My hot water seems far too hot 🥵

[Deleted member 120897]

Greetings members of plumbersforums,

Firstly - long post alert

I’m struggling with my hot water currently - I’ve only mainly dealt with combi boilers, even then, not really had hot water too hot. Anyway I digress, my system is an open vented pre-lagged cylinder, S-plan, Myson 2 ports with a Myson cylinder stat. My hot water seems far too hot, I haven’t measured it recently, but when I did a wholeness back, I was getting 68/69 from being set at 60. I reduced it to 40, and was manageable, but still seemed hot. I’ve been been trying to “test” it today, and wife’s has said it’s gone worse. Here’s what has been done so far:

• Replaced Myson with Honeywell temporary, still same issue (although couldn’t have wired it wrong)
• 2port isn’t firing boiler by itself (that I know of)
• Checked spindle on 2 port - is fine
• Immersion heater isn’t on - checked and double checked
• Checked lever on 2 port head - not fantastic resistance
• 2 port comes on when I turn dial up on stat, and goes off when I turn it down
• Noticed just now, cylinder stat no longer clicking
• When I tested for continuity between orange and grey it brought pump on? But was my multimeter acting as a link?
• Cylinder stat is making good contact, and worked fine for approx 9 years
• Felt tank by stat, seems cold?

I have checked wiring from cylinder thermostat and is as follows:

• Common from programmer goes to terminal 1
• There’s a blue wire (assuming switch live?) in terminal 2
• Nothing in terminal 3

I read it as 3 is call for heat? I asked elsewhere and told call is terminal 2? This is where I’m struggling - can anyone explain it better? Can anyone help me test this properly? Someone has mentioned stratification?

A thanks already in advance

 

[John.g]

So looking at the image one could say the stat location isn't really ideal?

Anywhere within the coil flow/return should be OK IMO.

 

[Deleted member 120897]

The stat location looks a little low, although I couldn't say for sure where it's best placed. Perhaps @ShaunCorbs can clarify.
With the stats location and setting of 40°c the water at the top of the cylinder will naturally be hotter.
Have you got a thermometer out and actually measured the temperature of water at the taps?

I did a while ago, and set at 50 iirc came out of the tap iirc over 60, possibly even close to 70. I need to service the boiler, so may do everything at once. Thanks.

 

[SJB060685]

You will always get layers of stratification without a destrat pump but a 20°c difference seems a lot.

 

[Deleted member 120897]

You will always get layers of stratification without a destrat pump but a 20°c difference seems a lot.

This is what I was thinking. I’ll try and move the stat up first. Thanks

 

[John.g]

You will but would expect 5 to 10C max depending on the distance between the coil flow/return, I have two coils in my cylinder, the bottom one, a solar coil has a very short distance between the flow and return and there is virtually no difference between any of the three probes, one just above the solar coil, the second about cylinder midway, the third at the cylinder top. The boiler coil which heats 100 litres of the 150 litre cylinder will gives 5/8C differential max. Would like to see the differential with a bottom mounted immersion element.

 

[SJB060685]

You will but would expect 5 to 10C max depending on the distance between the coil flow/return, I have two coils in my cylinder, the bottom one, a solar coil has a very short distance between the flow and return and there is virtually no difference between any of the three probes, one just above the solar coil, the second about cylinder midway, the third at the cylinder top. The boiler coil which heats 100 litres of the 150 litre cylinder will gives 5/8C differential max. Would like to see the differential with a bottom mounted immersion element.

I remember a post of yours a very long time ago about your cylinder and the testing you had done.
You would agree with me that a ∆T between stat and temperature at the taps of around 20°c seems a lot?

 

[Deleted member 120897]

Someone mentioned to me a few months back about checking the boiler switched live - does this sound plausible?

 

[SJB060685]

Someone mentioned to me a few months back about checking the boiler switched live - does this sound plausible?

The boiler switch live and pump live should ONLY become energised when either zone valve end switch is contacting. Using a multimeter you can prove correct operation in a couple minutes.

 

[John.g]

I remember a post of yours a very long time ago about your cylinder and the testing you had done.
You would agree with me that a ∆T between stat and temperature at the taps of around 20°c seems a lot?

Yes, couldn't see that deltaT (normally) unless the stat is faulty or dial mis positioned or cylinder motorized valve passing with boiler on CH. Other unlikely cause could be the bottom "1/2" of the coil immersed in a mountain of limescale but moving the stat up might tell alot.

 

[SJB060685]

Yes, couldn't see that deltaT (normally) unless the stat is faulty or dial mis positioned or cylinder motorized valve passing with boiler on CH. Other unlikely cause could be the bottom "1/2" of the coil immersed in a mountain of limescale but moving the stat up might tell alot.

The OP is adamant that the water in his area is soft and by the data he showed it does look very soft.

 

[ShaunCorbs]

Just adjust the stat to match 60dc outlet this might reduce the stored hot water you have eg at temp might only have 1/3 -1/2 of a cylinder of hot water

 

[Deleted member 120897]

Not sure if it’s worth mentioning, but I can put my dhw on timer, comes on approx 6am, goes off at 7:45am, will probably last until early to mid afternoon (nobody using it much then). During this timed period, I’ll hear the boiler fire up at least 3 times, is that normal?

 

[John.g]

Seems normal enough, as the cylinder temperature rises the coil emits less heat so the boiler may cycle a few times before the stat is satisfied.
Forgot to ask if there is a big difference between the coil flow and return temperatures, even by just feeling them when the boiler is firing.

 

[Deleted member 120897]

Forgot to ask if there is a big difference between the coil flow and return temperatures, even by just feeling them when the boiler is firing.

I haven’t checked this yet, might try it tonight. Would this help prove scale? Thanks

 

[ShaunCorbs]

Yea normal to cycle as the coil kw is lower than boiler kw normal tends to do it on hot water only

 

[John.g]

If the coil was very badly scaled then there would be little difference in the flow/return temperatures., Normally, it depends on the circulation flow rate through the coil, the faster it circulates the less the difference (deltaT) between the flow/return and the more heat emitted so the cylinder heats up faster. If the flow rate is very slow then you will get a very high deltaT but the cylinder will take much longer to heat up but you may get a higher temperature difference between the cylinder top and bottom. I can't see scale being your problem.

 

[Deleted member 120897]

I can't see scale being your problem.

Nor can I really, it heats up quick enough, circa 20 - 30 mins at the most

 

[Deleted member 120897]

Hi @John.g, the flow and return on the coils seem similar by hand, I’ll have to get my clamps out to be more accurate, both seem red hot, can’t keep hand on for long at all.

 

[John.g]

Assume you atre heating say 200 litres from 30 to 60C will require 7kwh (14kw coil) if heat up time is 30 mins and if the coil is circulating between 15 and 20LPM will mean a coil deltaT of between 13.5C and 10C so with very hot boiler water might be difficult to pick up by hand if 10C or less, measurement is best obviously.

 

[John.g]

Here, for interest, is a screenshot of readings taken from my twin coil 150 litre cylinder of which 100 litres is heated with the boiler coil, the solar coil heats either 50 litres (dedicated solar volume) or 150 litres for 5/6 summer months with no auxiliary heating.