I've been trying to understand what happens with the hot water when my wife screams that it has gone cold in the shower so I watched it the other day.
Turn shower on, burner comes on, led lights (Baxi 105HE) start to light up to a maximum. Everything fine for a few minutes then the burner turns off, leds go back, and wife screams. Burner comes back on - repeat and rinse.
Shouldn't the burner flame just adjust to maintain the temperature rather than cut-out? Obvious behaviour of the CH but not for DHW.
Otherwise is this indicative of a thermistor problem with hysterisis or similar? Is it easy to replace?
Has it always done this? As a mere user of the Baxi 105HE, not an engineer thereof, I'm wondering where the CH & Hot Water temperature knobs are set? Mine are set so the temperature never quite goes up to 70 (I was told to do this to reduce limescale deposits though I don't know if that really works.) I never have any thermal cutout problems.
When my oil fired combi went wrong I did think that with four multiple terminal relays it is to complicated for its own good. What I would quite like is a series of LEDs on the front panel that show the status of the relays, thermostats and other sensors. I was surprised to discover that it does not appear to care about the pressure in the central heating (and internal hot water tank) system.
It depends on combination of the boiler and the type of shower mixer. If the shower throttles back the flow of hot the temp rises and eventually the boiler will cut out.
Some thermostatic mixers and some combis play nicely together, others fight.
I had that sort of problem from new with both a Vaillant and a Halstead, admittedly in an older and rambling house with longish pipe runs. Given up and gone to a proper system boiler.
Some friends have a small holiday cottage that we use occasionally, this is a new compact house with very direct pipe runs and the shower is flawless, even with other demand on the small combi.
I've seen this when the secondary plate exchanger was scaled up. Boiler would throttle the burner back, but when the plate exchanger gets so bad it can't conduct the minimum burner power through the lime scale, then the burner ends up cycling on and off.
You might get a short improvement by turning the hot water temperature to max, as a higher temperature differential will enable more power to be passed through and it might be able to handle the full min burner power without cycling, but this will also scale up the plate exchanger faster, so it's only a very short term fix.
I took the plate exchanger out and descaled it. It required an enormous amount of descaler - fortunately I had a 2kg tub of Furnox DS-3, and it took quite a while. A plumber would probably just replace the plate exchanger.
If we assume for the moment that the boiler is working as it should then there is a likely cause that could result from the boiler DHW temperature going over the set point.
IIUC, your Baxi does not have any form of flow regulation. So all it can do in response to varying DHW flow rates is modulate its burner output.
So normally you set a temperature, say 60 deg, and the boiler will attempt to deliver water at that temp.
If you are drawing DHW at a high rate, then its unlikely to have the power to reach that temp - and so the temp will fall below the max.
If however draw at a low rate, then it will have to modulate its burner output power down to prevent the temperature exceeding the limit. If it can't keep under the limit even at minimum burner power, then it will start to cycle the burner.
You can test this theory, but running the shower as normal, but by adding some extra demand - say running the hot tap in the basin to increase the DHW flow rate a bit.
If that stops the burner cycling, then you may be able to fix the problem simply by increasing the max DHW temp on the boiler (the TMV on the shower should keep the person in there safe). If that still does not do it, then you may need to change the shower head for a more "thirsty" one, or use the basin tap trick for every shower.
I had some problem a few months ago that led me to deduce that having the knob set too low appeared to be causing cut out. I was set to about 2 'o' clock, I'm now on about 4 'o' clock. 6 'o' clock ie vertically down seems to be the max. I'll wind it back a notch or two and see what happens but I'm definitely hitting max temp at the current setting.
That I'd be happy with as even though the water will not get very hot it is simply a matter of turning the shower knob anticlockwise to reduce the amount of cold. As stated the problem is when the burner goes out.
That's what seems to be happening.
Good point. I'll give that a go.
No TMV afaia, simply turn anticlockwise to reduce the cold water.
Interesting, I'm wondering now when I did change the shower head because my wife found the other one too gentle. But that is probably the opposite to what you are proposing.
Yup, just including the description for completeness.
Alternatively you could try running the shower with just the hose (i.e. head removed)
But note also the comment from Andy and others. A scaled shower head could cause a similar issue as could a scaled up secondary HX in the boiler.
In which case you will need to be wary of setting the DHW top limit too high since you don't want to have a scalding risk. If you set it at 60 max, then by the time you allow for a bit of loss in the pipework etc that ought to be still cool enough to get out of the way should it start getting too hot.
Well a high flow rate head, with lots of wide jets may give a "softer" more rain like flow, that a lower consumption one with a smaller number of narrower more focussed jets. How hard the shower feels will be a product of the speed of the water and the weight of it hitting an area of the body.
Vaillant make Heating only boilers (vented and unvented), System boilers, and Combi boilers. Typically the 400, 600, and 800 series respectively. So a HE 624 is a 24kW System boiler and a 838 a 38 kW Combi.
Goes to show what I know about boilers. Too many years with coal fired heating.
I'm guessing our bungalow used to have a system boiler in the loft with supplementary immersion heating. In fact that's pretty well what I had in my previous coal fired property and to be honest that worked well as the immersion heater wouldn't need to do anything when there was a fire and kept a stock of hot water using E7 when heating was needed.
Also if I turned the temperature down the boiler started cycling a bit erratically and with bit of what sounds like valves or circuit breaker noises. If I turned the temperature up it was better but would still cut out.
It is a Mira 415. I suspect put in at the same time as the boiler so
2006?
I'm fairly sure the problem is in the boiler somewhere. The shower has worked fine in the 3 1/2 years we've been here. The only changes have been:
1) Flow reducer valve put in a couple of years ago
2) Change shower head to one that doesn't have such a fine spray
3) Replacement diverter valve about 18 months ago
It's definitely cycling when it shouldn't and as I understand it now the burner should simply adjust and not turn off/on though I presume that it correct operation for the CH?
Ah, ok the 415 is not a traditional TMV but a pressure balanced valve instead. These are specially designed for use with combi's since a traditional TMV can have unexpected results.
(i.e. if the temp falls, a normal TMV will compensate by increasing the flow of hot. On a combi that might actually lower the temp of the hot supply even more. A PB valve will keep the pressure on the inlets balanced, this tends to better reflect the effect of demands being made elsewhere in the house)
The main difference between CH operation and DHW is the addition of the secondary plate heat exchanger for the DHW.
Since we have now eliminated a lack of adequate DHW demand as a cause, it points the finger of suspicion at the Plate HX, and lack of adequate rate of heat transfer. The MO would be the system fires, but the primary side runs hotter since you are getting less cooling effect from the PHX.
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