Balancing pumped hot water system

[Jm66208]

...in a hot water system. 3500sf house. A couple of the cast radiators have been replaced with fin tube radiators at some point in the past. One of them doesnt get hot. I'm guessing the pipes are rusted. (80 y/o two story house)

 

[Dana]

When a radiator isn't getting hot it's usually because there is air in the system creating a vapor lock preventing flow. Blockage from rust is all but unheard of, nearly impossible do occur, since the heating system water becomes depleted of oxygen fairly quickly in a closed system.

Ideally there would be an air bleeder valve at every radiator or fin tube baseboard to purge air and get the system flowing again. On baseboards it's usually under the tin on one end of the baseboard or the other- looks a bit like a half-inch hex nut with a tiny tube or hole in one side, with a screwdriver plug in the middle. The center screw is actually a needle valve that opens up, a half or full turn is enough to let the air out, and when the thing stops hissing & spitting, peeing a steady stream you close it back up.

On 80 year old radiators it may have a different form-factor, but the form and function haven't really changed much. It's usually on near the top of the radiator on one end or the other. Some are a little more ornate than others, most antique versions have been replaced after 80 years.

If the system pressure is way low, sometimes the radiators at the top of the house can be at negative pressure. If the boiler is in a full basement with 2 floors above it it needs to read about 12 psi in the basement when the system is idling. If the top of the top radiator is more like 30' above the pressure gauge 15psi would be better, but there's no advantage (and some disadvantages) to running it any higher than 15 psi.

 

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[Jm66208]

Thanks Dana. Yes, we bled all the radiators, several times on the ones that aren't getting hot. The one in question is in the master bedroom. There is a sun room just off the master that has a smaller radiator in it. It get really hot. Also, the one in the master bath heats too.

 

[Jadnashua]

Did you get a good flow from the bleeder when you opened it on the one that does not produce heat?

For the use of PEX, it depends on your system operation temperatures...PEX does have max heat limits (many limit it to 180-degrees F maximum).

 

[Dana]

There may be a larger vapor lock on the loop somewhere preventing good flow, even if there isn't much air right at the radiator. Sometimes it requires running large amounts of water through the system along that path to fully purge a big air bubble. Mapping out a schematic of the plumbing to/from/between radiators can be important too. If it's a monoflow-teed system that has been monkeyed with it could be that the flow is going primarily through the bypass branch, with nearly none through radiator, or there could be other explanations.

If you can sketch the system plumbing and post it there might be other possibilities that don't immediately come to mind.

 

[Jm66208]

Ok, ill do the best i can to diagram the system when i get a chance. One other thought i had is that when they installed the fin tube radiators, they used dielectric fittings between the black pipe and the copper. Ive seen these nearly clogged with calcium before when used on water heater hookups. Just a thought. I would disassemble it and take a look, but i just dont know enough about boilers to tear into it. The system is still under pressure.

 

[Dana]

Closed heating systems don't get nearly the liming accumulation of potable hot water systems, since accumulating more minerals requires bringing new volumes of mineral laden water into the system, which doesn't (or shouldn't) happen in a heating system.

 

[rjbphd]

Come on ,Dana... the third picture isn't a bleeder valve.. it's a balancing / trottle valve..

 

[Jm66208]

Following up on this thread, i am wondering if it is possible to operate the circulation pumps without firing up the boiler, to help with purging any air. Its 100 degrees here today!

 

[rjbphd]

If the system properly designed, you need pump to purge the system. Furthermore, die electric unions have no business being on hydronic heating system.

 

[Dana]

Following up on this thread, i am wondering if it is possible to operate the circulation pumps without firing up the boiler, to help with purging any air. Its 100 degrees here today!

Yes, you can run the pumps, but to do so you may have to defeat temperature interlocks on the controls to force the circulation pump, or disconnect the pump from the zone controller/relay, and set up a temporary way to feed it power.

While the system is cold it's not unreasonable to purge any settled iron-oxides crud'n'mud in the boiler and distribution plumbing by opening up the system and flushing it it completely with a new charge of fresh water. This sort of maintenance is often done annually on systems with iron distribution plumbing and cast iron radiators. Fresh water is fairly corrosive and this shouldn't be done frequently, but build up of ferrous crud in the system can be hell on pump impellers & valve seats, so a once every year or two rinse can be worthwhile preventive maintenance.

 

[Jm66208]

OK, the heating season is here, so we are trying to get this system balanced once again. Here is a crude map of how this (crude) system is piped. What I have found out so far, is that everything on the left side works properly. I noticed that on the radiators on the right side, the return line gets hot, and the feed line is only luke warm, and one of the second story radiators stays room temperature. A real mess...
So far, I've replaced all the valves on the individual radiators (what a job!). Help!

 

[NY_Rob]

If on the right side of the boiler the returns are hot and the supply is luke warm... the return water from the left side is flowing into the returns of the emitters on the right side and returing to the system via the supply lines.

That diagram doesn't seem like it would work properly without multiple flow check valves.... where are they on your system?

 

[Jm66208]

NY_Rob, I think you are exactly right. Pump #2 on the left side is overpowering the system & forcing water to circulate backwards (or sit stagnant) on the right side. Pump#2 is a 3-speed pump, but setting the pump to low just causes a different imbalance elsewhere in the system. There are ball valves near the boiler on both the supply & return, but adjustments are hit & miss. There are NO check valves anywhere on the system!

 

[NY_Rob]

Did the system as drawn above ever work correctly?
It seems like it can't realy work properly as is...

 

[Jm66208]

Did the system as drawn above ever work correctly?
It seems like it can't realy work properly as is...

I doubt it, but i dont know for sure. house was recently purchased . Keep in mind that this house is circa 1926. I assume it was converted from steam to pumped hot water at some point.

 

[Jm66208]

Bump. Any ideas? I dont know enough about these systems to even ask the proper questions, but i dont see how hot water would have any incentive to flow through any but the furthest radiator on each branch. I would like to be able to tell the home owner if the system is even salvageable.

 

[Dana]

This really needs a site visit to know for sure what the right way forward is. Unless there are valves not shown in the sketch there is no way to balance the flows properly. If the drawing is correct they may have simply removed the steam traps & vents, installed a hydronic boiler with the mother of all pumps and crossed their fingers.

Installing thermostatic valves w/ bypasses on all the radiators there is a remote chance one could get sufficient flow to all radiators to get hot, with enough adjustment range to made to work & balance reasonably, but that's potentially expensive hackery, and not the same as designing it to work.


Any real solution starts with a room by room load calculation (either Manual-J or I=B=R is good enough for balancing purposes), then measure up the equivalent direct radiation (EDR) in each room, which may or may not be reasonably balanced in terms of LOAD/EDR ratio. Most fin-tube baseboard is equivalent to ~3' EDR per running foot of baseboard.

To get much further would require a lot more information, but doing at least the load/radiation arithmetic is worth doing up front, and will be useful in vetting contractors, and useful to the contractors for figuring out the quickest or best way forward.

 

[Jm66208]

"If the drawing is correct they may have simply removed the steam traps & vents, installeda hydronic boiler with the mother of all pumps and crossed their fingers." -Dana

Thats exactly what they did! Then someone came along and tee'd off of it to heat an addition. Half the house has cast iron radiators, and half has hydronic panels. Thanks for helping me confirm, and professionally explain that this system is hopeless. The house has two forced air a/c's. My recommendation is going to be to convert them to forced air heat & a/c, and abandon the boiler.

 

[Dana]

The hot air solutio may be the cheapest way forward, but not necessarily the most comfortable.

If you go that way be sure to do a Manual-J load calculation (not I=B=R) using aggressive or "optimistic" assumptions, (or run a fuel-use based heat load calculation), lest the hot air contractors oversize you by the typical 3x +. While the typical overizing isn't an efficiency hit for hot air systems, it's definitely a COMFORT hit, and it costs a bit more up front. Right-sized (= between 1.2-1.5x oversizing for the 99% load) and modulating heat pumps or gas-burners are definitely more comfortable than what most contractors would install.

The right indoor temp for the calculations should be a code-min 68F (even if you keep the place 75F), and KC's 99% outside design temp is +4F. Even if you up sized the system at EXACTLY 1.2x you'd be good down to -5F or so before it would begin to lose ground. At the ASHRAE recommended 1.4x oversizing factor you'd be covered down to under -15F. Equipment sizing steps are large enough that there isn't much risk in taking the first model that covers the load, even at 1.0x sizing from the calculated number, since even Manual-J tends to overestimate unless you are VERY aggressive in the U-factor and air leakage assumptions. In a fuel-use calc, the presumption that the as-used efficiency is the same as the steady-state efficiency of the boiler is also going to oversize slightly, since the standby & distribution losses are going to be on the largish side in your system.