Hi All I am musing about finally replacing our anciente back boiler (Baxi Bermuda, part of a Plan C system dating back ... quite a bit)
One possibility I am considering is having a system boiler fitted rather than a combi. On question - would it be necessary/advisable to replace the cylinder at the same time, or could this be kept?
Thanks & Regards J^n
I would say that it wouldn't be strictly necessary, but it would be highly advisable.
The cylinder will almost certainly be the indirect type with a separate coil inside through which the primary water from the boiler flows. Whilst would work with a system boiler, it may not work very well. If is was installed in Baxi Bermuda days it is pretty old and will likely be scaled up unless you are in a soft water area. Also, it's unlikely to have a fast recovery coil inside so the domestic hot water won't be heated anything like as fast as it would be with a new cylinder. Finally, all recent cylinders are covered with foam insulation which is likely to be better than fitting a loose jacket round it.
I'm assuming that you plan to alter the pipework and controls in order to make it a fully pumped system? Modern low volume boilers just won't work with gravity circulation HW systems like the old cast iron lumps did. Unless the pump maintains the flow until the boiler has cooled a bit after each firing cycle, it will overheat and trip out due to the residual heat.
If your cylinder is as old as the rest of the system it will be wasting a lot of energy in the summer (when heat leaking out isn?t useful). I personally prefer boiler/tank combinations. Combis are too ?eggs all in one basket? for me. A tank with an immersion heater is very useful back-up in case of boiler breakdown.
Tim
A system boiler still needs some form of hot water storage, so your existing cylinder could be used, assuming it is an indirect type. But if old, may well not perform as well as a decent new one.
The OP could consider S-plan. And if an issue to minimise alteration of pipework, the OP could have separate pumps for CH and DWH with non-return valves.
+1
Combis are good where space is a premium.
If the system controls are updated to modern standards (room stat, cylinder stat, fully pumped operation) then the existing cylinder would still work, but not as well as a modern one.
Your existing cylinder is probably a slow recovery indirect type, ideally suited to drawing a little heat when run in parallel with the rads. A modern condensing boiler would be better matched with a fast recovery cylinder run in S plan[1] configuration (so full output of the boiler directed at the cylinder or the rads, but not usually both at once).
If you have a decent cold mains supply, then you could also consider an unvented cylinder[2] that will give you mains pressure hot water all around the house, and no need for a cold cistern in the loft.
[1]
That would make it pretty difficult for pump over-run conditions, where the boiler needs to control the pump. Not impossible, but lots of relay logic!
Some heat only boilers don't have a pump over-run facility. The Baxi EcoBlue is an example.
I discovered that my fast recovery one will give a reasonable shower starting with a cold cylinder. Not quite as hot as I'd normally have, but OK if in a hurry. A sort of instant heater.
Fair enough if it can cope with the flow stopping as soon as the flame stops. But, in any case, the OP was talking about using a system boiler, which included a pump inside the casing.
S-Plan+ with a single pump is easy enough - many system boilers have an internal bypass valve, and so can cope with occluded flow. The control systems are also usually clever enough to overrun the pump and keep the required valves open for long enough after the flame is extinguished.
Hi All thanks a lot for all the useful comments. I wasn't really expecting to get the nod towards keeping the current cylinder, but it's useful to get more of an idea of the various considerations. I do also appreciate that there will need to be some replumbing...
Also thanks John Rumm as always for his wisdom, and for flagging the S Plan possibility.
It is partly a matter of location that it leading me to the System Boiler route - also that (like Tim+) I don't like putting all my eggs in one basket, as it were. Having eyeballed the current setup again, one concern I have is what to do with the condensate drain. Running a flue to the outside would be OK, but it is a long way to a drain or soil pipe etc. Are there any other options here? We have a 'French drain', for instance, and I wondered if that could be brought in operation...
FWIW one reason for doing this *now* is that the back boiler has starting making 'kettling' noises when it starts up. We've fairly recently set the controller to not put the radiators on, but I wouldn't have thought that that would have caused any such issues? It is looking to me that the heat exchanger is really sooted up - I actually fitted a new thermocouple, and cleaned the jet out, a month or so ago, and it seemed fine then. I'd quite like to fix/understand that even if we do finally sort out a new boiler...
J^n
A back boiler type - that gets its air from the room - would need regular cleaning IMHO. About once a year.
Well, it could do. Without the radiators acting as a ?heat sink?, the heat exchanger is probably running hotter, especially if your HW system relies on a simple thermo-siphon to circulate the water to your HW tank.
Sooting isn?t good. Suggests inadequate air supply but a certain amount in an old open vented boiler is probably normal. A good clean out and sweeping the chimney wouldn?t go amiss!
Tim
Actually, that is a fair point - thanks.
Yeah - we had similar kettling issues a couple of years ago and my service guy had to do a lot of soot cleaning, although no-one ever understood quite why it was so bad. Covid restrictions have meant that we didn't get them in last autumn. As I mentioned, I had separate reason to take a look inside a month or so ago. The thermocouple had burnt away and the pilot light was quite yellow. I replaced the thermocouple and cleaned the jet of the pilot light (about as far as I would go on my own). At that point, the flames were pretty clean and I didn't notice any great excess of soot. So it ... feels ... like this sooting up has occurred quite quickly.
Maybe I will (as a temporary measure) try leaving some rads on as an experiment.
Useful Tim+, thanks. J^n
or even W plan (i.e. 3 port diversion valve rather than a mid position one) has become more useful again with fast recovery cylinders that can take the full output of the boiler.
Yup cylinder (vented or unvented) with an immersion will give you options.
(although to be fair I have one in mine, and the only time ihas been used was in the first few days after installation of the cylinder before I had finished the boiler install and require replumb to convert from vented to unvented etc)
Yup there are a few - the main requirements for a condensate drain are that it won't freeze and it won't mind the very slightly acidic condensate. So taking and mixing it into an outflow from a sink or washing machine that then goes through the wall in a larger pipe (say
40mm) is fine, or a dedicated pipe. The thing to avoid is taking the 21mm plastic pipe straight outside where it could freeze, or letting it drip directly onto masonry where it could stain etc.Yup a soakaway is fine - you are not talking about huge amounts of water
- a few L/hour at most usually.
It might be just a lower load on it means its running hotter. Also scale in the HEX can result in kettling. You could add some "Boiler Noise Reducer", which normally calms it down a bit.
It probably also wants a good sweep out and clean - especially important if it's not a balanced flue design.
Fair enough. But I was replying to another poster who had suggested keeping the existing 2 circuit pipework with a pump in each circuit. Using a system boiler in such a setup would be a challenge!
What colour is the boiler flame? If the HE is sooting up it suggests that there's not enough oxygen, so the flame will be yellow instead of blue. Make sure that the air supply to the burner isn't blocked and - if possible - check the gas pressure at the burner when it's firing.
If the boiler is kettling, that's likely to be due to scale on the
*inside" of the HE rather than soot on the outside. As JR says, there are additives which will help with that.
screwfix is a good source - "noise reducer"
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