Temperature monitoring and legionella

[xanld]

Hi,

I have a new installation of an Air Source Heat Pump (Grant UK Aerona), and a horizontal Tempest 200l pressurised cylinder. It has a single immersion heater, which is controlled by a Timeguard FST77 7 Day Fused Spur Timeswitch, and the whole system is controlled by a ESI ESCTD Mechanical Dual Cylinder Thermostat (set to the maximum of 65 degrees currently)

I've also added a MyEnergi Eddi with PT1000 probes for monitoring the temperature in the top and bottom of the cylinder using the dry pockets.

My installer has set the immersion heater to come on once a week for an hour using the Timeguard timer. This is to ensure that the water temperature goes above 60 degrees to kill legionella bacteria, as I understand the ASHP is not capable of getting it to this temperature on it's own.

BUT, I'm not convinced it's working. My PT1000 temperature probes are showing that the temperature never goes above 51 degrees, and I can see that the immersion heater stops drawing power even when the green light is on the timeswitch, and I've set it to boost.

To verify this, I've also used a manual temperature probe in the cylinder pocket. I managed to get a readout of 53 degrees by pushing it hard against the pocket walls. I got a readout of 55 degrees from the hot water coming out of the bathroom tap (from an un-insulated run of about 5 metres). It's still a long way off the 65 degrees set on the cylinder thermostat though.

So my questions are:

1. Is my temperature monitoring reliable? Is there something better I can do to verify the temperature in the tank? Perhaps something to measure the temperature of the water flowing directly out the top of the tank? Is there an inline temperature monitor that I can install on the 22mm pipe that is more accurate than the dry pockets?
2. What would be stopping the temperature getting above 50-55 degrees? Does the immersion have a cutout?

As a separate question - is it OK, that I have 22mm pipe coming out of the cylinder for about 3 metres before it reduces to 15mm? 22mm pipe holds about 3 times the amount of water as 15mm, so I think this will increase the amount of cold-run-off, and the amount of hot water that I'll be wasting in the pipes. What's standard here?

 

[flameport]

What is the immersion heater thermostat set to?
1 hour of heating will not get 200 litres of water to 65C unless it was already fairly hot to start with.

 

[xanld]

The ESI ESCTD Mechanical Dual Cylinder Thermostat on the cylinder is set to 65 degrees that's why I'm confused. Unless this crude dial is not accurate. AFAIK this is the only stat, unless I'm missing something.

Yes, 1hr is not going to be enough, but it should already be up to 50 degrees from the ASHP, and at any rate, I'm manually boosting the immersion heater, and it's not drawing power once it reaches ~50 degrees.

 

[dilalio]

Try some thermal paste in the dry pockets.
Contact the cylinder manufacturers and ask what the delta is likely to be between the stored water and the dry pocket.

It only has to get to 65° as a peak to kill off the bacteria, not maintain it.
If the delta is close then overshoot the immersion stat for a longer period and set it for a time before any draw off will be made.

 

[xanld]

Thanks. The thermal paste and advice to find out the delta are great ideas. And the advice on peaking Vs maintaining is also reassuring.

It's already set at 2am on a Sunday. But I still have an issue that it stops heating before the timer period (or before the boost timer) before reaching temperature. I can only assume the temperature probes are inaccurate compared to the mechanical stat. I've contacted the installer for his advice too.

 

[flameport]

this is the only stat,

Immersion heaters have their own separate thermostat as part of the heater. Underneath the wiring terminal cover for the immersion heater.

 

[JohnD]

horizontal Tempest 200l pressurised cylinder.

so it is fed from the incoming watermain, with clean, cold, chlorinated water, and is not exposed to the air, dust or pollutants.

I got a readout of 55 degrees from the hot water coming out of the bathroom tap

Legionella grows at temperatures between 20C and 45C if nutrients are available

It will be killed at 50C, taking 2 hours to kill 90%. The hotter the water is the faster it dies, hence plumbers often say 60C which gives an extra margin of safety and only takes 2 minutes to kill 90%

If I understand you correctly, the hygene cycle heats it during the night, so the hot stored water will maintain its temperature for several hours.

 

[xanld]

Contact the cylinder manufacturers and ask what the delta is likely to be between the stored water and the dry pocket.

I did, and they said 4-5 degrees which is consistent with everything else.

Immersion heaters have their own separate thermostat as part of the heater.

This is the conclusion I was coming to and exactly what I needed to know. This thermostat is what I believe is the issue. It's way too low IMHO.

so it is fed from the incoming watermain, with clean, cold, chlorinated water, and is not exposed to the air, dust or pollutants.

Via a salt softener, but otherwise yes.

Legionella grows at temperatures between 20C and 45C if nutrients are available

It will be killed at 50C, taking 2 hours to kill 90%. The hotter the water is the faster it dies, hence plumbers often say 60C which gives an extra margin of safety and only takes 2 minutes to kill 90%

This is very reassuring. Thank you.

If I understand you correctly, the hygene cycle heats it during the night, so the hot stored water will maintain its temperature for several hours.

Correct.

---

So I'm less concerned now about leigionella (thanks JohnD) however, I would still like the potential to heat above 55 degrees so that I can take advantage of surplus solar when I have it. I'll continue to explore whether the immersion stat can be adjusted, as I think this must be limited. Otherwise perhaps I'll need an alternate immersion heater.

I appreciate I may need to consider a thermostatic mixer on the hot water outlet to avoid scalding if I go higher temperatures.

One other related issue is that Telford suggested it was necessary to have a de-stratification pump. There seems to be mixed opinions on this. The only logic I can see on this is that the immersion heater (which is at the top of the horizontal cylinder) is only heating it's local water, and without the de-strat pump, cutting off before the water near my temperature probe is up to temperature. This seems a little far fetched given they are only a foot or so apart along the top of the cylinder.

Perhaps it is necessary to have a de-stratification pump during the hygiene cycle to ensure the entire tank is up to temperature, not just the water near the immersion?

That said, the ASHP regularly heats the entire tank uniformly to 50-55 degrees, and from what JohnD days, this is probably good enough if it's held at this temperature for a number of hours.

Thanks again all.

 

[xanld]

OK, here it is. This is the immersion. And in this (which I assume is the highest thermostat position, it stops heating at about 53-55 degrees. Is this normal?

I will contact Thermowatt.

As I say, whilst legionella may not be so much of a worry (although it's still slightly disconcerting I can't get it to the plumber's recommended 60 degrees), I'm more disappointed I won't be able to use surplus solar, and without de-strat, I won't be able to heat the whole tank even to 55 with this immersion heater.

 

[ericmark]

I have oil central heating, and no thermostat on the DHW tank, so during the summer only control is time the boiler is running, it runs three times a week for ½ hour which is lowest time Nest Gen 3 will allow, but boiler only actually runs for 20 minutes, after that the return water gets warm enough to turn off the boiler.

However it is apparent the water gets cooler and cooler as summer progresses, it seems the energy can't transfer from the hot coil fast enough, so 20 kW for 20 minutes is not enough, so 20/3 x 20 = 133 minutes, so it would likely take 2 hours three times a week for the immersion heater to do the same job, my water never goes cold, so it seems likely one hour is not long enough to heat the water.

 

[MeldrewsMate]

Only a minor point, but your claim that 22mm pipe holds 3x that of 15mm pipe is incorrect; it's about twice, with 28mm pipe holding about 3 times that of 15mm.
15mm o.d - (2 x 0.7 wall thickness) = 13.6mm i.d. 13.6² x PI / 4 = 145mm² area, or 0.145 litre/m of pipe.
22mm o.d - (2 x 0.9 wall thickness) = 20.2mm i.d. 20.2² x PI / 4 = 320mm² area.....

Rather than a simple timeswitch, perhaps what you need for best energy conservation is a 168 hour timer which continually counts down when the water temperature is below 62°C. Once timed out the immersion heater will be invoked that night; thus imported electricity will not be consumed unless the renewables (PV or solar thermal) had failed to sanitise the cylinder in the past week.

MM

 

[Harry Bloomfield]

This seems a little far fetched given they are only a foot or so apart along the top of the cylinder.

A foot apart horizontally, or vertically? Stratification means that only water down to the bottom of the element will be raised in temperature, sue to the effect. Below the element will remain cool, because there is nothing to promote mixing. Might the element be too short for the height of the tank?

 

[xanld]

Only a minor point, but your claim that 22mm pipe holds 3x that of 15mm pipe is incorrect; it's about twice, with 28mm pipe holding about 3 times that of 15mm.
15mm o.d - (2 x 0.7 wall thickness) = 13.6mm i.d. 13.6² x PI / 4 = 145mm² area, or 0.145 litre/m of pipe.
22mm o.d - (2 x 0.9 wall thickness) = 20.2mm i.d. 20.2² x PI / 4 = 320mm² area.....

Rather than a simple timeswitch, perhaps what you need for best energy conservation is a 168 hour timer which continually counts down when the water temperature is below 62°C. Once timed out the immersion heater will be invoked that night; thus imported electricity will not be consumed unless the renewables (PV or solar thermal) had failed to sanitise the cylinder in the past week.

MM

Top drawer pedantry. Wall thickness and internal vs external diameter are indeed things I hand't accounted for. Reminder to self: there is always a smarter person.

Rather than a simple timeswitch, perhaps what you need for best energy conservation is a 168 hour timer which continually counts down when the water temperature is below 62°C. Once timed out the immersion heater will be invoked that night; thus imported electricity will not be consumed unless the renewables (PV or solar thermal) had failed to sanitise the cylinder in the past week.

MM

Wait what? This sounds amazing! A quick search for "168 hour timer" has yeilded no useful search results though. Do you have a link or make/model?

Also - I intend to replace the timer switch with a MyEnergi Eddi which will hopefully be smart about diverting solar surplus, and being aware of lower tariff times. However, the 168 timer sounds like it may be a useful addition. I don't have solar thermals though, and the ASHP won't heat above ~50 degrees, so this would only happen with surplus solar electricity invoking the immersion.

 

[xanld]

A foot apart horizontally, or vertically? Stratification means that only water down to the bottom of the element will be raised in temperature, sue to the effect. Below the element will remain cool, because there is nothing to promote mixing. Might the element be too short for the height of the tank?

Horizontally. It's a horizontal cylinder. So yes, I assume that a probe a foot away horizontally across the top of the tank would not require a de-strat pump to be accurate.

 

[Harry Bloomfield]

As I say, whilst legionella may not be so much of a worry (although it's still slightly disconcerting I can't get it to the plumber's recommended 60 degrees), I'm more disappointed I won't be able to use surplus solar, and without de-strat, I won't be able to heat the whole tank even to 55 with this immersion heater.

Are you sure the element is actually drawing current when it is supposed to be on? Might the stat have tripped out - I cannot see the trip, but they are fitted with a resettable over temperature trip, to prevent water boiling, if the adjustable stat fails/jams/weld up it's contacts. Yes, that stat is set at the maximum temperature.