most efficient HW setup ?

[Dave Jones]

Looking to maximise efficiency for our hot water setup for our in progress build. 

 

Will have 3 x 300L tanks, one of them has the heat exchanger for heat pump. It's a lot of tanks but that's how much is needed to cover peak useage + 25% margin. 

 

9kw solar on roof.

 

Time of day tariff.

 

The ASHP flow temp target is 25-30c (UFH temp) so that alone wont be enough to heat the tanks. Looking at a solar pv divert to dump excess solar into the main tank when its available and top up overnight on cheap rate.

 

Assume we need some sort of pump to mix all three tanks controlled by a stat ?

 

Whats the best scop heat pump out there at the moment ? Plumber is advising samsung and daikin.

 

 

 

 

 

 

 

[crispy_wafer]

13 minutes ago, Dave Jones said:

Whats the best scop heat pump out there at the moment ? Plumber is advising samsung and daikin.

 

 

 

 

 

 

 

Vaillant Aerotherm models worth a look?

[JohnMo]

16 minutes ago, Dave Jones said:

The ASHP flow temp target is 25-30c

That's just for heating, when the heat pump gets a signal for DHW heating, it changes its settings, normally to something like 55 degrees and circulation pump ramp up to max speed.

 

A modern inverter heat pump are all pretty close to each other efficiency wise. Download the technical handbooks from the internet, takes a bit of searching, but most are there. Compare output/CoP tables.

 

So in winter you will heating 900l of water during the night, as you may get little or no solar. Sounds expensive 

 

How have you sized your storage of hot water, at what temperature did you design it?  Why 3x cylinders and not one, less piping, smaller surface area for heat losses etc. Less complications. You can also get much bigger surface area coils for more effective heating.

[Nickfromwales]

You wouldn't mix them, but it is very dependant on your consumption patterns / peak DHW demand at any one given time.

Excess solar from 9kW will not leave much to go into hot water, as I am assuming this is a very big / multiple occupant dwelling (?) to need that much DHW?

I'd look at a pair of 400L cylinders, with the second for DHW and the first for DHW pre-heat. Putting pre-heated water into the 400L DHW tank will probably double its useful capacity.

Can we have some more information about house size / occupants / number of bathrooms etc?

[Dave Jones]

I need to provide for 4 showers happening at the same time, the washing machine and dishwasher on at same time.

 

The showers are decent 20L/min so wont take long to empty a small tank. Coming from limitless hot water I'll need to keep adding tanks until it doesn't run out from our usage pattern.

 

The mixergy tank looks good for a 'primary' tank but there doesn't seem to be a lot of reviews on it.

 

My heatloss calcs show minimal heating requirement of 6.6kw for an outside temp of -15C, want to make sure i size the ASHP for hot water production as efficiently as possible.

 

Not sure if it makes sense to use the excess solar to drive an immersion as if it were to drive the ASHP then each kw of solar would be 3 or 4 kw of heat right ?

 

Finding a plumber locally who knows enough to design this setup is seemingly impossible, ive emails into vaillant and samsung asking their recommendations.

 

 

[Temp]

Can your incoming mains supply 80 to 100L/min?

[Dave Jones]

2 minutes ago, Temp said:

Can your incoming mains supply 80 to 100L/min?

 

we are fitting a 32mm main to replace the old lead pipe. 1L/sec but accumulator tank may be needed your right.

Edited March 8, 2023 by Dave Jones

[Temp]

We built our house in 2005-7 and back then I found it impossible to get sane answers from heating/plumbing engineers. One company proposed two boilers at opposite ends of the house and a £60k bill for it all. 

 

Our builder said he knew an old boy that was an expert witness in court cases involving heating and plumbing. Turned out he didn't understand how thermal stores worked. I ended up driving him 150 miles to visit a company that designs and supplies them. We learned a lot but I still had to do my own trouble shooting after it was installed. Ended up redesigning part of the controls to avoid having a pump rumming 24/7. 

 

It's just an oil boiler, 300L thermal store and two UFH manifolds. Not exactly rocket science.

[IanR]

56 minutes ago, Dave Jones said:

I need to provide for 4 showers happening at the same time, the washing machine and dishwasher on at same time.

 

The showers are decent 20L/min so wont take long to empty a small tank. Coming from limitless hot water I'll need to keep adding tanks until it doesn't run out from our usage pattern.

 

Is this for a domestic install? +80l/min of 40° water must have taken some setup if that's what you are used to. What boiler did you previously have?

 

How frequently would x4 showers be running simultaneously?

 

1 hour ago, Dave Jones said:

Finding a plumber locally who knows enough to design this setup is seemingly impossible, ive emails into vaillant and samsung asking their recommendations.

 

It's an unusual requirement.

[JohnMo]

52 minutes ago, Dave Jones said:

Not sure if it makes sense to use the excess solar to drive an immersion as if it were to drive the ASHP then each kw of solar would be 3 or 4 kw of heat right ?

That is only true until the cylinder is about 50 degC.  The PV diverter can then drive the heat to max the cylinder can take say around 70. Vastly increasing usable hot water.

 

56 minutes ago, Dave Jones said:

I need to provide for 4 showers happening at the same time, the washing machine and dishwasher on at same time

Why, just copy your last design if that gave limitless hot water - find it hard to believe the whole house will have a shower that the same time, unless its a hotel.  Most dishwaters and washing machines only take cold water these days, so ensure yours takes hot also. Isn't the 20l a mix of hot and cold water, not just hot.

 

Again what is your design temp for the cylinders this will have a huge impact on the design capacity required.

 

1 hour ago, Dave Jones said:

mixergy tank looks good for a 'primary' tank

Why do you say that - it makes a big thing about being able to part charge - you need 3x300 or 900l?

[Mr Punter]

53 minutes ago, Dave Jones said:

the washing machine and dishwasher on at same time

 

These are normally cold fill only, so will not impact on your HW but may do on your mains flow requirements.  Your shower flow rates don't meet building regs.

[Dave Jones]

45 minutes ago, JohnMo said:

That is only true until the cylinder is about 50 degC.  The PV diverter can then drive the heat to max the cylinder can take say around 70. Vastly increasing usable hot water.

 

Why, just copy your last design if that gave limitless hot water - find it hard to believe the whole house will have a shower that the same time, unless its a hotel.  Most dishwaters and washing machines only take cold water these days, so ensure yours takes hot also. Isn't the 20l a mix of hot and cold water, not just hot.

 

Again what is your design temp for the cylinders this will have a huge impact on the design capacity required.

 

Why do you say that - it makes a big thing about being able to part charge - you need 3x300 or 900l?

 

previous house was 2 x gas system boilers, worked perfectly. Switching to electric only to save the planet etc

[JohnMo]

Think reducing water consumption will have a bigger impact on the planet than going all electric.

[Dave Jones]

1 hour ago, Mr Punter said:

 

These are normally cold fill only, so will not impact on your HW but may do on your mains flow requirements.  Your shower flow rates don't meet building regs.

 

agree ref shower its a preference as there is no shortage of water in the UK. 

[IanR]

29 minutes ago, Dave Jones said:

 

previous house was 2 x gas system boilers, worked perfectly. Switching to electric only to save the planet etc

 

A 50kW gas combi could perhaps get to 18l/min of 50°C water, so blended down to 38°C could get you to perhaps 25l/min. With just 2 you'd still be +30l/min below your target.

 

I'd suggest it's not reasonably practical to meet your unusual requirement with an ASHP, so go with Gas.

Edited March 8, 2023 by IanR

[Dave Jones]

30 minutes ago, IanR said:

 

A 50kW gas combi could perhaps get to 18l/min of 50°C water, so blended down to 38°C could get you to perhaps 25l/min. With just 2 you'd still be +30l/min below your target.

 

I'd suggest it's not reasonably practical to meet your unusual requirement with an ASHP, so go with Gas.

 

id like not to admit defeat quite yet and say ASHP aren't capable of the job.

[JohnMo]

54 minutes ago, IanR said:

50kW gas combi could perhaps get to 18l/min of 50°C water, so blended down to 38°C could get you to perhaps 25l/min

Just to stray of topic, I have a 32kW combi and can do 3 showers at once, the secret is a pre heat cylinder upstream of the boiler reheating the cold water going into the combi, this lowers the delta T required across the combi. Giving huge flow advantage and never ending water.

 

Best efficiency may be this.

 

Circa 500l heated to about 48/50 degs by ASHP. The flow out of this routed to a solar diverter valve, so anything above 45 degs goes direct to tap.  On a long draw of hot water, the cylinder temp drops, to below 45 and water diverted to combi for additional heating.  Most of the time cylinder provides DHW, sometimes cylinder and Combi.

 

Combi boiler that can take preheated water, Alpha, Atag plus others

 

Variable could use the CH combi circuit to lift cylinder to 70 from 50, at shower time.

 

Central heating by ASHP.

 

Use the strengths of both systems. Endless hot water, standing charge for could be lower than the losses keeping 3 cylinders hot.

Edited March 8, 2023 by JohnMo
Spelling

[Dave Jones]

my back of a fag packet design is for the ashp to provide low temp heat (45) during off peak hours until the tanks are all at 45, then immersion them upto 65/70 again off peak so that the ashp can return to providing heating. This way tanks will be ready for the morning showers. 

 

Daytime solar into the ashp to get the tanks back to 45, any exces solar into the tanks as well and when they done to the tesla powerwall.

 

During the winter when not much PV, id use the tesla to provide power during expensive times.

 

What size of heat pump should I be looking at, bigger is better ?

 

 

Workable ?

 

 

[JohnMo]

So immersion will be doing around 25 to 30kWh per day at a CoP of 1. Also assuming cylinder will have to be charged from 40 to 45 twice a day at an ASHP CoP of 3, another 10kWh, 5 at peak rate after showers and 5 at off peak, prior to showers and immersion heating.

 

So 35kWh off peak at 15p, is £5.25 per day and say 50p kWh, as your on a time of use tariff, peak another £2.50, so £7.75 per day, or £2800 per year.

 

So 900l of water 70 degC water diluted 50/50 with cold water, will give 1800l of water. At 20 l/min X 4 will give 4 showers for 20 mins. Why would you send 20 mins in a morning shower?

 

If we all did that, there would be hose pipe bad in the winter as well. 

 

Hope your on a water meter.

 

Heat pump size will also be affected by heating demand, and size of buffer required, bigger the heat pump compared to heating demand the bigger the buffer likely.

[Nickfromwales]

4 hours ago, Dave Jones said:

Not sure if it makes sense to use the excess solar to drive an immersion as if it were to drive the ASHP then each kw of solar would be 3 or 4 kw of heat right ?

The ASHP won't have a CoP of 3 or 4 when driving DHW at 65^oc, more like 2 if you're lucky, and that's if it's not winter and the air is freezing / damp and the HP freezes over from charging this HUGE volume of hot water.

This needs a serious re-think in honesty! Winter PV will do next to zilch too, unless you're installing north of 12kWp on 3ph. Do you have gas available? I think you'd be much better off with gas for this scenario. 

[Nickfromwales]

1 hour ago, Dave Jones said:

Daytime solar into the ashp to get the tanks back to 45, any excess solar into the tanks as well and when they done to the tesla powerwall.

During the winter when not much PV, id use the tesla to provide power during expensive times.

Sounds like a huge expenditure on equipment with limited service life.

 

1 hour ago, Dave Jones said:

What size of heat pump should I be looking at, bigger is better ?

Probably a high-temp split ASHP would be better CoP-wise?

 

How much PV / battery do you have? Or have you not bought and installed anything yet?

[Dave Jones]

already have the Powerwall, will be taking it from our existing place. will be fitting approx 9kw of solar as its relatively cheap and quick payback.

 

yes 3 phase (all new builds in western powers area are required to be 3 phase now).

 

must be a calculator somewhere that tells how long to heat X volume of water to X temp in kwh .....

 

 

[Nickfromwales]

2 hours ago, JohnMo said:

the secret is a pre heat cylinder upstream of the boiler reheating the cold water going into the combi, this lowers the delta T required across the combi. Giving huge flow advantage and never ending water.

Diminishing hot water? When the combi registers DHW flow the pre-heat circuit shuts down and the volume of heated water starts to diminish rapidly.  

[Nickfromwales]

5 minutes ago, Dave Jones said:

must be a calculator somewhere that tells how long to heat X volume of water to X temp in kwh .....

If you say @SteamyTea three times, he will appear.  

[Dave Jones]

2 minutes ago, Nickfromwales said:

the secret is a pre heat cylinder upstream of the boiler reheating the cold water going into the combi, this lowers the delta T required across the combi. Giving huge flow advantage and never ending water. 

 

all the showers waste water will be run through a waste water heat recovery column which feeds the tanks. All helps right