Reverse Flow in HW Cylinder

danjo

Just noticed last night that the feed to the hw cylinder seems to be going in on the lower pipe. It is an old system with an indirect cylinder, with two coils, heated from an oil boiler and a back boiler on the grate.

Just turned on the boiler last night and was checking around the rads and the found that the lower pipe feeding into the cylinder coil was hotter than the upper one.

The oil boiler replaced a solid fuel range about fifteen years ago. I suspect now that the plumber may have connected up the pipes to the cylinder incorrectly. The new boiler is not in the same place as the range so he had to plumb in some new copper piping.
Not easy to trace the pipes as they go up through the ceiling and under floor.
Since then I had a new f/e tank installed. Same position as the old one and the plumber did it without draining down the system. I don't think this has anything to do with the problem.
There are two pumps and both seem fine. Tried switching off the back boiler one but it made no difference to cylinder flow

The flow out from the boiler is fine and the rads upstairs and downstairs seem fine as well. Have not lit a fire yet.
Should I be concerned?

kscobie

it will be fine and really makes no difference, its not like a solid fuel set-up where its usual to have the flow to the upper connection, with the expansion pipe taken at that point

ntpm

Pipe layout to the cylinder is open to interpretation.

Good practise/ some people pipe the oil flow pipe to the top with the Open Safety Vent (OSV) taken from the top of the cylinder to your header tank.
Note OSV is usually incorrectly thought as the expansion pipe.
The return to the boiler is taken from the bottom of the coil.

This way the heat from the boiler is in contra flow with the convection current within the cylinder ( the hot water rises)... technically more efficient.

but some prefered to pipe it the opposite way.

I generally believe that solid fuel should be piped to the top and return to the bottom. This adds the gravitity flow.

The plumber may have just connected the pipes without tracing flow and return but as long as you have circulation it should be ok. Howeverr I would check the OSV pipe.

Buford T Justice

ntpm wrote: »

Note OSV is usually incorrectly thought as the expansion pipe.

Funnily enough, I would have thought that. Can you make a distinction for me?

danjo

Thanks for the replies. The pipe going to the f/e tank is connected to the top pipe on the cylinder. I assume this is the OSV as it is bent over the top of the f/e tank.

ntpm

danjo wrote: »

Thanks for the replies. The pipe going to the f/e tank is connected to the top pipe on the cylinder. I assume this is the OSV as it is bent over the top of the f/e tank.

With regards to heating systems: Utilising a F&E tank.
The feed (makeup) pipe ( usually 1/2")is from the base of the tank to a appropriate part of the system.(discusion for another day).
This pipe is actually where any expansion in the system should occur and will expand back in to the tank.

The pipe that is "bent" over the top of the tank is actually the Open Safety Vent. most plumbers incorrectly term it as the expansion pipe.
It is important to allow for the re-expansion of water in the F&E pipe and the positioning of the F&E inlet, float arm height and adequate distance to overflow pipes is criticle but once again over looked by most plumbers.
This OSV pipe will ,due to its high posistion, vent some air in the system but its main function is to act as a emergency pressure vent release should the system get exesively hot.

Also to note: some plumbers when seeing problems like "pitching" water comming out of the OSV tend to combine these to pipe and think they are resolving the problem... however, they could be restricting the pass of steam up the OSV pipework.
pitching is due to incorrect positioning of the pump, feed(makeup) pipe layout, Tee positions for feed to radiators and hotwater circuits and return pipework tees.
Basically the system is not designed and balanced properly.


To clarify with regards to the Hot Water Cylinder.
The hot water "take off" is at the top of the cylinder. This suplies domestic hot water to taps.
There is an expansion pipe tee'd of the "take off" which is bent over the Cold Water Domestic Tank. This pipe is indeed the expansion pipe, should the domestic water gets too hot and expand exesively. Remember the domestic cold water feed to the cylinder is gravity feed but is never considered as an expansion.


The above is open to argument and I am always open to correction.:D

ntpm

ntpm wrote: »

most plumbers incorrectly term it as the expansion pipe.

Maybe that's why they don't fit them, so they don't have to worry about the name.:rolleyes:

danjo

Thanks ntpm for a very informative reply. Very good details there to consider. Glad I did not chance fitting the f/e tank myself.

I need to clarify my previous post by saying that the OSV pipe is coming off the top pipe on the cylinder coil i.e. the one I believe is used as the return because it is cooler than the lower one. This OSV overhangs the f/e tank.

There is a separate pipe coming off the top of the cylinder i.e. the hot water out which has a tee connection with an OSV which is piped up to the attic and overhangs the main water tank.

So should I be concerned?

ntpm

Re; OSV from coil.
Not ideal but not too much of an issue.
Good practice is that the OSV should be taken from the flow (feed) pipe from the boiler.
This gives unrestricted path for expansion and any steam that may be generated from the boiler should there be a problem.


Re; Expansion pipe from cylinder take off. (Domestic hot water).
This seems to be correct.

What i would sugest that if you have any works carried out in the future that involves draining down the system, I would identify the flow and return pipes at the coil and switch them over.
Ideal time to do this work is if you consider any upgrading works like zoned heating / hotwater or fitting TRV's and cleaning/ flushing radiators out, refilling with inhibitor etc.
I would hazard a guess and say that if you had an old solid fuel stove, the system may have taken in air over time and you may have sludge and "black water".
This would give you a more efficient system.

danjo

That's good. Thanks for the info ntpm

regards,
danjo

ntpm

danjo wrote: »

That's good. Thanks for the info ntpm

regards,
danjo

No problem. My Pleasure.