Old School hydronics layout

CBRob

In this type of design how is the pressure to flow through the radiator generated?

I can see how there might be a slight pressure change on either side of the radiator but it would be slight.
I've seen similar setups with a pressure differential bypass that would send flow primarily through the radiator.
 
This one seems like it would only go through the primary loop when the bypass valve is open.
The secondary loop does not have a pump.

What am I missing here?
Is that bypass valve normally open?
Close it partially?
Thanks 👍

CBRob

Real life version.

GroundUp

It's a bypass valve, not an open loop. That valve stays closed or throttled, which forces flow through the radiation

hot_rod

From a cold start you adjust that bypass so the boiler sees at least 130 on the return connection within 10 minutes.

Lot of conflicting advice about piping those copper boilers. At one point they told us 120 return was adequate. But that was not always the case.

Finally RayPak , a competitor to Laars, added a bypass and valve from the factory.
The manual may tell you how to adjust the bypass?

CBRob

That sounds so confusing and easily messed up by one curious homeowner and a valve.
Makes me appreciate the simplicity of today's modcons.
I found some guidelines for adjusting the bypass valve, and even old school they liked the primary/secondary option.

CBRob

mattmia2

Primary secondary doesn't guarantee return temp protection, you could have a high mass zone that is cooling the boiler loop below condensing temps.. A thermostatic bypass is better than a fixed bypass.

hot_rod

What is the date on those installation sheets? I don't recall that bold P/S suggestion in the manuals back in the early radiant days? They cover return protection nicely in that sheet, offering options and precautions. All non con boiler manuals should include that verbiage

I agree with @mattmia2 primary secondary does not necessarily provide return protection. Their thermostatic "union" is still the best absolute protection, in my opinion.

CBRob

That was the closest manual I could come up with for this laars mini therm.
Belongs to a friend of mine, plans to replace it before winter. Has a mod con boiler and an direct water heater in the works to replace it.

CBRob

hot_rod

In some manuals I see it with two valves, one to restrict the supply after the bypass, and the bypass valve also. You had to play with both valves to gety the bypsss dialed in. it just depends on the system it is connected to.

Laars asks for a full size bypass. The RayPak had a tiny 1/2" bypass with a valve. So solutions are all over the map for return protection suggestions.

Copper tube swimming pool boilers have a thermostatic disc built into the header between return and supply. I recall some installers using pool boiler in the early days, for radiant, since they had protection built in. Cost less than hydronic boilers also. I think our local inspectors shut that down, no ASME tag on them.