Cast iron system not heating well

might be best to have primary secondary or a hydraulic sep.If it is a combi, it has a pump inside.

But it may not have the capacity to flow the boiler and distribution piping.

The boiler manual should have some piping info.

You need to make the system "primary secondary" to make it work.

The boiler loop has to flow enough water to keep the boiler happy.

The heating loop need to have enough flow to satisfy the radiation. You may need balancing valve on the radiation loops if you don't have them.

You need to have 2 circulators if you only have 1 you will have issues

too bad that the radiation furthest from the boiler is 1/2" which will make it more difficult to balence but it may be ok if sized properly.

Combis and condensing boilers have a large pressure drop.

need more info on the # of pumps and pictures of the boiler piping.

you probably need around 10gpm through the boiler to move the output of the boiler. yo may need more in the system to make it even although if parts are actually freezing it is probably air bound or has broken valves or something like that.

All the air is purged out of the radiation.

What you have is not primary/secondary, although it's not always necessary if the onboard circ can provide adequate flow to cover the load.

It sounds like you've tried balancing flow, but to clarify, if you close off every radiator besides the ones in question are they still cool? There is obviously a flow problem somewhere, but narrowing that down will require some diagnostics. Assuming the circ is adequately sized, there may be a blockage in the piping or rad(s) somewhere. While uncommon, I have seen a few times where sludge builds up in the base of the rads and inhibits flow. Several times I've found debris in the piping (usually against an elbow or valve) that was accidentally left in there during construction. Once upon a time, I found the metal end cap of a fitting brush wedged in the cup of a fitting that must have broken off of the brush during cleaning prior to installation.

if the boiler short cycles when the bypass is open either the system pump isn't moving much water on its own or the boiler circulator isn't working. though it isn't really quite p/s it should act more or less that way with the bypass open. probably the first thing to do is to do the math on the external circulator. also make sure the controls are set up right to run the external circulator on a dh call however that is set up.

some pictures of exactly what is there would help too because the sketch is simplified and may be missing something important.

what kind of emitters are these? are they standing cast iron radiators or are they something more restrictive?

there are likely multiple problems now.

i don't like the opening the bypass makes the boiler unable to distribute its heat energy. either one of the 2 circulators sn't running or is air bound or if the resistance of the system is great it takes a combination of the head of both circulators to move enough water. that is unlikely unless the emitters are very restrictive.

but that isn't the original problem i don't think, it is a problem created trying to fix the initial problem

If you are going to go to the trouble of re-piping the closely spaced Tees do it like the manual shows, not like your sketch.

while the boiler piping isn't great, it isn't the problem either. the fact that is short cycles if the "bypass" is open means either one of the circulators isn't working or is very undersized, the system is very high resistance, or it isn't actually piped the way it is drawn. figure that out first then work on better near boiler piping.

I went through the boiler manual pretty good.

The lowest TD this boiler will work at is 30 deg TD with 11 gpm of flow. They say 11 gpm is the max flow through the boiler.

The pump curve for the internal pump shows a max flow of 4.5 gpm @0' of head and 0 gpm @11' of head. This does not seem to match up

That pump curve I am not understanding

But I am confidant in saying that the secondary heating loop needs to have a proper primary secondary connection and the secondary loop needs to have a circ that is sized correctly if this is to have any chance of working.

"They say 11 gpm" does that include or exclude the DHW flow ?

Not a pump guy but "max flow of 4.5 gpm @0' of head and 0 gpm @11' of head"

Why couldn't a pump do 4.5 gpm with no head pressure and at a head pressure of 11' it will no longer move anything significant. Kind of stating either end of the curve.

Also we don't even know if the valve is a full port ball valve or something else, and if the pipe size also is 1".

Seems as stated earlier multiple issues promotes poor performance.

gate valves should not be used for balancing. the gat being partially open with flow will cause the gate to chatter and damage the screw that drives the gate up and down and it may erode the sealing surfaces of the gate. it should work in the short term though. globe valves or ball valves should be used for balancing.

which radiators don't heat? the way it is set up with all the flow through the lower radiators and only a bypass on the upper radiators, the amount of water needed to move the needed heat may only flow in to the lower radiators and dump all the heat there, by the time the water gets to the upper radiators it is cold.

since the piping is all open I would be inclined to tee each supply and each return connection from the main and connect separate supplies and returns to each radiator with either a globe valve or a radiator valve for each radiator. that being said, unless it is the radiators without the bypass that are the only radiators heating, it could be fixed by fiddling with the valves.

you need to sort out the circulator problem, one of them isn't running and you need to figure out which one and why.

oh, and it doesn't matter if the supply is top or bottom but air will want to move up so supply below and return above is easier to get the air out of.

while neither a gate valve or ball valve are ideal for balancing, They can work. The ball valve will have the most effect at the last 20% and be very sensitive to small movements.

However with manual balance valves when you make a change to one, all the others in the system are changed and need to be readjusted. So it will be a tedious process.

Ideally the valve would have test ports to connect a delta p meter to set and adjust each one to the required flow

But you have some flow issues to sort out first.

Seems pretty obvious that this is not piped right. The rad in the picture does not match the drawing. Looks monoflow with the valve doing the job of the venturi tee.

I see a lot of pipe and long runs of 1/2" pipe and a lot of 90 degree elbows making a lot of head that the circ can't overcome

This is all about flow and not having enough of it. looks like someone threw it together without any calculations at all hoping it will work.

You can't fix it with band aids it needs a complete redesign to find out if any of the existing piping can stay

Sounds like the boiler, radiation and heat load match up pretty well so that is a good thing