Water supply valve and back back flow preventer

Reply

  #1  
Old 11-02-13, 09:04 PM
Member
Thread Starter
Join Date: Nov 2007
Posts: 12
Received 0 Votes on 0 Posts
Water supply valve and back back flow preventer

I have a hot water radiator system. Two questions:

1. Should the valve on the water supply coming into the boiler be open or closed? The order of things going towards the boiler is water supply valve, back flow preventer, and then the pressure reduction valve that is currently set to 12 psi.

2. There is a vertical pipe from from the back flow preventer down to the floor. This is in addition to a vertical pipe coming from the TP relief valve. What is the purpose of the vertical pipe from the back flow preventer? It would seem to me that it is redundant to the TP relief valve.
 
Sponsored Links
  #2  
Old 11-03-13, 07:40 AM
Member
Join Date: Oct 2008
Location: USA
Posts: 2,916
Received 7 Votes on 7 Posts
Question #1 comes up about every month. There is no universally accepted answer. Personally, I think if you have a low-water cutoff that you regularly test, the valve can be shut - otherwise, I would leave it open.

Hot water boilers should not have a T&P relief valve - only a pressure relief valve. The relief valve and the backflow preventer serve separate functions, and both should have a discharge pipe down within a few inches of the floor.
 
  #3  
Old 11-03-13, 08:19 AM
Member
Thread Starter
Join Date: Nov 2007
Posts: 12
Received 0 Votes on 0 Posts
Thanks

Gilmorrie: Thanks for the response. Very helpful. And, yes, indeed I only have a pressure relief valve on my boiler. Was thinking instead of my water heater.

With regards to the discharge pipe coming off of the backflow preventer, under what conditions would it discharge water? In other words, what would cause a buildup of pressure between this preventer and the pressure regulator?
 
  #4  
Old 11-03-13, 08:27 AM
Member
Join Date: Oct 2008
Location: USA
Posts: 2,916
Received 7 Votes on 7 Posts
what would cause a buildup of pressure between this preventer and the pressure regulator?
The main risk is that your domestic water pressure would somehow drop below the boiler pressure. That could be caused in several ways, none of which occur during normal operation. That could suck boiler water into the domestic water system and contaminate it. Many city water utilities require such a backflow preventer and may inspect to see if it is installed. I get a notice every year or two from the water utility asking me to certify that I have a backflow preventer installed and to give them the model number of the preventer. But, I would install such a device even if it were not required because it is good practice.
 
  #5  
Old 11-03-13, 10:36 AM
NJT's Avatar
NJT
NJT is offline
Member
Join Date: Oct 2006
Location: USA
Posts: 23,539
Received 3 Votes on 2 Posts
The backflow preventer is a fairly complicated device. I 'dissected' one to learn how it works. Somewhere in one of the past posts there is a diagram I drew up to explain it's function.

Found it... read this:

http://www.doityourself.com/forum/bo...ater-line.html

I don't have a Low Water Cut Off on my system, and I run with the fill valve CLOSED...

BUT!

I regularly (meaning daily as I walk past the boiler every day) glance at the pressure gauge to check that all is well. I know where it usually reads and at what TEMPERATURE it reads that pressure (the two are linked in direct proportion) and if I see something amiss, I will check it out.

Most boilers are tucked in a corner somewhere and the gauge may not get looked at for years...

An OPEN fill valve can mask system LEAKS by constantly providing make up water to account for those leaks. A system could slowly leak for YEARS (DECADES EVEN! and I have proof!) before being discovered.

A CLOSED fill valve presents the possibility of running the system dry... the LWCO will prevent that.

Yes, it's an on-going debate with pros and cons on both sides.
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes
 
Ask a Question
Question Title:
Description:
Your question will be posted in: