Boiler Bypass

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Old 01-21-10, 06:00 PM
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Boiler Bypass

My next question but I'm sure not my last. I am going to install a new boiler for a relative who is on a budget and can't really afford the pros coming in. I will do the heat loss and size it right but after doing alot of research I think the system will need a bypass. It's a 3 story 5 bedroom house and my concern is the return water coming back to cold and also the flue condensation. It is radiators and gas fired 1 zone. My plan was to do a boiler bypass vs a supply bypass. Same size pipe between the supply and return. From what I have read I plan on putting valves both on the bypass and also on the return but it seems like the bypass valve should be wide open and you throttle down the return valve. I was going to put a temp gauge on the return to be able to monitor the temps coming back and adjust the valve accordingly. Questions are what temp should the water be coming back at? 140 or 40 different from the supply temps? Also does my plan sound ok to you guys? I am no expert but have done 5 boilers with some pretty good success, so far. Just want to try and get this right. Thanks in advance and let me know if I left something out.
 
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Old 01-21-10, 06:37 PM
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What boiler will be installed and type of radiation (cast iron radiators, fin tube baseboard, etc)? Return temperature is typically between 20 to 40 degrees less than the maximum system supply temperature. That temperature is dependent on your heat loss and amount of radiation.
 
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Old 01-21-10, 07:03 PM
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Here is a link explaining boiler bypass. For large cast iron radiation adjust the bypass for a 40f rise through the boiler and you will be good. The best way to adjust the bypass is let the system get to about 140 degrees or more first. The bypass valve is wide open and the return valve is going to be at least 1/2 to 3/4 closed. this will force more water through the bypass.
Bypass_Piping_Explaination
Also
ComfortCalc :: View topic - Cast iron boiler condensation
and lastly see pic's on this link
ComfortCalc :: View topic - Cast iron boiler thermal shock
 
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Old 01-21-10, 11:02 PM
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rbeck= Great stuff!

cookman you will see boilers come with installation instruction and most are very good.
drooplug makes a good point because obviously the types of boiler designs vary greatly.

A type that I have mostly sold and installed are the low mass, cold start. These are like a hot air furnace in that they sit at room temperature until there is a thermostat call, then heat up quickly. To aid this they require a SYSTEM BYPASS. Some examples are Energy Kinetics System 2000, Burnham LE etc. The instructions clearly describe it.

Another type I have been installing recently is 'Triple Pass Modulating boilers' such as Buderus and Burnham MPO. The instructions with these ask for a BOILER BYPASS. The MPO requests a simple bypass of the same size pipe as the headers. The Buderus asks for a motorized mixing valve on the System Bypass.
Then you have 'condensing boilers' and 'mod-con' as well as the old simple always hot cast iron hogs.

The best intro to hot water heat systems is Dan Holohan's Hydronic Radiant Heating, well worth the time to read before jumping in.
http://www.heatinghelp.com/products/...neer-Installer
 
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Old 01-24-10, 03:18 PM
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Thanks for the replies guys. I am going to install a Williamson boiler. From what I can gather they are made by Weil McClain but called Williamson. I looked up their install manual and it does show bypass. I'm just not sure which I want, system or boiler bypass? Which one would be best for my situation? I attached the link to the install guide.

http://www.williamson-thermoflo.com/...1-010_0703.pdf
 
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Old 01-25-10, 05:26 AM
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The only time a system bypass should be installed is if you use p/s piping or in a commercial system. A system bypass reduces the system flow which reduced heat output from the radiation.
You may want to look at the ES2 as it works with much lower return water temperature out of the box and can get an easy to install OD reset control which saves about another 15% - 20%.
Make sure the new boiler is properly sized. This is the key to fuel savings and lower maintenance bills. DO NOT SIZE OFF THE OLD BOILER OR THE AMOUNT OF RADIATION!!!!!!!!
 
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Old 01-28-10, 01:22 AM
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p/s ?

What is p/s piping?

Low mass, cold start boilers usually require system bypass to get up to temp fast: eg System 2000, Burnham LE etc.
 
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Old 01-28-10, 04:37 AM
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P/S = primary secondary

Basically, you don't want to use a system bypass on a residential system, opt for P/S piping instead.
There are may advantages that can be had once you switch to P/S piping arrangement, and it really only costs another circulator.

A system bypass does get the low-mass (copper tube) boiler up to temp faster, but at a cost of flow to the home. One would have to size the system flow to be a percentage larger than what the house needed to flow in order to satisfy the heat loss.
The results in bigger circulator, bigger pipes, higher power consumption, etc. Nothing we really want here.

Most installer 'round here just slam the copper tube boiler, well even cast iron boilers I suppose as well, on the ground and connect the system supply and return to it with out a bypass. Since it's copper there should be no need to prevent flue gas condensation. Luckily these same "installers" are retiring and / or want nothing to do with mod cons.
 
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Old 01-29-10, 06:41 AM
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I agree a cast iron boiler should not have a system bypass but the original post is a boiler bypass which I would strongly suggest on a cast iron boiler with large water volume systems. You could use p/s but have to reduce the flow through the boiler with a valve or install a system bypass with the p/s. A better choice is a thermic valve with system bypass and p/s piping. I know sounds contractive but the system bypass with p/s piping does not affect system flow as a boiler bypass does not affect system flow. The boiler bypass requires only one pump. P/S alone does not give boiler protection.
 
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