Gravity Reset

Tom R.

It is hard to control the entire building with sensors in two apartments on the same floor. Better to take sample apartments from all floors, and several exposures. Many controls manufacturers have inexpensive owner-programmable systems for this purpose. Rather than OST, use the least satisfied apartment and return water temperature to reset water supply temperature. Eventually, you will be better able to balance the risers between apartments as well. Forget what is going on outside - look at the inside.

Ted_4

Gravity Reset

I'm looking at a large 7 unit apartment building with the original gravity hot water system from 1915. The two American Radiator boilers have been converted to oil and test out at 84% combustion efficency. They are trying to control this beast with a single t-stat with remote sensors in two first floor apartments. It's not working well.

Does anyone have experience using outdoor reset on this type of system? It seems to me it would work as long as the water temp sensor were properly located and we got the reset ratio right.

With 6 and 8 inch mains, the owner is not anxious to spend anything on piping changes, so the reset control would be the way to go - if it works.

Brad White_9

Normally

gravity hot water has its own "reset" by default. I am speaking in terms of single-family homes not multi-family as you describe. So this is not meant to be a full answer but some background you may find helpful.

Specifically, in mild weather the boiler fires "x" times an hour in response to space temperature. In colder weather it fires "x+" times, hence has hotter water on colder days, cooler water on milder days. What is key is anticipator settings which I will admit always baffled the "x" out of me, to dampen the thermal flywheel affect. But normally I have seen this "automatic reset" just happen.

Just projecting now, if I were to employ automatic reset to such a large system, some progressive anticipatory response control would seem necessary. Something to gauge how long the burner fires and compare that to outdoor temperature with indoor influence and perhaps return water temperature (sort of a heat timer). Not definitive I know. I will just sit back and let the greater minds flow; this got me curious.

Ted_4

Reset

I agree with your analysis, but I think the residents would be more comfortable if there was constant water circulation. When the present thermostat control cycles off the conditions at the two sensors determine when it comes on again - conditions that could be vastly different on the first floor vs the third. With some constant circ the "hot" people could throttle their radiator valves, and the "cold" people could leave 'em wide open. At least I hope this will work well enough with reseting the gravity system on outdoor temps.

Mike T., Swampeast MO

If TRVs are in the budget, they'll work very well provided you change to forced circulator of course...

If still operating under gravity the piping might have roughened to the point that flow and balance are adversely affected.

Bill Nye_2

Yeah But,

I always thought gravity circulation worked on the principle of heated water being more boyuant. If you don't heat it enough will it circulate?

Some days I can heat my house with 110° - 130° water, will a gravity system operate at lower temps? Will it circulate to the furthest rads ? or just heat the nearest ones? Just thinking out loud, I could be all wrong.

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Brad White_10

Yes, low temperature water

will circulate and in proportion (more or less) to its temperature difference, supply to return. Even one degree warmer water is more buoyant although not by much. So it is not a matter of not heating it enough, but of heating it at all. If the piping is a good gravity flow design (reverse-return meaning all legs are approximately equal in resistance), yes the furthest radiator will get as much water as the first. Taller buildings will likely have the top floors heat before the bottom floors due to the stack effect of a tall column of warm water rising and cooler water descending.

That is why even in mild weather the reset effect occurs. Because the boiler fires in response to space temperature, it is always "just enough". Best thing to come out of Canada since hockey and stray moose.

Brad White_10

Mike, you are right

What you are proposing is an effective per-radiator compromise that will even out the forces. Constant circulation, especially with a low-head pump (really low probably in this case) and TRV's will give good gentle heat with true democracy. Controlling such a building by one or two thermostats is an oligarchy at best. Not sure why I am on a civics bent right now, just an analogical moment :^)>

BTW, Mike: I still browse the Sinopia web site you turned me on to; fun with fine art finishes.