Posted by: @jamespa

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Posted by: @ngillam

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@jamespa I tried to discuss with kids, I got a couple of eye rolls....and one tutted!

 

Exactly.

It strikes me that grey water recovery might be a simpler and more robust way to 'save' the water.  There is still some heat lost though.  A smart tap that turns the pump on but then only turns the tap on when the water comes through hot is the ultimate solution.  Are we overthinking this problem?

A shower directly next to the HW tank......or a shower head off the tank 😀 

Posted by: @ngillam

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A shower directly next to the HW tank......or a shower head off the tank  

Which is exactly how houses used to be designed (and I suspect mass designed houses still are).  Put all the plumbing as close together as possible.

I have a recirculation pump that we had fitted when the house was renovated 5 years ago; at the same time, we had our ASHP installed which provides DHW via our Daikin 'thermal store' (posh water tank). The thermal store is at one end of our single-story 300 sq m house; the master bathroom and our son's bathroom are at the other end. We were scared off using the recirculation pump by our installers who said 'it will use a lot of energy' but that was, we think, a ruse to get us to drop the requirement as the builders were scrapping with the plumbers about payment at the time (as subcontractors).

Anyhow, we re-visited the recirculation pump a few months back found it worked a charm. Prior to using the pump, our DHW was set to 55C and was always just warm enough in the morning, to the extent that the shower handle was on max; we have Grohe (Cosmopolitan 310) rain shower heads and we would wait a couple of mins (or more) for the hot water to arrive - at 9.5l/min we were wasting 80 or more litres of water a day waiting for hot water (2 showers a day in summer for three people - obvs if someone showers immediately afterwards somebody else they are not waiting around, so it was 4 'shower events').

With the pump now running the hot water is instant, and we were able to reduce to the DHW temp to 50C and it still feels hotter at the shower than when set to 52 without the pump. So, we are saving electricity on heating the water, and up to 80l of water a day. I checked the daily electricity consumption which I can see on an hourly basis and the morning (4am) DHW spike is less if anything - previously the spike was around 2.8kwh (incl background, non-DHW) and now its around 2ish kwh.

My setup is that the recirculation pump (a Dab Evosta2) is controlled by a mechanical DIN rail timer which I have set to run from 6-10am, and in the evening from 6-8pm. If I get round to it I may replace this with a digital daily timer so I can accommodate the shift in shower times at the weekends.

@marzipan71 Morning, it looks as though it's a very much a "must have" in spaced out residences, do you witness the same temp drop from DHW tank on initial start up of pump as the pipework is primed?  I can drop as much as 5 degrees typically, I may heat the tank to 50degs and set the circulation pump to 24hrs to see how fast the heat from the tank dissipates without a reheat over say a 12hr period.

I think my family want me to stop tinkering.......

Thanks,

Neil.

Morning @ngillam hmmmm don't think I've measured that tbh. I should probably do a test with a cup and thermometer at the shower and see what the temp diff is between what the ASHP sees in my thermal store (DHW tank) and the shower water temp  both with and without the pump running. I'm actually going to drop the DHW storage temp again (to 48, which will mean I've dropped 7C from where we were without the pump) as I'm a few notches below running the shower at max temp (my shower valve does not have a safety temp limiter - it really doesn't) when showering and my understanding is that the optimal scenario is to not have to blend cold water at the shower. Also looking at a Shelly DIN rail timer which would allow me to ask Alexa to run the recirculating pump on demand - like when someone randomly decides on a bath. It would be interesting to know how long it takes from starting the pump to get to temp in the master bath.

FWIW I had an (apparently) excellent conversation with ChatGPT about this subject (and, indeed, about setting up and fine tuning my ASHP for this winter cycle.

Hi,@marzipan71 I plan to use my melcloud output to show a day of circulation pump switched off and then a day of it on 24hrs and watch how quickly the temp trace drops throughout the day.  I just need the family members to obey which rarely happens.

Hi @ngillam I don't have anything as sophisticated for data with my Daikan alas - I've a digital controller that looks like my car stereo display ca. 2002 and that's about it. While you're consulting melcloud, I'll be in the bathroom with a cup and a thermometer. Bless my installers.

@ngillam @marzipan71 Did either of you test out the best way to run your pump to be as efficient as possible?  Our heating system is being fitted at the moment and the installers have fitted a loop for the hot water and are about to fit the pump. They said it was required by MCS if the furthest tap was a certain distance away.

They have suggested I fit a timer but after reading your experiences ngilliam I'm wondering if it is something else that will be wasting energy.  We don't have a very long run to the main bathroom but our ensuite is the furthest away from the DHW tank and that will be used the most.

@johnnyb Hi - not done any testing but we still have the recirc pump on timer for those few hours in the morning, and I run it manually when the boy needs an evening shower. There is no meaningful impact on electrical consumption from a DHW perspective.

We currently run a DHW cycle to 48C at 5am and 12pm. I'm actually going to trial running the 12pm cycle to a (calculated) 58C which allowing for evening use should leave the water in the tank at 48C at 6am which is actually a little hot for showering. This avoids the morning DHW cycle which currently spikes at 6kwh, while taking advantage of lunchtime solar.

An upgrade would be to switch out the mechanical timer on the DIN rail for an Alexa-connected wifi timer but I haven't pulled the trigger on that yet.

The portion of our grid consumption that relates to DHW is minimal compared to the heat pump's space heating appetite for power in winter, which has been up to 60kwh a day this year, so I dont stress too much about it really.

Good luck!

@marzipan71 @johnnyb Morning, My issue was watching my DHW dissipate from the tank if I left the pump running 24hrs (or on-demand at the tap or shower).  Basically my 48 degs of nice hot water would lose 10-15degs over the day even with nobody home.  To get around this I do the same as @marzipan71 and set the Heatmiser timer for a couple of hours in the morning for our showers and then manually switch on for the evening due to sporadic requirements depending on day.  I haven't found another way, if the timer isn't selected to run, the pump doesn't work, so no hot water throughout the house.

My COP is about 3.7 so not too bad.

Hope your install goes well, this site is fantastic for information.

Thanks,

Neil.

I'd agree with the more rapid 'loss' of hot water with the recirc pump running compared with not, but don't have any data to back this up - just a feeling that there's less hot water in the tank, or it runs slightly less warm, after the recirc pump has been running. For this reason, I have it on between 6 and 10am to cover morning showers (inc the weekend) - if I switch out the relay timer thingy for a wifi version I'll be able to set day specific runs to cover the Sunday morning lie-in. Internet tells me having the recirc pump running 24x7 is a bad idea. Our hot water use is massively skewed to the morning showers and my son's evening shower in any case.

For info, my tank is a 500L Daikin thermal store which has slightly different behaviour to a normal hot water tank - it stratifies the water - and allegedly loses 1-2C over 24hrs. The tank's thermistor is at the bottom of the tank - I understand normal tanks have them mid-way - so it shows a temp of, say, 30C prior to a DHW cycle. It will then build to our target temp of 48C over maybe 45 minutes. However, after an hour or so the thermistor will tell me that the 'tank temp' is back to 30. I thought I was going nuts but my AI assistant tells me that this is expected behaviour, as in my Daikin tank the hot water rises to the top and is then available for showers etc - the stratification effect. I thought I was losing heat somehow or the space heating was pulling heat, but testing the water temp at the tap showed it was exactly 48C, where the thermal store thermistor told me it was 30C due to the stratification and the placement of the sensor at the bottom of the tank. I've had my kit since 2021 and only worked this out this winter.

TMI for the current discussion but I mention only because its difficult for me to assess whether the recirc pump is pulling significantly more hot water or not. But - comfort wise, I love the recirc pump as previously 48C water would give us tepid showers, and now that same 48C water is almost too hot and we have to use the mixer at the shower to add cold. If we turn on the recirc pump, the water at the shower goes from tepid to hot almost instantaneously - and the shower is at the opposite end of the (300sqm single storey) house from the tank. We're also saving electricity on not heating the water to low or mid-50's of course. However - as above, I'm going to trial this single mid-50's DHW cycle at midday (using solar) and see if I can have the tank at 48C in the morning for the 6-7am showers.

My recirc pump is a Dab Evosta 2, running at speed 1 (of 3). I wouldn't go back to not having it, but being able to program daily schedules and also just ask Alexa to turn on the pump would be a big improvement.

@marzipan71 @ngillam 

Forgive me if I'm stating the obvious or have misunderstood, but undesirable cooling of the DHW is surely an inevitable effect of a system with a recirculating pump and thus to be expected.  You are constantly circulating the DHW around a house that is at a colder temperature than the water, and so heat will be lost from the water to the house.  Its the price you pay for the 'instant on' facility and the only way to avoid it is to switch the recirculating pump off.

In winter of course the energy lost contributes to the house heating, albeit at a rather lower COP than the space heating circuit.  In summer it contributes to unwanted overheating.

In days of yore houses were designed so that all of the outlets were close together thus reducing plumbing runs and in particular runs for hot water.  There was a good reason for this design choice!