Most efficient timed DHW?

[Andehh]

We have a 300L tank and a 12kw Samsung gen6 ASHP. Family of 5, 3 young kids. Large insulated hot water circulation pump timed for hour morning, hour lunchtime, 2 hours evening.

 

Joules set it to 50deg and a 24/7 on cycle, with it reheating if it drops by 10 degrees.  I changed this so it only heats up hot water before we need it. Joules said this would be less efficient - but why do I want 50 degrees sat that all night??

 

So current timing is:

 

• 30mins in the morning to heat up enough for breakfast washing up (gets to high 30s)
• 2 hours mid afternoon to maximise solar PV (gets to 50degs), and ready for 4.30pm dinner time for kids, our dinner time, bath time for kids and means we have approx 40deg water for showers. 
• The luke warm tank cools down over night. (lows 30s by morning)
• Disinfect cycle once a week or so - manually engaged when we need more hot water or sunny day!

 

This has been Ok over the summer, but now that it is getting colder, wife wants hotter showers. I was going to add an extra 30min temp boost 6pm ish to get it up to high 40s for showers.

 

However... I am now thinking I might be better off putting the hot water on from 8am to 7pm (ish) all day keeping the tank at 45degrees and being done with the faffing. Reheats when it drops below 35degs.

 

But I am keen to see what everyone does does/any thoughts?

 

Thanks

 

 

Edited September 27, 2023 by Andehh

[sharpener]

Well my plan for when the 12kW Vaillant gets installed is to charge my 210 l tank off E7 to 50C from 0500 to 0600, and again in the afternoons to capture any free surplus PV. This will allow 0600 to 0700 for the HP to recharge the thermal store and pre-heat the UFH.

 

This mirrors our current usage on an oil boiler. We usually have 2x showers when we get up, but sometimes more if we have guests, and sometimes in the evenings as well or instead if we have been gardening.

 

So 210 l is enough storage for us allowing for dilution to a comfortable temperature and I am a bit surprised by the amount you say you want though I suppose 3 kids baths might use 240l.

 

36 minutes ago, Andehh said:

now that it is getting colder, wife wants hotter showers

 

What, hotter than 50C (see above)?

 

 

36 minutes ago, Andehh said:

Joules said this would be less efficient - but why do I want 50 degrees sat that all night??

 

A modern 300l tank has losses under 2kWh/day so it will not make a great deal of difference if it is left hot overnight or not. But I see no particular point in re-heating it earlier than you need to and I think the advice that it is less efficient is plain wrong.  OTOH there is every chance even with a complicated schedule you will not achieve perfection, having it on just during the (entire) day may be the answer as you say.

 

 

Edited September 27, 2023 by sharpener

[TonyT]

To the OP what do you pay for electricity?

Do you  have a cheap night time tariff?

[Conor]

I have ours set to heat in off-peak only, 5c offset, 45c target. We shower in the mornings so no real issue if the temp drops off in the evening. At the weekends this often isn't enough (my parents stay with us most weekends) so I have it set to come in anytime before 6pm. With a heatpump you want it coming on as few times as possible, but for as long as needed.  

[SteamyTea]

53 minutes ago, Conor said:

With a heatpump you want it coming on as few times as possible, but for as long as needed.  

Want it coming on when the DHW is as cold as you can get away with. The heat transfer is better, the greater the temperature difference.

1 hour ago, sharpener said:

A modern 300l tank has losses under 2kWh/day

I would hope a lot less than that, if not, add more insulation.

[sharpener]

29 minutes ago, SteamyTea said:

I would hope a lot less than that, if not, add more insulation.

 

Well, not a lot. By chance I was looking at Megaflo earlier today, their current 300l is quoted as 1.77 kWh. But is has got better, my OSO from 1996 takes about 2kWh after the battery is full and another 0.5 or so two hours later when some mixing/diffusion has taken place and the temp has fallen back by the thermostat hysteresis.

[JohnMo]

36 minutes ago, sharpener said:

Well, not a lot. By chance I was looking at Megaflo earlier today, their current 300l is quoted as 1.77 kWh

You need check the temp the testing is done at, it is always the same temperature, introspective of storage temp. It will be done to a defined standard.

[SteamyTea]

Just now, JohnMo said:

It will be done to a defined standard

I seem to remember that the standard has an odd methodology. Heat and use the hot water right away, then measure the losses in the residuals.

So say the cylinder ends up at 30⁰C, the airing cupboard at 25⁰C, that is a delta of 5 at the start, may end up, after 21 hours at 27⁰C and 21⁰C, so delta 6.

Looked at it a decade ago when I added a load of extra insulation around my E7 cylinder.

[RobLe]

Our 240l tank runs for 3 hours in the ‘go’ period overnight, it usually just gets to 50C, then stops heating.  It’s enough for us, never runs out (2 adults, 2 teenagers, nobody takes super long showers).  It’s a 2.5kW gshp.

We have PV, but our export is close enough to the ‘go’ import price - so I figure it’s both more reliable to run on go power than PV (what if it’s overcast?) and it’s greener anyway to export elec when it’s needed more.

[sharpener]

1 hour ago, SteamyTea said:

I seem to remember that the standard has an odd methodology. Heat and use the hot water right away, then measure the losses in the residuals.

So say the cylinder ends up at 30⁰C, the airing cupboard at 25⁰C, that is a delta of 5 at the start, may end up, after 21 hours at 27⁰C and 21⁰C, so delta 6.

Looked at it a decade ago when I added a load of extra insulation around my E7 cylinder.

 

Yes, I was really only commenting on the order of magnitude involved vs usage. I imagine the standard methodology allows you to compare one make with another but not necessarily to have much relation to the real world, like EV range.

 

My observations are probably not directly comparable, and are for when the house is unoccupied. They do not seem to vary noticreably from summer to winter which is odd.

[ReedRichards]

My feeling is that in the real world DHW cylinders lose a lot of heat by conduction down the connected pipes and this isn't taken into account in tests.

[JohnMo]

2 hours ago, ReedRichards said:

My feeling is that in the real world DHW cylinders lose a lot of heat by conduction down the connected pipes

Would agree with that, I was looking for some info the other day and found some info on a USA site, and it reminded me you don't see many heat traps these days on cylinder installations.

 

[sharpener]

How is it supposed to work? Doesn't prevent conduction. Might prevent parasitic convection currents.

 

Also we would not allow a shut-off valve between tank and expansion vessel!

[JohnMo]

2 hours ago, sharpener said:

How is it supposed to work? Doesn't prevent conduction. Might prevent parasitic convection currents.

 

Also we would not allow a shut-off valve between tank and expansion vessel!

Basically hot water rises, due to density, it can only rise to the high point, the deliberate vertical fall arrests the heat rise. 

 

I tried it out on a temporary cylinder I have installed, its the cold water feed in to a DHW coil. At top of the cylinder the pipe exiting the cylinder is 46 degrees, while bottom of the insulation/vertical section to horizonal section is 23 degs.

 

[JohnMo]

2 hours ago, sharpener said:

we would not allow a shut-off valve between tank and expansion vessel!

Wasn't talking about the correctness of the other aspects of the sketch. Just the heat trap.

 

A video

https://www.youtube.com/watch?v=W5LWjhj098c

[sharpener]

28 minutes ago, JohnMo said:

At top of the cylinder the pipe exiting the cylinder is 46 degrees, while bottom of the insulation/vertical section to horizonal section is 23 degs.

 

Fortunately my cylinder has the outlet pipe running downwards from it, after about 3/4 m its surface temp under the lagging is approx 30C. Airing cupboard is only faintly warm and all within thermal envelope anyway. I agree a heat trap might be esp useful with a vented cyl where the vent pipe typically runs straight upwards but I have never seen it done in any house I have had.

 

BTW I note you use what look like Tectite fittings, what is yr experience of them? (I have only ever used the one but it is at mains pressure and has been fine.) However to judge from their pix of completed HP installations the professionals seem now to use crimped fittings instead and I have wondered why that is, it might be price but then the crimping tools themselves cost £00s.

[JohnMo]

1 minute ago, sharpener said:

Tectite fittings,

Two versions are available Sprint and classic. The sprint ones are single use. Only had one issue, where I tried to fit in a tight space, and got it miss aligned, had to cut the fitting off. Other than that, no issues.  Have some classic fittings on the new cylinder, they come apart again, which can be useful. No leaks so far. I like them as the are a nice radius curve, instead of a tight bend.

 

Cost would be an issue if you used loads of tectite, I.e. full time plumber and availablity can be iffy.

 

My new cylinder, starting to be plumped up, incorporating a few Tectite fittings, but transitions to Hep2O after the copper.

[sharpener]

29 minutes ago, JohnMo said:

I like them as the are a nice radius curve, instead of a tight bend.

 

Thanks. Didn't know about the Classic kind. Thinking of fitting my own upgrade rads for HP, Tectite Sprint look nice for that and as you say not constrictive unlike some cheap solder ring fittings e.g. Screwfix unbranded. Anyway am getting too old for lots of hot work (7 bigger rads will need min 28 elbows).

 

Haven't tried the Hep2O but am a fan of Polyplumb, my rainwater system has quite a few as (unlike JG Speedfit) you can be confident they are done up but are easy to dismantle without tools e.g. to service the pump and controller.

 

Nice looking job, I read somewhere that Ideal cylinders are in fact made by one of the major cyl mfrs but are nevertheless cheaper to buy from Ideal.

[JohnMo]

1 hour ago, sharpener said:

Ideal cylinders are in fact made by one of the major cyl mfrs

Made by Gledhill, only differences between a Gledhill and Ideal Heat Pump cylinder is Ideal supply a 3 port diverter, and Gledhill a 2 port valve, the temperature thermostat is different. I changed mine to a digital dual thermostat for £15.  Most heat pump installs will have a thermistor tied direct into the heat pump.

[sharpener]

1 hour ago, JohnMo said:

Most heat pump installs will have a thermistor tied direct into the heat pump.

 

Yes I have been wondering about this if I keep my existing cylinder, as I don't think there is a spare pocket for it. Maybe using the existing stat as the call for heat will be enough, IIRC the Vaillant VR71 interface box has terminals for that.

 

Otherwise since I will have a secondary circ pump I am hoping it will be OK if I strap it to the h.w. pipe where it leaves the tank - so long as I can arrange it that the pump keeps running for the entire programmed hw period and not just when the cyl is calling for heat. Else it might cycle once the pipework cools down, but maybe with enough insulation and big enough hysteresis it wouldn't be a problem anyway.

 

Have now got a date for a site visit from Vaillant's area tech man so hopefully this is the kind of wrinkle he can resolve. I think he has been persuaded by @JamesPa's thermal model to accept the existing cyl (or he would not be coming), main worry atm is what he will think of the 15mm flow to the coil. Though I can if necessary run it in parallel with the feed to the TS as the times will be the same and then the load on the HP will be split between cyl and TS.

 

Sorry to the OP for thread drift but it is all related!