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Thread: Hydronic radiant thermostat

  1. #1
    DIY Junior Member birchlake's Avatar
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    Default Hydronic radiant thermostat

    Last fall, I built a small 5 room 1008 sq. foot patio home. Single level, slab on grade with hydronic radiant heat supplied by an Electro Industries 9kw mini boiler (model EMB-H-9). The entire house is on one zone. The home is located in Minnesota. I use a ductless mini-split for heat in the spring and fall and the hydronic heat for the coldest weather.

    The hydronic system is currently being controlled only by the slab sensor using a Ranco single stage Electronic Temperature Control unit.

    I love the in floor heat, but the one thing I am not crazy about is having to program the Ranco control unit slab temperature sensor setpoint frequently throughout the winter whenever there is a significant weather change. If I fail to do so, it results in either the air temperature of the house being too cold or too warm. This was particularly obvious when I left on vacation for a week and when I came back the outdoor temperature had risen significantly and because no slab setpoint changes were made during my absence, that resulted in a very warm house upon my return. And of course, extra energy consumed.

    The installer of this system had originally told me that air thermostats are not a great way to control hydronic radiant systems because of issues like passive solar gain, overshoot, undershoot, etc. I kind of get that, but after Google searching and seeing frequent nods towards Tekmar controls, it appears that they manufacture stand-alone thermostats that utilize floor slab temp input plus the air stat input TOGETHER, along with the ability to control minimum slab temperature. I was wondering if a stat with the ability to control these three things instead of simply slab temperature might not be a better choice for my application.

    In hindsight, I know that it would have been better if the system would have been designed with more sophisticated controls like a whole house unit and outdoor reset control, etc. But the house is built and my budget does not allow for a major upgrade at this time.

    So while I do realize "optimum" and "budget" don't go well together, my question for you guys today is this:

    If I replace the Ranco slab temp control with a more sophisticated hydronic thermostat like a Tekmar 552 and one of their slab sensors, do you think that would "improve" my control? I'm not looking for specific room control; I am fine with one stat for the entire house. If I feel that I have a shot at "improving" my control, I'm willing to try it. And I thought it would be best to get one of their higher end stats that would integrate with their whole house control unit network in the event that I am still not happy and want to make further upgrades down the line..

    If any of you have any experience with Tekmar or can make any specific recommendations for my application that would be much appreciated!
    Last edited by birchlake; 05-08-2012 at 04:18 PM.

  2. #2
    In the Trades Tom Sawyer's Avatar
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    Go with the Tekmar set up. It is much more accurate and you will be happy with the results.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

  3. #3
    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    Most radiant floor heating installers know precious little about radiant floor design. The vast majority of radiant floor heating systems we design and/or install here in Minneapolis employ a simple air sensing thermostat for perfect control and comfort. Slab sensors are strictly for advanced radiant floor designers, the people you should start with before you buy your first foot of PEX.

    http://www.badgerboilerservice.com/helpme.html

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    DIY Junior Member birchlake's Avatar
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    Thanks for the input guys.....

    Tom Sawyer, from my research, Tekmar seems to be one of the top dogs in hydronic radiant controls. When you said go with the Tekmar setup, do you mean that the Tekmar stat alone will be an acceptable improvement over what I currently have?

    Badger, I agree with you that my system was not designed optimally. I checked out your website; looks great and a lot of good information in there, but starting fresh is not an option for me. My mini-boiler is adequate for my house, I'm just looking to get a little better control of it.

    So what I'm specifically looking for comment on is this guys....................

    I am considering installing a Tekmar 552 thermostat and sliding one of the matching Tekmar slab sensors into the conduit into my cement slab (there's room in the conduit) so I can set a min/max floor temp. Will this be an improvement in my control?

    Link to the Tekmar stat I'm looking at....

    http://tekmarcontrols.com/zoning/552.html
    Last edited by birchlake; 05-07-2012 at 10:10 AM.

  5. #5
    In the Trades Tom Sawyer's Avatar
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    Yes, that is what you want
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    DIY Junior Member birchlake's Avatar
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    Quote Originally Posted by Tom Sawyer View Post
    Yes, that is what you want
    Thanks Tom.

    Along with input here, in the last couple of days I talked to applications specialists at both Tekmar (hydronic radiant thermostat supplier) and Electro Industries (mfg. of my mini-boiler) to see what they thought.

    Electro Industries said that they have had many customers using Tekmar thermostats with this mini boiler with good results. Tekmar also thought I would be happy with the improved control using the Tekmar 552 thermostat which has an integral air stat, uses input from the floor stat and allows me to keep a minimum slab setpoint temperature.

    Both Tekmar and Electro Industries reminded me that while use of an "outdoor reset control" would be optimum, what I intend to do should be quite an improvement over just using a slab stat.

    So I will be purchasing the Tekmar 552 and the matching Tekmar slab sensor and we'll see how it goes. Worth the $210 investment for improved control. I'll let you know the results, but will be next winter until I know!

    Thanks for the input guys.....

  7. #7
    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    And why do you want to keep a "minimum" slab temperature?

  8. #8
    DIY Junior Member birchlake's Avatar
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    Quote Originally Posted by BadgerBoilerMN View Post
    And why do you want to keep a "minimum" slab temperature?
    Badger: this is the recommendation that was made to me regarding minimum slab temperature.........thoughts?
    -------------------------------------------------------------------------------------------------
    As a starting point you may want to keep the Floor Minimum set = Off and the Floor Maximum set high e.g. 90F.. Then adjust your Room temperature setting to a temperature you find comfortable, e.g. 70F. Make sure the rest of the system is adjusted to allow the thermostat to reach the room temperature setting, IE. adequate supply temperature.

    Let it run like that for a few days. Monitor what the slab temperature is running at during this time to get a feeling for what floor temperature is required to heat the space. During colder weather, coming out of setback, or in the night/mornings, you may see the slab temperature running at it's hottest. How hot does it feel on your feet, too hot, not hot enough? Use this information to help determine what a warm floor feels like to you and consider that as a 'Floor Minimum' temperature setting for a couple of hours in the morning "Wake" period.

  9. #9
    DIY Junior Member birchlake's Avatar
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    Update on this one guys; sorry for the delay as I wanted to get a winter under my belt to see how things went.

    Last fall, I installed the Tekmar 552 thermostat (in place of the simple Ranco slab sensor) and I just love it. Basically, I have just set my desired room setpoint and haven't touched it all winter. The 552 thermostat is fully programmable. I've also been impressed with Tekmar's customer service with answering questions.

    As this is the simplest of systems (single zone, no outdoor reset, etc.) I have found that during quick outdoor temperature swings, I do experience a bit of overshoot and undershoot of temps, but it is very acceptable.

    So now that I'm happy with my controls, I do have one boiler question for you guys. I have an Electromate 9KW mini-boiler. The two temperature options that are viable for this house in the winter are 102F and 114F. Any less than 102F doesn't keep up. 114F does fine, but I've noticed that when temps are not ultra-cold at the beginning and end of winter, that the boiler obviously will cycle on and off more frequently than when I have it set at 102F.

    As I have to manually select my boiler temperature, would it make sense to use 102F at the beginning and end of winter so that the boiler runs longer? And then bump up to 114F in the mid-winter when temps are the coldest?

    So, in short.....

    1. Is it better for the boiler to run more continuous at a lower temp of 102F, than to cycle more frequently at a higher temp of 114F?

    2. What are the energy ramifications of the above?


    Thanks for any thoughts....
    Last edited by birchlake; 03-06-2013 at 04:23 AM.

  10. #10
    Retired Defense Industry Engineer jadnashua's Avatar
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    Each time you shut off, some of the internal heat of the boiler is wasted. Each time you start up, it takes it a (fairly short, but it's there) time to come up to full efficiency. Cycling also means more wear. Things like to just run, not turn on and off. So, efficiency and longevity will increase if you do that. Now, if it was integrated with an outdoor reset controller, it would adjust itself and be more efficient all the time, rather than when you notice it enough to make the change yourself.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  11. #11
    In the trades Dana's Avatar
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    Uh, Jim, the EMB-H-9 is an electric boiler. If it's inside of conditioned space almost nothing is wasted. With solid-state controls there is also very little wear & tear from cycling on and off, unless it's EXTREMELY short on cycles, and dozens of cycles per hour.

    But sure, outdoor reset and a continously variable output type electric boiler would be more comfortable.

    I'm curious as to why you just don't run the slab at the lower temp and let the mini-split make up the difference, unless it's a mini-split that cuts out at too high a temp.

    The Mitsubishi xxxFExxNA units have a specified output at -13F (where it's running a COP of about 1.8, which is WAY better than 1, like your electric boiler), but they don't auto-shut-off until about -18F or so, which would cover you at least down to your 99% design condition and beyond in Melrose. But if it's lowest rated output is at +5F or higher, it'll probably self-abort operation well before the coldest temps. But at whatever temp the mini-split quits, given it's much higher efficiency figuring out how to make the radiant and mini-split "play nice" together is probably worth it. A 73-74F slab is pretty cushy even if something else is carrying the lion's share of the load.

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    DIY Junior Member birchlake's Avatar
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    Dana,

    I'm gone quite often, sometimes for extended periods of time, so it just seems easiest and best for peace of mind (not knowing what temp swings may occur during the week) to leave the Mitsubishi mini-split system off and rely solely on radiant heat during those times.

    I do agree with you though that I am under-utilizing my Mitsubishi mini-split system. On my model mini-split (outside unit MSZ-FE18NA, inside unit MUZ-FE18NA..........20 Seer, 14.2 EER, 10.3 HSPF), the owners manual says that it has a 100% rated heating capacity at 5 degrees farenheit and an 80% rated heating capacity at -13 F.

    *Is there any real downside or danger in trying to run my mini-split system down to that -13 degree range? What low temp range would you be comfortable with for my mini-split?

    *Yes, boiler is electric. Went with electric as no gas line out this way and the electric cooperative offers "dual fuel" program where I get my electricity for heating at "off peak" approximately half price. Would you choose 102F or 114F during the "marginal" times of the winter, assuming the outside temps are warm enough for 102F with an extended boiler run time?

    Thanks.
    Last edited by birchlake; 03-08-2013 at 01:45 PM.

  13. #13
    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    We design radiant based HVAC systems for many homes up "Nort" WI, MN, ND and beyond. Most get a Misub. mini likes yours and an Electro with outdoor reset. This with a simple ambient-only thermostat is all that the average home will need.

    Your use of an exclusive slab sensors is very rare and does not improve comfort, efficiency or reliability in most applications. In hydronic heating systems outdoor reset, not to be confused with thermostat set-back , will maintain comfort, save fuel without complicated programming. I operate my personal mini-split down below freezing and then prefer the comfort of my radiant floors.

    We beg the folks up Nort to let us design and specify the systems and let the local mechanics install and do the maintenance with full warranty of a viable, efficient system.
    Last edited by BadgerBoilerMN; 03-19-2013 at 09:15 AM. Reason: typo

  14. #14
    DIY Junior Member birchlake's Avatar
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    Badger,

    I agree completely about the use of outdoor reset. And if I had a do-over, my house would definitely include an outdoor reset/modulating boiler system. But that is hindsight and for now I need to make do without those items. In the future if I need to swap out the boiler for any reason, I'll make the upgrade at that time.

    With the new Tekmar thermostat though, I'm really satisfied (and a little surprised) with how well my system performed this past winter. And we did have plenty of cold weather to test it out this winter!! The Tekmar 552 is basically an air sensor thermostat, with a slab sensor for the programmable features. I really like it; it's more stan than I really need, but I do like the programmable min/max floor features and it has helped me learn more about what the new stats can do. It also hosts an extra large touch screen display. My original Ranco stat was "slab only stat". I was not happy with the Ranco stat as I was constantly having to tweak the stat setpoint to accommodate the changing weather to get the desired room temperature; it wasn't user-friendly at all. But with the Tekmar stat, once I set the room temp setpoint, I haven't had to change it....ever.

    Thanks for the insight on how you use your mini-split. After studying the specs and hearing some feedback from you guys, I've learned that the mini-splits are more efficient than I thought into the cold weather! I think I will try to utilize my mini-split more in the colder weather; just haven't figured out exactly what temperature where I leave the mini-split off and rely solely on radiant.
    Last edited by birchlake; 03-08-2013 at 08:50 AM.

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    In the trades Dana's Avatar
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    Quote Originally Posted by birchlake View Post
    Dana,

    I'm gone quite often, sometimes for extended periods of time, so it just seems easiest and best for peace of mind (not knowing what temp swings may occur during the week) to leave the Mitsubishi mini-split system off and rely solely on radiant heat during those times.

    I do agree with you though that I am under-utilizing my Mitsubishi mini-split system. On my model mini-split (outside unit MSZ-FE18NA, inside unit MUZ-FE18NA..........20 Seer, 14.2 EER, 10.3 HSPF), the owners manual says that it has a 100% rated heating capacity at 5 degrees farenheit and an 80% rated heating capacity at -13 F.

    *Is there any real downside or danger in trying to run my mini-split system down to that -13 degree range? What low temp range would you be comfortable with for my mini-split?

    *Yes, boiler is electric. Went with electric as no gas line out this way and the electric cooperative offers "dual fuel" program where I get my electricity for heating at "off peak" approximately half price. Would you choose 102F or 114F during the "marginal" times of the winter, assuming the outside temps are warm enough for 102F with an extended boiler run time?

    Thanks.

    I'm very familiar with the -FE18NA, even had one installed at my mother's place last year, and they're one of the most popular units for heating Net Zero Energy homes in New England as well as other high-performance houses. A friend installed three of them on a 3-family that I helped him do a deep energy retrofit on last year. They are reasonably "smart" units, and designed to take it at low temp. As I understand ti the automatic shut down at about -18F is to self-protect from damage related to mechanical tolerances of the metals in the bearings and compressor. They have modulating internal resistance heaters to keep them in mechanical spec, but not enough to run the at any arbitrarily low temperature. I have no issue with just letting them bounce off the bottom of their spec rating, since they're designed to self-protect. I believe Daikin has similar cold-temp auto-protections built in, but I don't know at what temp they kick off. If they work for central ME and northern VT (where excursions below the self-protect temp are common enough) they'll work in Melrose MN. (IIRC BadgerBoilerMN has an FE18NA too, but only uses it primarily for air conditioning, his being a hard core wet-head & all. :-) )

    You're not really losing efficiency by letting an electric boiler cycle, and mechanical stress & degradation from thermal cycling isn't really all that much either, at the temps you're talking. How often does it cycle during the shoulder seasons at the 114F setting, and how short are the "on" cycles?

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