(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

[IainH]

I was just about to post about our hot water problem, but then saw this post.

 

Brief update of where we are -

 

In 2018 we got change of use and then full planning for three barns, started our build in March 2019 with the intention of building a 2000 sq ft barn, selling our existing house, move into small barn then build the "grand design" 7700 sq ft with glass gable ends etc etc.

 

During the build we had further thoughts on the end product, ceased the build for nearly a year to get planning for linking two existing barns with a third new build barn - giving three sides of a rectangle layout, with courtyard in the middle. Despite being in year six of a two year plan, the end product is much better than the original and we are very happy with it ....but, we have a hot water issue.

 

Our Kensa GSHP cannot heat the house and hot water at the same time with DHW having priority. This means that when the water is being heated, the building is not. After tweaking the room stats and the water heating times three times a day to 50c, we have a working system BUT, we cannot get two showers out of a 305L tank. The bottom of the tank seems to be constantly cold plus, due to the distance between GSHP and kitchen etc, we have a secondary hot water circuit pumping from 0600 to 0200. This was running in mode 3 (4.3M3 / hour) which we have reduced to mode 1 (no idea of water volume, but made little change). So, 4300 litres of water an hour through a 305 litre tank means that the water is being circulated 50m - 60m fourteen times an hour. The circuit is very well insulated but, if each circuit drops 1 deg c then the water is cooled well below shower temperature, and if two showers are required we have to boost the water to ensure there is enough hot water for the second shower.  

 

Is there such a thing as an on demand pump that will only pump only when water is demanded or is there any alternative solution please? Boiler installers and the plumbers are both out of ideas. 

 

Thanks in advance.

 

PS. There are only three on and three off options on the secondary pump timer. 

 

 

Edited yesterday at 09:34 by IainH

[ProDave]

32 minutes ago, IainH said:

Is there such a thing as an on demand pump that will only pump only when water is demanded or is there any alternative solution please? Boiler installers and the plumbers are both out of ideas. 

Pumping water such a long way so many times a day when it will be used twice is wasteful.  I would turn off the water circulation pump.

 

I would look at a local solution like a small electric under sink water heater to provide hot water instantly for hand washing, and accept that when showering, the hot water will take a while to arrive from the main tank, so turn it on as you start to get ready for your shower. 

 

A custom solution like turn the circulating pump on for a timed period with a motion sensor might work, but you will still be circulating water many times whenever someone enters the bathroom, even if they don't use any hot water.

[JohnMo]

40 minutes ago, IainH said:

Kensa GSHP cannot heat the house and hot water at the same time with DHW having priority

That is the same for ALL heat pumps and boilers that run priority water.

 

41 minutes ago, IainH said:

water heating times three times a day to 50c

That seems a lot reduced to 2?

 

41 minutes ago, IainH said:

secondary hot water circuit pumping from 0600 to 0200.

You really need to reduce this time. And reduce pump speed.

 

Time it when you are likely to be in the kitchen only - and actually need hot water, definitely not all the time. We have our down to 2 hrs per day only. The other thing to add is a pipe thermostat, on the return close to the cylinder, as soon as it senses the return temp getting to say 35 (adjust to suit) it switches the secondary pump off. The pump then comes on as the temp drops about 4 degs.

 

If the bottom of the cylinder is always cold, check the position of the cylinder sensor - it should be near the bottom of the cylinder not in the middle.

[IainH]

Thanks for both replies. My responses are below -

 

1. By "on demand" pump I was thinking of a pump activated by impeller movement, not just by someone entering the bathroom. Turn on the hot water, impeller moves, pump kicks in?

 

2. On underfloor heating and DHW I understand NIBE GSHP's can dual function?

 

3. I will try DHW  twice a day only and

 

4. I have already reduced pump speed from max (3) to min (1).

 

The problem with timing is I that am a lark and regularly up before 5am whilst my wife is an owl who rarely gets in bed before 2am to awake at the crack on noon. Children were conceived by correspondence!

 

I will also check the cylinder thermostat - but I'm not 100% sure the tank is communicating to the Kensa. 

[JohnMo]

11 minutes ago, IainH said:

By "on demand" pump I was thinking of a pump activated by impeller movement, not just by someone entering the bathroom. Turn on the hot water, impeller moves, pump kicks in?

Someone has already opened the tap, not sure what the pump brings to the party at that point. You have a full 3 bar pushing hot water down the pipe.

 

13 minutes ago, IainH said:

On underfloor heating and DHW I understand NIBE GSHP's can dual function?

There is only one flow temp out of any heat pump or boiler. Two different flow temperatures is only done by mixing down the hottest flow to the make it colder. Again why would you bother.

 

If you are on UFH and your house is cooling down while the heat pump is DHW you have some issues with insulation that need addressing. Our house is around the same age as yours, our UFH can be for Manny many hours and house temp doesn't change. We also have a whole gable end fully glazed.

15 minutes ago, IainH said:

will also check the cylinder thermostat - but I'm not 100% sure the tank is communicating to the Kensa. 

Will obviously be communicating or it won't do hot water. Check what position the sensor is installed in. Needs to be lowest sensor pocket to enable whole cylinder to be heated 

 

[IainH]

Thanks again for replies.

 

The house is holding it's heat fine but when there was long periods of DHW heating during the winter when the timer was wrong, it would cool down.

 

Please see attached photo of our hot water tank. Thermostat pockets are in the middle and top of the tank - both unused. The electric immersion at the bottom of the tank was only wired very recently ("we forgot that bit"- even though it was MCS inspected) and, as far as I understand, is only an override should the boiler fail?

 

The hot water works at the moment only on the three times a day timer (soon to be twice a day) and seems only to be on or off - or is the GSHP getting a temperature reading via the wiring for the electric immersion heater?

 

We just has two showers and only achieved that with a DHW boost. Something ain't right so we will try with the secondary pump off & see what happens.

[Wil]

Can I come see it, I’m near Titchmarsh and being nosy about a 9000m2 Barn conversion? 🙂

 

Sounds like there’s lots to consider with GSHP and secondary circuit. Do you have an PV or anything you can divert into the newly wired immersion?

 

 

[JohnMo]

1 hour ago, IainH said:

We just has two showers and only achieved that with a DHW boost.

Only thing I can think of is the secondary return is just sucking the heat from the cylinder. 

 

1 hour ago, Wil said:

9000m2

715m² only (!). But still way more than I would like to keep. Pity the poor person that has to clean the place.

[IainH]

14 hours ago, Wil said:

Can I come see it, I’m near Titchmarsh and being nosy about a 9000m2 Barn conversion? 🙂

 

Sounds like there’s lots to consider with GSHP and secondary circuit. Do you have an PV or anything you can divert into the newly wired immersion?

 

 

It's not 9000 M2 (and sorry for any confusion). The 7700 Sq ft barn has planning (with some works commenced, so now in perpetuity) but is still undeveloped. For our build we joined two existing 2000 sq ft barns with a new 1600 sq ft link to give total 5600 sq ft so around 520 M2.

 

Also, as we are on the DRHI scheme, we are not allowed secondary energy producing equipment. I don't like the looks of PV but would have a wind turbine when the DRHI scheme is finished.

[IainH]

12 hours ago, JohnMo said:

Only thing I can think of is the secondary return is just sucking the heat from the cylinder. 

 

That, I think is the problem. Will try switching the pump off / playing with different pump timings.

 

Thanks.

[SteamyTea]

17 hours ago, IainH said:

"on demand" pump I was thinking of a pump activated by impeller movement, not just by someone entering the bathroom

My shower pump works similar.

Only works safely/reliably with gravity/vented systems though.

 

Not read the rest of this topic I  detail, but I assume you have an invented cylinder. If so, is your hot water flow adequate for your needs and is it possible there is a dynamic flow/pressure imbalance between the hot and cold e.g. PRV on the hot set lower than the cold.

[IainH]

2 hours ago, SteamyTea said:

My shower pump works similar.

Only works safely/reliably with gravity/vented systems though.

 

Not read the rest of this topic I  detail, but I assume you have an invented cylinder. If so, is your hot water flow adequate for your needs and is it possible there is a dynamic flow/pressure imbalance between the hot and cold e.g. PRV on the hot set lower than the cold.

 

Thanks for the reply.

 

There was a serious pressure imbalance between hot & cold as we had a second (unregulated) cold water feed direct from mains at 12 bar into the original build that took some working out as to why the hot water at 3 bar (set with PRV) was not getting anywhere! 

 

That was resolved, but the running out of hot water issue still remains with the secondary circuit being the prime suspect. 

 

Still working on it.