Boss trouble

[bobdee]

After ages cogitating on the merits of alternate systems, and ploughing through countless threads here, I'm now committed - rightly or wrongly - to the idea of a 300l vented cylinder configured as a heat bank. Heat will be loaded by a WBS w/backboiler; existing gas boiler backup (for flick-a-switch convenience); and solar by next spring. WBS is in place but not commissioned.

Even after all the reading I *still* have a couple of nagging questions about bosses. Given their major contribution to heat loss my inclination is to reduce the number where possible.

1. The gasser and the WBS won't ever be firing at the same time, so is there anything against joining both their flow and return near to where they connect to the HB? (My plan currently has them separate, but at the same level on the cylinder). Should I fit non-return valves on either or both? (Or is this a Bad Idea, and I should keep them separate?)

2. On the same theme... the vent will rise from the top centre tapping of course; but since DHW should also exit at the highest point, anything wrong with Teeing the DHW from the vent pipe just at it leaves the HB?

3. I'll have a separate F&E tank in the loft. Am I right in thinking I can run a 15mm from there and T it into any return to the HB, such as a boiler return?

I so hope these idiot questions have simple answers... Advice much appreciated.

[desperate]

Hi Bob, not sure I have a complete idea of what you plan, can you post a pipework ciurcuit diagramme?

1  Are you planning to connect the gasser and WBS to the cylinder direct? that would seem at odds with the domestic hot water.

2 This is what makes me wonder about hot water and direct connections

3 Probably not, it is very important to get the feed and expansion pipes in the correct relation to each other, otherwise you risk "pumping over" through the vent pipe or drawing air down the feed pipe into the system..

We use a gasser as back up to a WBS with a heatstore, but we also have a hot water cylinder, check out CRAC in show us yours for details.

Desp

[bobdee]

not sure I have a complete idea of what you plan, can you post a pipework ciurcuit diagramme?

Plan below. Sorry it's taken so long - this project got shelved for a year and I've mislaid some of the bits.

The gasser and WBS  are direct to the cylinder. My original had separate 28mm bosses for boiler and WBS; I'm just wondering about rationalising all that copper if poss. DHW is via a PHE, of which the heatbank side comes directly out of the cylinder - hence the notion to tap it from the vent.

Take your point about pumping over/drawing down the f&e, but the idea was prompted by another thread here which said it could be done. Again, any saving on copper/bosses is worth doing providing it works of course.

[wookey]

Intersting questions. In general it's best to send everything tothe neutral point of the tank, but as you say, more bosses is bad, so some amalgamation is tempting. It's pretty normal to combine vent pipe and DHW takeoff on vented cyclinders.

The thing to think about is all the ways it could possibly go wrong and water go the way you aren't expecting due to pressure or thermosiphoning. The 'how not to make combisystems' doc has some examples of how to mess it up: http://www.elle-kilde.dk/altener-combi/dwload/List_of_problems_030519a.pdf

Careful treatment of connections to avoid thermosiphons out of the tank is one thing that often gets forgotten.