replacing / repairing back boiler

I currently have a baxi bermuda 551 back boiler central heating / hot water system. It has a gas fire on the front which is no longer used.
When I turned on the central heating - it's been off since last winter - the there was no characteristic gush of water surging round the system and the radiators don't work, apart from one in the back bedroom.
I bled the radiators, checked the valves on the thermostatic radiator controls weren't stuck, but still no joy.
After looking at the boiler and reading some previous posts, I am coming round to the idea that the pump is not working. The boiler still heats water for the hot water system but it isn't coming on and staying on for any period of time as it usually did when the central heating was on. I can't quite work out why the back bedroom radiator is working ok, unless water is reaching it via the hotwater system.
* How easy / expensive is it likely to be to replace the pump on this 'antique' boiler (getting someone qualified to do it that is)
* If I replace the back boiler, could I use a combi boiler but basically keep the same hot water/ central heating setup. The back boiler currenly heats water in a hot water tank in the airing cupboard.
* Are there any reasons other than aesthetic why I can't put a combi boiler on the fire place using the chimney as a flue. i.e need to be certain height above ground, thereby minimising changes in the plumbing which is all currently situated there.
* If I went for a condensing boiler, other than finding a way to run off the condensate, could this be situated on the fire place as above.
many thanks angus
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On Tue, 08 Nov 2005 17:07:47 GMT, angusclaydon

You probably don't have a fully pumped hot water system. The water is heated by gravity (relying on the fact that cold water is heavier than hot water, so hot water rises around the circuit. This happens in the radiator circuit as well - which explains why the upper radiators are hot while the lower ones aren't.

The pump isn't in the boiler - it's a separate unit something like this: http://www.screwfix.com/app/sfd/cat/pro.jsp?idr726&tsw549 That's easy to replace. 100 quid max for a plumber to do it.

There's no point. You can use a system boiler in place of the back boiler and still keep the tank but the whole idea of a combi boiler is that it heats water instantly as you need it. You'll get much better showers from a combi, but if you choose a "cheap" one you might not get the flow rate you want when filling up the bath.

The flue needs to go outside - you can't flue into the chimney. Modern combi boilers are room sealed appliances. They take the fresh air they need for combustion from the flue.
I think it wouldn't be easy to get the flue all the way up the chimney and out the top. There are maximum limits on flue lengths anyway specified in the boiler's installation manual.

Chances are that you will be forced to have a condensing boiler. The condensate output must fall at around 2 degrees or more to an internal trap or external drain or soakaway. If you can't get the fall for whatever reason you'll need a separate condensate pump which collects condensate in a tiny tank and pumps it out when the tanks is full. A big boiler might produce 2 litres/hour condensate.
Apart from the condensate I don't think there is any reason why you shouldn't put the boiler on the fireplace.
Mr F.
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In an earlier contribution to this discussion,

As someone else has said, the pump is a stand-alone unit - not part of the boiler. If the pump hasn't been used all summer, it may simply have seized rather than failed completely. If you can find it, unscrew the endcap (very little water will escape) and try to turn the shaft with a screwdriver. You may be able to free it. If not, it is not too difficult to replace - particularly if it has functional isolating valves either side, enabling it to be removed without having to drain the system.
The pump is probably located fairly near to the boiler - it could even be inside the chinmey breast, or it could be under the floorboards if you have a wooden floor downstairs.
If an upstairs radiator gets hot without the pump running, it means either that the radiator is connected to the gravity hot water rather than heating circuit, or that there's *some* gravity circulation within the pumped circuit, as sometimes happens.
If you *do* decide to replace the boiler, you will need a lot of plumbing and control system changes to bring it up to modern standards - and meet current building regs - and you need to find a more suitable location for the replacement boiler.
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angusclaydon wrote:

Thanks for your help
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In an earlier contribution to this discussion,

<snip>
Have you fixed it - or decided what to do?
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Well, I have found it, and got the end cap, haven't managed to get a screwdriver in as its awkwardly positioned next to floor joist :(
Looks like I will be able to get it off as its got taps either side of it! Also been trying to get my head round the timer wiring as I replaced that last year too with electronic one, as it doesn't appear to be independantly fused ...
rgds
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In an earlier contribution to this discussion,

Sounds like you're making *some* progress. Is it possible that - due to your wiring changes - the pump isn't being told to run? Can you get at its connections to make sure that they're live when they should be, or can you temporarily wire it into a 13A plug to see whether it runs when you *know* it has power? [Even if you can't get a screwdriver in, you may be able to get a finger in to check whether the spindle is going round.]
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Set Square wrote:

It doesn't make a sound when wired directly... its a dodo
Just sourced a wylo (30 something) locally as its cheaper than the original grundfos (57 yikes) ...
Do I have to bleed air out once the new one is in? Getting to the back of the boiler is nearly impossible ? or maybe I can just bleed it out of the radiators
rgds
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In an earlier contribution to this discussion,

If the valves work ok, you'll only introduce a pumpful of air - which isn't very much. It should bleed itself with the end-cap removed. Any remaining air should either go up the vent pipe or collect in the radiators. Try to mount the new pump in a way which enables you to get at the end of the shaft - or drill a hole through the joist for screwdriver access.
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You do not have to have a condensing boiler if replacing a back boiler. My mate has had his 1970's Baxi Bermuda with 70's style gas fire replaced with a new Baxi back boiler and coal effect gas fire. Fitted by a Corgi installer who totted up the "points" (major problems, no easy condensate drain and no space for external flue not near windows or external doors) so a conventional back boiler was fitted. Also converted to fully pumped in the process. About 2.5k I seem to remember.
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