Boiler low return temperature explanation.

TimZ

Hi. I am new to boilers and need some help explaining low return temperature. I have a Weil MCLain cga 3 boiler and the manual is adamant about return temp being above 130 degrees. Corrosion and damage to the boiler. My thermostat is set at 70 degrees and during the day today it was 15 degrees outside. From about noon to 2:30 the boiler didn’t run. The thermostat actually went to 71 degrees. Sunny day outside with a few big windows. The radiators were cool to the touch. The boiler temp dropped to 90 degrees. This evening the temp is 5 degrees outside. Burners were on for 10 minutes off for 25 minutes and the boiler temp dropped to 127 degrees. The thermometer on the return manifold was 110. Is a low return temp when you hit 175 degrees at the boiler but the return temp is lower than 130? Is it bad for the boiler to cool down and the restart at a low temp? Thanks.

nicholas bonham-carter

As I understand the 130 degree limitation, this would be while the boiler is firing, and not when it is off. When it is firing, and the returns are consistently below that temperature, the flue gases can condense on the metal, and eventually rot them out.
Does the boiler get up above the 130 mark while it is firing?
Congratulations on reading the manual for the boiler, and wanting to inform yourself on this subject, and I hope the installer also read the manual!—NBC

JohnNY

Yes. <130° is only a problem while the boiler is firing. The hot flue gases against the cool metal cause the condensing problem.
That said, at this outdoor temperature, I'm in NYC and it just dropped one degree to 7°, your boiler should probably not be shutting off.

EBEBRATT-Ed

Sounds like your boiler is a little oversized.
How old is the boiler?

It probably wouldn't be a bad Idea to install a three way valve with a temperature sensor to keep the return temps above 130.

It is normal for the boiler to be below 130 while not firing. When it does fire the return temp should reach 130 within 15 min you probably ok.

Continue monitoring it and deal with the three way valve in the spring if you need it

TimZ

My boiler does go up to 175 on the boiler and like 160 on the return thermostat. It just doesn’t run for like a half hour and the boiler temp drops to sometimes 120-125 degrees. The boiler is brand new. I got 5 quotes and they were from 88,000-120,000 btu. The 88,000 btu quote used the manual j calculation and it came up with a 38,000 btu heat loss. I was told the next one down would be too small??? And the 88,000 would be fine. The Weil MCLain is net 51,000 and the old unit I had was 66,600 btu a 60 yr old Bryant. I was worried with the old one and the quotes I got it would be too small. Damn. I will be adding baseboard radiators to my kitchen and possibly basement. Would that help?

KC_Jones

Actually the next one down is exactly 38k and is the one they should have installed with that heat loss number.

Welcome to the world of bigger is better, completely wrong, but its the world that many contractors create.

Adding baseboard is usually done to lower operating temperatures in support of a mod con boiler.

hot_rod

The important number us the return temperature, measured with an accurate gauge, is what you should watch
Within 10 minutes and for the entire on cycle the return should be above 130

A cold high mass zone could pull it down for a period, like a slab radiant, but keep non condescending boilers above condensing temperature as much as possible

TimZ

Sorry. What do you mean a high cold high mass zone?

hot_rod

do you have cast iron radiators? More than one zone?
A cold slug of water returning to the boiler could temporarily pull return temperature down

Again, 10 minutes or less with return below 130- 140 is a good rule

TimZ

Yes cast iron baseboard radiators. Baseray 9 3/4” tall. I have a manifold. Each room has a supply and return from the manifold. Here’s a picture.

hot_rod

looks like a nice installation. Maybe switch gauges on the manifold to see if they read accurately, or the same. They should slip in.

TimZ

I’ll try that thanks. So do I have any options. Mixing valve more radiators. Anything. I read about a buffer tank. Thanks guys for the help.

EBEBRATT-Ed

Your probably fine the way it is. If you get a day when the boiler can be shut down for a while, then turn it on and monitor the return temp to the boiler. See how long it takes to get above 130

Gordy

Nice install. Clean simple. Pumping away from PONPC. Air removal in hottest part of system.

hot_rod

The

TimZ said:

I’ll try that thanks. So do I have any options. Mixing valve more radiators. Anything. I read about a buffer tank. Thanks guys for the help.

I agree with Ed, it should wore fine as is. Get some tighter data

SuperTech

Looks like a really nice installation. I'm a big fan of cast iron radiatiors and baseboards as well. I would install a Taco I series mixing valve, that would take care your problem and help improve efficiency.
If you are the adventurous type you could go with a variable speed injection pumping setup, that's what I did when I had the same problem.

TimZ

If I did put in a mixing valve where would it go? I do want to add some heat in the basement in the future if that would affect mixing valve placement and piping.

TimZ

So I put in this mixing valve.

TimZ

TimZ

And my numbers for burner run times are.

TimZ

TimZ

TimZ

Burners then stayed off for 1 hour 10 min. 25 F outside. Thermostat at 70 F.

Solid_Fuel_Man

Nice piping job!

TimZ

Thanks. Now if I could only figure out why I hear air and knocking in the boiler near pipes.

08GeezerGlide

Tim- A couple of late questions if you are monitoring your post as I am assembling my own system-
1. What is the rationale for the high-mounted drain valve on your supply from the boiler? I am noticing that it is just above the horizontal location of the down stream supply piping. Just for purging? Or for venting a future boiler drain?
2. In your first photo of your system, I noticed that the system feed water ball valve is turned off. Is this the normal position when your system is in service? Always was of the understanding this valve stays on, which allows the pressure regulator to automatically fill the system should there be any loss of water. If these systems today assume no sporadic small losses of water, and a normal “OFF” position for the feedwater valve, what would be the need for a back flow preventer?
Thanks for any insight. Trying to get a better grip on the purpose and location of some of these hydronic system components. Seems like every system I see has a different approach. After doing some research here, your system hit me as well thought out and professionally installed. And I too am trying to design to prevent low return temps.

STEVEusaPA

2.5 years later? You should start your own post and reference this one if needed.