Help with Grant Vortex combi overheating & water heating issues

[JEB1986]

We have been having problems with our Grant Votrex Pro Internal Combi boiler since we moved into our recently renovated rental property in Nov 2016. Here is it's story - I'm hoping someone might have come across something similar, or that someone with better knowledge of boilers than me might be able to shed some light on what could be going wrong.

We initially noticed that we could not run the shower at full pressure and maintain a warm temperature - it would slowly run cold after a few minutes. We didn't think much about it, and just used it at the lower pressure.

In early 2017 (by March) we noticed that the boiler was switching itself off, and we had to press the overheat over-ride button to switch it back on again. A plumber came to see what was happening, and said that we needed a Grant engineer. The Grant engineer replaced the circuit board (May 2017).

At this point we had solved the pressure problem with the shower, and it was running at full pressure and heat with no problems. The overheating problem though, continued. The Grant engineer came back again at the end of May, but we couldn't replicate the problem.

During the summer we weren't using the heating, and we had no issues with the boiler during this time. In September, we starting to use the heating again and the overheat issue returned. Only this time we couldn't always press the over-ride button - we'd have to wait several hours before we could press the button and see the boiler come back to life (but sometimes it would work right away). The Grant engineer visited again, and still could not replicate the problem, but asked us to log every time it happened (as follows).

The boiler would only go off after the shower had been in use. It didn't seem to matter the length of the shower, the pressure, the switching the shower between the two shower heads, whether the heating was also on at the time, or if someone else ran a tap while the shower was in use. But it never went off while the shower was actually in use. It would usually go off after a hot tap had been run after the shower had been used. Sometimes, however, we could do exactly the same routine and it would not go off at all - but I did noticed a potential pattern with the outdoor temperature. The boiler was generally overheating when running a hot tap after the shower had been used when it was less than about 10'c outside. When it was milder, there weren't any issues. This was true for 14 out of 17 occurrences (the other 3 occurrences were consecutive days).

Oct: 11 times off after shower during colder weather (12th to 19th and 24th - 26th Oct not off at all - above 10'c outside)
Nov: 3 times off after shower during colder weather (2nd - 5th Nov not off for 3 showers despite lower outdoor temperatures - not consistent)
Dec - not off at all (despite colder temperatures, potential start of hot water problems. Oil tank refilled at end of Nov)

However, recently we have gradually seen a return of the original issue; not being able to have a warm shower at normal pressure. This time it seems worse - it has to be a very weak pressured shower to get any heat at all. There is nothing on the boiler that indicates an error. At normal pressure, the hot water gradually fades to a luke warm temperature after about 6 minutes of continuous running. Once the tap/shower is turned off, hot water returns again a short time after - probably after about 5-10 mins but this is difficult to test without running the tap!

Other observations:
- when running one hot/cold tap and then turning on another hot/cold tap (such as running a bath or flushing the loo while running the bath/shower) there is a significant drop in pressure. Although this is understandable, I don't remember this being quite so significant when we first moved in, especially with the hot tap affecting the cold and visa versa.
- with the current issues, the hot water we do get seems hotter than usual.
- the boiler does not have a gap around it for air circulation - it is surrounded by walls on 3 sides. I expect this could well be causing an issue, but it doesn't explain the intermittent nature of the problems.
- There is no 'timed' option installed on the boiler. The heating is controlled by a wall mounted thermostat.
- The pressure is fine, the dial reads 1.5.
- The boiler appears switch between water & heating as it should, and continues running after the hot water has been used (to get the water back to temperature).

Any thoughts would be welcome - even our Grant engineer has now drawn a blank! Thanks

 

[MeldrewsMate]

Not familiar with the Grant range (or any oil boiler), but the principles here are common to all boilers.
It seems the Vortex is a storage combi, i.e it heats up a small volume of water to a high temperature, and this is circulated through a secondary heat exchanger to produce your domestic (bathing) hot water. It also seems that your boiler is overheating during the regeneration phase, i.e. bringing the internal storage water back to temperature. You stated that sometimes you have to wait several hours before you can reset the boiler.

From what I know of oil boilers, they are quite meaty devices, the main heat exchanger (HX) walls are thick steel, and therefore can take a long time to heat up AND COOL DOWN. It follows that an overheat of the main HX will take a long time to cool enough to allow a reset to occur. Generally speaking, when in combi hot water mode the burner will be on high output, so lots of heat. If that heat cannot be dissipated (to the storage tank) quickly enough the boiler case may well overheat, and it is here that I believe the fault may lie.

Firstly, have the main HX pump inspected and cleaned. If its performance is poor or it's blocked then the heat cannot be dissipated quickly enough, and the temperature of your stored water will reduce when on hot water demand. You will not notice this initially, because there is a 50C mixing valve on the DHW outlet.
Secondly, check and clean the main HX and associated pipework, so that the boiler heat can be dissipated effectively to the store.
At this stage there seems no reason to suspect the secondary HX is clogged or scaled, as you have adequate hot water performance when in use.
My suspicion is that your heating system may not have been adequately cleaned when the new boiler was fitted, and now this debris is clogging your HX and primary transfer pipework.

Also, your water pressure from the mains may vary greatly throughout the day/season, so you will get more/less interaction between multiple water outlets with time, the only thing you should do is check that your mains stopvalve is fully open (minus 1/4 turn to reduce chance of seizing), and the same for your street isolator, whether it's a stopcock of water meter. You should be able to fill a 1 litre container from the cold mains (kitchen tap?) in 10 seconds or less.

I hope this helps.
MM

 

[oilhead]

As Meldrews Mate says, the problem seems to be tied in with poor cold water supply. The boiler thermostat cannot operate quickly enough to cut the burner off before it goes to overheat, although you may benefit from turning down the DHW temperature. What is it set at? You still can't rule out a PCB sensor fault, but look at the other points first.

 

[oilboffin]

I would say it's a thermostat fault.Bob

 

[Hairymelon]

Overheat fault .most likely an intermittently sticking dhw flow switch, I sometimes have had to reduce the diameter of the "paddle" with a file, as the housings seem to vary in internal dimension and can stick. Poor hot water temperature is plate hex clogged on primary side ( maybe scaled as well ) depending where you live. Your grant guy seems a litttle inexperienced.

 

[JEB1986]

Thanks for all of your feedback & suggestions - it's been very helpful. I'm glad I'm not alone in thinking there must be a fix for it!! Fingers crossed

 

[snb]

As the problem didn’t happen during the summer months and reoccurred after the heating has been on would possibly suggest air in main or slave exchanger, are the auto air vents closed?