How to Set Aquastat for Gravity Hot Water Heating

beer_foam · November 2022

I bought a house this summer and I’m trying to learn a little more about boilers so I can make sure everything is running correctly but not wasting fuel. I have a relatively modern oil fired boiler (2017 Buderus G215) hooked up to an old school gravity feed hot water system with cast iron radiators. To further complicate it, there was a circulator pump added on that supplies water to some rooms which have newer baseboard radiators. (Everything is on 1 zone)

The aquastat is a “Fuel Smart Hydrostat 3250” currently set to 130F low limit and 180F high limit. When the Tstat isn’t calling for heat it will cool to ~115F, fire until 130F, and repeat. My problem with having a low temp limit is the gravity feed radiators will continue heating the house when there is no call for heat. Obviously this is suboptimal with oil at $6/gal.

I think I could fix this by just turning off the low limit so it won’t fire until there is a call for heat but I’m worried this will cause too many cold starts and damage the boiler. Is this a valid concern on equipment like this?

The boiler’s operating manual states it’s a low temp boiler and there is no low limit requirement if using a fancy "Logamatic 2107" controller with outdoor reset but the manual also says if using an aquastat the min boiler temp is 150F which is even higher than the current 130F low limit.

Any insights would be appreciated. Thank you!

mattmia2 · November 2022

You can turn off the warm start but you need some way to keep the boiler from condensing, either a bypass, thermostatic bypass or an aquastat that shuts down the circulator when the boiler is too cold. Some of this might be built in to the boiler.

130 f is the minimum recommended return water temp. Unless it is very cold out it is unlikely your converted gravity system will get that hot which is why you need a return water temp protection scheme.

It may be very difficult to balance the baseboard and cast iron on one zone.

EdTheHeaterMan · November 2022

Someone has unfortunately attempted to use a modern boiler to do gravity heating, but that never goes well.
Next, you can not have cast iron radiators and copper tube/alum fin baseboard on the same zone operated by one thermostat. That never works out well either.

Let's start out with the fact that someone will need to redesign the near boiler piping to accommodate the old radiator system using some type of bypass to maintain 130° return water temperature to the boiler. and to also provide a higher temperature zone for the baseboards. Once you have agreed to that, you need to go to the Find A Contractor link at the top of this page and look for someone that can resolve your problems.

here is a booklet that can help you to become better acquainted with your system https://www.xylem.com/siteassets/brand/bell-amp-gossett/resources/technical-brochure/fh-z100b-bg-zoning-made-easy-2.pdf

Page 18 in this book gives you an idea of how you might get 2 temperatures from the same boiler. The bypass pipe on the lower temperature zone for the old cast iron radiators can also be configured to provide the 130° minimum return temperature you need to protect the boiler from condensation of the flue gasses, which can result in premature boiler failure.

While the boiler is drained to do the repipe, you might also want to check on the top floor radiators to see if there are orifices in the valve union. Some old gravity systems had them to balance the system properly. If they are in there you want to remove them. Once the circulator repipe is completed The gravity design will be all cattywampus when the circulator starts moving that water faster.

beer_foam · November 2022

Thank you both for the replies. I can confirm there is no bypass piping. The aquastat does have a feature to keep the circulator from running when the boiler temp is below 125F but this circulator is only for the baseboards.

It sounds like I will need at least 2 more circulators (1 for the bypass loop, and 1 or more for the cast iron radiators) as well a mixing valve set to maintain 130F in the bypass loop. Should the proper piping look something like what is covered in this article?

https://www.supplyht.com/articles/100650-myths-and-methods-for-protecting-boilers-against-flue-gas-condensation

mattmia2 · November 2022

So the supply and return to the gravity system connect to the boiler before the circulator?

Was the boiler that was just replaced a converted coal boiler?

hot_rod · November 2022

beer_foam said:
Thank you both for the replies. I can confirm there is no bypass piping. The aquastat does have a feature to keep the circulator from running when the boiler temp is below 125F but this circulator is only for the baseboards.

It sounds like I will need at least 2 more circulators (1 for the bypass loop, and 1 or more for the cast iron radiators) as well a mixing valve set to maintain 130F in the bypass loop. Should the proper piping look something like what is covered in this article?

https://www.supplyht.com/articles/100650-myths-and-methods-for-protecting-boilers-against-flue-gas-condensation

As that article shows, it is best to have a protection piping, valve, system, that can react to the return temperature.

A hydronic system is very dynamic, temperatures are always changing. A valve or bypass pump cannot adjust, it is set for one specific condition. IF you go to the trouble of adding a protection, consider one of the methods that includes temperature sensing.
A dynamic solution for a dynamic problem

beer_foam · November 2022

The Buderus was installed a few years ago, before I owned the house, so i'm not sure exactly what it replaced. The boiler is down in a pit about 2 ft below the basement floor, so maybe it was steam once upon a time?

Yes, the circulator for the baseboards is sort of tacked onto one of the supply pipes that feeds a cast iron radiator.

How to Set Aquastat for Gravity Hot Water Heating

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