Boilers - Steam and Hot Water Systems - pipe temperature?

I think I have my hydronic system thermometer in a bad place. I was thinking, maybe I could compare its reading to the surface temp of a pipe?

Can anybody tell me about how much cooler the surface of a copper pipe would be than the water inside it?

On a similar note shouldn't the pressure in the system be the same everywhere? Because the pressure gauge has always read much higher than it should. (It was a brand new Honeywell, and in our two story house the end radiator doesn't fill without it saying something like 35psi.)

Thanks!

Are you talking about a clip-on, or strap-on type of 'contact' thermometer? If the pipe is un-insulated, there may be some difference, but not a heckuva lot... insulate the pipe at least a foot in either direction and it will be closer.

When you say, 'in a bad spot', what do you mean exactly?

What is it that you are trying to achieve? a little background about what you are doing might help get the correct answers.

No, the pressure is not going to be the same throughout the system, but if you are able to pressurize to 35 PSI, then something is wrong.

STATIC pressure in the system will decrease by about 0.43 PSI per foot as you go up. If you have a system that is 20 feet high, the pressure will be about 8.6 PSI LOWER at the top of the system than it is at the bottom.

Your boiler must have a pressure relief valve installed on it. There are some exceptions, but it should probably be a 30 PSI unit. If you really have 35 PSI in the system and that relief has not opened and spewed some water, there's a problem. Look for a relief valve on the boiler and tell us what the tag says.

If you have no relief valve, or it is plugged, you are in danger.

A temperature measuring point may be in a bad spot if it's after an 90 elbow, too close after an injection Tee etc.

I have seen instances where the flow in a pipe has not been enough to stop laminar flow and there can be quite noticable differences in water temp from one side of a 90 to the other.

Hi. Thanks for the replies.

Here's a pic of the current spot:
http://ewtkop4.fileave.com/sm_thermo.jpg

(It's a combo thermo/pressure gauge. Don't ask me why I put it in facing those pipes... I guess it was to face the place I usually walk.)

Yes, the pressure relief valve is a 30psi unit. Both it and the gauge were brand new when installed and the valve has never popped. Right now the gauge says 39psi.

I just added a baseboard loop and increased the boiler water temp to 180 but the thermometer doesn't get up that high.

Thanks,

Laz

p.s. I guess I should stick it in the end of the far right tee in the pic? And maybe go buy a new gauge? I always thought Honeywell made good stuff, but this one and another new one both seem to be off, at least for psi. (Maybe both were from the same batch of defective gauges.)

Right now it's in somewhat of a dead area for temperature accuracy. Yes, in the end of the tee would be much better.
Dumb question - please don't be offended - are you sure you're reading Pressure and not Altitude? :)

Good thougt ya Old Grouch! :D Thanks for that...

39 FEET is what, like 17 PSI ?

That sure does sound much more believable!

increased the boiler water temp to 180 but the thermometer doesn't get up that high.

Perhaps the thermostat is being satisfied before the boiler hits the high limit?

OldGrouchy was right! I was reading altitude! Doh!

Thanks guys!

Laz

Laz, you ain't the first, and you won't be the last! So don't feel TOO 'Homerish'... I'm kinda there with you myself, becuz for all the times it's happened, I keep on forgetting it! DOH!

So you should be all set then...