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Should I “Re-do” a Radiantec system

Elmo101Elmo101 Member Posts: 12
edited December 2018 in Radiant Heating
Recently installed a system I bought through Radiantec in a 32x40x12 pole Building. They did a heat calc. I have two insulated 10x10 doors. An 8x8 insulated. Two windows and, a Man door. R19 walls, R25 ceiling with steel ceiling and walls. 2 in foam under slab and 2 in foam on exterior of the slab (except where garage doors are) They sold me on a American standard 160,000 btu tankless hot water heater based on 96% efficiency. Ups26-99fc Circe pump getting 3gpm. 4 loops at 300 ft (two maybe a few feet shorter) I have spent hours reading on here after I already bought their system that I should not be using a tankless heater but a boiler. I like everything, it heats great but seems to be using a lot of propane. Around 20 gallons a week @ 55degree floor temp. Is that high or is that normal? That’s around $200 a month. Should I replumb for primary/secondary before buying a boiler? Mixing valve? Recommendations on a boiler? Supply temp is 100 degrees and return is 69 degrees. I live in northeast Ohio. Any input would be greatly appreciated

Comments

  • GordyGordy Member Posts: 9,343
    How much propane you are using is hard to judge with out knowing the outdoor temps for the period.

    That’s about 11, 000 btus an hour input on average with your usage for 7 days. Or 8.5 btus a sf.


    Your shop is 1280 sf with 160,000 btu input tankless. 96% efficiency is irrelevant, and I highly doubt You are getting that.

    The heatloss is no where near that.


  • Elmo101Elmo101 Member Posts: 12
    Most all of November was high on average around 40 and lows of around 30. For not being that cold it just seems like a lot. I still have to go through Jan. And Feb. yet. I don’t have anything to compare it to but it just seems high to me.
  • GroundUpGroundUp Member Posts: 667
    At an outdoor temp like that your heat loss is likely less than 10k which the input reflects. With a 31 degree delta T and only 55 degree floor temps, your 46,500 BTU output (at 3 GPM) seems to be doing a whole lot of short cycling and could benefit from lower SWT as well as a buffer to cut fuel consumption
  • Elmo101Elmo101 Member Posts: 12
    Would primary/ secondary piping help? I’m no plumber by any means but wouldn’t installing a buffer tank make me lose all system pressure? From me just reading it seems that these tankless heaters like to run at a min of like 15 psi.
  • wcs5050wcs5050 Member Posts: 126
    Buffer tank is still part of your system with no division in pressure. Buffer tank is a version of pri/sec piping, and probably the best one for your application. Indeed it will squelch your short cycling that is causing excessive fuel consumption...to a point. Even though you have a domestic tankless water heater it should perform as a boiler here, as it will understand the deltaT rate passing through it and modulate down as it satisfies the buffer tank. Heat emission side pulls heat off the tank at its own rate. Check out Argosy tank being “suggested” on these pages. HTP now making one as well as a few others.
  • GordyGordy Member Posts: 9,343
    Tankless water heaters are most efficient with a large delta t through the HX. Which is not what is going on in a heating application.

    That aside unless you want to change the heating plant to something more applicable. Then use a buffer tank to curb cycling.

    I think if you observe the system while heating you will find out how much it turns on, and off to reach set point. Especially since your minimum modulation is much higher than the load it is seeing.
  • Elmo101Elmo101 Member Posts: 12
    I’m not opposed to changing over to a boiler. Just not sure which one to get. I just want it done right that’s why I went with a system basically already done. Man was I wrong. The heat calc called for 38,500btuh based on a 60 temp difference.
  • GordyGordy Member Posts: 9,343
    The Smallest modulating condensing boiler modulates down to 8k input. The question is do you want to spend the dollars for the right piece of equipment, or use the one you have until it fails in the future.
  • GordyGordy Member Posts: 9,343
    The HTP uft 80 is a good one budget priced with excellent features for the price point. There are others also.
  • Elmo101Elmo101 Member Posts: 12
    Few question about the HTP.1 It says it sill work without p/s piping. Thoughts? 2 all my piping is 3/4 is that ok? I have a sp-81 relay box controlling everything. Is that still usable with this boiler? I’m using a d- 508 thermostat with the in slab floor sensor.
  • GordyGordy Member Posts: 9,343
    There are some things to think about here.

    That 26-99 will end up being huge. Since that pump was part of the package because of the high head heat exchanger tankless has.

    The tstat and slab sensor are usable. Everything else is onboard.

    So in the end you would be ditching the tankless, getting a different pump, and buying a boiler. With fuel savings unknown.

    Anything savings would be a guess right now but say 30%. Is that worth it to you? Think about the ROI for the money.
  • GordyGordy Member Posts: 9,343
    60 dollars a month Times 6 months. 360 bucks a year. Times how many years to recoup the dollars.
  • GordyGordy Member Posts: 9,343
    It isn’t like what you have is not working, but what it will do when temps get colder as far as fuel consumption, and performance is yet to be seen.
  • GordyGordy Member Posts: 9,343
    Another thing that was not asked. Are you letting this present system run, or are you setting back when not there?
  • Elmo101Elmo101 Member Posts: 12
    I leave it at set at 55 and that’s where it stays
  • GordyGordy Member Posts: 9,343
    Good
  • SuperJSuperJ Member Posts: 555
    Will the tankless heater let you run less than 100degF?
    How long are your heating calls, or what percent of the time is there a call for heat?
  • GordyGordy Member Posts: 9,343
    Your tankless propane model will modulate down to 13000 btus.
  • GordyGordy Member Posts: 9,343
    100 degrees is as low as it will go.
  • Elmo101Elmo101 Member Posts: 12
    edited December 2018
    I never timed it but I would guess it runs for 20 or 30 min. I do have the thermostat set to kick on after 1 degree temp drop. Should I make that bigger and let the system run longer? 100 deg is the lowest it will let me turn it to. The “design” temp is supposed to be 120.
  • CanuckerCanucker Member Posts: 563
    edited December 2018
    Design conditions are 1% of the heating season, typically
    You can have it good, fast or cheap. Pick two
  • GordyGordy Member Posts: 9,343
    I think you need to make some observations of the system running. Burner fire time, and off time for a heat call. Modulation rate during those times.

    Are all dip switches in their proper position. Especially for venting?
  • Elmo101Elmo101 Member Posts: 12
    Honestly never checked the dip switches. That’s something I’ll have to look into
  • GordyGordy Member Posts: 9,343
    You do have the I/O manual right?
  • Gman66Gman66 Member Posts: 18
    I think we need to go back to first principles to try and determine if Elmo is really burning an excessive amount of propane. Its possible that the problem is really just a poor guess on how much propane would be needed. The details are thin here but let me give it a shot.

    Gordy points out that 20 gallons of propane would is a per hour input of 10.9K. Elmo suggests highs averaged 40 and lows 30 for the month so lets say 35 degree average temperature. Elmo does not say what temp he kept the building but does says the slab is at 55 degrees, so I will estimate 50 degree interior temp so a 15 degree temperature differential. One final clue provided by Elmo is design BTU/hour of 38.5K based on a 60 degree differential. Based on those rough numbers I get an 88% efficiency. Lots of room for error in the calculation but I just don't see an obvious usage problem.
  • Elmo101Elmo101 Member Posts: 12
    Slab temp is 55 and room temp hovers around 50. If I were to go the buffer tank route, how big of a tank and pump would I need? Can I control the new pump with the new buffer tank?
  • SuperJSuperJ Member Posts: 555
    edited December 2018
    I'm questioning what value the buffer tank will give. I'm a big fan of buffers, but if thermal mass and min flow are already present, the buffer won't improve things much, and potentially can make things worse by raising the entering water temp to the boiler. Ideally the boiler should see the nice cold 50-70degF water coming out of the slab, not a tempered buffer tank temp.
    The slab offers a lot of thermal mass, and as long as the flow rate is reasonable then adding a buffer will not improve things much. Better to save your pennies towards a real modcon someday.
    You could probably improve the controls and pumping to get some unrealized efficiency gains.
    It sounds like there is less than 10% efficiency gain left on the table, at 55 degrees I bet most boilers would run above 98% efficiency, as long as you can keep the modulation down.

    Here's an idea to get the most out of your tankless:
    -Pipe the slab as a secondary and pump it constantly with a dedicated pump (probably smaller than your 26-99)
    -Pipe the boiler in a primary loop (closely spaced tees, and dedicated pump).
    -Set the boiler set point to 100f
    -Run the boiler pump based on a boiler outdoor reset control on the slab return temperature (this should ensure longish cycles). Since the boiler is a hot water heater it will look at it's flow switch which will engage every time the pump starts.
    -balance the boiler flow to less than secondary flow to allow for nice cool return water temps. Hot water heaters are setup for fairly low flow and high temp deltas, and that's what you need to get max efficiency. If you can't get enough heat on the coldest days allow a bit more boiler flow by increasing primary pump speed, opening balancing valve, or increase boiler setpoint.
    -You could put a room thermostat in to act as a high limit to prevent heating if the room gets to maybe 55-60f on a sunny day. But your primary control would be a tight outdoor reset, that would raise the target return water temp as it gets colder outside.

    For the Outdoor reset control you could use some like the Tekmar 256 or 260. Caveat is you may have to use a manual differential when using them on a return line, since the auto differential setting seems based around a supply temp control.
    256 is pretty barebones
    http://www.tekmarcontrols.com/products/hvac-systems/256.html
    The 260 give you indoor feedback which would be nice.
    http://www.tekmarcontrols.com/products/hvac-systems/260.html
  • Elmo101Elmo101 Member Posts: 12
    edited December 2018
    They said I did not need a mixing valve but another company similiar says ya do due to flow restrictions through the Takagi. Maybe start there?
  • GordyGordy Member Posts: 9,343
    So long as you are getting the flow rate needed for the tankless, and the slab. I see no need to do primary secondary. You are getting .75 gpm to each loop. 3gpm through the tankless.
    So long as that gpm reading is accurate. It's a 30 delta, but it's also a shop floor. Is the readings off of a flow gauge at the loop manifold? Is the flow adjusted for max flow at the loop manifold?

    The reason for primary secondary on a tankless, is because the hx on a tankless is very restrictive, and then add the head of the system sometimes the 26-99 just isn't enough pump. So you run a pump for the tank loop ( usually a 26-99 ), and one for the system loop, primary secondary which is usually a smaller circulator. The two flows are then independent.

    You might narrow the slab delta due to a higher loop flow rate, but I don't see how that will help the tankless efficiency.

    It's still a 160k tankless that modulates down to 16k for a 38k load at design. Adding another pump, and a constant flow just upped your electric bill also.

  • Elmo101Elmo101 Member Posts: 12
    Will less flow to the tankless let’s say 2gpm help efficiency while turned all the way down to 100deg? I guess that’s what I was thinking a mixing valve could accomplish while “ tighting” up the delta T. More flow through the slab while using less hot water? I’m extremely new to all of this and can’t thank all if you enough for the responses. My thoughts are to get the garage finished the way I planned then next year switch to a boiler. Just want to try to make this system work the best it can for this season without dumping money into it that could just be used toward a boiler next season. Just wasn’t sure what everyone else’s propane usage was with a shop size similiar to mine.
  • GordyGordy Member Posts: 9,343
    How are you determining the flow rate? Is it with the flow meters on the loop manifold? If so is the flow wide open to the loops?
  • Elmo101Elmo101 Member Posts: 12
    edited December 2018
    All loops are full open ( there are ball valves on both manifolds for each “run”).I have been going by the display on the tankless unit itself. It shows supply temp, return temp, and flow. I do have a thermostat probe on each manifold to double check supply and return temps. I do not actually have per say a flow meter. Sounds like a good investment to make. I just used everything that they supplied in their kit.
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