What's correct water temp??

D. Brown

I have a primary-secondary boiler system with the boiler located in one secondary loop, an injection system controlled by a 371 Tekmar for the main floors on a second secondary loop and a Taco mixing valve on another secondary loop for the basement and garage. The heat loss calcs say the correct supply temp for the garage and basement is about 100 deg. What happens if I set the mixing valve to, say, 130 deg?? It seems to me, 130 deg will heat the 4" concrete with 18" OC tubes faster and therefore have the boiler on less time than if I set it to 100 deg. I'm only keeping the garage temp at about 50 deg. With a setting of 100 deg, it seems to take forever to start heating the slab and then have any effect on the room temp. This is my first winter with this system so I am unsure of the proper settings.

Thanks for any help!

Tundra

Are you sure it is 18" OC for the tubes? that seems rather far appart. I would expect closer to 8" OC. I was unclear do you want to keep the garage at 50 degrees or is that all you can get? Radiant is by far my favorite way to heat but it does not have the fastest response times. In a garage situation where you open the large door and let all the hot air out it will take awhile to warm the air. If you require faster response add a unit heater to the system. A boiler that is off is zero percent efficient, longer run times are better. If you still want to raise the water temperature try raising it five or ten degrees at a time, beware, you can make the floor uncomfortable to walk on.

Weezbo

Radiant has to be installed correctly to ramp like a leer jet.

were you to use 75¡ãF water it would step up to the plate a bit slower ,thats all.radiant heat is more about distribution of btus.an old saying is" Good things come in time...." think of ramping the slabs over days and set your parameters after you have it stepped to the room temps your interested in maintaining....right now i have one of my tools on the job with a 4.5 kw "heat source"it isnt 90 in the place ,its like 15 20 below zero outside,you may be sure a heat loss would indicate no way could it work...:)right now the slab we poured last Friday and the walls are insulated with a complete vapor barrier no ceiling sheet rock or insulation...where are you located? 100¡ãF sounds like a reasonable temp for a super cold day .radiant doesnt work like a blower fan and coil thats more akin to bang its on bang its off.the "unit heater can be rated and used with steam temps .radiant stradgey says put a little bit of heat, here and there and every where:)to further answer your question,thicker slabs require more time to heat than thin slabs,the temps for one are usually higher than the other. however,lower temps can and do indeed work ...higher temps force loading slabs, can work also. only thing is ,Then You are the evil deamon when the slab cracks due to over exhuberance in the proceedure:) next time look to closer placement of the tubing ,and if you want the slab to ramp faster use two loops that return and a loop that supplies in opposite directions and install them closer to the surface in thick slabs ."careful with the Ramset". or, you can go another way. jack higher temps into the slab and watch the Mood swings...

Unknown

If your garage is up to temperature, and you open the big doors, and close them again, the room should be back up to temp in no time, unless you've left them open long enough to actually cool the surfaces in the room down appreciably.

radiant's response time in high mass is only on changes in temperature of the mass. Air temp is nearly trivial if that's the only thing that has dropped.

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Unknown

Is that mixing valve motorized? if so, the tekmar should be the best thing you could possibly use there. It should be resetting your boiler and intelligently running the spaces you are trying to heat. That is, the tekmar should be already ramping up the water temperature to a max of 140 or so (safe maximum for concrete) when it's not keeping up, and ramping it back down as it gets to its setpoint.

However, you can't be changing the thermostat settings and expecting instant response. Set the RTU to 50 and leave it, and it will run as efficiently as it can.

Boilers run best when they run long and smooth firing times.

If that mixing valve is NOT motorized and controlled by the tekmar.. well you're wasting a big part of why tekmars are great to have, but go ahead and turn your water temp up as long as you don't exceed 140.

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D. Brown

Thanks and here's more info

Thanks for all your help. My main question has to do with the cost of heating the slab. Which will do it cheaper- a low supply temp(100 deg) for a longer time (boiler is up to 180 deg longer) or a higher supply temp (130 deg) for a shorter time (less boiler time at 180 deg?).

I understand the air temp changes will not be quick, since that's the nature of radiant - I'm only trying to optimize the boiler on time.

NRT.Rob: No it's not a motorized mix valve. I am already using the 5 zones on the Tekmar 371 to control the main house so rather than put in another Tekmar, it's wired in as a setpoint command to the Tekmar which in turn turns the boiler on, keeping the boiler temp at 180 deg while the mix valve supplies the garage loops.

Tundra: The loop O.C. is 18" presumably because it's a garage and doesn't need uniform heat nor are higher floor temps a problem to walk on.

Unknown

If uniform heat isn't a big deal, I would make the garage a slave zone to one of the house zones so it can't turn on the boiler at all, it only gets heat if a main zone is calling and if its thermostat is calling at the same time. If you have another high loss zone you could slave off of that could be effective. Or, if say your basement isn't finished living space, you could make that a slave zone off of a fully RTU'ed garage zone. You'd get some variance in temperature but the 371 would always have its boiler reset feature working, instead of bypassing it for the setpoint demand. The big trick is making sure the slave zone needs less heat than its master zone.

That would be probably the best you could do without a seperate tekmar. With what you have and no other changes, I would turn up the mixing valve to 135-140 to shorten the time the tekmar's boiler reset is being overridden.

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hr

Actually radiant energy

travels at the speed of light, 186,000 miles per second. Unless you completely cool the slab, the warming effect should be instant when the doors close.

18" is not uncommon in shops. The key is the heat load calc. If the design states the load can be handled with 18" on center, so be it.

Sure the supply temperature could be ramped up for quicker response. Watch that the surface temperature doesn't become uncomfortable is all. I doubt it would with that wide spacing.

Plenty of 50 and 60 vintage slabs running 180 supply temperatures

The best efficiency will be meeting the load with the lowest supply temperature. Unless you have a condensing boiler the supply from a mixing valve driven by a 180° boiler will not be that noticable, efficiency wise. Has a bit more to do with the boiler operating temperature than the mixed down temperature when looking for efficiency gains. Outdoor reset will help with this, but the conventinal boilers need to run above that 130-140 return temperature.

hot rod

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