Controls or Buffer

Tom_133

Hello all,

I have a new customer who has a large building with air handlers some in the attic and some wall units, but all are coils and fans, the boiler is a New NTI TFT 175. The whole thing works very well, except one problem. It cycles, a lot. The heat loss on the building is 110K it has 25% Glycol mixture due to the attic piping, the boiler outputs around 140 minus the glycol effect. I stopped in yesterday, (it was 40 degrees outside) and what I found was the main auditorium which has 1 sensor that tells the Honeywell Tstat the temp, (which doesn't seem to have a built in differential) basically drops maybe a half of a degree (the stat set for 69, calls at 69) and calls for heat. The boiler is set for a anti short cycle of 5 minutes, the stat calls every 5-10 minutes. This cycles the boiler A LOT. I made the adjustment to the anti cycle to push it to 20 minutes, now when the stat calls the boiler won't start but gives off it's left over temp and that is typically enough to satisfy the call. The problem is the boiler the runs for 2 minutes every 20 minutes, to satisfy a minor call at least at 40 degrees outside and that drives me crazy. I want to be able to adjust the ASC for occupied and unoccupied, if I could then I could do a solid hour on the ASC during unoccupied which would allow the boiler to run a bit longer and catch up. The boiler control can't do an unoccupied and occupied. My fear is during occupied that a long ASC would make it blow colder air on people and the complaints will be endless. Should I do a tekmar or a Buffer? I will be changing the auditorium thermostat to find one with a 2 degree differential and adding a couple of sensors to allow it to average better.

nicholas bonham-carter

Could an aquastat control the blower, so that there would not be a cold air problem?--NBC

njtommy

How is the building piped? Does it have a main supply and return pipes that goes through the entire building and every thing branches off of it?
Or is the pipping direct runs from the mechanical room?

A combination of a buffer tank and controls is probably not a bad idea any way. Buffer tank would keep the boiler running longer and have less cycles which is a win win.

Tom_133

Agreed,

NBC, yes that would definitely help.

NJtommy, let me show you the boiler piping.

Each pump goes to a coil. I installed based off someone else's recommendation and now I kick myself that I didn't do the proper homework. I removed a 400K Btu boiler, that was piped wrong.

njtommy

A buffer tank looks like it wouldn't be too hard to install with a 2 pipe configuration.

What do they have for controls?

Tom_133

A 6 zone taco relay control is the only controls it has right now, with Tstats wired to each one. I was considering a Tekmar and upgraded Tstats. I do really like this boiler it's really well built.

njtommy

How many heating cycles per hour are the t-stats set for?
Also are you running odr?

Tom_133

Yeah ODR is hooked up and is why I stopped in they said they had no heat when it got real cold out, I figured it was the ODR messing with it, but it was just a fuse at the fan for the main auditorium.

The Tstat is a touch screen honeywell and there is no runs per hour setting, it just calls as it needs to, which seems like every half a degree lower than setpoint

njtommy

If it's a touch screen Honeywell it's most likely a vision pro 8000. They have a ton of options depending on model some won't have a differential in less it's a two stage t-state. They only thing you can do is lower the amount of heating cycles per hour to 2-3.
All the info on the t-stats is on line or you can call tech support. They are pretty good.

This could really be a simple t-stat program problem or even t-stat is in the wrong location. I.E. Breezy door way, sun light hitting it or drafty window.

I would imagine it was a problem before you replace the boiler, but it went unnoticed.

Tom_133

Yes I agree. I don't have a boiler problem, as I have said the boiler is excellent but according to the display it has cycled on and off 4100 times in 3 months. I do have a short cycle problem. Part of the problem is the other zones are much smaller, most are single K84's in the wall and even at 180 they only put out 10K. I wish I had looked around the building before doing this job. It was spec'd by someone else and I listened, oh well lesson learned. I will be changing Tstats. The other zones tend to call far too often as well. I may put a deeper anti short cycle and see how it goes.

njtommy

If the zones are 10k each a buffer tank would still be on the table in my mind.

Rich_49

What jumps out at me is the fact that these small units each have a dedicated circ . Probably very low head loss which would suggest a really high flow of really hot water . Take a look at the standard aquastat on these and the corresponding outputs at various temps (swt) . A 2 pipe buffer may very well be a giant help , after the ODR is utilized and the header is reconfigured , possibly with one right sized circ for the load and zone valves . Heat that cannot be dissipated is not heat at all , in fact it is waste . Zone by zone or room by room heat loss is in order to determine what is actually needed .

Needless to say that this boiler in this config. can never condense and will suffer from high limit issues . That aquastat on the boiler loop should be a sensor capable of telling the boiler that temp based on ODR .

Please see next post .

Rich_49

See the attached for flow and temp specs .

Tom_133

Rich, thanks for the input, I completely agree with your thinking and recommendations. I was doing this job as a favor to a friend who couldn't get it done. I went by his spec's assuming he had it all figured out. I was hoping to avoid a buffer tank due to cost and time. Possibly do a tekmar that would allow more cycles when necessary during occupied so the main lobby entry stays warm during use, and the auditorium wouldn't cool too much during ASC delay. Then unoccupied I would set a deep delay in ASC at least an hour and let the boiler run on a tight outdoor reset configuration. With a new Tstat and some fine tuning I will see how it goes. Thanks all for the time.