Operating temperature for hot water heat??

Hi, I'm new here but have dropped in a few times to check it out. Seems like a great site. <P>
I have a question about a good operating temperture for hot water heat with baseboard rads. I ran into one yesterday at a lady's house that was operating at 230 degrees F and then I went to check one that I own on a rental house and it was running at 210 degrees. These temps seemed high since our boiling point here at 6000 ft. is about 200 degrees f. I turned the one I own down to 190 degrees. My user manual does not seem to have a suggested temp. and I havn't been able to find much googleing around. Any advice would be greatly appreciated. HS. The pressure on both of these units was less than 20 psi.

temp

Many years ago when I did wet heat, we usually ran it at about 190 degrees, but if a system is underdesigned increasing the temperature is a "cheap" way to compensate for it. The boiling point is determined by the pressure in the system, not your altitude. But once you pass the point for your altitude, then you can get into the area of "superheated" water and that would be a factor if the system were to suffer a catastrophic rupture.

Thanks hj. That starts making sense to me. I will keep mine at 190, I think the system and amount of radiation can handle that. I also have a steam system on my own house which must be running quite a bit higher than 200 degrees. I've always heard that steam heat was hotter than hot water heat and so was a little confused when seeing these temps on the hw systems. I don't have a decent thermometer on the steam system but will get one soon. Thanks again. HS.

You car's cooling system can often get to 230-240 in stop and go traffic on a hot day...if you don't have a proper pressure cap for the system, it will boil over. Same basic thing for the hot water heat - as long as it has adequate pressure, you will not turn it into steam.

steam

You do not need a thermometer on the steam system. If you know the pressure then there is a specific temperature that will create steam at that pressure. Less pressure, less temperature, more pressure, higher temperature. It is the nature of a steam system. At sea level, 5 psi steam = 227 degrees.

hj said:

You do not need a thermometer on the steam system. If you know the pressure then there is a specific temperature that will create steam at that pressure. Less pressure, less temperature, more pressure, higher temperature. It is the nature of a steam system. At sea level, 5 psi steam = 227 degrees.

Click to expand...

Oh, so my steam system is not running much higher than a hw system. I was always told that steam needs 1/3 the rads of a hw system. It sounds like that 1/3 rule might not be true. HS

Not really...if I remember my physics, it takes 540 calories to change water to steam at the same temperature. To move it one degree in the same state (i.e., water heating) it only takes one calorie, so an equal weight of water vs steam (albiet much larger in volume) contains MUCH more energy.

Pounds vs PSI

Steam has a latent heat of about 1000 BTU/pound. Once it gets to steam, the pressure of saturated (not superheated) steam depends only on temperature. One pound of steam at 212F has 1000 more BTUs than one pound of water at 212F. When you are talking about steam pressure you should talk about gauge pressure (what you read on your pressure gauge) relative to atmospheric pressure (or absolute pressure but we won't go into that here).

The heating effect of steam in the radiator is almost all (about 90%) a result of it giving up that latent heat when it condenses.

Steam systems require less radiator area because the heat transfer rate of steam to metal in the radiator is much greater than water to metal, and the temperature inside the radiator is greater than the average temperature for the water in a hydronic system. On the air (room) side the rates for steam benefit from the fact that the radiator is at higher temperature.

Steam radiators are almost always piped to return the condensate to the boiler by gravity through the steam pipe, and must always have an air vent because any air in the "radiator" cuts down the heat transfer rate.

Thanks again, some of that will take a couple of readings to digest.<P>
I had one very good heat man tell me that my 1952 Bryant boiler was probably about 40 -50% efficient and I would not do much better with a new boiler and the cast iron Bryant is pretty much bullet proof. One of the plumbing/ mech inspectors said the new ones have to be 70% efficient. I hate to put out the bucks for new equiptment and find not much change in heat costs. The costs of steam seems high compared to forced air systems I own but about even with the 5 year old Slant Fin system in one of the rental properties. Any thoughts on new boilers vs. my old Bryant? HS.

I'm not a pro, but have been looking over boilers (hot water, not steam). Viessmann has a condensing boiler that is rated at a minimum of 94% and as high as 98% if you use lower temp water and don't need the full output (it can modulate the burner down to 20% of max). After the first of the year, the Feds will be giving tax incentives to people who replace heating systems that are energy star compliant. These new systems are computer controlled, and might not be as simple as the old systems...time will tell how reliable they really are. Viessman is a German company with a US factory in RI.

As a rule of thumb, I usually set baseboard systems at ~190 degrees, radiators at ~165 degrees.

IMO, settings in excess of the boiling point of water are a danger. You cannot guarantee that the system will get the needed maintainance to insure proper pressure to handle temperatures over 212 degrees. The 190 gives a bit of freeboard just in case.

The 165 on radiators is to help prevent little ones from getting burned.