Posted by: @lakey

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Our ASHP install has just completed, however we seem to be consistently running out of DHW. I know it takes some adjustment but I was hoping for advice on managing that transition. We have a 200L cylinder for a 2 adult and 1 toddler household which I wouldn't have thought we would use it all when filled at 55 degrees. 

What's the best way to measure flow rate from the DHW cylinder? A water meter or just a couple of additional thermocouples closer to the top of the cylinder to give me a nudge to reheat the cylinder? 

You could do a sensor near the top (it would need to be inside the insulation so can you get at it and how would you use it to signal you) but there is probably a solution that doesn't involve any additional hardware or manual intervention

How is it currently set up (time schedule, target temperature (55?), location of sensor).  What is your typical hot water usage pattern?  Is the water coming out of the shower faster than previously - you can measure that with a measuring jug - and if previously you had a combi its very likely quite a lot faster.

I wouldn't have thought it would be a problem either unless one of the adults has very long showers (just saying!) or you do not actually heat the whole cylinder or you have a lot of heat loss from it.

It might be as simple eg as doing reheat (only if necessary) twice per day or giving it a bit more time to reach temperature.  Do you know it gets to 55?

@jamespa Thanks for the reply. 

I'm going to time the showers etc today, I know there have been once or two longer than usual showers but didn't expect them to empty the tank. Perhaps that combination with a rainfall shower head does actually use a significant amount of water! I'll try and check the output from the shower, I hadn't noticed it was faster than the old combi. 

I believe the system is measuring the hot water temp through the cylinder coil return temp when heating. I can run a check cycle on the cylinder but as some water is consumed that result only lets me know how warm the base of the tank is. Cylinder is a 200L Elite HP cylinder, so ~3m^2 surface area. 

Currently I've set the system to heat to 55 deg C at 5 am. Usage pattern varies but we usually have 2 showers and one toddler bath a day with minimal washing up.

If the showers will really use that much water I don't think I have much of an option other than getting a flow meter fitted to the hot out of the cylinder to measure total volume used getting a notification/refill when we've used a fixed amount. 

Posted by: @lakey

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@jamespa Thanks for the reply. 

I'm going to time the showers etc today, I know there have been once or two longer than usual showers but didn't expect them to empty the tank. Perhaps that combination with a rainfall shower head does actually use a significant amount of water! I'll try and check the output from the shower, I hadn't noticed it was faster than the old combi. 

I believe the system is measuring the hot water temp through the cylinder coil return temp when heating. I can run a check cycle on the cylinder but as some water is consumed that result only lets me know how warm the base of the tank is. Cylinder is a 200L Elite HP cylinder, so ~3m^2 surface area. 

Currently I've set the system to heat to 55 deg C at 5 am. Usage pattern varies but we usually have 2 showers and one toddler bath a day with minimal washing up.

If the showers will really use that much water I don't think I have much of an option other than getting a flow meter fitted to the hot out of the cylinder to measure total volume used getting a notification/refill when we've used a fixed amount. 

 

 

Rainfall showers can be 20l/min and stored water at mains pressure/3bar generally flows faster (possibly a lot faster) than anything coming out of a combi.  200l at 55 will yield about 300l at 40C, about 15mins total at 20l/min.

I have never heard of fitting a flow meter to trigger reheat and, unless there is a time gap between the showers, it wont reheat fast enough anyway, so I cant see that this is a good option.  You already have a temp sensor probably half way down, so you could change the heat pump to 'DHW priority'and it will then trigger reheat as soon as the temperature at that sensor drops below some defined temp (probably 10C below the set temp, but this is usually adjustable), or alternatively schedule a reheat between the showering periods if they are at reasonably fixed times.  If that doesn't do the job its because your showers are too close together in time to reheat between, in which case you either shower for less time, fit an aerating shower head or space the showers out more.

One way or another this should be soluble using the existing controls using a combination of time periods and temperature triggers, all of which your heat pump controller should have, unless you have a fundamental problem that your showers are so long and fast and so close together that reheat part way is not possible.

You may also want to think about the cost of all that hot water you are now using! 

@jamespa Thanks, looks like it's just going to take an adjustment. I'm happy waiting for a refill for a shower, even planning around cheap TOU price drops, we never shower back to back so that's not an issue. Our shower time usually varies due to shift work/exercise etc so again, easy if I can get a trigger but not to set a schedule around. I'm reluctant to do it day by day. 

Given I should be able to get two 4 or 5 minute showers from the tank even with a rainfall shower head, I shouldn't need the extra cycles daily

I don't have a thermocouple in the tank, I have had the Adia system installed which measures the return temperature from the heating coil. I could ask for  thermocouple added in to an existing port but given that will only indicate once the system is half cold (still possibly enough for a shower but possibly not!). Hence asking about a smart flow meter to measure total daily usage from the tank. That would also allow me to consider storing the tank at 43 ish if I'm going to need to refill regularly then why waste energy heating higher? Plus no legionella concerns if I have a high enough daily turnover. 

Posted by: @lakey

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@jamespa Thanks, looks like it's just going to take an adjustment. I'm happy waiting for a refill for a shower, even planning around cheap TOU price drops, we never shower back to back so that's not an issue. Our shower time usually varies due to shift work/exercise etc so again, easy if I can get a trigger but not to set a schedule around. I'm reluctant to do it day by day. 

Given I should be able to get two 4 or 5 minute showers from the tank even with a rainfall shower head, I shouldn't need the extra cycles daily

 

I don't have a thermocouple in the tank, I have had the Adia system installed which measures the return temperature from the heating coil. I could ask for  thermocouple added in to an existing port but given that will only indicate once the system is half cold (still possibly enough for a shower but possibly not!). Hence asking about a smart flow meter to measure total daily usage from the tank. That would also allow me to consider storing the tank at 43 ish if I'm going to need to refill regularly then why waste energy heating higher? Plus no legionella concerns if I have a high enough daily turnover. 

I think you first sentence is spot on, stored DHW is just different to on tap DHW from (presumably) a combi.  Not worse, not better, just different.

Half a tank should be enough for a shower, if you have a thermocouple (eg attached to a multimeter) you could always push it into the pocket and 'do the experiment'.  Push a bit of insulation behind though so that the thermocouple is at water temp.

I dont know of any system designed to use a flow meter but of course it could be rigged up if you felt so inclined.  I believe that there are showers that can be programmed automatically to switch off after a certain time, that or wind up timer might be a simpler solution!

Alternatively perhaps Adia 'sniff' the water temp every so often by temporarily switching the diverter valve.  Unless they do this I cant see how they can ever operate DHW priority mode (but there again - perhaps they dont offer that).  Maybe a question for Adia!

@lakey Hi, I had a quick read through and wanted to see if I can help a little.

Most new high gain ASHP cylinders loose very little heat so its worth heating the cylinder 24/7 not on timer, this way the water you want is available, I would only advise changing this is you were on a lengthy holiday and if you do it would be advisable to force a legionella purge before retuning to normal operation. 

Most ASHP control probes stuffed in the side of the cylinders are not done correctly and I have seen this many times make a diff of 8 Deg, so you think the water is 50ºC but it is really 42ºC which will still feel hot, you can purchase a decent food probe on amazon for £8 and just hold that under the water on a fully heated tank and check. If you find it is out by more than 2ºC it can often be adjusted in the controls as an offset and if not, pull the probe out, take some insulation foam and place it back in making the foam squash it against the pocket wall. If as you say above you dont have a probe in the tank just do the thermometer test on the near and far hot water outlets to get an accurate hot water temperature, in this case if it is low offset the set point to compensate.

Sometimes existing shower mixers and other mixers don't work as well with the lower temp, I have had this twice, replacing these fixed the issue and what seems to have happened is the water was on 60 for a very long time when fed from the boiler, the spring in the TRV cartridge is slightly warn but not noticed on a higher water temp, water temp is now 50 and the spring is not releasing as much as it should. (this is less often the case)