How low can I go. Aquastat on gravity HW system.

Tom Hopkins

Hello,

I have a 1929 Closed gravity HW system that still has no circulator. The boiler originally was coal and was converted to gas many years ago. I think it is original to the house.

In the mild weather the system tends to overshoot the setpoint of the thermostat after it shuts the boiler off. I was wondering if I could set the high limit control a little lower in the mild weather. If I can set this lower how low can I go and still get good water circulation through my boier and radiators? I know I wouldn't want to go any less than 140 to 150 degrees. It's currently set at around 180 degrees.

If it is OK to reset this thing, Is there any reason that I couldn't put an automatic reset on it?

Thanks for anything,
Tom Hopkins, Bismarck, ND

Unknown

Easy first step

Check under the cover for a heat anticipator setting. It'll usually have numbers, and/or an arrow marked "longer" meaning longer cycles.

Adjust the slider to a lower number, or "shorter" cycles.

That thing is an antique.

Noel

Tom Hopkins

I want to reduce the supply water temperature

Thanks, Noel.

The control you see in the picture is the high limit. It's a little hard to see but it's in one of the two hot water supply lines above my boiler.

I'm using an older Honeywell Magicstat setback thermostat for the main control and I'm not sure if it has anticipation or not. The main thermostat is the one that's overshooting. I'm just wondering how low a setting I can go on my high limit.

will smith_2

control

I don't think you would have any problem resetting down to 160. You might consider hanging your old limit on the wall (I've got a collection of antique controls and boiler plates myself) and getting a new limit-this one could be a little out of calibration. If you don't want to fork over the dough to install a reset control, you could use an outdoor air lock-out to keep the boiler off altogether on mild days. I use the Johnson A419 (I've even used one to act as a reset using the no/nc sets of dry contacts with a dpdt relay. Good luck!

Unknown

In a gravity system

The room thermostat controls the water temperature by the length of run cycle. It only runs long enough to bump the water temperature up enough to maintain temperature.

It is indoor reset/constant circulation. The THERMOSTAT gives the boiler the feedback on how much water temperature is needed to maintain temperature. The THERMOSTAT is the delicate measuring device on a gravity system. The anticipator is the "fine tuner" on a gravity system.

A setback thermostat is like a cement block on the balance beam of your system.

The setback thermostat is designed for a pumped, or a fanned (on hot air) system. It will give you nothing but grief on a gravity system.

Think of it this way. A gravity system, after it has run a few hours, seeks to put the MINIMAL amount of temperature into the water, by only bumping the water temperature up enough to MAINTAIN the temperature in the room. This is the variable water temperature controller. This is the savings that you say you are looking for. If you realize that you don't have a system that can be set back to save fuel, because it already RUNS that way all of the time, you won't make it run those long, overheating cycles that the clock thermostat causes each day.

The clock thermostat doesn't give THIS system short, moneysaving cycles, it causes LONG, OVERHEATING cycles.

The cure is to put in a single temperature thermostat, set the anticipator, and enjoy the shortest possible run cycles, with the lowest possible water temperature; night AND day, and comfort that can't be beat.

If you try to reengineer your gravity system, you'll be buying parts till the cows come home, trying to duplicate what the system would do if you put it back the way it was.

Noel

Mike T., Swampeast MO

Short answer: Go as low as you can where you still get circulation of water during moderate weather. Problem is that during cold weather that same setting likely won't be high enough to get good circulation. The average water temperature will have risen and there likely won't be enough differential for flow. A reset control (dual-adjustable mechanical should be all you need) would be a good addition. Curve will likely be significantly flatter than in a forced system.

Long answer:

Does the boiler reach the aquastat setting during a typical cycle? (e.g. does the flame cycle before the t-stat is satisfied?)

If the flame doesn't cycle, I'd write down the supply temperature at the time the thermostat is satisifed, subtract 15° and use that number to help establish the low end of the curve for a reset control. Also in the case that the flame doesn't cycle, adjustment of the thermostat anticipator (your first reply) can probably help.

For the high end of the curve you'll have to wait for cold weather. If the flame still doesn't cycle during a call for heat set similarly. If it starts cycling around 180° use that temp as your high at that outside temp.

Slight overheating during moderate weather was mentioned long ago as a fault of gravity systems.

As the system ages the pipes get progressively more rough--this adds flow resistance so it starts taking hotter water to achieve circulation.

If the burner is cycling during a call for heat at moderate weather then likely the resistance in the piping has grown significantly over the decades. In that event, it's probably time to convert to forced circulation (unless you want to replace all of the piping In this case, a lower aquastat setting (or reset) probably won't help much as it's taking that hot water to achieve circulation.

If the home has been insulated/weatherized that old boiler (with its large water content) is likely massively oversized. It will still take the same temperature differential to achieve circulation, but with the water moving more slowly and all of that heat in the water, overheating during moderate weather is just going to happen.

Matt Undy

The thermostat probably has adjustable anticipation. You should be able to get instructions on how to set it off honeywell's web site. You also might do better with some of the newer stats that average over the past few days and shoot for a given number of cycles per hour instead of an anticipator. Esentially on a mild day the thermostat will have to stop calling for heat before the thermostat sees any heat from the system.(i had a furnace that was the last of the gravity furnaces that was installed origianlly with a conversion blower. on mild days the bolwere wouldn't even come on until after the themostat stopped calling for heat, the heat dint' reach the fan controler until after it was time to stop calling for heat)

Matt

Matt Undy

Could he just set his setback thermostat at constant tmepurature? Does it have an anticipator that will work with a gravity system if set properly(shorter cycles i think?)?

Matt

Tom Hopkins

Thanks!

Thanks for the replies everyone. I’ve been out of town for a couple of days.

I put the night setback stat in a few years ago mostly for comfort at night. We prefer slightly cooler temperatures for sleeping. We never set the stat back more than 3-4 degrees. One of the times it tends to overheat is in the evening when the system calls for heat as the house cools down at night after a warm day. I still have the Honeywell round stat on hand and I may try re-installing that. I think it has an anticipator. The system also tends to overheat quite a bit in the morning after coming off of the night setting.

Mike T, You point about the high limit possibly not reaching set pint on a mild day is a good one. I hadn’t thought about that. I’ll do some experimenting and see how hot the water really gets on a mild day.

My wife is getting to the age of hot flashes and she’s extra sensitive to changes in temperature!

Thanks Again, Tom

Mike T., Swampeast MO

Please let us know what you find out and do! It really helps!

Mike T., Swampeast MO

Should you find evidence that your gravity system is slowing down...

...and if you are willing to part with an amount of money proportionately less than the dead man who originally owned your system...

...and if you want to significantly reduce both consumption and fuel bills...

...and if you want that system to have the best chance of lasting your lifetime...

...install TRVs on EVERY radiator and use a Vitodens boiler by Viessmann.

Sorry, but even the extremely tight temperature control of TRVs on oversized standing iron can't deal with hot flashes.