# Thermal snap disk control question



## dogwood (Oct 29, 2012)

I want it to turn on and off a small Laing re-circulation pump that will circulate DHW from my standard propane fired 40 gallon water heater through a flat plate heat exchanger. The twenty plate hx will be heated by water from my Tarm Solo Innova or storage. I am looking to purchase a thermal snap disc control that will turn on the Laing pump when the boiler or storage water coming into the flat plate is hot enough to heat the DHW; and like-wise shut it off if or when the water becomes too cool to heat the DHW.

What set point temperatures do you think I should have on the snap disk? One, http://www.pexsupply.com/pex/control/search/~SEARCH_STRING=snap disc, I was looking at, has a cut-in temperature of 150 degrees and cut-off of 130 degrees. Should I be going higher or lower on either temp setting? 

One other question. Many of the snap discs are listed to turn on fans. Any issue using one of these for a small pump? Thanks in advance for any help.


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## maple1 (Oct 29, 2012)

This place works with voodoo sometimes, I swear. I just logged on after coming from here, like 2 minutes ago:

http://www.ebay.com/itm/180814051518

while looking for something I could use to control my sidearm & it's zone valve. I was going to see if anybody has used something like this. If you go to their store they have different ones in n/o & n/c configurations with different setpoints.

Will be watching this thread.


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## Robby (Oct 29, 2012)

I use snap disc,s all the time. If you look in old furnaces, and some new as well, you will see snap disc,s that have been operating for 40 or more years, called a fan switch, over heat switch, etc. A lot of gas appliances use non automatic resetting snap switches as safety. Your dryer, especialy if gas may use them to regulate temperature.

Because I don't completly trust electronics I use snap disc's on chimney, over 180F pump is running(old boiler). If water temp at bottom top of tank not 140F domestic water (coil in tank) zone won't turn on. On my old system, coal/wood boiler, I used snap switches on the chimney to completly shut off the system when using for batch heat.

Also they are very inexpensive.


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## maple1 (Oct 29, 2012)

Guess I now have another reason check in at the supplier.


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## dogwood (Oct 30, 2012)

Appreciate your explaining that a snap disc is not an electronic device, Robby. I didn't know that. If I read your post right, sounds like you use 140 degrees as your cut-in setting for the pump from your boiler.

Would you or anyone have any idea of what cut-in and cut-out temps the snap disc should have that will turn on and off a Laing recirculation pump circulating water between my DHW heater and the flat plate hx? I am going to mount the snap disc sensor just before the flat plate's boiler/storage water input port.

I want the Laing recirculation pump to start circulating DHW from the water heater through the flat plate only when hot water from the boiler or storage arrives and remains at temp. The water to heat the flat plate travels about sixty feet from the garage boiler room. The flat plate is next to the DHW heater inside the house. When hot boiler water is no longer arriving at the flat plate, the snap disc will sense the drop off in temp and shut off the Laing recirc pump.

I also have to figure out what cut-in and cut-off settings to use on an aquastat turning on or off a Grundfos pump which will circulate the hot water from the Solo Innova or storage to the flat plate when called for. Unlike the snap disc's, the aquastat's temp settings are adjustable, so I can play around with them. Any suggestions on those settings would be welcome too.

The aquastat will turn on the Grundfos circ pump when the water in the DHW tank cools to a predetermined temperature, maybe 130 degrees or so I'm guessing. I'll mount the aquastat to a pipe coming out of the DHW heater port near the top of the tank, currently used only by the pressure relief valve. The aquastat will also turn off the Grundfos when the DHW tank gets up to temp.


Mike


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## Nofossil (Oct 30, 2012)

Before I built my controller, I did everything with snap switches and relays. Don't remember where I got them but they had a wide selection of both normally open and normally closed over a big range of temperatures. They're not adjustable, and may not be as close as you's like to the listed cut-in and cut-out temps, but as mentioned above they're very reliable and inexpensive.

I MIGHT have gotten them from Digi-Key - worth a look.


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## dogwood (Oct 30, 2012)

Nice to hear from you Nofossil. Hope the hurricane doesn't hit you too badly up in Vermont. We missed the worst of it here.

I was thinking of using 150 degrees at the flat plate to turn on the Laing DHW recirc pump, and 130 for it to shut off. And maybe 140 at the DHW tank prv port at the top of the DHW tank to turn on the Grundfos circulating the Solo Innova's/storage's supply to the flat plate, and 160 to turn that off. Do these numbers sound in the ballpark to you. I will have a tempering valve on top off the A.O Smith DHW tank, so can run somewhat higher temps. I really don't know what the stratified temperature range within a standard water heater is, so am guessing about the 160/140 figures at the top of the tank, maybe they're too high.

I'll leave the current, lower, propane burner's settings in place, so the burner can heat the DHW when I'm not home to fire the boiler.

Thanks,

Mike


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## Nofossil (Oct 30, 2012)

Sounds pretty good now that you've mentioned the tempering valve. I use a Honetwell AM101 valve for my DHW and have it set up very much like you're proposing - I heat it to 160 with the wood boiler and let the aquastat control the backup heat source at 120.

Of course, my approach is nowhere near as simple and reliable as snap switches ;-)


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## dogwood (Oct 30, 2012)

Appreciate you getting back to me so quickly Nofossil. Thanks for the reassurance on those temperatures. I'll go ahead and order that 150 on/130 off snap switch from PexSupply. 

Mike


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## Robby (Oct 30, 2012)

I have no experience with your set up, sounds like "Nofossil" does. I still use the snap switch to cut power to domestic hot water tank. It is dual coil, solar on bottom coil and storage tank to top coil. Solar just runs as sun shines. I let it get as hot as it wants and have a tempering valve. Solar works good, but as temperature goes up efficiency plummets (evacuated tube). When 100 gal tank get up to 160F + solar input is real small to nil.
 I changed a snap switch a couple of months ago on a Lennox furnace that was installed in 1962, it was the original (I think) and replacement was on the shelf. Sometimes wonder why we don't change things tha need change and leave the rest alone.


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## dogwood (Oct 30, 2012)

Thanks for your advice Robby. It would be good to add some solar collectors to the system in the future if I ever get this wood boiler project done.

Mike


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## Bob Rohr (Oct 31, 2012)

dogwood said:


> Thanks for your advice Robby. It would be good to add some solar collectors to the system in the future if I ever get this wood boiler project done.
> 
> Mike


 

You can buy adjustable temperature versions also.  I get them from Johnstone Supply.  I have not seen any with adjustable differential however.  Both 20 and 40° differential asre available.

For extra protection you can but one with a resetable over-temperature button.


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## dogwood (Oct 31, 2012)

Thanks for the info and pictures of the adjustable snap discs Bob. Here's a link for anybody who'd like to look at them in the Johnstone catalog: http://www.johnstonesupply.com/storefront/search.ep?keyWords=snap switches&filters=b201269&sorter=popularity-desc&filters=b20 and for pricing: http://catalogs.johnstonesupply.com/3511_john_203/full.asp?page=366

If you don't mind my picking your brain, Bob, what do you think the minimum temperature difference should be between the boiler/storage water, and the DHW being heated, for effective heat transfer to still occur in a reasonable time in a flat plate hx. I'm working with a 20 plate Bell & Gosset flat plate hx. being supplied on the boiler/storage side by a Grudfos 3-speed circ, and on the DHW side by a small Laing recirc pump. If the Solo Innova boiler is firing I'd be heating the water to the flat plate directly from that, if not then from storage.

The charts I've looked at always seem to assume a 180 boiler water input. If the boiler water was 180 and the DHW was 100-120, the flat plate should quickly transfer heat to the DHW. But if the water heating the flat plate was coming from storage at only 140 degrees, and maybe the DHW is 100-120, only a 20-40 difference, would the DHW still be getting heated at other than a miserably slow rate, even if the Grundfos was on the high speed setting.

I 'm trying to figure at what temperature difference between the DHW, and boiler or storage water both the pumps should switch off. And how low I can go with the storage supply temp to the flat plate. What do you think?

My plenum w/a hx is designed to work down to 140. It would be nice if the flat plate could heat the DHW in a reasonable time at that temp. Then I could maintain 140 as the minimum temperature for storage for heating both the house and/or the DHW. Maybe I'd need a much bigger flat plate for that to be able to happen. The w/a hx is over-sized.


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## Bob Rohr (Nov 1, 2012)

Ideally you could produce DHW with the lowest possible temperature on the "A" or boiler side. I have been putting this to the test in my shop this fall. I went with a 5X12 30 plate HX. I can still take a shower with about q 1.5 gpm flow with the tank at 108°F. Basically you just reduce the "B" side flow rate as the "A" side temperature drops. So in the shower I can reduce the flow rate on day where my solar tank temperature is low. so far I have not used any additional heat for the DHW. Winter time solar gain may require me to add a little.

You should be able to size a close approch HX within 10 degrees, even 5 degrees of A to B side. Of course the size of the HX grows with closer temperatures.

Some good DHW info and drawings here www.caleffi.us/en_US/caleffi/Details/Magazines/pdf/idronics_11_us.pdf

I still have issues with the flow switch I used. It sometimes sticks on after the shower stops flowing, which keeps the pump running. Not a big deal for the hot water, but it consumes electrical energy.


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## dogwood (Nov 1, 2012)

Thanks for the link on the Caleffi Idronics article Bob. I see what you have going on with the point of use water heater, flat plate hx and storage. Excellent article. 

I'll first try the 20 plate I already have. I hope that not having to directly heat incoming cold water at 50 degrees, like a point of use water heater might, but instead just having to raise the temp of the 40 gallon standard water heater as it cools below 120, maybe the twenty plate could keep up, if the storage water is at the 140 design minimum. If not I guess the propane burner would have to kick in to finish the job. Maybe I could hook up a second flat plate in series with the first, if the 20 plate doesn't get it.

By the way, maybe somebody reading this, or better yet, if you post your flow switch problem in a separate post, someone can provide you with a fix on it. You've provided so much help to everyone on this site, I'm sure somebody would be glad to return the favor. Unfortunately I wouldn't know a flow switch if I fell over one, so I've no suggestions to offer. Couldn't even find a picture of one in the Siegenthaler book. Thanks again.

Mike


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## Bob Rohr (Nov 2, 2012)

dogwood said:


> Thanks for the link on the Caleffi Idronics article Bob. I see what you have going on with the point of use water heater, flat plate hx and storage. Excellent article.
> 
> I'll first try the 20 plate I already have. I hope that not having to directly heat incoming cold water at 50 degrees, like a point of use water heater might, but instead just having to raise the temp of the 40 gallon standard water heater as it cools below 120, maybe the twenty plate could keep up, if the storage water is at the 140 design minimum. If not I guess the propane burner would have to kick in to finish the job. Maybe I could hook up a second flat plate in series with the first, if the 20 plate doesn't get it.
> 
> ...


I'm sure the 20 plate will work , just expect a lower flow rate.

I can tell when my solar tank temperature is dropping to 100°F or lower as the shower is luke warm   So I just throttle down the shower valve to coax my way through one my shower.

Any way to pre-heat the water first.  Solar is idea, even an un-insulated galvanized tank will pick  up some room temperature and lessen the load on the HX.

I also like the drain pipe pre-heat HX shown in that I-dronics.  if you have an application where it will fit it is actually an excellent ROI,considering the cost, simplicity and life expectancy.


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