Simple design for a cast iron boiler install

jesmed1

Thanks to all here who have taught me much as a novice in home heating systems. I've posted before about our 4-unit condo building with a converted gravity hot water system with cast iron radiators and two Weil-McLain WGO-5 oil fired boilers and received much helpful feedback.

The system is now working as well as it can, with the recent addition of an Airtrol tank fitting on the steel expansion tank and improved piping to direct air into the tank. But the boilers are 25+ years old and won't last forever, so I'm planning ahead for when they do need replacing.

@EdTheHeaterMan has been very helpful in suggesting improvements such as primary/secondary piping, and/or combining both systems (one for each side of the house) into one that a single boiler can run. After a lot of weighing the pros and cons or various options, I'm back to thinking that the best solution for us is also the simplest, ie keeping the two cast iron boiler setup, but going to smaller boilers better suited for our relatively low heating load.

Based on our total heating load of 95,000 BTU/hr for a zero degree design temp, we can downsize to two WGO-2's with 75,000 BTU/hr output IBR and still have plenty of margin. So my concept below is based on two WGO-2's replacing the current two WGO-5's.

I've found that the head in our converted gravity system with large pipes is so low that our existing Taco 007's are pumping about 21 gpm through the WGO-5's, resulting in a low delta T of just 12 degrees. Since these are cold start boilers that were installed with no bypass (and yet somehow have survived 25+ years in good condition), the high volume of cold return water is a concern. The boilers typically run for about 40 minutes, starting at 65 degrees and ending around 140, which means that more than half the boiler run time experiences sub-120-degree water that makes people worry about condensing.

In order to reduce condensation concerns on the WGO-2's, we will need to install a bypass and also a lower-volume circulator like the Taco 006. Ron Beck published a useful article with tips for installing cast iron boilers in which he said that the delta T across a cast iron boiler should be a minimum of 20 degrees F, and preferably more like 35-40 F.

https://www.usboiler.net/tips-successful-cast-iron-water-boiler-installation

He says to calculate flow rate through the boiler for a 20 F delta T by taking the boiler's DOE output and dividing by 10,000. So for a WGO-2 with a DOE output of 84,000 BTU/hr, the desired flow rate is 8.4 gpm. And since our old converted gravity system has such large piping, the total head is only about 1 foot water. (This is derived from the observed delta T across the existing WGO-5's with known input rate, and the Taco 007 pump curve).

Since we need at most only 8.4 gpm through our new WGO-2 (and preferably even lower flow to get the higher delta T's Ron Beck advises), a Taco 006 running at 1 foot head would give us about 11 gpm, still too high, but if we had the bypass valve set to recirculate half the flow back into the boiler, we could get that delta T of over 20 F.

Also, since our total head is so low, there's virtually no pressure change across the pump, which means it's OK for us to keep the circulator on the return, pumping toward a new diaphragm tank. This helps us because there's very little headroom in the basement and there is a maze of supply branch piping directly above the boilers, which makes it very difficult to find room to move the circulator to the supply as some would advise. But the delta P across the pump is so low that it shouldn't matter much where the pump goes. So I'd prefer to leave it on the return.

The Weil-McLains have built-in air separators, a fact also pointed out to me by @EdTheHeaterMan. We now have the air separator ports piped into the expansion tank via the Airtrol fitting, but in future we could just put an auto air vent on that fitting as I've seen done one other Weil-McLains. What brand/model air vent would you advise?

I chose an Amtrol SX-30V even though it's oversized for our application, because it's not that much more $ than an EX-30, and my theory is that an oversized tank will experience less flexing of the diaphragm and hopefully last longer. Also I liked that it's floor mounted, so we can stand it on the floor next to the boiler without having to hang it from the ceiling.

Would appreciate constructive criticism. This is a KISS design (keep it simple, stupid), with the objective being maximum compatibility with our existing near-boiler piping with minimum rework, to get us something that may not be what you'd do if you had a clean sheet build with an unlimited, but that will hopefully work well enough given our many contraints on space, etc.

STEVEusaPA

Why in the world you put in two WM wgo's? 2 triple pass boilers or EK's would be better.
Your expansion tank in relation to the circulator is incorrect.
Your air vent is basically worthless, but at least you are putting a shut off valve on it for the easy, frequent replacement.
Your theory of 'an oversized tank will experience less flexing of the diaphragm and hopefully last longer'...well nm. Just make sure the cold fill pressure, the boiler fill pressure and diaphragm pressure are all the same.
If you're doing any re-piping, why not do it right with the boiler replacement, with modern components, set up to make replacement easier?

EBEBRATT-Ed

I would do primary secondary but don't do the series primary secondary pipe the boilers so they get the same return water temp. I would put the expansion tank on the house loop with the make up water. I would use a three way valve on each boiler set to maintain a maximum of 20 degrees across the boiler. I would not do more than 20 degrees you can shock the boiler with the sensor to operate the valve on each boiler return

jesmed1

STEVEusaPA said:

Why in the world you put in two WM wgo's? 2 triple pass boilers or EK's would be better.
Your expansion tank in relation to the circulator is incorrect.
Your air vent is basically worthless, but at least you are putting a shut off valve on it for the easy, frequent replacement.
Your theory of 'an oversized tank will experience less flexing of the diaphragm and hopefully last longer'...well nm. Just make sure the cold fill pressure, the boiler fill pressure and diaphragm pressure are all the same.
If you're doing any re-piping, why not do it right with the boiler replacement, with modern components, set up to make replacement easier?

Thank you for your thoughts.

1. We don't have the budget for EK's. We could do a triple-pass like a Buderus 115/3, but the rear venting and rear water connections mean we'd have to move the boiler a few feet from where it is now into a position where access is awkward. I realize no one likes to clean pin-type Weil-McLain boilers, but our oil company has been doing it every year with no problem. Also, the Weil-McLains we have now have proven themselves very durable and resistant to abuse (like being run 25+ years with low return water temps). So while we could pick up maybe 2% efficiency with a triple-pass, that's not a huge incentive.

2. I understand the principle of pumping away. But in this case, we have around 1 foot head, so the circulator is adding maybe 0.4 psi. I can't see how the expansion tank is going to care.

3. LOL re the air vent. I did put the valve there for frequent replacement. But why is it worthless? Isn't that built-in air separator eventually going to catch all the air?

4. Re oversized expansion tanks, I honestly don't know. But if given the choice between a tank where the membrane expands 50% per cycle vs a tank where the membrane expands 25% per cycle, I'd choose the latter if the marginal cost was minimal.

jesmed1

EBEBRATT-Ed said:

I would do primary secondary but don't do the series primary secondary pipe the boilers so they get the same return water temp. I would put the expansion tank on the house loop with the make up water. I would use a three way valve on each boiler set to maintain a maximum of 20 degrees across the boiler. I would not do more than 20 degrees you can shock the boiler with the sensor to operate the valve on each boiler return

OK thank you. I have thought about the 3-way valve, but they seem to be prone to clogging, and we have a very old debris-filled gravity conversion system, so I worry about that. Then we need a filter, etc.

WMno57

EBEBRATT-Ed said:

I would do primary secondary but don't do the series primary secondary pipe the boilers so they get the same return water temp. I would put the expansion tank on the house loop with the make up water.

Ed, @jesmed1 didn't say this in his post, but I assumed he is keeping keeping the existing one boiler per two units configuration. So two separate house loops, each with their own boiler. No interconnection.

jesmed1

WMno57 said:

EBEBRATT-Ed said:

I would do primary secondary but don't do the series primary secondary pipe the boilers so they get the same return water temp. I would put the expansion tank on the house loop with the make up water.

Ed, @jesmed1 didn't say this in his post, but I assumed he is keeping keeping the existing one boiler per two units configuration. So two separate house loops, each with their own boiler. No interconnection.

Correct, thanks. The other factor here is that one boiler is likely to need replacing before the other, as they are unlikely to fail simultaneously. So the simplest approach is to keep the piping the way it's set up now, and just remove the failed boiler and plug a new one into the same place with minimal piping changes.

hot_rod

Good idea on the upsize expansion tank if you don’t know the actual system capacity.

A good air sep keeps efficiency up, any micro-bubbler.
IF you really want a specific gpm, use a balance valve instead of a bypass. The bypass increases return temperature and lowers boiler efficiency.

The delta T will change based on the distribution load, regardless of what you design to. The heat emitters dictate boiler operation.
Only when the return stabilizes will you be at thermal equilibrium, that is when you confirm delta T.

I’d series instead or primary secondary, no need to have unknown blending going on in the close tees. If you want 180 SWT, direct pipe. Unless you have a high pressure drop boiler.

High mass, high volume systems are a good candidate for boiler return protection

A dirt mag on the return also. Would you run your car without an air or oil filter?

jesmed1

@hot_rod Thank you for the good tips.

hot_rod said:

Good idea on the upsize expansion tank if you don’t know the actual system capacity.
A good air sep keeps efficiency up, any micro-bubbler.

A micro-bubbler would be easier to install if there is one that works on a vertical supply. Is there such a thing? The ones I've seen all seem to need a horizontal run which complicates the install in our tight envelope.

hot_rod said:

IF you really want a specific gpm, use a balance valve instead of a bypass. The bypass increases return temperature and lowers boiler efficiency.

Maybe it's the same result by different means, but the CI boiler installation instructions I've read for Weil-McLain, Buderus, Biasi, etc, all say specifically that the reason for the bypass is that they do want higher return temps in order to minimize condensation.

hot_rod said:

The delta T will change based on the distribution load, regardless of what you design to. The heat emitters dictate boiler operation.
Only when the return stabilizes will you be at thermal equilibrium, that is when you confirm delta T.

Understood, but my point about our low observed delta T throughout the burn is that (1) our gpm is unnecessarily high, flooding a CI boiler with cold return water for most of its life, and (2) the resulting low delta T just recycles colder water back to the return faster.

hot_rod said:

I’d series instead or primary secondary, no need to have unknown blending going on in the close tees. If you want 180 SWT, direct pipe. Unless you have a high pressure drop boiler.

OK, you lost me there. Series what? We don't need/want 180 SWT. We only want return temps high enough to make the CI boiler mfrs happy (130-140), and we can do that with a bypass. And the thermostat typically satisfies by the time SWT has reached 140 anyway.

hot_rod said:

High mass, high volume systems are a good candidate for boiler return protection

Isn't that what a bypass does, or is there some other form of protection?

hot_rod said:

A dirt mag on the return also. Would you run your car without an air or oil filter?

I understand the need for a dirt mag if we have clog-sensitive things like micro-bubblers and thermostatic mixing valves. But I was hoping we could get by with a simple manual bypass and an auto air vent on the air separator port.

dko

Caleffi series 5517 for vertical air separator

Onepipe

Caleffi makes a boiler protection valve that would be piped into your bypass line with a set mix temp. I use the 140f models all the time and they work great. It ensures 140 back to the boiler but shuts off when return temp is higher. Basically a thermostatic mixing valve. It’s there 280 thermoprotec valve. Caleffi also has a dirt mag that can be mounted on a vertical pipe. It’s also fully serviceable.

hot_rod

The problem with a bypass valve or even a bypass pump is it has no control logic. It works properly only under one specific condition, the condition you set it up at. When would one set the valve? Cold start, mid way through the heat call, when the return reaches 140?

A thermostatic valve or a properly applied setpoint pump have the ability to watch and respond to the ever changing return temperature and give you absolute protection under any condition.

If you need or want 140 supply, running a 20 delta, the boiler needs to run 160 or so to give the boiler the 20 delta. Off at 160, on at 140 on the control.

As others mentioned Caleffi and others have vertical air, dirt and mag sep. These protect the boiler, pumps, control valves and keep debris out of the boiler base where it reduces efficiency

The key to a balanced system is to match the boiler output closely to the heat emitter capacity.

IF in fact 140 SWT is all you need, a mod con is the best option, both for modulation and able to thrive at low return

Another option is one mod con and one conventional, best of both worlds.

jesmed1

hot_rod said:

The problem with a bypass valve or even a bypass pump is it has no control logic. It works properly only under one specific condition, the condition you set it up at. When would one set the valve? Cold start, mid way through the heat call, when the return reaches 140?

I'd just set the bypass once manually to give us a better delta T than we have now. Now we have a constant deltat T of 12 F throughout a 40-minute burn. So maybe we set the bypass to double that and give us a constant delta T of 24. It's not a perfect control, but it's better than nothing, which is what we have now.

The irony of this situation is that we have cast iron boilers in a cold-start gravity converted system that has been running ridiculously low return water temps without a bypass for 25+ years, in a way that everyone tells me should have ruined our boilers from condensation, and yet the boilers are perfectly fine and the techs tells me we could get another 5-10 years out of them, or more.

This is one of the reasons I'd lean towards installing another Weil-McLain, because even with a basic manual bypass, I'd have some confidence that it would last another 25+ years.

The key to a balanced system is to match the boiler output closely to the heat emitter capacity.

LOL, we have massive cast iron radiators in a 1920's house, so you know we are way over-radiated. In a perfect world I'd be installing a big ATW heat pump out back with a heat exchanger buffer tank to deliver 110 degree water to all the cast iron radiators, and that would heat the house on a zero degree day. Unfortunately we don't have the budget for that.

A mod-con would be the next best thing, but the disadvantages of higher up-front cost, shorter life, and that we don't have a good place to run the vent pipe all weigh against it. So we are pretty much stuck with non-condensing 85-87% AFUE boilers.

dko

Assumes the cast iron of the newest WGO is of the same caliber and quality as of the past.

Specs say they weigh the same but maybe the composition of the cast iron is slightly different and the new ones won't be so kind to your low return temps.

But I don't know anything about that, just speculative. playing devil's advocate.

jesmed1

dko said:

Assumes the cast iron of the newest WGO is of the same caliber and quality as of the past.

Specs say they weigh the same but maybe the composition of the cast iron is slightly different and the new ones won't be so kind to your low return temps.

But I don't know anything about that, just speculative. playing devil's advocate.

Maybe so, but as they say, better the devil you know than the devil you don't.

The Buderus has a reputation for being able to tolerate low return temps too, and I'd love to be able to install one of those, but the geometry of the connections (rear flue, rear water connections) are awkward for the physical limitations of the space where our boilers are. Maybe when the time comes, our oil company will be able to figure out a way to make a Buderus fit.