Adv/disadvantages of pumped open system v sealed system??

[jobloggs]

A lot of posted questions deal with blocked feed pipes and pumping over into f & e tank.

Am I right in thinking that the problems that cause these symptoms don't occur in sealed systems?

What are the advantages disadvantages of these systems relative to each other? Is it just the cost of the pressure vessel against a f & e tank?

I'm into life-long learning!

 

[chrishutt]

I'm into life-long learning!

Looks like I'm into life-long answering questions...

Basically sealed systems are better because they eliminate the interaction of air and system water. Expansion vessels and PRVs are not expensive, and often save having to take pipework into roof spaces with attendant risks of freezing, etc.

However sealed systems do require more maintenance - PRVs leak, expansion vessels lose precharge pressure - but this maintenance can be incorporated into regular (?) boiler servicing.

The only other significant disadvantage of sealed systems is that water loss is not made up automatically (it could be, but stupid water regulations don't allow it). Therefore pressure will need to be manually restored at intervals (perhaps once a year on a sound system).

This can be a problem, particularly on rented accommodation where tenants often don't know what's involved, giving rise to lots of call outs for landlords. However it has the advantage of alerting the owner to water loss that might otherwise go unnoticed for years until structural damage results.

 

[jobloggs]

Thanks Chris.

Could one replace a F & E tank with a pressure vessel if the feed and vent pipes were tee'd together and led into the PV? What size of a PV would be needed for a typical system (15 rads)?

Thanks

 

[chrishutt]

You can only convert an open system to sealed if the boiler is a suitable design, i.e. one with an overheat stat built in. What type is yours?

The expansion vessel, filling loop, pressure gauge and PRV should be near the boiler connected to the return pipe. The existing vent and feed pipes can be cut back and may need to be fitted with AAVs.

Higher system pressures may cause leaks from radiator valves. It might be advisable to replace any old, leaky ones.

Not as simple as you thought, is it?

 

[Slugbabydotcom]

A typical system I would say is what you would find in a three bedroomed semi detached house ie 8-10 rads.
Manufacturers say that if you are fitting a compact model boiler then you will likely need another PV for more than 10 rads.
If you are into learning then you should know the general gas law and the fact that water expeands a maximum 1/20th of its volume up from 4ºC to its point of latent heat. Figure out how much water you have in your system then do some sums!!
Or you could cheat and just fit two of the cheapest standard combi PV's you can get your hands on.

Do you want to do this because your system has gone wrong? or because it might go wrong?

 

[jobloggs]

I've an oil fired boiler with a boiler stat fitted. That is the only control I have at the moment other than recently fitted TRV's on all but a couple of radiators. I was thinking of fitting a cyl stat and a 2 port valve on the HW circuit but the vent pipe comes off the HW circuit just before the cylinder 2 floors up (feed connects to HW return just after cylinder).

When I isolated the coil and closed of all the rad TRVs to simulate a heated house and hot water in cylinder the system pumped over into the F & E tank. Putting a bypass valve in the across the HW circuit wouldn't solve this as I don't think there would be enough pressure difference to open it rather pump over when the 2 port valve closes.

I suppose one solution would be to disconnect current feed and vent pipes from the flow and return of the HW cylinder coil and run them the whole way back down two floors to the boiler house and put them on the return side of the pump. Obviously a bigger job.

As you said Chris not as simple as it seemed and I bet you are sorry you asked about my boiler/system.

 

[chrishutt]

There is a simple solution to the pumping over problem (if only you'd stated this clearly at the beginning...).

At present the vent pipe branches off the cylinder flow pipe and the cold feed & expansion pipe branches off the return. There is therefore a pressure differential between those two points in the circuit (especially if a closed motorised valve is inserted), and consequently between the top of the vent pipe and the feed & expansion tank. If this pressure differential is greater than the height difference of the tank water level and the invert of the vent pipe, then pumping over can occur.

The easiest solution is to raise the invert of the vent pipe, if there is sufficient headroom available. Failing this, one can change the connection point of the feed & expansion pipe from the return pipe to the flow pipe adjacent to (but downstream from) the vent pipe connection. This will eliminate the pressure differential. If this is impractical, the feed & ex. pipe can be joined to the vent pipe at some higher point, even near the feed & ex. tank.


Of course the problem you describe would not arise if you placed the HW circuit motorised valve after the feed & ex. pipe connection. Admittedly this would block the direct feed route to the boiler when shut, but the alternative rout via the flow pipe would exist and when filling the motorised valve can be manually opened.

 

[jobloggs]

Thanks for all your efforts Chris.

No headroom to raise vent pipe.

Will try and get my head round the other suggestion over the weekend. (away to in-laws) Will get back to this on Monday.

 

[Slugbabydotcom]

Fitting a de-aerator is one way of sorting out the problem as it brings the feed and vent pipes to one location on the same side of the pump.