Short cycling zone valve

Wayco Wayne

It has been going through ignitors unusually fast. The customer is a very smart guy and he decided to monitor it closely since it worked well every time I was there. What he discovered was interesting. ( I keep imagining him down in the boiler room in a lawn chair with a beer in hand keeping track of the boiler operation. I would have built the system differently if I knew wheat I know now,but... It has 3 zones, Indirect, radiators and radiant floor. each zone has a zone valve and they all run off the boiler circulator. apparently when the indirect and radiator loops are not calling for heat the radiant floor, controled by a Tekmar 352 control is short cycling. Since this job I have done primary secondary and used variable speed circ controls, butthat was then this is now. What is the least exspensive way to slow this short cycling down???

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Gary Fereday

Tekmar 352

http://www.tekmarcontrols.com/acrobat/d352.pdf
There are eight pages of instruction here for that control. Only you know how your control is configured. I bet that there is somethong you have missed in the configureation, Just start at the biginning and leave out what you think and follow the instructions. Sometimes thinking and instructions get confused! I know I have faced many things that way and solved the kink!
bigugh

hot_rod

Generally

the problem is a mismatch between the heat generator (boiler) and the heat emmiter (radiant floor) Two of the best ways to deal with it would be a modulatinf heat sourse, or add some capactiance, in the form of a buffer tank.

What type of heat sourse do you have? BTU output? Some control adjustments may help a little, but they won't fix a large mismatch. Ignition systems, ignitors, and vent dampers take the biggest hit in a short cycling heat generator. Worse case could lead to flue condensation, plugged HX, and on and on.

I'm adding buffers on all my new installations. It is easy to do, on the original install, and the cost of the tank pales in comparison to the headaches, callback, and in efficiencies generated by short cycled equipment.

hot rod

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Wayco Wayne

Yes

I've been thinking buffer tank also. The boiler is a Dunkirk condensing boiler. 100,000 input.The radiator loop is sized close to the output of the boiler and runs long when it's cold outside, but the radiant floor when running by itself is a small load. Whenever I've been there the radiators were always running and the radiant floor would just cycle on and off to no consequence. Apparently there are times whenthe radiators are off and the rad floor short cycles the boiler. I was thinking a buffer tank on the rad floor loop only to separate boiler and floor. Should I pipe it in series to gain the thermal mass, or should I pipe it in paralel and draw off the tank as a resevoir???

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hot_rod

Is there a

mixing system for the radiant? With a condensing boiler you could pipe in series, since cold return temperatures would not harm the boiler. HTP makes a nice 20 gallon tank that is for buffering the Munchkin. 4 tappings makes it nice for series or parallel.

I have done my last two cast iron with a parallel tank and the injection mix, or Danfoss ZCP taken off the extra 2 taps. I have also used electric HW tanks. A 30 or 40 gallon electric WH may be just right for your system.

Also check the differential setting on the boiler aquastat. Spread it out as much as possible to help with the cycling. Heck, with that condensing boiler you could run 130- 180. That would be a simple, inexpensive thing to try first, before piping a buffer.

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Wayco Wayne

Hey ho

thanks for the help HR. I was thinking of putting the buffer tank only on the radiant floor run. It runs from the Zone valve in the boiler room out to the addition where it injects into a constant circ loop for the radiant floor. That way I could keep the temps in the buffer tank lower than the 180 needed for the rads and indirect. I think that way I would get longer cycles. Just run the supply pipe through an electric water heater tank on it's way to the loop. Any holes in my thinking? I was also thinking of taking the end switch wire off the zone valve so the zone valve would open but not call for the boiler and would only inject when the radiator loop calls for heat. That way I could piggyback on the thermal mass of the cast iron radiator in the older (less insulated) part of the house. I would be sacrificing the indepedence of the system but it may not be needed.

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hot_rod

A cup of Delta T together?!

If this in in fact a common occurance (the radiant zone calling without the other zones) then the wider the delta t and the higher the buffer temperature the less the cycling. In other words, you can store a lot more BTUs in 180 degree water than 100 degree water.

First lets size a buffer tank. In this example lets use a 100,000 boiler with a 20,000 BTU/hr radiant zone calling. Lets pick a 15 minute burner on time. As you see a 60 gallon tank would give you a 15 minute run time WITH the 20,000 BTU/hr zone calling.

That 60 gallons of water would store 10,000 BTUs at a 20 delta T

5,000 BTUs at a 10 delta T or

2,500 BTUs at 5 delta T

But look at a 100 degree delta T (run the buffer to 180 and draw it down to 100 degrees) A whopping 40,000 BTUs stored. Or 2 hours of radiant zone run time without firing the boiler.

Use the force, (delta T) my son

Thanks to Siggy for the formula. DVW also did a fine article on a buffer sizing and installation. He used a 100 degree delta T to size a buffer tank for a job. If I get his permission I could post his formula, example and math here.

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