Gravity to Circulated water

PaulStraface_2

Hello,
Quick Question looking for the best pipe method to convert an old gravity heating system to circulated hot water with new cast iron boiler?
Thanks in advanced
Paulie

Ironman

If you don't need to remove the large iron pipes, I'd leave them; they make a great buffer which prevents short cycling.

Why a cast iron boiler? A mod/con would work great if done correctly. Probably about 20% more efficient with outdoor reset.

Either way, make sure you thoroughly flush the system. Also, open the unions on the upstairs rad's and see if there are any orifice plates. If so, move them to the first floor rad's. The entire dynamic of flow will change when the system is pumped and the plates need to be downstairs, if you have them. A lot of systems don't, they just have smaller risers going upstairs.

If you're set on removing the large pipes, I'd use a home run system with manifolds and connect pex-al-pex to the risers in the basement. We're doing one just like that now.

PaulStraface_2

Thank you for the response, were doing cast iron due to the budget we have to work with. probably going to leave the large piping at this point. But do you think its worth doing primary / secondary off the boiler or just pipe with a thermal bypass?

Ironman

I'd do p/s so that you can tweek the flow rate in the system. Also, with the large pipes, the boiler's gonna see low return water temp for longer periods.

A new cast iron boiler will require a metal chimney liner if placed back on a masonry chimney and the liner will have to be insulated if it's on an outside wall.

A CI boiler plus that, and you've spent more than an HTP UFT fire tube would have cost and you've got less efficiency.

GW

I’d recommend finding some money and getting a mod con. But if it needs to be cast you may wish to have a 4 way mix valve installed to keep boiler happy

newagedawn

i would definitely primary secondary your system, i just did one with a cast boiler due to finance issues and it works great!!!, you might need a larger pump on the secondary due to all that water to push, i used a taco 3450 variable speed and it works great, but alpha makes a great variable speed also, do some calculations to determine the water content and that will give an idea of what size pump you need, i would also recommend a peerless boiler, there still made in the US and they are great boilers, good luck and make sure to pump away from the point of no pressure change and you'll be a happy man!!!!!

leonz

PaulStraface said:

Hello,
Quick Question looking for the best pipe method to convert an old gravity heating system to circulated hot water with new cast iron boiler?
Thanks in advanced
Paulie

=====================================================


Why are you intent on converting it??? is it due to remodeling the home or business?

Is there any reason you cannot use 2 bell reducer fittings and 2 short nipples and unions to connect the new boiler to the old piping?

Have you talked to a boiler builder to ask if they accept change orders to weld in the same size tapping in a steel shell boiler?

jumper

leonz said:

PaulStraface said:

Hello,
Quick Question looking for the best pipe method to convert an old gravity heating system to circulated hot water with new cast iron boiler?
Thanks in advanced
Paulie

=====================================================


Why are you intent on converting it??? is it due to remodeling the home or business?

Is there any reason you cannot use 2 bell reducer fittings and 2 short nipples and unions to connect the new boiler to the old piping?

Have you talked to a boiler builder to ask if they accept change orders to weld in the same size tapping in a steel shell boiler?

I have similar questions. If gravity worked well before why is secondary pump required? If boiler requires circulator you can join old pipes where old boiler was and use diverter fittings to attach new boiler. Another method to consider is to bridge in attic instead of in boiler room and then you may not need any circulator.
Gravity burns more fuel but it has its advantages. With no pumps you don't have to continuously add water and eliminate air.

newagedawn

with the water mass of the gravity boiler a 007 pump will not move all that water the pump will come on and in 3 minutes run the hot water out of the boiler it will run and run and run, primary secondary helps that situation, the primary loop will keep hot water supplied to the secondary loop and your house will heat fast, word to the wise= DO NOT USE ONLY ONE PUMP it will not work without running all day, its your house do as you wish,... but do some research and you'll be happy you did

Gordy

I'll disagree. Gravity systems have very little pressure drop in the piping. You want to mimic the gravity flow the system once used. Mining pumps are not the answer.

Do large pumps work? Sure at a cost in the larger pump, energy, and unnecessary excessive flow rates. If you can't pull off a 30 delta on the system side you are over pumping a gravity Ci rad system.

leonz

A larger gallon volume expansion tank with a Tee in the base of the tank will solve all of this as the greater amount of water in the steel expansion tank will create more pressure in the system without pumping.

If the steel expansion tank is piped as I described with a tee in the base thus creating a hot water loop to let the hotter water travel faster back into the basement and boiler sump to be reheated.

Gordy

The expansion tanks purpose in a hydronic system is to provide enough pressure to raise the water to the highest point of the system. The circulator moves water by pressure differential created by the circ.

FranklinD

@Gordy is correct on all counts.

I have a converted gravity system in my home, as does my mother across town. Her system is twice the size of mine (I have 425edr vs her 800). Her system has been using an NRF22 circulator for 20 years with no problems, and I use a Bumblebee on the “Constant Speed 2” setting - there is so little friction in my system that speed 2 moves 8-10gpm and heats up the radiators very evenly (and relatively quickly - as quickly as 130 gallons of water can be heated).

I read the article on this site about circulator sizing for converted gravity systems long after I played with settings to find what worked the best...and the article was dead on. I wish I had found it earlier and saved myself all of the time spent looking around.

As for adding water/eliminating air, I haven’t had to add water to my system since early fall 2016 - I had drained it in August to repack all the handvalves and replace all the bleeder screws on the radiators. I keep my water feed valve closed just in case I spring a leak upstairs, but I’m also the kind of person that glances at the pressure gauge at least once daily.

Ironman

> @leonz said:
> A larger gallon volume expansion tank with a Tee in the base of the tank will solve all of this as the greater amount of water in the steel expansion tank will create more pressure in the system without pumping.
>

I'm sorry, but that's not correct. A 1 gal. tank would have the exact same static pressure as a 100 gal. tank if the water line is at the same height.

Gordy

leonz said:

A larger gallon volume expansion tank with a Tee in the base of the tank will solve all of this as the greater amount of water in the steel expansion tank will create more pressure in the system without pumping.

If the steel expansion tank is piped as I described with a tee in the base thus creating a hot water loop to let the hotter water travel faster back into the basement and boiler sump to be reheated.

Adding more pressure to the system beyond what is needed to raise the water to the highest point in the system 12-15psi for an average two story home with boiler in basement is all you need. The relief valve on the boiler is 30 psi.

The only thing higher pressure will do is keep oxygen in solution, raise the boiling point of water, and shrink trapped air bubbles to move along to the air removal device. This is usually temporary in initial purge.,except for raising the boiling point of water.

There is really nothing to solve. The dead men did gravity circulation at its finest for decades before circulators were invented. The invention, and implementation of the circulator allowed for smaller piping, and deviation from normal gravity piping schematics.

Lance

Keep it simple. Any boiler will do, the key is your design choices regarding performance vs efficiency and air and flow control. A hot water system allows use of an outdoor reset control. This is a no brainer. 90 plus boilers work best at low temps and don't care to see low return temps. But they are still good. Don't size on radiation, size on heat loss J calculation. Return temp control is a must on a non-condensing boiler. Pick your type. Hydraulic separation is great but not critical if certain specifications are done. Its all in what you design. A good B&G100 is a good choice for circulation, volume and flow must be considered. Friction loss is nil. Usually no complaints when converting from gravity to forced. Best advice is get something not to complicated. Sad it is tech are not educated enough to fix complex stuff. Also the more complicated the more service required. Not too many good designers today, but that is where to start. Pick a good designer then build it.