Cylinder wiring

[LondonFoy]

Thanks, not heard of EPH before, I’ll go look. I was reading another post on here about flow rate. Am I right thinking if the flow rate from the HW pump is too slow, then the entire coil won’t get hot, only the top. That way, only the top of the tank gets hot. If the flow is increased, then the entire coil get hot and therefore transfers more heat into a wider surface area of the tank.

…. Which takes me back to my earlier comments about the bottom of the tank is really really cool while the taps are scalding.

It’s got one of those Trident eco/smart pumps…. CP1,2,3, PP1,2,3, speed 1,2,3, Auto/eco and night modes. I note it’s currently on CP3 which gets 33 L/m. The manual speed3 gets 50 l/m.

Just a thought…

 

[Johntheo5]

One might normally expect around 8 to 10C difference between the cylinder top and just above the bottom coil and a little less between the cylinder top and the cylinder stat location. The greater the coil flowrate the greater the heat transfer and less difference between the cylinder top/bottom. Your pump on CP3 will be running at a very high output, feel the coil at the top and bottom and note the temperature difference. Also look for a wheelhead gate valve on the coil return, this, if installed, may be throttling the flow.

 

[LondonFoy]

Yes, cp3 is giving 30-35 litres/minute flow rate. The difference in temp between too and bottom of tank is more like 25c or greater. I’m tempted to drain the tank and crack it open to see how badly it’s scaled up etc. it’s an old open system and the CH was in a shocking state last year before we power washed it, dosed, magnaclean added, pump changed etc. 1 whole loop was almost fully blocked.

This then takes me back to the stat not getting enough temp that far down, if the tanks either full or junk, or the flow is transferring heat at the top, and not sufficiently at the bottom of the coil that might add to the lack of satisfied cutting the HW, as it’s just not getting to temp, hence it having to set the stat so low?

Stat position is right. No gates anywhere, only either side of the main pump. Cold flow from lift tank sufficient diameter for flow input. It’s a 140l Hercal.

 

[Johntheo5]

Some circ pumps give very useful info including the flowrate in m3/hr, does yours show at least the power in watts?, if so, note it, also post any info you may have on the pump like a e manual.

 

[LondonFoy]

This is the one. https://www.tridentpumps.co.uk/product/energy-efficient-pump/

It shows the watts/litres/

 

[Johntheo5]

Well, what is it showing (on HW only) in watts and litres?, its normally litres/min but we can figure that out. I think yours only shows watts , W.

 

[LondonFoy]

On cp3 it is showing 33 watts

 

[Johntheo5]

Unfortunately, I cannot find any pump curves for that pump, if CP3 is, for example, 4.5M then the pump will be only circulating ~ 12LPM but what you might do is just change the setting to CC3 for a minute or so on HW only as most pump's CC3 is set to the maximum head, 6M with this pump, and I can then get a reasonably accurate flowrate from this.

 

[LondonFoy]

Okay, on cc3 it’s at 45 watts.

 

[Johntheo5]

I can say for certain that the circ rate is at least 16/18LPM and possibly far higher but because its running at max power on that setting I cannot say how much more it may be circulating, I have emailed Trident for the pump curves, if I get them I can then deduce exactly the circ rate on CP3 at 33W.

A circ rate of 16/18LPM is quite high/good so except that coil is fouled by limestone or the like then there should only be a fairly small dT between the coil flow and return, (maybe 10C or less).
Can you feel the coil (pipe) where it enters at the cylinder top and compare it with the coil return pipe where it exits the cylinder bottom. You said above, 25C difference between cylinder top and bottom.

Does the boiler fire continuously on HW only, (assuming gas fired), and what make/model.

 

[LondonFoy]

The boiler is a grant vortex oil. It’s set to 75%, which is around 70c. It’s the one part of the system that is fully serviced and in excellent condition. I haven’t noticed it cycling, but I’ll carry out diagnostics tonight and I’ll measure flow and return of coil, and also properly measure top and bottom of tank and l water temps, as I’ve got all my thermometers now.
Thanks for this support!
I am leaning towards dirty tank as through elimination, I’ve resolved most other things…

 

[Johntheo5]

That Range cylinder has massive flow and return to the coil looks like at least 25MM so wonder is it a gravity coil but is circulated by a circ pump, its hard to imagine a oil fired boiler using gravity flow HW circulation.

 

[LondonFoy]

It’s 28mm flow and return. It’s an open gravity fed system, but circulation pump with a 3 way valve. So, HW is pumped on that circuit. 2 CH loops and then he HW loop.

 

[Johntheo5]

Your next so so is to try and measure the coil flow and return temps, do you detect much difference just by touch?

 

[Johntheo5]

Paul from Trident Pumps kindly sent me on the pump curves, it doesn't show the power but once knowing that CP3 is 4.4M I got the flowrate from my own Wilo 6M pump curves/power, you said the power on CP3 was 30/35W, say 32.5W which gives a flowrate of 15.8LPM and would give a flowrate of 18.5LPM on CC3 much as I said above.
A flowrate of 15.8LPM should give a reasonable coil flow/return dT, maybe 10C, so can't see it at 20/25C which it should be if the cylinder bottom is 25C lower than the top immediately after a reheat.
If you've got all your thermometers now then take a few measurements.

Don't know the cylinder volume?? but if say 200L which will require 11.63kwh if reheated from 10C to 60C, a coil dT of 5C (& 15.8LPM) will give 5.51kw so a reheat time of 2.1hrs, a 10C coil dT gives 11.02kw, a reheat time of just over a hour.