Zoning advice?

varadhammo

Hi,

This building is a community bath house, 4 full bathrooms roughly 9' x 9', utility/laundry room ~ 16' x 18'. All radiant, 1/2" PEX in 4" slab, 2" XPS under whole slab and perimeter. The two bathrooms in the front of the building have 2 exterior walls each, two in the middle have one exterior wall each. My calculated required slab temps for each of those three are 84.5, 83, 81 degF. (29, 26, 22 Btuh/ft^2 respectively at design).

Should I control these as separate zones w/ 3 thermostats? Or use 1 master thermostat for the building and try to balance flows on the manifold? If the latter, where should I put the thermostat? Would sensing the slab temperature be better than sensing air temp? (I'll definitely use PWM radiant thermostat(s), Tekmar or Wirsbo.) Please advise!



Thanks,
Jake

hot_rod

sounds like a good application for a dual stat. I like the tekmar 500 series dual stats.

Why not zone it, just an addition couple actuators and stats. Put a well into the slab for the sensors, pex with a copper stub on the end for good conduction, bring it right to the stats location.

varadhammo

Which tekmar stats are you referring to? Do you mean multiple thermostats with a wiring center, or is it actually a single unit with multiple sensors that controls multiple zones?

Ironman

What's your heat source? The total for the entire building only comes to just under 7,500 btu's. Trying to micro zone that (or the entire structure) is gonna cause major short cycling without some buffer.

hot_rod

varadhammo said:

Which tekmar stats are you referring to? Do you mean multiple thermostats with a wiring center, or is it actually a single unit with multiple sensors that controls multiple zones?

This is the versionI like, also available as a programable model.

Good question regarding the heat source, if you have a boiler will it also supply DHW? Sometimes a reverse indirect can buffer and produce DHW, Thermomax for example.

varadhammo

Ironman-

I'm not sure how you calculated that... I get 2 rooms x 9' x 9' x 29Btuh/sf + 2 rooms x 9' x 9' x 26Btuh/sf + 1 room x 16' x 18' x 22Btuh/sf = 15246 Btu/h total load... (adding infiltration and downward loss and without rounding off my actual total is 16645 Btu/h)

The source is wood boiler with storage (GARN) so short cycling shouldn't be an issue. DHW will be a plate exchanger recirc loop to electric tank.

If I use a thermostat w/ slab sensor, wouldn't that not be able to respond to different heat loads at different times? I.e. the floor needs to be warmer to keep up in January than it does in September...so change the floor setpoint according to the season? Or does the tekmar consider both the floor & air temps in some smart way?

Ironman

I mis-read 16x8 as the total. 16k btu's is still a micro load since you only need that at design temp.

If you want to regulate slab temp to match the load, then you need variable speed injection mixing or a smart valve using outdoor reset. The Tekmar with the sensor will compensate some, but not to the degree that's needed to adjust slab temp to the load.

wrxz24

80 degree floor temps are pretty warm? I ask because I have radiant floors as well and the tekmar stat with the floor sensor. The only time my floor temps go above 80 degrees is during the real cold days. You can set min and maximum floor temps so you can can heat to the floors despite the air temp and I believe you can disable the air temp and control just the floor.

hot_rod

I don't think the micro load will cause an issue in this case. The Garn wood boilers hold between 980- 3200 gallons, depending on the model.

The stat mention can impose limits on the slab and air temperature.

Remember whenever the slab surface is warmer than the ambient, heat energy is emitted. Roughly 2 btu/ square foot for every degree difference.

It all comes down to how much you want to $$ to have how much control. Indoor feedback, outdoor feedback, ODR, and even t-stats that tie to weather channels and adjust based on incoming weather, are all available currently.

I have had really good feedback from customers that use the combo air and slab stats. It is an affordable solution.

It's the unknown and ever changing load in an application like that making "exact" control a challenge.

I'm assuming there is a mix down device and the slab is not seeing 180°F from the Garn

varadhammo

Yes, for mix down I have a caleffi mixcal (the one with thermometer & check valves).

I found a good price on a Tekmar 509...that's the older model, now replaced by the 519. I can't find any info whether there's any significant difference between the two, any idea?

hot_rod

I've used the 509 version. The 519 looks like a newer design and style, maybe larger screen?

I used the 012 adapter plate also and mounted a 2X4 electrical box so the sensor well tube would end up in the box, behind the stat. That allowed you to insert and remove the sensor if it ever fails.

The adapter plate covers the electrical box perfectly.