Return Temperatures

Kickbundler

Mike,
Thanks for the info. My boiler was changed out from oil to gas circa 1960s (not really sure).

What about changing the system pressure to get increased flow? I had the boiler and system inspected last year. The guy said not to exceed 30 psi on the system. I have an auto fill valve and my pressure goes up to about 23 - 24 psi at high temp (I keep the high tmep limited to 170 F).

I think that my boiler is way oversized but hate to get rid of a working part (especially considering I don't have the change to drop on updating boiler and possibly circulator). I'm trying milk out what bits of efficiency I can considering the way gas prices are going.

Kickbundler

Return Temperatures

I have a 1922 house in Michigan with a gravity heated system. The sytem works well (reasonably good ramp up speed).

I would like to get more efficiency out of the system. My gas bills have been high. I know that the insulation in the house is mostly 1920s era (i.e. virtually none) and the house is bigger than I'm used to, so I don't have a good reference for just how high they are. I have done some experimenting with the system and replaced a few of the old frozen radiator valves with TRVs, so I've been putting some thinking into how the system works (This website has been a big help). I have been wondering why the return lines are insulated. I got wondering about this because some of the previous owners have removed the insulation from a few locations in the basement (I assumed to provide some heat down there). I did come calcs on just how much heat is being lost in the basement runs and have concluded that those pipes radiate a lot of energy.

My thought was that it would be a good idea to reinsulate the supply lines but leave the returns bare. This seems to make sense since then the return lines (in the basement) would radiate their heat and lead to faster flow through the system. However, I wonder why they (supply lines) were ever insulated in the first place and I have seen on this web site that the target delta T should be about 20 deg F. Why is this? It seems that the hot water would stay in the bottom fed radiators and that having colder return water would increase the circulation rate in the system.

Any insight on the delta T = 20 reasoning would be appreciated.

Mike T., Swampeast MO

The system is still under gravity circulation (no circulating pump ANYWHERE), you've installed TRVs and those radiators still heat?

Presuming forced circulation:

Why the insulation on the returns?

Likely because there were freezing problems. If the boiler is typically oversized and if previous owners used deep daily setbacks in cold weather, it's VERY possible that branch return lines from remote radiators froze. Not enough to burst pipes, but enough to stop circulation.

Why 20 degree delta-t?

Because it makes very convenient math for a full design load. In reality, delta-t is highly variable.

Kickbundler

> The system is still under gravity circulation (no

> circulating pump ANYWHERE), you've installed TRVs

> and those radiators still heat?

>

> Presuming

> forced circulation:

>

> _I_Why the insulation on

> the returns?_/I_

>

> Likely because there were

> freezing problems. If the boiler is typically

> oversized and if previous owners used deep daily

> setbacks in cold weather, it's VERY possible that

> branch return lines from remote radiators froze.

> Not enough to burst pipes, but enough to stop

> circulation.

>

> _I_Why 20 degree

> delta-t?_/I_

>

> Because it makes very convenient

> math for a full design load. In reality, delta-t

> is highly variable.



Mike,
Absolutely no forced circulation.

So if I understand you, the circulation is added mainly to prevent freezing? The insulation is the original asbestos wrap and was not added after construction.

Also, you mean the 20 degree drop is an assumed value for calculation, an ideal drop for the system?

Thanks for your help.

Kickbundler

> The system is still under gravity circulation (no

> circulating pump ANYWHERE), you've installed TRVs

> and those radiators still heat?

>

> Presuming

> forced circulation:

>

> _I_Why the insulation on

> the returns?_/I_

>

> Likely because there were

> freezing problems. If the boiler is typically

> oversized and if previous owners used deep daily

> setbacks in cold weather, it's VERY possible that

> branch return lines from remote radiators froze.

> Not enough to burst pipes, but enough to stop

> circulation.

>

> _I_Why 20 degree

> delta-t?_/I_

>

> Because it makes very convenient

> math for a full design load. In reality, delta-t

> is highly variable.

Kickbundler

Mike,
The system is still under gravity circulation (no

> circulating pump ANYWHERE), you've installed TRVs

> and those radiators still heat?

>

> Presuming

> forced circulation:

>

> _I_Why the insulation on

> the returns?_/I_

>

> Likely because there were

> freezing problems. If the boiler is typically

> oversized and if previous owners used deep daily

> setbacks in cold weather, it's VERY possible that

> branch return lines from remote radiators froze.

> Not enough to burst pipes, but enough to stop

> circulation.

>

> _I_Why 20 degree

> delta-t?_/I_

>

> Because it makes very convenient

> math for a full design load. In reality, delta-t

> is highly variable.

Kickbundler

> The system is still under gravity circulation (no

> circulating pump ANYWHERE), you've installed TRVs

> and those radiators still heat?


Absolutely no circulating pumps. Yes, the TRVs have worked well. Eliminated overheating in the rooms I installed.

> _I_Why the insulation on

> the returns?_/I_

>

> Likely because there were

> freezing problems. If the boiler is typically

> oversized and if previous owners used deep daily

> setbacks in cold weather, it's VERY possible that

> branch return lines from remote radiators froze.


The insulation is the original asbestos type. So, you're saying that insulation is added to prevent freezing in the lines? I guess I was operating under the assumption that it was to hold heat until it reached the radiators.

> _I_Why 20 degree

> delta-t?_/I_

>

> Because it makes very convenient

> math for a full design load. In reality, delta-t

> is highly variable.




So the 20 degrees is an assumed temp drop, not an ideal operating drop? Is there a limit to how much temp drop I should have?

Thanks

Mike T., Swampeast MO

Learn something every day--I never would have thought that TRVs would work with gravity.

Some gravity systems I've seen have the mains (supply and return) and perhaps some branches insulated. Presume they were trying to avoid heating the basement.

Other times only a few branches are insulated and they're inevitably near the end of the mains--have always assumed that it was because of freezing problems in the past.

Others have absolutely no insulation. In that case the basement is often used as living space. (Lots of basement rentals in this college town.)

Different old texts quote different assumed temp drops--usually from 20° - 30°. As gravity systems age, the pipes get rougher and flow restriction (slight though it is) increased. As that happens, velocity will slow and temperature drop will increase.

Presuming you're no longer using solid fuel and have a wall thermostat, you'll probably get some really hefty temperature drop--particularly in slightly cold weather when the system cools significantly between cycles.

As to what to expect? Whatever works so long as you can heat the place in the coldest weather without too high of a supply temperature--not good to go much over 190° supply at the boiler unless your system has one of the old "heat concentrators" still installed and functioning.

As to more efficiency--sorry to say, but there's probably not a lot you can do to the system itself. If the basement is warmer than you like, insulate mains and/or branches. I'd start with the supply lines as they are hotter. Insulate and weatherize the house itself as much as practical. Get your windows/doors in good shape and insulate the attic well. Wall insulation usually isn't as cost effective unless you're gutting things out or doing extensive renovation anyway. I'd be very cautious adding any more TRVs as you're almost certain to hit a point where gravity can no longer give enough circulation force without excessively high supply temp.

If you really want to cut fuel bills and are willing to plunk down some $$$, replace the boiler with a condensing/modulating unit and install TRVs on all the radiators. It's quite safe to say that you can expect a dramatic reduction in fuel use.

Mike T., Swampeast MO

Pressure in the system has absolutely nothing to do with flow. In fact, most gravity systems are open to the air via an open expansion tank (usually in the attic but perhaps above a closet ceiling on the highest floor) so they are not even pressurized. There is pressure at the bottom of the system because of the weight of water above, but there is zero pressure at the top of the expansion tank. (Occasionally, the small amount of air that's normally trapped in the very top of radiators is used as rather goofy "expansion area" but most gravity system have the tank.)

Even with a closed system that uses a compression tank and is under pressure, the job of the static fill pressure is merely to lift the water to the highest point in the system. Increasing the static fill pressure does absolutely nothing to increase or decrease the ability of the circulating pump to move water.

In a gravity system the slight difference in density between hot and cold water creates a force similar to a circulating pump, but that force is so small that enormous, non-restrictive piping must be used.

Since gravity systems very gradually corrode and restriction builds, it can take hotter and hotter water to establish circulation. You [might] benefit slightly by converting the system to forced circulation; likewise you [might] benefit slightly with a good boiler cleaning and air/fuel mix adjustment. But, particularly if this is a really old solid fuel boiler converted to oil then to gas, (they're frequently round and encapsulated with white--sorry to say asbestos-containing--insulation) then it's woefully inefficient. It may last forever, but you'll pay through the nose to fuel it.