Is my dual water temp system something that is heard of?

  11. #11
    Join Date
    Jan 2004
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    Increasing GPM, will actually also increase return water temp. Defeating the reason you increased it.

    Are your baseboards piped in one seires loop. Or are they piped in parallel loops.
    1 large loop will often times have more temp drop then parallel loops.

    Outdoor reset is used on baseboard systems as well as other hydronic systems, so multiple temps are quiet common.

    In order for your booster heater to be used, and not shut down the heat pump. In a standard baseboard system. You would need to slow the water flow down, so the water gives up more heat while in the baseboard.
    EG: Water enters baseboard at 125°F with booster energized, and leaves at 110°F.

    At 1 GPM(500 punds per hour), that would leave 3,500 BTUs for the HP to add to the water before reaching 117°F.
    You just have to figure out if 1 GPM is enough volume for the baseboard to be able to heat the house.

    You need to know what your water temp drop will be at a flow rate and water temp high enough to heat house with the baseboard existing in the house.

    As simple as this sounds. You also have to keep in mind, weather the house had hard wood floors originally. If it did, and they now have carpet, is the gap between the cover, and carpet enough to leave enough air flow through it.

    Over the years, I've been on more thenone call where carpet was installed 3 or 15 years later, and the customer calls and says the system doesn't heat right.
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  12. #12
    Join Date
    Jan 2004
    Location
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    Increasing GPM, will actually also increase return water temp. Defeating the reason you increased it.

    Are your baseboards piped in one seires loop. Or are they piped in parallel loops.
    1 large loop will often times have more temp drop then parallel loops.

    Outdoor reset is used on baseboard systems as well as other hydronic systems, so multiple temps are quiet common.

    In order for your booster heater to be used, and not shut down the heat pump. In a standard baseboard system. You would need to slow the water flow down, so the water gives up more heat while in the baseboard.
    EG: Water enters baseboard at 125F with booster energized, and leaves at 110F.

    At 1 GPM(500 punds per hour), that would leave 3,500 BTUs for the HP to add to the water before reaching 117F.
    You just have to figure out if 1 GPM is enough volume for the baseboard to be able to heat the house.

    You need to know what your water temp drop will be at a flow rate and water temp high enough to heat house with the baseboard existing in the house.

    As simple as this sounds. You also have to keep in mind, weather the house had hard wood floors originally. If it did, and they now have carpet, is the gap between the cover, and carpet enough to leave enough air flow through it.

    Over the years, I've been on more thenone call where carpet was installed 3 or 15 years later, and the customer calls and says the system doesn't heat right.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

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