Will an outdoor reset benefit me?

Discussion in 'Quotes and Proposals' started by minnesotajoe, Dec 4, 2011.

  1. minnesotajoe

    minnesotajoe New Member

    Bear with me I am new! I am getting a water to water geo unit to heat a buffer tank. With the tank I will be heating 2 concrete slabs, one in the basement and my garage floor. I will also use it to heat the water coils in my two air handlers which both have variable speed blowers The duct work will carry the majority of the load. The unit has a desuperheater that will be hooked into it own buffer tank before my water heater. Primarily heating here in Minnesota. Should I be using an outdoor reset in this situation? My quote doesn't include one. What is included is just a thermostat on the buffer tank to control temperature. Any advice would be appreciated!
  2. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    I insist

    Well, I strongly encourage outdoor reset with all our radiant systems.
  3. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Yes and no.

    The problem is that you might need different supply temperatures for your radiant floors and your air handler. If you designed your radiant floors with 85F supply, how much lower can an outdoor reset get you?
    You air handlers might need a certain minimum supply temp. However, your floors turn on and your cold slab might render a return temp of 75F, way too cold for the air handlers.

    So it depends a bit what your designed supply temp is, your flow rate and how you control all this. At the end you might not realize much savings from outdoor reset.

    Resets are no brainer at higher temperature radiant systems, like radiators, which require a higher temp at colder outside weather, and the supply temp can be lowered significantly at warmer outside conditions.
  4. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Different temperatures

    I understand how different temperatures for air handler and floor slab, but not sure how outdoor reset would effect that? Just gear down to the higher one required. Or another way of asking - not sure how it would be different from not having one (mixing valves?).

    But I do see if you had a system designed for 85F then you wouldn't need outdoor reset.

    I also like the "off" that outdoor reset gives us in the summer.
  5. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    You might want to consider looking at the design again and consider a hybrid unit, such as the hydron combo or waterfurnce synergy 3d.
    That way you can run the hydronic portion at a different temperature e.g. 85F, and then switch over the forced air and have the desired delta T over the coil. That way you do not blow colder air around your house, and would not have a need for an outdoor reset.

    This can work extremely efficient, here is an example. You could nicely zone it with Intellizone.
    Temperature and Energy logging by: Web Energy Logger
  6. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    I am not sure if mixing valves would make much sense with geo. First you pay with less efficiency for the higher temperatures, than you mix it down again. Not a good way.

    The higher one required should be at least 105-110 degrees, so you do not blow cold air around your house. 110 degrees can be very hot for radiant floors.

    Radiant floors and air handlers is not a great combination if you want to run things efficiently. Again, spend your money on designing your floors with a lower off center space between the pipes, allowing for lower supply temperature and consider a hybrid with 2 forced air zones.
    Last edited: Dec 5, 2011
  7. minnesotajoe

    minnesotajoe New Member

    The geo is a retrofit, added on to the house twice in the last 4 years. Radiant slabs are 12" on center with 1/2" pex. The water coils are suppose to be for low temperature, i assume that means they are just oversized. Anyone have any luck with this setup on all water?
  8. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    So the radiant slabs would nee higher temps anyway, given that they are 12" OC. What is low temp for the coils? Is 110F low temp, or is 130F low temp?

    The question was not if the setup will work, but if you would benefit from an outdoor reset. The setup would work nicely, but your minimum temp would be may be around 105F and your maximum around 120F.

    I am not sure if you would benefit from an outdoor reset. It depends on your sizing, the load, what kind of heatpump etc.

    More info needed about your system.
  9. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    I am typing to let Joe know

    I will be watching this thread two of the best in the business are on this thread now.

    I have nothing to add since we do not have enough information.

    Last edited: Dec 24, 2011
  10. minnesotajoe

    minnesotajoe New Member

    Here is some information about the system

    Sorry for the wait, I did some research on my current electric bill and the heat pump capacity. Both my blowers are variable speed. Unit is a Econar GW 871 it is rated for 66400 btu @ 32 degrees of loop temp and a 104 degree output. It has been a fairly mild December so far but the coldest day I used a total of 320 KW for the day, it was a high of 19 and low of 1 that day. I am on all electric heat currently. If my math is right I come up with 45,400 btu/hr. A schedule J (?) was done, and it was 90k btu for the whole house and my large garage heated to 50 degrees. It was done at a 90 degree temperature differential, so 20 below zero. I wasn't planning on heating the garage to 50 when it is negative 20. But it would be nice to use the geo when I have the capacity to run it. Let me know what other information you need.
  11. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    I guess I am confused here! Are you on geo or electric resistance? 320 KW is way too much for 8 ton geo, so is the geo system in the process of being installed? I sthe question stil if outdoor reset makes sense? As mentioned earlier, that depends on the efficiency of your distribution systems. What is the BTU of your radiant floors at a certain supply temperature? Same question for your air handlers.
    Merry x-mas!
  12. minnesotajoe

    minnesotajoe New Member

    Electric now, committed to the econar unit, install will be in January. I should have left the electrical usage out of the conversation, I apolgize.

    Yes my question is still about the outdoor resest. The 2 air handlers in the planned system are each rated at 54,300 btu @ 10 gal per minute with an ewt of 110 degrees and 1600 cfm. I have no idea design temps for my slabs other then sizing, 1092 and 1215 square feet, 1/2" pex 12" on center. 2" foam insulation under slab and around the perimeter. The heat load for the house is broke up between the 4 zones, 35% for each air handler and 15% for each slab. Hope this is the information you need.
  13. minnesotajoe

    minnesotajoe New Member

    Forgot to add that I am using a temporary 9 kw electric boiler now that I have adjusted to have a lwt of 110 degrees and it has always kept up with the load. Had it set this way all last winter and never had a problem. I think it could get turned down farther but 110 is the lowest setting on the unit.
  14. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    I guess I would put an outdoor reset in. I also would put more pipe in, at 8" OC, lowering the needed supply temperature. Then the air coils would get a lower supply temp as well, running longer and using more electricity for the blower. So the outdoor reset's range would be quite limited, probably between 105F and 120F supply temp to buffer tank, resulting in 97-112 F going to the zones. Keep in mind, at 120 F your heatpump is not making the 66 KBTUs anymore, but more likely below 60KBTU.

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