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Discuss Help and advice with bypass on ufh with combi boiler & variable pump in the Water Underfloor Heating Installations area at PlumbersForums.net

Phantom 1

Messages
2
Hi All,
hoping for some advice and pointing in the right direction.

I have a Worcester Bosch 40he condensing boiler, supplying hot water as priority, central heating radiators using TR2 thermostat in one room.
no auto valves on radiators or storage tank.

I have built a small extension and installed Wunda UFH system single zone 2 loops, which will be with a heatmiser neo ultra thermostat for ufh only.
my Plan is to
1). install a 2 port zone valve approx 1m from boiler, for the existing CH
2) install a 2 port zone valve approx 3meters from boiler for the UFH,
3) Install a bypass and Honeywell ABV in the bypass Approx 3 meters near the UFH zone valve.

just come across a line in the WB instruction book stating,
B This appliance uses an automatically variable pump. An auto-bypass must not be fitted.

My questions,
A, Is this statement for normal central heating directly to radiators or still applies considering no1) above with installing a zone valve closing that system down
B, Do I still install a bypass with a ABV considering zone vales and potential pressure build up, although if not required I will probably pipe the bypass in but put a ball or gate valve in and close it In case I need a bypass in future when the boiler needs replacing

l’m not sure how the variable pump in the boiler would handle this with zone valves fitted and without a bypass
.
C, Is 3m distance from boiler ok for a bypass if it is used.

thanks in advance

Paul
 

Ric2013

Plumber
Messages
3,483
I don't don't really understand your desciption of your system, but never mind.

I can see what Worcester Bosch is getting at. Nearly all normal central heating will have motorised valves and TRVS too, so as you are obviously aware, it is conventional to fit a differential pressure ABV placed on the boiler side of all zone or radiator valves to ensure a minimum flow through pump and boiler at all times. However, the old fashioned pumps had a fixed speed so pressure was greatest when zone valves were closed and this would open an ABV when there was not another route the water flow could take. Your boiler contains a variable speed pump that is probably programmed to keep the pressure slightly higher when the valves are open so an ABV wouldn't open when you needed it to and might open when not needed. So automatic variable-speed pump + differential pressure ABV are not a suitable mix.

Does WB not show what kind of bypass, if any, it wants installed?
 

Ric2013

Plumber
Messages
3,483
Actually I've had a quick look at the IOM myself. It's a combi boiler (which you hadn't told us). I suspect (wait for others to confirm) the boiler uses the secondary heat exchanger as a controlled bypass and, as such, needs no external bypass at all.
 

Phantom 1

Messages
2
Hi Ric2013,
thanks very much for the reply,
yes about the heat exchanger, pgs 7&8 item 355 shows a plate exchanger.
if a bypass isn't required that is good news, although i will still pipe one in for future, with a valve closed.
the WB 40HE is a condensing boiler as i mentioned second line of post.
cheers
Paul
 

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Ric2013

Plumber
Messages
3,483
Hi Ric2013,
thanks very much for the reply,
yes about the heat exchanger, pgs 7&8 item 355 shows a plate exchanger.
if a bypass isn't required that is good news, although i will still pipe one in for future, with a valve closed.
Glad to have helped!

the WB 40HE is a condensing boiler as i mentioned second line of post.
cheers
Paul
Yes, a condensing boiler, but also a combi boiler. Nearly all modern gas boilers are condensing (i.e. very efficient), but you can have condensing system boilers, condensing heat-only boilers, and condensing combi boilers. (Combi boilers are the only ones that give hot water on demand rather than needing a separate cylinder). The terms 'condensing' and 'combi' are often confused but have completely different meanings.
 

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