Post and beam Posted March 19 Share Posted March 19 Youtube link: = https://youtu.be/GuhOogbQo5o I am having an argument with Worcester Bosch about this boiler that has misbehaved almost since day one. Occassionally it refuses to fire up when asked to give hot water. I believe the nonsense their technical people are giving me is them trying to wriggle out of their warranty obligations. So i would very much appreciate the opinions of people that know what they are talking about. If i am wrong please feel free to shoot me down. Link to comment Share on other sites More sharing options...
JohnMo Posted March 19 Share Posted March 19 6 minutes ago, Post and beam said: Occassionally it refuses to fire up when asked to give hot water. A combi boiler uses a flow switch on the DHW to startup the boiler. It either isn't seeing enough flow or is a defective flow switch. Do you have a combi save devise installed, if you don't know you probably haven't. Is it the same for all taps or outlets, i.e. it never starts up? I would go around each outlet and log what happens to the boiler. Also your flow temp for the central heating seems very high. 1 Link to comment Share on other sites More sharing options...
Post and beam Posted March 19 Author Share Posted March 19 Hi John Mo Central heating temp is this high becuase i have not turned it down from the winter. This is still in our current house of 1986 Vintage, not a new build. Yes when it plays up it is all outlets its just that the shower is the most obviously annoying place for it to leave us with freezing cold water. When my furious playing around with everything on the boiler display eventually gets it to fire up it will be good for perhaps a couple of days or weeks. The flow switch was replaced a year ago on a previous engineer warrany visit. Pressure is very good so i doubt its flow related but who knows. The most recent engineer tried to claim that it the boiler was behaving perfectly normally, ( on watching the video) because ' you called for heat and it fired up'. Seriously? Dont know anything about a 'combi save device' Thanks keith Link to comment Share on other sites More sharing options...
John Carroll Posted March 20 Share Posted March 20 You may have a xover problem with a mixer. Can you turn off the cold water supply to your shower and mixing taps, reduce the DHW temp setpoint to 40C and see if combi fires up when you switch on the shower. 2 Link to comment Share on other sites More sharing options...
Post and beam Posted March 20 Author Share Posted March 20 I do have a reported crossover bleed issue. Have you watched the video? I cannot see how a bleed over can reduce the boilers sensed temperature down to the same as what comes in directly off the street. And, why that low temperature does not induce the boiler to fire up. This makes no sense to me. I can imagine the possibility of an affect on the shower supply temperature via a bleed over to a lower than desired delivery temperature. But the onserved scenario i cannot imagine. Link to comment Share on other sites More sharing options...
John Carroll Posted March 21 Share Posted March 21 If the mains flowrate through the boiler isn't greater than ~ 2.5LPM then the flow switch won't activate to fire the boiler, this can happen if the crossover in any mixer is preventing this. The mains or boiler low temperature has nothing to do with firing up the boiler in DHW mode, its only the mains flowrate that calls for burner operation. Link to comment Share on other sites More sharing options...
Post and beam Posted March 25 Author Share Posted March 25 Knowledgeable Worcester engineer visited this morning. Nothing to do with cross bleed via the mixers ( didn't believe it was) and apparently nothing to do with flow switch ( i thought it likely was). Basically it comes down to the PID control of the heater when water temperature is falling just below the set point from an already high winter set point temperature. It does not appear to be able to modulate enough to prevent overshoot of the target. A greater engaged water volume in the radiators would allow the excess temperature to have somewhere to go. Upshot is a high temp 'blocking code' that needs about 3-4 minutes before it will allow any more heat to be produced. Bloody software engineers. 1 Link to comment Share on other sites More sharing options...
ProDave Posted March 25 Share Posted March 25 9 minutes ago, Post and beam said: Knowledgeable Worcester engineer visited this morning. Nothing to do with cross bleed via the mixers ( didn't believe it was) and apparently nothing to do with flow switch ( i thought it likely was). Basically it comes down to the PID control of the heater when water temperature is falling just below the set point from an already high winter set point temperature. It does not appear to be able to modulate enough to prevent overshoot of the target. A greater engaged water volume in the radiators would allow the excess temperature to have somewhere to go. Upshot is a high temp 'blocking code' that needs about 3-4 minutes before it will allow any more heat to be produced. Bloody software engineers. That sounds like a lot of waffle that translates into: "There is a bug in the software that under certain heating situations, the controller is unable to switch over to hot water heating and just heat the hot water. And we don't know how to fix it" Anyone disagree? 1 Link to comment Share on other sites More sharing options...
JohnMo Posted March 25 Share Posted March 25 11 minutes ago, Post and beam said: Basically it comes down to the PID control of the heater when water temperature is falling just below the set point from an already high winter set point temperature. It does not appear to be able to modulate enough to prevent overshoot of the target. So your heating system is locking the boiler due to short cycling, in plain English A symptom of too many zones then. Reducing the zones would therefore decrease your energy consumption also. Link to comment Share on other sites More sharing options...
ProDave Posted March 25 Share Posted March 25 13 minutes ago, JohnMo said: So your heating system is locking the boiler due to short cycling, in plain English A symptom of too many zones then. Reducing the zones would therefore decrease your energy consumption also. But surely the basic issue is when you open a hot tap the boiler should forget heating mode, and switch, without issue, to hot water mode and heat the hot water to the required temperature. I would be saying you have had plenty of chance to fix this, and you have failed, so i want the boiler removed and a full refund given and I will replace with a different boiler. Link to comment Share on other sites More sharing options...
Post and beam Posted March 25 Author Share Posted March 25 15 minutes ago, ProDave said: There is a bug in the software I agree which is why i made the comment about software engineers. Little point in me demanding a replacement boiler under the warranty. But i will use this to try to get some kind of software update. However, i think we are on their latest and if they dont have a better algorithm then i am screwed until/unless they do. Link to comment Share on other sites More sharing options...
Post and beam Posted March 25 Author Share Posted March 25 18 minutes ago, JohnMo said: A symptom of too many zones then This is a traditional 1986 house with a gas boiler and radiators. Link to comment Share on other sites More sharing options...
JohnMo Posted March 25 Share Posted March 25 Just now, Post and beam said: This is a traditional 1986 house with a gas boiler and radiators. Do you have trv's on every radiator? Link to comment Share on other sites More sharing options...
Post and beam Posted March 25 Author Share Posted March 25 7 minutes ago, JohnMo said: Do you have trv's on every radiator? All but 2 yes Link to comment Share on other sites More sharing options...
SimonD Posted March 25 Share Posted March 25 1 hour ago, ProDave said: That sounds like a lot of waffle that translates into: "There is a bug in the software that under certain heating situations, the controller is unable to switch over to hot water heating and just heat the hot water. And we don't know how to fix it" I think you've got that spot on. 1 Link to comment Share on other sites More sharing options...
John Carroll Posted March 26 Share Posted March 26 8 hours ago, Post and beam said: Knowledgeable Worcester engineer visited this morning. Nothing to do with cross bleed via the mixers ( didn't believe it was) and apparently nothing to do with flow switch ( i thought it likely was). Basically it comes down to the PID control of the heater when water temperature is falling just below the set point from an already high winter set point temperature. It does not appear to be able to modulate enough to prevent overshoot of the target. A greater engaged water volume in the radiators would allow the excess temperature to have somewhere to go. Upshot is a high temp 'blocking code' that needs about 3-4 minutes before it will allow any more heat to be produced. Bloody software engineers. As a matter of interest, was this problem occuring during the summer when there was no demand for CH or is it only happening (occasionally) when switching from CH to HW? Link to comment Share on other sites More sharing options...
Temp Posted March 26 Share Posted March 26 (edited) Have I understood the problem... It occurs when the boiler has been delivering 76C water to the rads so it's nice and hot. Then you call for DHW and the thermal mass of the boiler momentarily heats the DHW to 76C (or perhaps a bit less) aided by the boiler flame not modulating down enough. The boiler detects the DHW is too hot and shuts down for safety reasons. Is that what the engineer was saying? I would start a diary with date and time and description eg Date, Time, No HW for shower. Also any call outs and bills. Backdate it if you still have earlier call out bills for same issue. If the spec says the boiler can deliver CH and DHW at certain temperatures and it can't then that sounds like grounds for asking for your money back. At some point you will have to write to the supplier/installer giving them reasonable opportunity to rectify the problem after which you expect a refund or initiate court action. That's what the diary is for. Edited March 26 by Temp Link to comment Share on other sites More sharing options...
JohnMo Posted March 26 Share Posted March 26 5 hours ago, Temp said: Have I understood the problem... It occurs when the boiler has been delivering 76C water to the rads so it's nice and hot. Then you call for DHW and the thermal mass of the boiler momentarily heats the DHW to 76C (or perhaps a bit less) aided by the boiler flame not modulating down enough. The boiler detects the DHW is too hot and shuts down for safety reasons. Is that what the engineer was saying? I would start a diary with date and time and description eg Date, Time, No HW for shower. Also any call outs and bills. Backdate it if you still have earlier call out bills for same issue. If the spec says the boiler can deliver CH and DHW at certain temperatures and it can't then that sounds like grounds for asking for your money back. At some point you will have to write to the supplier/installer giving them reasonable opportunity to rectify the problem after which you expect a refund or initiate court action. That's what the diary is for. From what he says below, I don't think there is an issue with DHW only. The DHW issue is related to the boiler being in a safety lockout, due to an uncontrolled over temperature experienced during central heating duty - short cycling. The simple fix is make a larger engaged radiator circuit volume. Remove more than two trv's for example. Basically in central heating heat has a huge boiler trying to heat a cup full of water, it can't do it in a controlled way. Don't believe there is a software issue, it's a system setup issue. 15 hours ago, Post and beam said: does not appear to be able to modulate enough to prevent overshoot of the target. A greater engaged water volume in the radiators Link to comment Share on other sites More sharing options...
John Carroll Posted March 26 Share Posted March 26 (edited) 38 minutes ago, JohnMo said: From what he says below, I don't think there is an issue with DHW only. The DHW issue is related to the boiler being in a safety lockout, due to an uncontrolled over temperature experienced during central heating duty - short cycling. The simple fix is make a larger engaged radiator circuit volume. Remove more than two trv's for example. Basically in central heating heat has a huge boiler trying to heat a cup full of water, it can't do it in a controlled way. Don't believe there is a software issue, it's a system setup issue. There must be thousands of combi boilers which have to deal with a similar cycling problem on CH, a combi boiler is sized for its HW duty, you may have a 37kw boiler sized to give a DHW flow of 15LPM at a dT of 35C but the CH load might fall to 5kw so the boiler just has to cycle, a 37kw boiler may have a minimum output of 6kw so not a big deal, older ones like the Vaillant ecotec plus 837 can only modulate to 12kw but still have to deal with very low CH demands. The main challenge, especially with Vaillants, which (used to at any rate) maintain the ignition conditions for 60 seconds after recycle/refire, is that they fire at ~ 65% of their max output and if they don't modulate down fast enough then the burner will trip at the target temp +5C leading to anticycle and refiring, nearly all boilers can be range rated right down to their minimum output which doesnt affect the DHW output but they will still refire at this very high output, the range rating does help but not directly. Overshooting the target temperature is a quite normal event and all boilers will then not refire until the anticycle time has elapsed and providing the flowtemp is target temp -5C or lower. I wouldn't have thought that changing to HW demand even if the boiler is in anticycle at the time should delay the (DHW demand) refiring as long as the flow temperature is less than the HI limit trip which is probably as high as 90C. The (primary) flow temperature then changes based on the HW demand and the set DHW temperature. Edited March 26 by John Carroll 1 Link to comment Share on other sites More sharing options...
SteamyTea Posted March 26 Share Posted March 26 18 hours ago, Post and beam said: Basically it comes down to the PID control of the heater when water temperature is falling just below the set point from an already high winter set point temperature Did it show an error code? And just as importantly, did it not show one? Link to comment Share on other sites More sharing options...
Post and beam Posted March 26 Author Share Posted March 26 9 hours ago, Temp said: It occurs when the boiler has been delivering 76C water to the rads so it's nice and hot. Then you call for DHW and the thermal mass of the boiler momentarily heats the DHW to 76C (or perhaps a bit less) aided by the boiler flame not modulating down enough. The boiler detects the DHW is too hot and shuts down for safety reasons. Is that what the engineer was saying? Yes, basically. It appears the boiler overshoots its upper limits and goes into a blocking mode for perhaps 5 minutes. My thoughts are that this is a failure of the P.I.D algorithm not being very sophisictaed, well not sophisictaed enough. The code is very specific '2965' to be precise. Link to comment Share on other sites More sharing options...
Post and beam Posted March 26 Author Share Posted March 26 4 hours ago, JohnMo said: Don't believe there is a software issue, it's a system setup issue. With respect i disagree. The boiler should be able to deliver the needs of my house and its occupants. In fact the engineer agreed and said that we should not have to bend to the will of the boiler. If the house is warm enough by the criteria we set that that is how the house operates. Stone cold showers every now and again are not part of my expectations. Link to comment Share on other sites More sharing options...
Post and beam Posted March 26 Author Share Posted March 26 1 hour ago, SteamyTea said: Did it show an error code? And just as importantly, did it not show one? Not to me. The engineers can see them. The previous engineer in january denied that they are time stamped, which i find unbelievable. Mondays engineer said that error codes and blocking codes are time stamped but that it is ' days since last reset' in some format. so not easy to decipher. Probably why the january engineer said what he did. By the way the boiler is a Greenstar 4000 rated at 25kw. Mondays engineer did say that when it fires up it will be at a % of max that makes the modulation very difficult when engaged water is low. Low as far as the boiler wants to see anyway. Link to comment Share on other sites More sharing options...
JohnMo Posted March 26 Share Posted March 26 27 minutes ago, Post and beam said: boiler should be able to deliver the needs of my house and its occupants Agreed, but trying to shove 20kW of heat via a couple of radiators and does not/isn't working, your boiler overshoots and locks you out. Once you are locked out, the boiler does not fire for heating or DHW. Nothing to do with pid controllers - just physics. Basically you have a circulation pump on the boiler it's trying to move a load of hot water, if it cannot flow through the system the boiler overheats. Is your heat system done in micro bore? Link to comment Share on other sites More sharing options...
Post and beam Posted March 26 Author Share Posted March 26 3 minutes ago, JohnMo said: micro bore No its 15mm. 4 minutes ago, JohnMo said: Nothing to do with pid controllers - just physics But the PID algorithm controls the physics. My contention is that if the boiler cannot do what i need it to then it is deficient. I am not after all asking it to do anything odd. Specifically, heat a 1986 vintage 3 bed semi and deliver hot water when asked. The boiler manual does say that in older houses it might be necessary to set the temp to as much as 82 degrees C. There by recognising the need. It does not say ' but we wont always be able to give you hot bathing water if you do' 1 Link to comment Share on other sites More sharing options...
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