Adding a boiler bypass

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Old 07-19-18, 02:19 PM
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Adding a boiler bypass

I have a slant fin cast iron boiler with a hydrolevel 3200 and indirect hot water installed last year. It is straight piped supply, return with no bypass. Hydro level is set to 140 min. I noticed over the winter it would purge then fire, run for a while then shut off, refire about 30 seconds later, run for 2 minutes then shut off and stay off until another call for heat. I'm thinking of adding a bypass but not sure if I should add a boiler bypass or a system bypass.
 
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Old 07-19-18, 06:48 PM
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There is no dedicated pro in this forum right now but a few of the members are aces when it comes to this work. Pictures of your current setup and piping can be a tremendous help in the discussion.
How-to-insert-pictures
 
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Old 07-20-18, 12:08 PM
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l,

A bypass is typically installed on a low water content boiler replacing an older larger volume boiler heating a large water volume system such as cast iron rads to temper the water in the boiler preventing thermal shock.

Why do you think you need a bypass. More info such as boiler size and system such as heating baseboard or rads and is boiler properly sized for system.

If you're getting short cycling, you most likely have an oversized boiler. What would you hope to gain by installing a bypass. With tempered water reentering the boiler it's only going to cycle that much faster.

If you are looking for longer run cycles it may be possible to down fire your boiler.

Pics would be helpful if possible of boiler and piping.

Hope this helps a little.
 
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Old 07-20-18, 12:27 PM
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In on my phone and can't find the advanced editor to insert pics. It is a cast iron radiator system. A heat loss calc was done and the boiler was downsized from 180k to 110k. This is where things get tricky. I know if I add a system bypass it will be to prevent thermal shock but would increase short cycling. If I add a boiler bypass it would lower the short cycling but up the thermal shock.
 
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Old 07-20-18, 01:23 PM
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With the cast iron rads it should have had a bypass installed to prevent thermal shock. Bringing down the heat load figures by 70,000 btu's how is possible you are short cycling, especially with the high water content of a rad system. What size boiler did they use.

How did they figure your heat loss and what the rads heat value are.

Is your control set up for Hi limit only (cold start) which is all you need since you have an indirect and no tankless coil.

I would worry more about thermal shock than short cycling. Possible down firing could help that or control adjustment.
 
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Old 07-20-18, 02:59 PM
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Heat loss done using slant fin app. I believe it is set to cold start. I think I realized one of the problems today. I was down my basement for a few hours and noticed the boiler firing about every 20 min even though nobody was using the hot water. I'm wondering if there may be a problem with the dhw aquastat.
 
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Old 07-20-18, 07:01 PM
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If your heat is not calling and your hot water is satisfied they may have activated your Low limit which would have your boiler maintain a temp as if you had a tankless heater in your boiler.

If you have the instructions for your control you can shut it off. You only need the Hi limit for your application.

Check also how they wired your indirect. There are separate terminals for that if you are using a separate pump and not zone valves.

http://www.supplyhouse.com/

Type in your control in the search box and then scroll down to install instructions and it will tell you everything you need to know.
 
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