Baxi 105e DHW problem

Xsparkie · Jan 4, 2012

Hi all
I'm having trouble with my daughters 10 year old Baxi 105e, the CH works fine but DHW cuts out after 45 seconds to a minute and comes back on again after 12 to 18 seconds, this cycle is repeated ad infinitum the fan and the pump continue to run after the gas shuts down. I changed the DHW sensor as the resistance readings weren't within spec but no change. I'm not a plumber, I'm an electrician with very limited experience of boilers but from what I have read on here I'm beginning to think the problem is going to be the plate heat exchanger. I'm clutching at straws here, but is there any way of proving this part without removing it. It looks like a pig of a job, especially as the boiler is in her attic and you have to sit side saddle on a joist to get at it. I suppose that as the boiler is 10 years old there is a good chance this is going to be the problem, although lime scale is not a problem in this area. From memory I can't remember if the side panels come off, but if they don't I imagine lots of other bits may have to come out first. My daughter is a bit hard up at the moment (single parent with two teenagers) so I would really like to fix this thing for her if I can, so any help that enables me to do so would be gratefully received, by both of us.

Regards X Sparkie

diamondgas · Jan 4, 2012

:welcome: Xsparkie. Most likely suspect is the plate hex specially as it's doubtful anyone has messed with it being fitted where it is 🙂 They take less than an hour to replace if you can get the LH side cheek off.

To verify though isn't so easy. I'm a touchy feely kind of guy and i like to check out the temp of the return pipe above the pump and also see if the water is pumping down the heating flow. A touchable return and a hot flow to rads is a good indicator that the hex is faulty. No guarantee's however that it isn't the diverter valve sticking until you replace the hex IMO 🙂

Xsparkie · Jan 5, 2012

Thanks diamondgas for that rapid response to my question, I'll try your common sense tests before I pull the thing out. As I keep telling one of my cocky grandsons "There is no substitute for experience". I suppose it wouldn't be a wasted effort to change a 10 year old plateX, even if its not the problem at the moment.

Thanks again X Sparkie

diamondgas · Jan 5, 2012

Actually there's an easier test for P-Hex when i think about it. You'll need a thermometer, stop watch and some kind of measuring jug. Also the manufacturers instructions, scratch that i have the details at hand.

Basically the 105e will guarantee 10ltr/min with a 35Deg C rise and 14 Ltr/min 30 DegC rise. So what you need to know:

Flow rate first at the hot tap ... Measure in seconds the time to run off 1 Ltr then divide it into 60 to give you ltr's/min. eg 6 sec 1 Ltr = 10ltrs/min

Next temp difference ... Measure how hot the hot water is getting then how cold the cold water is, subtract cold from hot to give you the temp rise.

If any of the figures match the above guarantees then you do not have a problem and the plate heatex is working as its meant too! However if as suspected it isn't transferring the right amount of heat then expect lower temp rise than manufacturers guaranteed ... By-the-way if the hot flow rate is too high just crack the tap back until you get the desired flow then measure its temp.

Hope that helps

It doesn't necesseraly mean the plate hex is the cause as other factors can bring about lower temp rise but at this time of year you'll be suprised at the amount of boilers that are working as per MI's yet the 'hot' water isn't that hot!! The temp of the cold up here in Newcastle is 9DegC which means any combi will only guarantee 44DegC. You can hold your hand under that!!!

Xsparkie · Jan 8, 2012

Hi diamondgas once again thanks for prompt reply. I carried out the tests you suggested, I marked a bucket at 10 litre, (borrowed wifes litre jug) I thought it would be more accurate than timing 1 litre, it took 1 min. 6 sec. to fill so that was convenient. Cold measured 7 degrees hot measured 49 that was with my ebay £3.89 turkey testing thermometer, which when tested with boiling water and ice cold water is within a couple of degrees so it looks as though the plate hex is OK. I also short circuited the safety stat but still the same pattern of running and shutting down. Other than the PCB I'm running out of things to blame, it does suggest to me an electronic problem as the on off periods are the same length of time every time, though I'm loath to stick my daughter with the price of a board without being certain, especially as I fitted a new board last May. Like you I have never been a fan of sticking new parts into anything as a fault finding technique. So any more ideas, I'm getting to the hair tearing out stage

Gordon T · Jan 8, 2012

If you fitted a new board last May, take it back as its still under guarantee.

Xsparkie · Jan 8, 2012

I'm waiting to find out if my daughter still has the receipt.

diamondgas · Jan 11, 2012

Xsparkie said:
Hi diamondgas once again thanks for prompt reply. I carried out the tests you suggested, I marked a bucket at 10 litre, (borrowed wifes litre jug) I thought it would be more accurate than timing 1 litre, it took 1 min. 6 sec. to fill so that was convenient. Cold measured 7 degrees hot measured 49 that was with my ebay £3.89 turkey testing thermometer, which when tested with boiling water and ice cold water is within a couple of degrees so it looks as though the plate hex is OK. I also short circuited the safety stat but still the same pattern of running and shutting down. Other than the PCB I'm running out of things to blame, it does suggest to me an electronic problem as the on off periods are the same length of time every time, though I'm loath to stick my daughter with the price of a board without being certain, especially as I fitted a new board last May. Like you I have never been a fan of sticking new parts into anything as a fault finding technique. So any more ideas, I'm getting to the hair tearing out stage

Are you getting a fluctuation in hot water temp, Hot/Cold/Hot?cold? 32DegC may be a p-hex that is on it's way out? No guarantees alas 🙂 & I'd be wanting to check other switches for there operation too. If the main burner is cutting out all together then relighting, that usually points to the o'heat kicking in but you've overridden that with the same results! Did you check the readings on the c/h thermistor? I'm not sure if it, at all, plays a part in checking circulation temps when in hot water mode, I don't think it does but am not sure. If it does, however, then it's possible that it is shutting the burner off prematurely! As said though I don't think it does!!

There are factors that need checking regards the burner modulation and flame sensing that really requires a trained person, and certainly not something that I'd share here... 🙂

However, back to your findings. If the hot water is being kept at a constant and not fluctuating hot/cold then I'd suggest you run the hot taps on a slower flow though. If the temp is fluctuating then there's is little else you can check apart from what I've suggested. You can't knock being present at the appliance Xsparkie alas. My assistance here is limited when not experiencing the appliance first hand ... 🙂

baxi-moldova · Jan 13, 2012

Hello, try to lower the dhw temperature by turning the knob counter clockwise and see what happens. I also noticed you said that that there is not a lime scale problem area, the deposits on the secondary heat exchanger might come also from radiators especially if those are made of steel. Sorry for my English which is not my matern

armourer · Jan 13, 2012

As diamond gas mentioned earlier don't forget to rule out the diverter passing before going deeper.

The easiest way I find is put your hand on the heating flow pipe (when cold) and run a hot tap.
If the pipe gets hot the diverter is passing.

Other than that as diamond said again you will be probably into the realms of high/low gas rates.