Loop and duct sizing

Discussion in 'Quotes and Proposals' started by kajinka, Mar 29, 2012.

  1. kajinka

    kajinka New Member

    we would like to finalize our decision which contractor to go with for geothermal installation in our home but there are still a few things hanging in the air that we would really appreciate some independent views on.
    We are retrofitting an existing home in Maryland (4000 sqft on 2 levels + basement) with the Waterfurnace Synergy 3D. We plan to use the existing ducts for cooling, but we plan to install radiant floor heating as the only heating source. Manual J calculations seem to point to a 5T unit to satisfy the heating load (the cooling load is only around 35,000 BTU).

    The first question we have is about the loop size. One proposal we got promised 160' of vertical 1.25'' loop per ton. Another contractor noted that based on a software that takes into consideration the location-specific factors, he would only install about 600' of 1.25'' loop total. We live in an area with solid rock about 40' below surface and average temperature of 56F. We understand from reading this forum that loops should not be sized by a rule of thumb, but we've seen too many mentions of the 150-200' per ton before not to be a little worried. A software can be wrong too... Of course the lower cost of the shorter loop is very tempting, but we want to do this right. Could anyone let us know whether the shorter loop makes any sense?
    One of the reasons we are worried is because another contractor mentioned that using radiant floor heating should be actually paired with a loop sized for a 6T unit. That really confuses us because we thought radiant floor heating was more efficient than forced air heating.

    The second question relates to duct sizes. For cooling, we want to have three rooms of the house on a separate zone from the rest of the house. We would like to create this zone using the three existing 7'' round ducts and just branch them off in the basement from the main supply. On one side, we are being told that additional ducts would need to be added between the basement and the second floor because the three existing ones together are not sufficient for the 5T unit output even when the unit runs in the lower stage, and thus we would be damaging the unit whenever the small zone calls alone if we did not add more ducts. On the other side, we hear that this is not a problem when a Waterfurnace zone controller with a variable speed fan is used. Could someone please shed some light into this for us?

    Any insights would be greatly appreciated.
    Thanks a lot for reading this.
  2. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader


    Software design is not the answer to all. I've seen similar outputs that I've had to argue against. I have the benefit of installing what I design so I see what works, but I've seen some pretty short looped proposals that I've had to compete against.

    Just ask them to explain the big % reduction in their design and their results warranty associated with it.

    Basically, a shorter loop may make sense, but not that short.

    2nd question
    The distribution method of the btu's (radiant and/or air) does not directly influence the size of the ground loop. But there may be a bit of "fluff" being suggested with a 6T ground loop to deal with a hybrid unit as it is a bit confusing as a designer to size one. You have run times typically associated with a heat pump to satisfy a house requirements. When it is one unit doing both, but in an alternating fashion, those run times can be difficult to predict. Put another way, a forced air unit running 1500 hours a year would have a different sized ground loop than one running 2000 hours a year. Software is not very good at working with hybrid units.

    But that is likely a bit more than you wanted to know:).

    Duct sizing
    The most important side of the equation is the return (or supply) air to the unit. Zoning can constrict this obviously. Pressure bypasses and ducting designed for expected flows help to make it all work. The intellizone does work well in situations like yours, but you can only step down a unit so far in zoning. I think this is 25% of full house size, but I'd be careful about taking it that far.
  3. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    Hi and welcome,

    At the end you need to trust the installer that has a long term relationship with installs in your area, the loop contractor, and is willing to put his money on the line to back up the design, not yours.

    Software is a great tool to add to the tool box, but the tool box should have a lot of "what works here" in it as well. Let your common sense guide you and get some refferals or check their pedigree may suffice.

    hope this helps
  4. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    RE loop size: the 150 to 200'/ton you see bandied about is generally in reference to 3/4" pipe. 1.25" utubes should collect 10+% more. The other factor in vertical loops is average depth. 2- 300' wells may colect more heat than 4- 150' wells.

    RE ductwork. 1st stage of a 5 ton is loosely 3tons and will require 1000-1200 CFM. That won't work in 3 branch ducts so you either by-pass (short circuit supply and return ducts), bleed (spill some of the air into other zones) or increase zone size. Be wary of anyone that suggests you can do what you described.

    RE radiant being more efficient- it's all about design. Depending on the water temps you hope to achieve it may or may not be.

    Good Luck.
  5. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    Here is an idea

    That could solve all three points.

    Take the whole system water to water.

    In our area, (Northern Ohio), and beyond, like Joe says we are talking about 3/4" pipe, the going rate is 300' per ton. I tend to go towards 500' per ton if there is any question about heat transfer. (BTW that number comes from how they package the pipe. Why throw away a bunch of 100' scraps?). So I feel your loop sizing is low but I do not have boots on the job site.

    My systems are more water to water as I find it easy to move BTUH with water. To give you spot on equipment sizing requires a heat loss and gain calculation. With those numbers one is then able to make an educated decision as to what size to buy.

    Now comes the radiant heating which ASHRAE, claims as a delivery system, to be 30% more efficient than forced air. Their thinking does not take into account the added comfort and air quality using radiant delivery. Google "Robert Bean" for more information and understanding. I am not sure if the RPA website is still working.

    So feed the heat pump into a buffering tank. The buffering tank tells the heat pump when to start and stop based on water temperature. It does this in heating and cooling, so when a zone calls for service the tank is full of BTUH and ready to go.

    I have not found a customer to let me try radiant cooling yet. The issue with radiant cooling is when the panels reach a temperature below the dew point they will condense water vapor. The controls are out there to do this. You will still need moisture and latent heat removal and that means equipping the system with an air handler.

    My favorite system is a high velocity air handler equipped with an electronically commutated motor, commonly called an ECM drive. Since you have standard velocity duct work such a system would waste what you have.

    Rather than throw away the duct work, I would spec what is know as a four pipe air handler. They are or soon will be equipped with ECM blowers. What does that gain for your system? First when you have a call for cooling there is a buffer tank full of chilled water ready and waiting. Turn on a pump and end the call. Second there are no issues with taxing the heat pump in a zoned system. Joe's comments about air flow are still an issue, but not one that will damage the heat pump. Third, with the warm water coil in the air handler you may add additional heat to the home if the geo is unable to keep up with the heat loss, and still keep the geo system running and contributing to your comfort with the lowest cost fuel.


  6. Howard Ek

    Howard Ek Member

    I would go with a ClimateMaster 5 ton water-to-water unit and a ClimateMaster 27 3 ton unit, if it was me. Climatemaster gives you back $900 cash in your pocket!:)
  7. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Temperature and Energy logging by: Web Energy Logger

    The above linked system operates with 133ft/ton of 1.25" pipe, is heat only and has a deep ground temperature of 54F in Buffalo, NY. However, it never drops below 30F entering water temperature (EWT). Rock starts at 2'. Going only with 2 boreholes should help.

    Your system appears to be similar. Your contractor seems to use Waterfurnace software which now auto sets to 28F min EWT due to a wider operating range of the heatpumps. Nothing wrong with it, no reason your loops should not work fine.

    Hybrid units need to sized larger when used for dual heat mode (forced air and radiant) in order to satisfy the load of either the radiant or the forced air zone in a timely fashion, and then switch over, but in your case that is not necessary since you use radiant as your only way to heat.
    The problem with radiant only is that you cannot bail yourself out with electric supplement heat easily, although we utilize electric heat elements in the buffer tanks sometimes. So usually, the heatpump needs to be designed to carry the entire load, including the most extreme temperature.
    Last edited: Apr 4, 2012
  8. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    Given the relatively low cooling load in a mild Mid-Atlantic climate AND the desire to operate a small zone I'd look long and hard at the decision to go with 5 tons.

    A 5T is gonna wanna move every bit of 1200 CFM in low gear and attempting to feed even half of that into just 3 sevens will make it sound like the registers will be blown off the walls / ceiling.

    A 5T meeting a 3T cooling load is unlikely to do so with good humidity control.

    Either investigate ways to bring system size down to 4T or less or go with some combination of dual systems.

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