Negative pressure in 2nd floor radiators?

jesmed1

I'm still working on how to fix the poor "air control", or lack thereof, in our 100-year-old hot water radiator system. I have an Airtrol expansion tank fitting on order and plan to install that on our plain steel compression tank. Meanwhile, I'm reviewing our system pressure along with circulator and tank locations, and trying to understand the dynamic-vs-static pressure situation.

I have the make-up water regulator set to 20 psi. I know that's considered high for a 2-story building, but now I'm not so sure that's sufficient. Because the Taco 007 circulator is installed on the return side, upstream of the tank, the tank is going to maintain about 21 psi at the circulator outlet (see diagram below). I figure that, based on our longest loop, we have about 6 feet of head, which means the pressure drop across the circulator is about 14 psi. (That's right in the middle of the Taco 007 curve, which shows a flow rate of 13 gpm at 6 feet of head).

So when the circulator is running, with an assumed pressure drop of 14 psi and an outlet pressure of 21 psi, that means the inlet pressure is now 7 psi. So when I look at the pressure in a second-floor radiator 18 feet above the circulator inlet, I subtract 8 psi. So now the pressure in the radiator is minus 1 psi?

What this tells me is that, because our circulator is pumping towards the expansion tank instead of away, we're pulling zero pressure in second-floor radiators, which is naturally going to suck air bubbles right out of the water and make them collect in the second floor radiators.

So shouldn't we be running even higher static pressure at the regulator, like 25 psi?

STEVEusaPA

I thought you said you had the book "Pumping Away". Your issue is a well known one.
If you keep pumping into the steel expansion tank you'll never solve your problem. Higher pressure is a lousy way to handle this.
Fix the piping and the relationship of circ to expansion tank.
And toss the steel tank and put in a bladder tank.

jesmed1

STEVEusaPA said:

I thought you said you had the book "Pumping Away".

I said I had "Classic Hydronics." I see that "Pumping Away" is now my next assignment.

STEVEusaPA said:

Fix the piping and the relationship of circ to expansion tank.
And toss the steel tank and put in a bladder tank.

I'll try to do that at the end of the heating season. Meanwhile we're stuck with how this system was piped 25 years ago, and if raising the pressure would make it work marginally better, I guess it's better than nothing.

EBEBRATT-Ed

@jesmed1

I am going to go off track for a bit as usual. I got sent to a job in Worcester 3 floor office building with constant air problems and they sent a Worcester guy to give me a hand. They told me to install a 4" Spiro Vent air separator. They had the 4 boiler Hydrotherm modular set up that was the hot item back in the 80s. I had never been to this job. Salesman sold him the Spirovent to "get rid of the air"

I walked in and noticed the pumps on the return. But they sent me to install the Spirovent so I did. When we were filling and bleeding we finally got water pressure to the third floor until we turned on the circulator. My radio chirped and the Wosta guy said. "WE got a problem , these vents is sucking instead of blowing"

So I had assumed your job had a low enough head pressure (system resistance) with the oversized gravity piping that you would not run into that issue.

The only way to know for sure is to put a pressure gauge on a rad on the top floor. You can run a higher pressure until you get too close to the boiler relief valve setting. I know your calculations are probably right but knowing the actual pressure would be better.

As @STEVEusaPA said pumping away from the expansion tank is the way to go. Pumping toward an expansion tank the system resistance is deducted from the pump suction pressure forcing the pump suction into a negative in some cases . Pumping away from the expansion tank puts the system resistance on the pump discharge.

jesmed1

EBEBRATT-Ed said:

@jesmed1
So I had assumed your job had a low enough head pressure (system resistance) with the oversized gravity piping that you would not run into that issue.

The only way to know for sure is to put a pressure gauge on a rad on the top floor. You can run a higher pressure until you get too close to the boiler relief valve setting. I know your calculations are probably right but knowing the actual pressure would be better.

OK, interesting. I don't know for sure what the pressure drop across the circulator is. I just did what the Bell & Gossett "hydronics" pamphlet said to do by adding up the longest loop (100 feet), adding 50% (for 150 feet total) and multiplying by 0.04 to get 6 feet of total head, or 14 psi.

I would do as you suggest and put a pressure gauge on a second-floor radiator, but those are different condo units owned by other people who aren't keen on me intruding.

I could turn the regulators up to 25 psi or so and hope there's enough margin to stay below the 30 psi relief valve setting, but I would worry about making some radiator valves leak, so I'm not sure I want to try that.

EBEBRATT-Ed

Yeah you may find you are ok with the PD. I can't imagine a gravity system having much restriction.

Did you use the 1 1/4 pipe size off the boiler for your calculations? That would show you more PD than you have

jesmed1

EBEBRATT-Ed said:

Yeah you may find you are ok with the PD. I can't imagine a gravity system having much restriction.

Did you use the 1 1/4 pipe size off the boiler for your calculations? That would show you more PD than you have

The Bell & Gossett total head calculation method that I mentioned above didn't make much sense to me, as it didn't even account for pipe diameter, so instead I've used an online pressure drop calculator that lets you input the pipe size. Using that online calculator, I figured the pressure drop across the circulator can't be more than 6 psi or so, and is probably less.

Then I looked at the Taco 007 pump curve and realized the max head it can produce is 10 ft, or 4.3 psi, at 0 gpm. And since the pumps are definitely creating flow, ergo the pressure drop across the pump must be less than 4.3 psi, probably closer to 3 psi.

In which case, the dynamic pressure at the top radiators will be around 10 psi. So I think we're OK with the regulator set to 20 psi.

leonz

Hello jesmed1,


One question with several parts;

1. Since your circulator is pumping into the boiler sump it most likely has a punchout plug in the top skin
sheet and it most likely has a cast air baffle in the steam chest to catch air bubbles.

2. If the boiler has this sheet metal punchout in the top of the stream chest it has a pipe plug in a tapping
in the top of the steam chest which is very good as it will let you plumb in the ATF-12 Airtrol fitting with
ease.

3. The pipe plug in the top of the steam chest will allow you to install a riser pipe and a sloped pipe run to
the steel compression tank with a 3/4" female/female globe valve piped to the ATF-12 Airtrol to shut
the water off in the steel compression tank if its ever required.

Using this tapping in the steam chest if it has one will be very simple to do to add air elimination to your existing system and you will have heat.

Please check your boilers top skin sheet to see if it has a punch out sheet metal plug before you go any further as this air vent tapping will work well for you and you will be able to use the steel compression tank more effectively for air control in your heating system and your place will have plenty of heat.

jesmed1

@leonz Thank you for the comments. Yes, we have that air-trap casting in the boiler, which EdThe HeaterMan first pointed out to me. In fact, that port is already pumbed to connect with the tank, but it doesn't have an Airtrol and the piping is only 1/2" and not sloped correctly. I posted a pic with description here:

https://forum.heatinghelp.com/discussion/193840/tricks-to-bleeding-a-problem-radiator#latest

I just received a new-old-stock Airtrol ATF-12 fitting which I bought on eBay for 1/3 of the list price, and am going to re-plumb that into the tank with properly sloped 3/4" copper as you suggest.

Thanks again for your help.

EBEBRATT-Ed

@jesmed1

Thought you were going with a bladder tank?

jesmed1

EBEBRATT-Ed said:

@jesmed1

Thought you were going with a bladder tank?

We will eventually, but I can't do that until at least until the end of the heating season, and even then I may not be able to convince the other owners of the immediate need. They may decide to postpone a diaphragm/bladder tank until a boiler needs replacing, and then it can all be done at the same time.

Meanwhile, I can install an Airtrol now fairly easily and inexpensively, so I'm going to try that as in interim solution until I can convince everyone else to spring for a new bladder tank.

neilc

don't forget an isolation valve on that 3/4 run so you can service the tank without pulling down the rest of the system

jesmed1

neilc said:

don't forget an isolation valve on that 3/4 run so you can service the tank without pulling down the rest of the system

Yup, thanks. Each boiler has a 1/2" iron nipple and a 1/2" brass iso valve coming out of those ports already, so I'm going to keep the valves and then transition to the 3/4" copper at the valve outlets.

I know the Airtrol instructions say to use 3/4" minimum pipe, which I am doing, but the boiler casting only has 1/2" ports, so I figure I may as well use the 1/2" nipple and 1/2" valve that's already there. The rest of the piping will be 3/4" copper.

neilc

yup, the 3/4 is for better air, bubble, flow, if and where you have horizotals, any horizontal should have good pitch up to the tank,

jesmed1

neilc said:

yup, the 3/4 is for better air, bubble, flow, if and where you have horizotals, any horizontal should have good pitch up to the tank,

Thanks, I have room for good pitch on the horizontals, so I can do like a 3-in-12 rise/run.

leonz

Once you have the piping and the Airtrol installed you will not need to replace it with a bladder tank as it will last for many decades and any future tenants won't have to worry about being cold either.

jesmed1

leonz said:

Once you have the piping and the Airtrol installed you will not need to replace it with a bladder tank as it will last for many decades and any future tenants won't have to worry about being cold either.

OK, thanks. It would probably be a few years before I could convince the other owners to cough up the $$ for a bladder tank, so we'll have that time to see how the Airtrol performs.

I see that one key instruction with the Airtrol is after installation to run the boiler(s) very hot to force all the air out at once, something we've never done before. And since our water temps are so low, the air never gets completely driven out. So doing that "hot boiler" air purge and not draining/filling the tank every year could make a big difference.

leonz

The way your piping is set up now would require a great deal of fussing to change over to a using an air scoop as your air scoop piping would have to be 1" or 1 1/4" to reach the air scoop and the same size piping to the circulator and then to the homes in the building.

The top of the air scoop could be connected to the bladder tank with another piping run to a floor mounted bladder tank but that would only cost more money and using 3/4" refrigeration grade copper tubing and flare fittings at both ends to meet the female N.P.T threads on the air scoop and the long pipe nipple and street ell under the bladder tank would be expensive and is not needed as you would need to buy a high quality flaring tool, a coil spring pipe bender, pipe straps and screws PROVIDED you have room in the ceiling joists for a job you would do once so the economy is not there, OH, and you would have to buy fifty feet of refrigeration grade copper and no less.

Your on the right track with what you are doing now and it will work.

Steamhead

leonz said:

Once you have the piping and the Airtrol installed you will not need to replace it with a bladder tank as it will last for many decades and any future tenants won't have to worry about being cold either.

This.

The advantage here is that the plain steel tank has no moving parts. In a bladder/diaphragm tank, the bladder or diaphragm is a moving part and will eventually fail. Depending on the size of the tank, replacement can get quite expensive.

jesmed1

leonz said:

The way your piping is set up now would require a great deal of fussing to change over to a using an air scoop as your air scoop piping would have to be 1" or 1 1/4" to reach the air scoop and the same size piping to the circulator and then to the homes in the building.

My eventual plan was that, when a new boiler is needed, we would change from the "air control" system we now have to an "air elimination" system by (1) moving the circulators to the supply, (2) installing air eliminators on the suction side of the supplies, and (3) installing a diaphragm tank. So I wouldn't be keeping the steel tank and trying to install air scoops.

But as I said, we'll see how the AIrtrol tank fitting does for a while. Maybe it surprises me by how well it works.

Steamhead said:

leonz said:

Once you have the piping and the Airtrol installed you will not need to replace it with a bladder tank as it will last for many decades and any future tenants won't have to worry about being cold either.

This.

The advantage here is that the plain steel tank has no moving parts. In a bladder/diaphragm tank, the bladder or diaphragm is a moving part and will eventually fail. Depending on the size of the tank, replacement can get quite expensive.

I understand the tradeoffs, and if it were my home under my complete control, I'd stick with the plain steel tank and a new Airtrol fitting that, hopefully, works as it should.

However, given that our boiler service company evidently knows nothing about air control systems as they keep draining and refilling the tank every year, my concern is that, when I move away and leave the Airtrol in place, even if I leave instructions not to drain the tank, it's 50/50 whether the next boiler tech will follow the instructions or know anything about how to maintain the Airtrol system.

So, while more expensive, diaphragm tanks will be better understood by boiler techs, and the automatic air eliminators will make bleeding radiators a permanent thing of the past. And I think the other condo owners will be willing to pay a few hundred bucks each every so often to replace the bladder tank for the convenience of never needing to bleed a radiator again.

Pros and cons on both sides.

EBEBRATT-Ed

@leonz

Question:

Why do you need refrigeration grade tubing, a spring bender, a flaring tool and flare fitting when you can more easily and less expensively use sweat fittings with L or M copper tubing?

leonz

The thing is from my personal experience I have had to live with and deal with water loss in a heating system and having to bleed every *&^%^&*&^baseboard vent when we lost our water and had trouble keeping water pressure in our heating system with a diaphragm tank and automatic water feeder.

I was referring to using the refrigerant grade copper with no sweating joints and connecting a floor mounted diaphragm tank with no soldering if it comes to that. Myself I not use soft copper or sweated copper for near boiler piping based on copper pipes blowing apart and losing my well water.

Jamie Hall

There are pros and cons to compression tanks vs. bladder tanks. The biggest pro for the com[ression tank is that if they are properly installed they will last, without maintenance, for decades. The bladder tanks won't. The biggest con is that most modern boiler techs. don't know how compression tanks work and can't be bothered to learn.

jesmed1

Jamie Hall said:

The biggest con is that most modern boiler techs. don't know how compression tanks work and can't be bothered to learn.

This is the most surprising thing to me, but apparently true. It's kind of like a car mechanic who doesn't know how a piston works.

At least two things were done wrong in our boiler/expansion tank plumbing: (1) pipe from boiler to tank not pitched correctly, so air bubbles get trapped in horizontal pipe, and (2) no bypass installed for low-temperature water conditions. Both of those are explicitly required in the installation instructions. Then in annual servicing, the tech routinely drains the tank and refills it, introducing more dissolved air.

So I'm finally realizing why our "air control" system is so messed up. There really isn't any "control" at all, except to bleed the radiators. However I do have an Airtrol tank fitting ready to install and am hoping to get that done this week.

hot_rod

jesmed1 said:

leonz said:

The way your piping is set up now would require a great deal of fussing to change over to a using an air scoop as your air scoop piping would have to be 1" or 1 1/4" to reach the air scoop and the same size piping to the circulator and then to the homes in the building.

My eventual plan was that, when a new boiler is needed, we would change from the "air control" system we now have to an "air elimination" system by (1) moving the circulators to the supply, (2) installing air eliminators on the suction side of the supplies, and (3) installing a diaphragm tank. So I wouldn't be keeping the steel tank and trying to install air scoops.

But as I said, we'll see how the AIrtrol tank fitting does for a while. Maybe it surprises me by how well it works.

Steamhead said:

leonz said:

Once you have the piping and the Airtrol installed you will not need to replace it with a bladder tank as it will last for many decades and any future tenants won't have to worry about being cold either.

This.

The advantage here is that the plain steel tank has no moving parts. In a bladder/diaphragm tank, the bladder or diaphragm is a moving part and will eventually fail. Depending on the size of the tank, replacement can get quite expensive.

I understand the tradeoffs, and if it were my home under my complete control, I'd stick with the plain steel tank and a new Airtrol fitting that, hopefully, works as it should.

However, given that our boiler service company evidently knows nothing about air control systems as they keep draining and refilling the tank every year, my concern is that, when I move away and leave the Airtrol in place, even if I leave instructions not to drain the tank, it's 50/50 whether the next boiler tech will follow the instructions or know anything about how to maintain the Airtrol system.

So, while more expensive, diaphragm tanks will be better understood by boiler techs, and the automatic air eliminators will make bleeding radiators a permanent thing of the past. And I think the other condo owners will be willing to pay a few hundred bucks each every so often to replace the bladder tank for the convenience of never needing to bleed a radiator again.

Pros and cons on both sides.

Properly sized with good water quality you should get 15- 20 years from a diaphragm tank.

Some other other options, the lined tanks made for non barrier systems and DHW expansion, no steel exposed to the water.

Or a full bag type tank where the fluid is inside the EPDM bag instead of the steel vessel.

Or. Stainless steel tank.

You can use multiple tanks also to keep cost down

The diaphragm tank will be 1/3 the size of the required compression tank also.

jesmed1

I just got Dan Holohan's book "Pumping Away" and read it. Very informative. On page 58 he talks about boilers with built-in air separators (which we have, and thanks to @EdTheHeaterMan for pointing it out to me) and shows a diagram of how to optimize it using a plain steel tank with an Airtrol, and the circulator on the supply side.

So after I install the Airtrol, we'll have the main elements in Dan's page 58 diagram except the circulator on the supply side. So maybe when we get new boilers, we can just swap the circulators to the supply, and maybe that's good enough for air control.

EdTheHeaterMan

Anyone that installs a new boiler and does not include a new expansion tank along with new zone valves or zone circulators, air scoop/air separator, flow control valves, Feed valve, backflow preventer, thermostats relays and anything else needed to make the system complete, should not be considered for that job.

I never had to say that the reason for your no heat call on this new boiler is because of the old whatchamacallit and it will cost you $$$.00 for that repair. Because my customer would reply, "I just spent $$,000.00 for an new heater and you are going to charge me to fix it. What does the warranty cover, anyway?

A new heater is more than the boiler or the furnace... It's a SYSTEM.

SuperTech

EdTheHeaterMan said:

Anyone that installs a new boiler and does not include a new expansion tank along with new zone valves or zone circulators, air scoop/air separator, flow control valves, Feed valve, backflow preventer, thermostats relays and anything else needed to make the system complete, should not be considered for that job. I never had to say that the reason for your no heat call on this new boiler is because of the old whatchamacallit and it will cost you $$$.00 for that repair. Because my customer would reply, "I just spent $$,000.00 for an new heater and you are going to charge me to fix it. What does the warranty cover, anyway? A new heater is more than the boiler or the furnace... It's a SYSTEM.

Ed I agree with you that new hydronic components should always be installed and piping errors corrected when the boiler is replaced. However in my experience customers who need an unexpected replacement often choose to have a "direct replacement" performed to keep costs to a minimum.

Pretty much every boiler I work on with a compression tank is a result of this. I don't mind replacing not air vents and bladder tanks on these systems. 

EdTheHeaterMan

Sounds like you are giving them the option for the lower price job @SuperTech. I never give that option. I would have my competitor take those jobs, then I don't need to deal with "I just paid you $??,000.00 for a new heater and you are going to charge me extra for a new thermostat because it wasn't included in the price of the new boiler?" I'm too good for those customers.