Fly Wheel Effect

keith

I,m looking for some feed back on a problem I encountered on a radiant installation for a office/ fabrication shop project. It was an addition to a existing building. The office expansion was added on to the back of the existing building and a shop stacked in back of that. The problem is in the office area. We used out door reset for the radaint water temp and on off hours every thing was great. However during the business day by about 10:00 am the space temp would over shoot the set point and the A/c would have to be turned on to be comfortable. The problem went away with a time clock to shut down the system during this time period. Do you guys have any thoughts on other options? A combination air sensor/floor sensor that has a minimum floor temp feature was suggested. Any experience with this set up?

Mark Eatherton1

Late -n- heat -n-

Did you say the place had a night time set back? Is there a substantial increase in internal gain? Solar gain? Radiant with an outdoor reset is usually pretty steady.

If it has a night set back, get rid of it. The controlled conservation effort is being done in the mechanical room. Mass intensive radiant is not conducive to deep night time set backs.

If there is a substantial change in internal gains (body, light , equipment etc.) may be you can anticipate that and shut down the system four hours before those gains come on line.

If it is compounded solar gain, a slab and room air tmep sensor will do you good.

Sometimes an oversized pump can cause blow by at the zone valves and cause overheating. Those valves closest to the pump usually bypass. A PAB (pressure activated bypass)will cure that.

The pump prophecy "If a little pump does a little good, a lot of pump will do a lot of good" couldn't be further from the truth. A lot of pump causes a lot of wasted energy and comfort control problems. It's so easy to do it right, the first time...

ME

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keith

set back

Mark, there is no night set back or solar gain. The entire problem is the additional heat gain from computers,people and lighting during the business day. On off hours the temp is perfect. The pumps are sized properly and there are no zone valves on the system. The day time time clock is the way I "corrected" the problem. thanks for any input.

steve gates

so right about those circs

my first boss sized this way' small medium and large. small house and so on. When I learned how to size a cic he'd still use his system of sizing. On my own now and have replaced several of the oversized ones because of problems like this.

Mike T., Swampeast MO

Your \"fix\"

While not elegant seems to work. Two ideas spring to mind if you want something a tad less clumsy--but still similar in nature.

1) If your reset can be reset (dropped in temp based on time) you could recompute the heat loss given the typical occupancy load and determine the water temp you need with full occupancy. Then reset your reset from around an hour or two (or whatever you need) before occupants arrive until 11:00 a.m. or so, then resume the "normal" setting. The resetted reset temp should be UNABLE to deal with the heat loss of the unoccupied space. I know you're never supposed to calculate occupant load in a heat loss, but it's only for a small time until the occupant load makes it into "full swing." You'd still be safe during non-work days as the temp couldn't drop low enough to get you in trouble. Don't know though if your (or any) reset control can do this.

2) Use a programmable setback thermostat to do the same thing--lower the setpoint before arrival then resume normal setpoint around 11:00 or so.

With some reasonbly careful adjustment of time, both schemes should be able to keep the space from being too cool first thing in the morning without overheating mid-morning. Once the occupant load has built up and you return to "normal" settings, the t-stat will run the slab at a lower temp until the occupants leave.

Hope this makes sense.

keith

set back

Your 2nd option will be a better fit for my situation. The 1st option would impact the main radiant zone loop temp. in the shop area. Thanks for the imput.