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Workshop with in-slab hydronic - temp control

Voyager
Voyager Member Posts: 258
edited December 2018 in Gas Heating
Ironman said:

What's your tubing size, spacing and length? 110* SWT seems high for a shop.

My shop is 48’x32’ with a 5” slab. There are three loops of 3/4” tubing spaced 12” and roughly centered in the slab. So, each loop is about 500’ long including the runs back to the manifold. The contractor told me the loops were 400’ each, but when I looked at the few pictures I got before the slab was poured, it appears to me that this number is low and the square footage and tube spacing suggests that also. I wish 4 loops had been used, but the length is still within, barely, the maximum length recommended for 3/4” pex.

The boiler has never even approached 110 SWT so I could easily set the upper limit back. In operation, I have never seen SWT higher than 82. I had hoped for a deltaT of around 10 degrees, but given the loop lengths the contractor installed, the actual deltaT runs closer to 20 degrees even though I have a Taco 0011 driving the three loops to handle the high head of loops of this length. The RWT is typically 62 or slightly higher.

I wish TT had a display that showed the firing rate, but I assume the little cc50s is running flat out most times it fires given the 20 deltaT and the fact that the boiler temp never approaches the limit. I think the highest boiler temp reading I have seen is 92 which seems odd to me as the temp at the exit from the boiler has never exceeded 84 and this is within 2 degrees of the inlet temperature of the pex manifold. I think any difference there is just thermometer error as there is only about 4’ of 1” copper pipe between the boiler supply and the manifold inlet.

I am not sure where internally the cc50s is taking its temperature measurement, but it consistently reads 6-8 degrees more than the temperature gauge I placed at the outlet of the boiler.

Comments

  • kcopp
    kcopp Member Posts: 3,669
    The sensor is on the left hand side of the block towards the back. You could use a strap on thermistor w insulation to sense what is coming and going.

    https://www.supplyhouse.com/Azel-Technologies-DS-60P-Dual-Zone-Digital-Temperature-Gauge-with-Two-Universal-Sensor-Probes

    Are you using the outdoor reset?

  • Ironman
    Ironman Member Posts: 6,336
    Was there any actual design work done to determine the heat loss, tubing layout, components sizes, etc?

    How much insulation under the slab? Around the perimeter?
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • GroundUp
    GroundUp Member Posts: 1,142
    Where are you located and how warm are you keeping this building with supply temps that low? The 0011 is grossly oversized for 400ft loops of 3/4" tubing, it's only ~5 ft of head and an 007 would perform the task just fine.

    What is your question, exactly?
  • Voyager
    Voyager Member Posts: 258
    kcopp said:

    The sensor is on the left hand side of the block towards the back. You could use a strap on thermistor w insulation to sense what is coming and going.

    https://www.supplyhouse.com/Azel-Technologies-DS-60P-Dual-Zone-Digital-Temperature-Gauge-with-Two-Universal-Sensor-Probes

    Are you using the outdoor reset?

    Yes, using ODR.
  • Voyager
    Voyager Member Posts: 258
    Ironman said:

    Was there any actual design work done to determine the heat loss, tubing layout, components sizes, etc?

    How much insulation under the slab? Around the perimeter?


    Design work was done after the fact. I hired a GC who said he was familiar with in-slab hydronic, but when I saw the tubing layout, I became skeptical. Unfortunately, I was traveling a lot and did not get to the design work until after the slab was poured so the tubing length could not be changed. I did the heat loss calculations, sized the boiler and did the boiler installation myself.

    2” of rigid foam under the entire slab. Unfortunately, the edges were not insulated beyond the 1/2” asphalt fiber expansion material. The stub walls from the footing up to the wood frame walls are concrete block so the end of the slab is not exposed to outside temps, but has only an 8” concrete block and air cavity or grout as insulation which is pretty meager. I did the edge loss calculation using the assumption the slab edge was uninsulated, so it should be conservative given that the concrete block and air cavity will provide some resistance to heat transfer.
  • Voyager
    Voyager Member Posts: 258
    GroundUp said:

    Where are you located and how warm are you keeping this building with supply temps that low? The 0011 is grossly oversized for 400ft loops of 3/4" tubing, it's only ~5 ft of head and an 007 would perform the task just fine.

    What is your question, exactly?

    Not a question per se, this is a continuation of a thread that became off-topic in another thread titled “Boiler takes long time to heat house.” The discussion was related to avoiding overshoot in a hydronic slab heated structure when the night to day temp swings might be 20-30 degrees. This is very common here in PA.

    My suspicion is that you can’t totally prevent that simply due to the physics involved with a high mass emitter. Others have suggested it is simply a control issue, but I think it is a physics issue also. Certainly, sophisticated control is needed, but unless you also have a chiller, when a warm slab meets a warm day, the building is going to overshoot as you simply can’t make the heat in the slab disappear. The only control scheme I can see which could address this is a “look ahead” thermostat that used temperature forecasts to start dialing back the heat input long before the outside air temperature, let alone inside air temp, had even begun to respond.

    To answer your questions:
    1. Located in north central PA just 1/2 mile or so from the NY border.
    2. Currently keeping the building at 55. I had it at 60 for a while, but that was a little warm when doing physical work.
    3. I used the Zurn Radiant Heating Design and Application Guide as it has a lot if information in one place. I calculated needing 40,700 BTUH at design conditions (10 F). This is 13,500 BTUH per loop in round figures. Using a DeltaT target of 10 degrees, this gives 2.71 GPM per loop with 100% water. I wanted to allow up to 30% antifreeze which would require 2.82 GPM. According to the Zurn tables, this yields 2.54 fps velocity which gives a head loss of 25 feet in 425 ft of 3/4” tubing, which is what I used in my calculations. I now suspect the tubing length per loop is closer to 500 ft. So, unless I have erred in my calculations, the head is quite high in my system. That is why I chose the 0011 to handle the design condition of 8.47 GPM @ 25 ft of head.

    I have only seen a couple of nights colder than the 10F design outdoor temperature, but the system had no trouble maintaining 60 inside on those two nights.

    I can’t find a maximum flow spec on the Zurn manifold that I have (QHPM3-S), but the flow meters only go to either 1.3 or 1.6 GPM (both values appear in two different Zurn documents and the meters themselves are hard to read), so I suspect that is the most they will flow per loop. Since I am seeing actual deltaT values closer to 20 than to the 10 I designed for, this is consistent as I had planned for a maximum flow of about twice that. I am running 100% water at present and may not add the antifreeze as I read more bad than good about its affect on pumps and seals. So, this gives a little more BTU capacity per gallon. My zone valves are currently manually controlled and are wide open.
  • GroundUp
    GroundUp Member Posts: 1,142
    With a 500ft loop, you won't likely see 2.82 GPM regardless of circ and with your 20 degree delta, I would estimate you're in the 1.5-1.8 range which makes ~8-10 ft of head with straight water. I had missed something earlier when I said 5, that was my mistake. 40,700 BTU seems awful high for a 1536 sq ft building of modern construction at +10F design also, I can't see it being more than 25k unless you are lacking in insulation or have a whole lot of overhead door area. Unrelated to the subject at hand, so my apologies for bringing it up, but I was mostly curious as to why some of these choices were made. Sounds like you have a good handle on it
  • Ironman
    Ironman Member Posts: 6,336
    26.5 btu's per sq. ft. does seem a little high unless there's some unusuall heat loss.

    According to my radiant calculator, you should only need 78* SWT to maintain 65* inside with a 26 btu per sq. ft. output.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Voyager
    Voyager Member Posts: 258
    GroundUp said:

    With a 500ft loop, you won't likely see 2.82 GPM regardless of circ and with your 20 degree delta, I would estimate you're in the 1.5-1.8 range which makes ~8-10 ft of head with straight water. I had missed something earlier when I said 5, that was my mistake. 40,700 BTU seems awful high for a 1536 sq ft building of modern construction at +10F design also, I can't see it being more than 25k unless you are lacking in insulation or have a whole lot of overhead door area. Unrelated to the subject at hand, so my apologies for bringing it up, but I was mostly curious as to why some of these choices were made. Sounds like you have a good handle on it

    No apology necessary. These are good discussions. This is my first hydronic design and installation. I am retired now so I thought it would be a fun learning exercise and it has been. I am sure I have made a few mistakes along the way and I undoubtedly do not yet have the reset curve set optimally.

    I may have been overly conservative in my assumptions. Part of the reason the loss is a little high is that I have 14’ wall heights in the workshop. In addition, I have 8 good sized windows, 2 man doors and a 12’ x 12’ overhead door so that I can fit my camper in to work on it. My slab is almost entirely above grade, and the lower side of my building drops about 3’ to make a shed roof area where I store my camper and other equipment under roof. So the slab on that side is 3’ or so above grade, though with the insulation under the slab this should not make a huge difference. I also was conservative on the air infiltration rate as while the building is insulated, the outside walls are not tightly sheathed, but have metal siding which allows air to move up the outside of the insulation given the ribs in the steel. I can feel air come through the electrical outlets when the wind is blowing. I just added foam gaskets behind the switch covers, but likely will need to spray expanding foam around the boxes help this. But I know that the stud cavities are simply not sealed as with a house, so the insulation effectiveness is reduced somewhat.

    So, the actual heat loss may well be less than 40K, but I will know once we get a few below zero nights. It didn’t matter much as I could not find a 30,000 BTU boiler. I am sure someone, somewhere makes one, but the cc50s was about the smallest one I could find. And it should turn down to 12,000 or so which isn’t a bad match most of the winter. I considered using a water heater, but I really wanted the quality of a purpose built boiler and money wasn’t really a serious constraint. I wanted a solid system that would last for a few years and hopefully a few decades, though I hear the new modcons don’t hold up like the old cast iron boilers did.

  • Voyager
    Voyager Member Posts: 258
    Ironman said:

    26.5 btu's per sq. ft. does seem a little high unless there's some unusuall heat loss.

    According to my radiant calculator, you should only need 78* SWT to maintain 65* inside with a 26 btu per sq. ft. output.

    My estimate may well be high as I just posted. I currently am running about 80 SWT and the building stayed right at 60 on a couple of nights recently that dipped down to near zero. I have been looking for a decent thermometer to measure slab temperature, but haven’t picked one up yet. That would allow an accurate calculation of BTUH/square foot.

    Even though I used the recommended 70 inside and 10 outside design temps for my area, I likely will never run the shop above 60 so I have some conservatism built in right at the get-go. Then again, the 10 degrees is based on the 97.5% probability ASHRAE assumption as I recall, which means that 2.5% of the time the temps will be below 10 degrees. This happens fairly often here and temps of -15 aren’t that unusual, but generally last only a night or two. So, I think with the 5” slab, I have the thermal inertia to ride through a few nights of temps below the design value.
  • DZoro
    DZoro Member Posts: 1,048
    Even with all the calculations, in real life there usually is room to move the water temps. Seems like they also have a buffer built in because they too don't want their numbers called out as wrong.
    Don't be afraid to lower your water temps, you probably will be surprised how much lower you can go and still maintain comfortable heat and heat bill.
    Always let the system adjust to your adjustments, this will take a few days, so don't be in a hurry when doing this.
    D
  • Voyager
    Voyager Member Posts: 258
    DZoro said:

    Even with all the calculations, in real life there usually is room to move the water temps. Seems like they also have a buffer built in because they too don't want their numbers called out as wrong.
    Don't be afraid to lower your water temps, you probably will be surprised how much lower you can go and still maintain comfortable heat and heat bill.
    Always let the system adjust to your adjustments, this will take a few days, so don't be in a hurry when doing this.
    D

    Very true. And thought, as an engineer, I have a fair bit of trust in the calculations, there is still a lot of judgement and Kentucky windage involved. Air infiltration is a big one. Unless you spend the money to have this measured, you are making a judgement and likely are off by 50% or more. The good news is that the heat load methodology is fairly conservative, so it is unlikely that an undersized boiler and emitters will be selected.

    I am still lowering the ODR curve and may end up with the upper end under 100 degrees. I started at 130 and am down to 110. I have been giving it a couple of weeks in between each tweak.
    DZoro
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