Combi Boiler and one-pipe system

Discussion in 'Plumbers' Talk' started by Willy Duwitt, Oct 4, 2005.

  1. Willy Duwitt

    Willy Duwitt New Member

    Hi chaps

    I've just had an installer round to price up replacing our old boiler with a new W/Bosch Combi. Everything went as expected, until he mentioned that as we had a 1-pipe system, we would have to get the pipework changed to a 2-pipe system in order for the combi to work properly yet he guy we had round a couple of nights ago had not made mention of this.

    Does anyone know whether he's right, that a combi system needs a 2-pipe system ?

    Any advice much appreciated.

    Cheers
     
  2. Dreadnaught Heating

    Dreadnaught Heating New Member

    At a risk of sounding 'know it all' YES you do need a two pipe system, apart from the fact that its likely that ANY old system with components such as a U/B valve and centre tapping rads will be under MUCH MORE internal pressure than they were originally designed for and even if they hold up under the strain for a few hours, your poor old installer will be back time after time to try to patch it up.
    Also, it is my experience that one pipe systems clog-up with deposits much more than a two pipe system and even though a powerflush is always recomended to clear it out before fitting a modern combi boiler with miniscule sized waterways, the black sludge we all know about will block those up in time. So, start with a new system, cold/hot flush and treat with an inhibitor - like it says in the instruction book, because manufacturers will not replace your heatexchanger due to blockage by magnetite without a water sample being tested.
    Phew.
    There could also be issues with automatic by passes etc.

    You might also care to look at a sample Worcester installation manual to see if they don't recommend a two pipe system too.
     
  3. Its VERY easy for an installer to miss noticing a one pipe system.

    We see these so infrequently that it almost never! In my case once in the last three years!

    Theoretically a one pipe system will work very well on a condensing boiler but the last few rads will be very cool. Not necessarily a problem in bedrooms IF its the same owner who has got used to that!

    Some one pipes have no rad valves. Repiping can add about £500 to £1000 to the cost so its very significant.

    All this assumes the one pipe is 15 mm. If its 22 mm or 3/4 then it can work very well. A minor downside is a thermostatic rad valve mey not work properly.

    Tony Glazier

    Agile Services
     
  4. Willy Duwitt

    Willy Duwitt New Member

    Thanks very much guys. That's given me something to think about. Sounds like I might as well bite the bullit and get a 2-pipe system installed.

    Cheers

    Carl
     
  5. devil's advocate

    devil's advocate New Member

    Hi BAAD.

    Yes, a combi will work fine on a one pipe system, but there's a couple of things you need to be aware of.

    1) As mentioned above, the last rads on the loop could run cooler than the others unless you keep the supply temp up. Unfortunately, if you are fitting a condensing boiler (I'm guessing you are?), you really want to keep the supply (and therefore return) temps as low as possible for maximum efficiency. So, it's likely you won't get the maximum efficiency benefit of a condensing boiler on a one-pipe system.

    2) Modern (low volume exchanger) boilers need a minimum constant flow rate through them. It's likely that a one-pipe system will provide this since it's effectively a loop of (probably) 15mm pipe, but it's something to be aware of. If the boiler doesn't have an auto by-pass built into it, one may have to be fitted in the system.

    A 2-pipe system is definitely more effective at feeding all the rads equally, but only you can decide if it's worth the extra money!

    (I kept my one-pipe system when I fitted a GlowWorm condenser last year because, with a new baby and a 1 year old in the house, I couldn't face ripping up all the floors! A Corgi I was thinking of using at the time checked with Worcester Bosch who said it would be 'ok but not as efficient'. A recent ground floor extension (and my new loft conversion when finished) are on 2-pipe systems, with separate zone valves. These rads heat up far quicker than the ones on the old system!)
     
  6. Willy Duwitt

    Willy Duwitt New Member

    Cheers D.A.,

    The more I read about this topic the more convinced I am that the 2-pipe system is the way to go. I dont want to spend £2k+ getting a new boiler fitted, only to then find that the system is not working efficiently and having to get the guys back to finish the job. Might as well get it all done in one go.

    Thanks again everyone for the sound advice.

    Carl
     
  7. kaboom

    kaboom New Member

    DA I'm a bit baffled as to how you fit a bypass into a one pipe system- surely its one big bypass? BAAD there's no reason why you cant fit the boiler and see how it goes, agreed it shouldn't be as efficient though. Pipework can always be done later. You can also get trvs especially for one pipe systems (bigger bore internal I believe). I've seen loads of one pipers(22mm loop), mainly because the bloke who taught me fitted them that way, and they always worked fine. Came as a shock when I first saw a design guide I can tell you- I was previously blinded by the moonlight!
     
  8. devil's advocate

    devil's advocate New Member

    Hi kaboom.

    I mentioned in my post that it's likely that the single pipe loop would allow adequate flow. However a long length of 15mm pipe does have a fair resistance and might not give the boiler the flow rate it needs; hence the possible need for a bypass (if the boiler doesn't have one built-in).

    My single pipe system takes a fair while to get up to temp - the flow in a single 15mm pipe has to supply the heat to all the rads!

    The boiler behaves differently when it's supplying the 2-piped zone. Here, it fires up to near max and gets the rads piping hot in no time, and then modulates down to the level to keep them going. When supplying the one-pipe system, it never really fires up to max - it just builds up the temp slowly, and then keeps it there!

    Thank the lawd it's a modulating boiler, else it would be cycling on 'n' off all the time.
     
  9. kaboom

    kaboom New Member

    Must say I've never seen a one pipe in 15mm!
     
  10. Willy Duwitt

    Willy Duwitt New Member

    No, D.A.'s right. Ours is 15mm throughout.
     
  11. devil's advocate

    devil's advocate New Member

    Hi BAAD.

    If you can stretch to it, I'd certainly recommend you replace with a 2-pipe system. Don't just allow the plumber to add a second pipe either, make sure he prices for completely new pipes throughout. (The existing pipe will be fairly old and is bound to have some sludge in it, and the old joints might struggle to cope with the pressure of a sealed sytem).

    Good luck with it. I'm sure you won't regret it. (I wish now I'd bitten the bullet and created havoc in the house for a week and replaced it all!)
     
  12. devil's advocate

    devil's advocate New Member

    Oh, and if any rads need replacing, it's worth going for ones with a slightly higher output; eg: one having convector fins if none on original, etc.

    The reason for this is that you'll get the same heat out of them as the original, smaller, ones when running your condensing boiler at a lower (and more efficient) temp.

    Just something else to think about!
     
  13. DA, a one pipe system SHOULD be VERY efficient with a condensing boiler.

    If the boiler is set to give a fairly high output temperature, say 70*C then the nature of the single loop is that the return will be at a very low temperature and enable the boiler to condense.

    Whilst I appreciate that adding a parallel circuit will upset the operating conditions, it should still be very efficient but only if you have reduced the flow by closing the lockshield valves until the return is at say 55*C when the flow is at 70*C.

    Are you able to measure or even estimate the actual temperatures on your system?

    The only downside of the single loop is that the last rads will be at a much lower than ideal temperature. If it had been properly designed it would have had larger rads to compensate. Unfortunately most single pipe systems were installed on a budget and many as a coal back boiler system.

    Tony
     
  14. devil's advocate

    devil's advocate New Member

    Hi Tony.

    You are probably right in that it could be efficient if the return temp was in the order of 55oC. Unfortunately, you have also recognised the main drawback of the system - the last rad on mine is in the living area, so I can't allow it to be too cool! (The first rad in the system is also in the living area, at the far end; so no benefit in reversing the system).

    I did replace the last rad with the very best one I could find (a much larger 'Trendline'), but it's still only just adequate.

    As soon as it becomes cool outside, I have to crank up the boiler to deliver in the mid-70's.

    Once the extension (on a 2-pipe zone) calls for heat, it's piping in seconds!

    The boiler gives a flow temp readout, but I haven't measured the return - I just look at the flue to see how much plume there is! (A lot less when set high).

    My main gripe with the 1-pipe layout, is that the boiler doesn't seem to be able to fire up to max even from cold. I suspect it's because the single 15mm pipe run just can't take the heat away fast enough, or perhaps the flow rate through it is at the margins of what the boiler likes. It does get there in the end, and what I've done is to set my prog stat (strongly recommend one, BAAD!) so's it maintains at least 18oC even through the night. It then gets up to proper temp relatively quickly.

    In short, the one-pipe system does work, but not as effectively as I'd like.
     
  15. kaboom

    kaboom New Member

    Tony,
    I can see the principle that it could be efficient on 15mm but unless a 22mm loop had say ten rads (based on est 2* loss per rad)I reckon it has little chance with the majority of the water getting back to the boiler too easily and too hot. And I suppose the more you throttle back the rads to balance the hotter the return loop gets. Its all facinating- would be great to sneak in to one of your dinner meets!
     
  16. Bahco

    Bahco Guest

    Don't forget part L, lads.
    Regs are that a 1 pipe must be upgraded to a 2 for new works.
     
  17. Scooby doo

    Scooby doo New Member

    I'm not a heating engineer but we're in the process of updating our one pipe heating system. This is what I have learnt. Don't simply plug a modern combi to a one pipe a) they are not compatible and b) if its an old cast pipe system chances are muck will wreck your lovely new boiler even if the pipes are cleaned out. We had three engineers round - one didnt have a clue and didnt spot it was a one pipe. The other two spotted it in about 30 seconds and gave proper advice, so dont be rushed.

    We had all new copper pipe work and a Vaillant system boiler fitted as we are planning to convert the loft in the future and the house reasonably large. We bought the boiler through the green boiler scheme at PTS - good price and government cash back £30 - every penny helps as none of this was cheap. If you are going this route may sure your engineer gives notice to building control as building regs apply for unvented systems.
     
  18. blahblah

    blahblah Guest

    Scooby, feed and return may be preferable but a one pipe is not 'incompatible' if done properly.
    Problems with iron pipes and dirt apply to all types of system.
    Building control does not have to be notified of a sealed system boiler (unless you're diying). Unvented hw storage over 15 litres, yes.
     

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