(ASHP-based) Hot water tank gets cold surprisingly quick

[puntloos]

So my current tank is 400L. Pretty sizeable! 

Yet when it starts at 37C, having one shower on, drops the measured temperature down to 29 in 5 minutes. 

Is that normal, or could there be something plumbed wrong?

 

To be clear:

- I know 37C is pretty low to start with, it was semi-intentional (I figured leave it cool a bit during the expensive period)

- Because 37 is already low, the shower user (my kid) probably tried to set it to max temp, so no other water mixed in.

- The temp sensor is halfway up the tank, so presumably there's hotter water above the sensor, yet the kid complained the water was (slightly) too cold indeed after 5 minutes.

 

Is this about within expectations? Of course the ideal tank temp is perhaps more like 53-55 (or way higher if gas-heated, 65?) but I still feel that having 400L of 37C water should.. in theory.. last a pretty long while - 12L/minute, so maybe we lost (and replaced with cold water) 60-100L over those 5 mins...

[joth]

How many temperature probe pockets does your tank have? IIRC it's 3 or more. Perhaps worth loading up a probe in each one to get a more complete picture of the stratification.

My tank just has the one pocket in the middle and I have similar annoyance at the fairly binary nature of the data it gives me...

[JohnMo]

37 is low, so no surprise you get comments. It is also in the optimal growth temp range for legionella. So not a good temperature to store water at. Big cylinder, so not a full turnover of water and storing at 37, wouldn't be my choice. Even with straight water from the cylinder it's a cool shower.

 

If the sensor is half way up the cylinder, assume the temperature control comes from the same sensor? If so you are only heating the upper half of the cylinder, below the sensor could be cold. So your 400L is now only 200L of semi useful water. So a 34 Deg shower is nearly all water from cylinder. The thermocline will just rise so water temp drops pretty quickly at the sensor.

[marshian]

21 minutes ago, puntloos said:

To be clear:

- I know 37C is pretty low to start with, it was semi-intentional (I figured leave it cool a bit during the expensive period)

- Because 37 is already low, the shower user (my kid) probably tried to set it to max temp, so no other water mixed in.

- The temp sensor is halfway up the tank, so presumably there's hotter water above the sensor, yet the kid complained the water was (slightly) too cold indeed after 5 minutes.

 

Thermostatic mixer showers can really struggle when the HW side is quite low - Both our showers really lose the plot control wise when the HW temp drops below 40 deg - above it it's fine and it will manage to control the shower temp nicely with no changes to the mix

[ProDave]

Why so low?  I heat my HW to 48 degrees, the ASHP has no trouble achieving that.

 

My cylinder only has 2 sensor pockets and I use the lower one of the two which is roughly mid tank.  Because of the nature of an UVC as water is used it tends to just fill up the bottom with cold with little mixing occuring.  So it is quite possible I have half a tank of hot and half a tank of cold before the ASHP turns on to re heat it.

[puntloos]

Just for clarification, I'm still tweaking my tank and heating etc. Of course (?) 37 is not my normal target temperature, for indeed just comfort, but also legionella reasons. Instead I have pretty close control over the target temperature, as indicated in yellow, vs the actual temperature, where indeed at 18:15 my kid showered, and at 23:00 two adults showered. 

 

The point of the curve was to raise the temp a little before 16:00 (elec is expensive between 16:00 and 19:00) to just have enough for that one shower), and let it cool a good amount between 0000 and 1600. 

 

 

even though the exact shape of the curve isn't right yet, the drop for my kid's shower was more than I expected, but as @JohnMo noted, perhaps it isn't incorrect or behaving poorly, it's just a lot of water that might not be as hot on average as I'm expecting. 

[John Carroll]

The above temperature is 1/2 way up the tank.

Does this temperature probe also control the heat pump/heating coil on/off?, normally the cylinder stat/control probe is installed say 150mm or so above the bottom of the heating coil, then depending on the coil dT (which depends on the flowrate etc) the temperature at the cylinder top might be ~ 10C higher than at the "bottom" so maybe 5C or so at mid point but if the control is from midpoint then less of a gradient.

where are the coil inlet/outlet connections? and what is the HP flow/return temps and flowrate with cylinder heating only on?. 

[John Carroll]

3 minutes ago, John Carroll said:

 

 

Double post.

Edited February 21 by John Carroll
Double post

[ProDave]

I see you have quite a bog gap at times between target and actual temperature.

 

On my ASHP you can adjust a parameter so set the hysteresis, i.s how for below the set point it has to go before the ASHP starts re heating.  You might want to investigate if you have something similar.

[JohnMo]

Or just heater hotter and have a time window when it's allowed to heat. I operate on two time windows, early morning on cheap rate, early afternoon (only heats if needed) to make most of any solar energy going spare.

 

I found letting heat as it thought was needed (as you are) used way more energy than timed windows.

 

Notice your heating the cylinder 5 times a day. Is that correct?

[RobLe]

That’s a big tank!  We heat our 220l tank at night with gshp to 50C, then for 30 minutes with a 60C immersion cutout just at the top - again at night, all in the ‘go’ time.  Used to do the immersion once a week, but that 50C drops - and Mrs Robl starts the day happy when the water is super hot, never really noticed myself.  Some battles are not worth fighting 🤗

[puntloos]

OK it's been a while but I have a bit of extra research, what I need to know: Is the current behaviour reasonable or should I complain?

 

In short:

- If tank temperature (as measured at the lowest slot (*) in tank) is ~49C or above, bath tap water is ~43C. This is hot enough. 

- If tank temperature is ~43C, bath tap water is ~39C (**). This is not hot enough. I'd say 42 is the lowest tap temp that works for filling a bath.

 

In conclusion the mixer valve is probably set to 42-43C, but is not able to provide fully unmixed hot water, since if the tank is 42, it only produces 39. (does that mean it might be broken? or just 'cheap' or 'configured incorrectly'?)

 

So- the question is, is the mixer, and its setpoint reasonable for a house with ASHP-temp water? When looking online, the NHBC Guidance suggests "bath taps will need a thermostatic mixer to ensure the temperature of the water leaving the tap will not exceed 48oC" so I don't understand why anyone would set the mixer to 43 unless it's (as per Health and Safety gov uk) "where vulnerable people can get access to baths or showers and the scalding risk is considered significant, a thermostatic mixing valve (TMV) Type 3 must be fitted to prevent water being discharged at more than 44°C"

 

To my read the TMV for my bath should've been set to 48C, just like my shower right next to it, but they set it to 43 which is incorrect.

 

What do you think? Is it actually incorrect, or at least 'unusual' to set a TMV to 43C for normal houses where there are no vulnerable people that could get scalded without being able to react?

 

-M

 

(*) while it's measured at lowest slot, I was sure to measure after temp stabilised a bit so I think it's likely fairly accurate. As you can see below I did my test at 14:30 so it's probably correct. 

(**) after 5 minutes of running it reached 39. 39 is barely enough to be a comfortable bath, but the bath itself is cold initially, so when you actually get in you're going to be a bit chilly fairly, not to mention that if the bath is (say) 36C, adding 39C water is barely moving the needle up again. 

 

s $

Edited March 20 by puntloos