hit hi-limit, get a little PRV release


  #1  
Old 01-23-06, 05:55 PM
R
Member
Thread Starter
Join Date: Dec 2004
Posts: 89
Upvotes: 0
Received 0 Upvotes on 0 Posts
hit hi-limit, get a little PRV release

Hi,

I tried finding this in the lists, but couldn't get something specific enough. Apologies for rehashing old news if I have.

Have a gas-fired boiler (water baseboard radiators). Put new water filler, air bleeder (although not one of those diverter units), Taco pump, and pressure relief valve on this fall. Things seem to be fine. Gas consumption isn't crazy.

Noticed a behaviour that I thought might be not right. Boiler ignites, pump pumps. After a while (about 10-15 minutes) the boiler temperature hits the hi-limit temp and shuts down. At this time, a little water is released via the PRV.

I'm wondering two things: 1) should my boiler be hitting the hi-limit as a regular part of its operation? and 2) should the PRV be letting water out at this point?

On the first question, I replaced my B&G with a much lower power Taco. It's hot to the touch during operation. Several plumbers have assured me that both things are okay. Just wondering if I'm not circulating water fast enough.

On the second question, just about the only thing I didn't replace this fall was the expansion tank. It was good at the time. I inflated it with my bike pump and checked the pressure (can't remember what it's set at offhand...).

Thanks for any info.
 
  #2  
Old 01-23-06, 06:02 PM
KField's Avatar
Member
Join Date: Dec 2002
Location: Easton, PA
Posts: 3,015
Upvotes: 0
Received 0 Upvotes on 0 Posts
The expansion tank would be the first place to look for your problem. The proper way to check it would be to turn off the water feed to the system, take out water until pressure is zero. Then check the pressure in the tank. It should be 12psi. If it is low, either it is bad or you didn't put enough in last time.

Ken
 
  #3  
Old 01-23-06, 08:15 PM
Who's Avatar
Who
Who is offline
Member
Join Date: Jan 2006
Location: San Jose
Posts: 2,066
Upvotes: 0
Received 2 Upvotes on 1 Post
My opinion is that of the things that you changed which could result in behaviour like this, that new Taco circ is the most likely suspect. Is there some reason that it is not actually circulating at all? Is it backwards between the flanges and circulating the wrong way? Does is have insufficent GPM flow, with regards to the system's head pressure, to adequately move the heat from the boiler to the baseboards? Anyone of those items could allow your boiler to run, but then fail to distribute the heat and lead to your observations. That would be where I would start liking if I were you.

Happy hunting...
 
  #4  
Old 01-23-06, 08:54 PM
KField's Avatar
Member
Join Date: Dec 2002
Location: Easton, PA
Posts: 3,015
Upvotes: 0
Received 0 Upvotes on 0 Posts
I don't believe any of the items you changed is responsible for the problem with the relief valve and here is why I think that. The Taco pump probably flows more water than your old pump because that is usually the case if your old pump was a B&G model 100. The new pump is water cooled not air cooled loke the old motor so the pump will always feel hot. It is being cooled by water that can be as hot as 225 deg. F so it will almost always be hotter than that temp. If you put it in backwards, it might not flow at all and you would have no heat in the house. If you put it in backwards and did have flow(no flow check in the system), you probably would not know the difference. The fact that your boiler gets to high limit puts it in a category with about 95% of all boilers ever installed. If the outdoor temperature is not at or below the temperature the system was designed to maintain, then it will reach high limit. If it was designed to hold the house at 70 deg. when it was 0 deg., outside, it will reach high limit on any day where those conditions are more favorable. The fact that your relief valve opens after a run that reaches high limit shows that there is a larger volume of water being heated and expanded than just what is in the boiler. Check the pressure in the expansion tank using the procedure I mentioned above and pressurize it to 12psi if it is low. Then let us know what happened.

Ken
 
  #5  
Old 01-26-06, 10:20 AM
R
Member
Thread Starter
Join Date: Dec 2004
Posts: 89
Upvotes: 0
Received 0 Upvotes on 0 Posts
Thanks for the feedback! I tend to agree with Ken about the pump. I definitely raised the issue of the lower hp Taco pump replacing my big red B&G (I think it was in fact a 100 series) at the time I bought it, but was assured that the power band (or whatever) was much stronger for the Tacos and that the model I bought was a suitable replacement. There is a directional arrow stamped on the unit, so unless I have the whole system backwards, I think it's in the right direction. I do in fact get plenty of heat in the baseboards. Sorry for not describing that the first time around (no point in making folks guess!). I'm glad to hear that hitting the hi-limit is a relatively normal phenomenomenom...that was the only behaviour that made the worry about circulation. Sounds like a non-issue.

We have some mild weather at the moment. I'll drain the boiler down this weekend and double-check the pressure on the expansion tank. I'm assuming that if I get water out of its schraeder valve, the tank is absolutely shot.

Thanks again and I'll post a follow up on Monday for the record.
 
  #6  
Old 01-26-06, 02:31 PM
KField's Avatar
Member
Join Date: Dec 2002
Location: Easton, PA
Posts: 3,015
Upvotes: 0
Received 0 Upvotes on 0 Posts
It sounds like you understand the system pretty well. The only clarification I would make is to make sure you know that you don't have to 'drain' the boiler to check the tank. Just take pressure off. You shouldn't have to drain more than a gallon of water to do that. You are correct about getting water out of the schrader fitting. If you do, replace the tank.

Ken
 
  #7  
Old 01-27-06, 02:39 PM
R
Member
Thread Starter
Join Date: Dec 2004
Posts: 89
Upvotes: 0
Received 0 Upvotes on 0 Posts
good books

Oh good catch! I was definitely going to drain. Thanks for the follow up.

It's definitely taken a bit of hacking to figure things out. I was wondering if you or anyone else had a recommendation on good text books on this (and/or forced air furnaces and A/C). Even if the books cost a few bucks, I'll end up saving that on repair bills.
 
 

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