Hot water system design - have I got it wrong?

[knobblycats]

Have a bit of downtime from the build due to an injury so I thought I'd try to design the hot water system. I'm planning to go with a manifold system of different pipe sizes suited to the end-use for each pipe run.

 

• I've chosen a direct unvented cylinder, probably around 250/300 litres. I did consider a thermal store and a Sunamp, would one of these be better?
• The laundry room, en-suite, and family bathroom are all within 4m of the cylinder. The shower room and ground shower room are up to 15m away but would only have occasional use, so I'm proposing running 15mm out to a small manifold within the room to serve the shower/sink. The idea is that running the shower then 'primes' the sink hot water, but not sure if that would really work? Run time for hot water to wash hands would be high without shower use.
• To maximise the amount of hot water, I'm proposing running the tank at about 85 degrees C and then blending the output down to a safer level before the manifold.
• Both the dishwasher and washing machine will take hot water, I'm hoping that by heating all the water on cheap night rates (and eventually PV), the running costs of washing/dishwashing can be reduced.

 

Any other flaws with my design? 

• 

[Mr Punter]

Will you have another manifold for the cold water?

 

Bear in mind that lots of washing machines and dishwashers are cold fill only, so don't restrict future options by not piping cold as well.

 

You could use an electric instant hot water for the distant sink for hand wash, but I wouldn't bother.  Another thing to go wrong.

 

How much does the Power Pipe cost and how much will it save per year?

[JohnMo]

10 minutes ago, knobblycats said:

maximise the amount of hot water, I'm proposing running the tank at about 85 degrees C

Why bother and have a bucket load of wasted heat with heat loss, will be ok in winter but overheating house in summer.

 

12 minutes ago, knobblycats said:

The idea is that running the shower then 'primes' the sink hot water, but not sure if that would really work? Run time for hot water to wash hands would be high without shower use

That makes no sense, so get in morning to wash and brush teeth and run the shower for no reason?

 

13 minutes ago, knobblycats said:

washing machine will take hot water

Are you sure about that. Even if they do the slightest of cold they receive first is what will be heated by the machine.

 

My general comments are

 

UVC at 300L and store at around 50 degrees, heat twice if you need to. Always install a heat pump cylinder with a 3m² plus coil. Fast recovery main reason. Future proofing if you are not installing a heat pump now.

 

Don't bother with thermal store unless oil heating. Thermal battery don't bother, cost to purchase daft.

 

Cold water manifold that also, good for easy isolation. All tap water is taken from the combined valve on the UVC no where else.

 

I would install a secondary return loop on the furthest room, the have a timer thermostat pump, or link a simple smart plug to Alexa and tell it to start pump when you want.

 

I would simplify the manifold to one port and pie run to each wet room and branch in the wet room. Do the long runs in 15mm.

 

What the heck is a power pipe?

 

 

 

[knobblycats]

35 minutes ago, Mr Punter said:

Will you have another manifold for the cold water?

Yes, omitted from this design for simplicity and definitely will be running cold to washing machine/dishwasher.

 

36 minutes ago, Mr Punter said:

How much does the Power Pipe cost and how much will it save per year?

 

Good question, costs between £450 to £600 and between £80/100 claimed savings per year. A few other forum users seem happy with their power pipes.

[Redbeard]

1 hour ago, JohnMo said:

What the heck is a power pipe?

Power-Pipe UK | Waste Water Heat Recovery Systems

A Power-Pipe^® recovers the heat energy lost down the drain during showering and uses it to pre-heat incoming mains freshwater. Saving £ + energy.

 

No, I didn't know either!

[knobblycats]

46 minutes ago, JohnMo said:

Why bother and have a bucket load of wasted heat with heat loss, will be ok in winter but overheating house in summer.

I'm anticipating 5-6 adults using the showers every morning. With that usage, most of the hot water will be at a much lower temperature by morning! Like you say, we can always run it at a lower temp if it turns out to be too much.

 

50 minutes ago, JohnMo said:

That makes no sense, so get in morning to wash and brush teeth and run the shower for no reason?

If you wash or shave using the basin (and wait for the sink hot water to come through), then the hot water is now in the shared pipe ready for you to use the shower if you choose to use it (and vice versa). If we run two separate supplies (one for basin, one for shower) from the central manifold, then running one wont 'prime' the other and if you wanted to use both, you'd have to wait twice.

 

It makes sense to me, I just don't know if it's better to do it that way.

 

1 hour ago, JohnMo said:

Are you sure about that. Even if they do the slightest of cold they receive first is what will be heated by the machine.

Yes, https://ebac.com/washing-machines/e-care-10kg-hot-fill

 

1 hour ago, JohnMo said:

All tap water is taken from the combined valve on the UVC no where else

What does that mean?

 

1 hour ago, JohnMo said:

I would install a secondary return loop on the furthest room, the have a timer thermostat pump, or link a simple smart plug to Alexa and tell it to start pump when you want.

Would that work if we're mixing the hot water down to a lower temp with a thermostatic valve? It won't circulate unless we're also adding cold water, and without any taps open, there's nowhere for that mixed water to go?

 

1 hour ago, JohnMo said:

I would simplify the manifold to one port and pie run to each wet room and branch in the wet room. Do the long runs in 15mm.

Why? 10mm pipes use 0.034 litres per meter vs 15mm at 0.103 litres per meter. 

 

If we use 10mm single supply to a basin 10m away from the manifold, you'd have 0.34 litres in the pipe. If we use 15mm, we'd have 1.03 litres in the pipe. 

 

Theoretically, if you use the figures as above for a basin mixer at 4l/m, it'd take 15 seconds to heat up for a 15mm pipe. With a 10mm pipe it'd take 5 seconds to heat up. So a smaller pipe saves 2/3 the time, water and heating energy.

1 hour ago, JohnMo said:

What the heck is a power pipe?

Waste water heat recovery - https://powerpipehr.co.uk/

[ProDave]

2 hours ago, knobblycats said:

I've chosen a direct unvented cylinder, probably around 250/300 litres. I did consider a thermal store and a Sunamp, would one of these be better?

What is heating the house?  If you have a boiler or ASHP then an indirect cylinder would make a lot more sense.

[knobblycats]

12 minutes ago, ProDave said:

What is heating the house?  If you have a boiler or ASHP then an indirect cylinder would make a lot more sense.

We're using mini splits for heating/cooling. The heat load for the whole house is 2kw at -5 degrees C outside so it doesn't make sense to drop £££ on either a boiler or ASHP.

Edited December 31, 2024 by knobblycats

[ProDave]

11 minutes ago, knobblycats said:

We're using mini splits for heating/cooling. The heat load for the whole house is 2kw at -5 degrees C outside so it doesn't make sense to drop £££ on either a boiler or ASHP.

My heat loss at -10 is only a little over 2kW and my 5kW ASHP is working well to heat the house and hot water.  I bought one cheap and self installed, I doubt I could have bought the the A2W kit for what i spent.

[JohnMo]

1 hour ago, knobblycats said:

What does that mean?

Your UVC has a valve that reduces mains water to 3 bar as well as doing other things. It also provides a balanced cold water flow outlet, so mixing taps function correctly. If you take water before this and use on a mixing tap, you may not get any hot water flow, depending on pressure before the cylinder.

[knobblycats]

11 minutes ago, JohnMo said:

Your UVC has a valve that reduces mains water to 3 bar as well as doing other things. It also provides a balanced cold water flow outlet, so mixing taps function correctly. If you take water before this and use on a mixing tap, you may not get any hot water flow, depending on pressure before the cylinder.

That's good to know, thank you.

[JohnMo]

1 hour ago, knobblycats said:

Would that work if we're mixing the hot water down to a lower temp with a thermostatic valve

Can do if you design correctly. But more to the point, store so you don't need to blend down. Install a bigger cylinder? But just install a heat pump, you will be paying a fortune just for DHW.

 

1 hour ago, knobblycats said:

you wash or shave using the basin (and wait for the sink hot water to come through), then the hot water is now in the shared pipe ready for you to use the shower if you choose to use it (and vice versa). If we run two separate supplies (one for basin, one for shower) from the central manifold, then running one wont 'prime' the other and if you wanted to use both, you'd have to wait twice.

 

It makes sense to me, I just don't know if it's better to do it that way

Secondary return heats the manifold also. You take the secondary loop to the furthest wet room, I then flows back to cylinder via manifold. Manifold is hot all user get hot water quicker, even you hot water appliances. Without it the hot water appliances will just be getting Luke warm water at best.

 

 

[Iceverge]

4 hours ago, knobblycats said:

I've chosen a direct unvented cylinder, probably around 250/300 litres. I did consider a thermal store and a Sunamp, would one of these be better

 

Yes to UVC. 300 litres is too small for that many people if you plan to heat exclusively on a TOU tariff. Get 2*300l and be done with it. 300l is the most bang for your buck the last time I checked. 

 

We manage to store enough for 2 adults and 3 kids with 300l and 70deg storage on a TOU tariff  30mins showering and a large bath and a bit of washing up I about what we get from it. 

 

 

4 hours ago, knobblycats said:

The laundry room, en-suite, and family bathroom are all within 4m of the cylinder. The shower room and ground shower room are up to 15m away but would only have occasional use, so I'm proposing running 15mm out to a small manifold within the room to serve the shower/sink. The idea is that running the shower then 'primes' the sink hot water, but not sure if that would really work? Run time for hot water to wash hands would be high without shower use.

 

Run everything in 10mm Hep2O except the kitchen/utility tap and baths/showers. 

 

The delivery time will be small even at 15m run length. 

 

Don't bother with anything fancy like return loops or external manifolds or remotely heated sinks. Just flick the tap for a few seconds. It'll be way cheaper on the long run. 

 

4 hours ago, knobblycats said:

Both the dishwasher and washing machine will take hot water, I'm hoping that by heating all the water on cheap night rates

 

Don't bother. The dishwasher will be already almost full of cold water from the pipe by the time it gets any warm water from the tank. 

 

 

 

[Iceverge]

3 hours ago, JohnMo said:

Why bother and have a bucket load of wasted heat with heat loss, will be ok in winter but overheating house in summer.

 

When you have a cylinder with heaps of fittings and coils and extras this is an issue. Jeremy had a problem with this. 

 

Ours is a simple direct 300l UVC living in a 2m2 cupboard and its never much above the house +2deg at a guess in there. Even in the height of summer.

 

3 hours ago, JohnMo said:

Always install a heat pump cylinder with a 3m² plus 

 

I would argue against this by default. The price different of the cylinder Vs a direct one could pay for a solar diverter and a few panels if you're committed to immersion only heating. 

 

 

[Iceverge]

 

 

That's ours. 

 

More pipe clips would make it prettier. 

 

Note the thermostatic mixing valve screwed straight into the cylinder and then the manifold directly above it. The. The first pipe away is the kitchen tap (10mm for us). The water in the manifold is preheated by convection so theres zero dead leg before the manifold. 

 

I've since removed the second stopcock and lowered the inlet group as it too was warming by convection. 

 

 

The only changes I think I might make to this if starting from scratch would be to lower the inlet control group more and move the tundish to the space between the tank cold feed and the cold manifold for tidyness. 

 

[knobblycats]

 

16 minutes ago, Iceverge said:

 

 

That's ours. 

 

More pipe clips would make it prettier. 

 

Note the thermostatic mixing valve screwed straight into the cylinder and then the manifold directly above it. The. The first pipe away is the kitchen tap (10mm for us). The water in the manifold is preheated by convection so theres zero dead leg before the manifold. 

 

I've since removed the second stopcock and lowered the inlet group as it too was warming by convection. 

 

 

The only changes I think I might make to this if starting from scratch would be to lower the inlet control group more and move the tundish to the space between the tank cold feed and the cold manifold for tidyness. 

 

 

That's fantastic! Thanks for sharing - are you happy with your system?

[knobblycats]

This is all really interesting, and at the risk of opening a can of worms, I wanted to share my thinking in going immersion only vs an ASHP for DHW. These are approximate numbers and it'd be good to understand what other people think and if there's a flaw in my logic?

 

A 250/300l UVC is around £800.

 

An ASHP with heat pump UVC would be approx £3,000 (can you get it cheaper?)

 

Running the immersion on a night tariff would cost approx £1.20 per day. The ASHP on a night tariff would be £0.40 per day.

 

The ASHP breaks even with the immersion after 8 years assuming no additional costs.

 

If we use lower hot water temps with the ASHP, we'd also have to run it during the day on day tariff to top the hot water up. That's hard to work out, but let's assume we do 100% night 50% day it could cost £2.24. At that rate, due to the high cost of the install, it'll never pay back vs an immersion on night tariff only.

 

Figures above use £0.08 night rate and £0.24 day rate. Assumes 3kw heating for 1kw electricity for the ASHP

Edited December 31, 2024 by knobblycats

[JohnMo]

7 minutes ago, knobblycats said:

An ASHP with heat pump UVC would be approx £3,000 (can you get it cheaper?)

You can do this, but I would not.  I would do a normal UVC and external ASHP like @ProDave mentioned. I paid £800 (incl VAT) for my 210L HP cylinder. Plus £1300 for a 6kW heat pump. Does all the house, summer house and DHW. Cylinder came with diverter valve, one direction to cylinder other straight into UFH manifold.

 

You are going to be getting through way more water than you seem to calculate.

 

10 minutes ago, knobblycats said:

Running the immersion on a night tariff would cost approx £1.20 per day. The ASHP on a night tariff would be £0.40 per day.

Very much doubt it would be that cheap for 6 adults even at £0.08 per kWh

 

12 minutes ago, knobblycats said:

Assumes 3kw heating

We use around 6kWh for 2 of us, so you could be around 18kWh with 6 adults

[knobblycats]

2 minutes ago, JohnMo said:

We use around 6kWh for 2 of us, so you could be around 18kWh with 6 adults

 Apologies, perhaps I didn't phrase this bit correctly- "Assumes 3kw heating for 1kw electricity for the ASHP"

 

To rephrase, for the ASHP I'm assuming 3kw of heating out for 1kw of electricity in, not total heating load.

 

If we take the £1.20 immersion cost per day and divide that by 0.08, it comes out as 15kw of heating, not far off what you're suggesting.

[marshian]

18 minutes ago, knobblycats said:

Running the immersion on a night tariff would cost approx £1.20 per day.

 

Figures above use £0.08 night rate and £0.24 day rate.

 

300 L to 85 Deg C from say 20 Deg C only uses 15kW of electric?

 

If the tank is being fully consumed daily and it sounds like it will be with that level of occupancy then to get 300 Litre of water from 20 Deg C to 85 Deg C is going to need a heat input of 22 kW

 

So by my rough math your calcs are understating you usage by 45%

 

With a 3kW immersion it's going to take 7 and a bit hours which is quite a chunk of time to recover

[knobblycats]

2 minutes ago, marshian said:

 

300 L to 85 Deg C from say 20 Deg C only uses 15kW of electric?

 

 

I'm assuming the DHW doesn't drop below 35 degrees - my thinking is that by the time the water is at that temperature it's already too cold for most usage so we stop using it.

 

Is that right, do you think? Or do UVC work differently than I imagine?

[marshian]

3 minutes ago, knobblycats said:

 

I'm assuming the DHW doesn't drop below 35 degrees - my thinking is that by the time the water is at that temperature it's already too cold for most usage so we stop using it.

 

Is that right, do you think? Or do UVC work differently than I imagine?

 

 

Yes they do work differently - it's called stratification

 

The hot water stays at the top of the tank and loses a small amount of temp due to tank convection losses - it's replaced by incoming water at anywhere between 5 deg and 20 deg depending on the season - there is minimal mixing between the two

 

You heat a well insulated UVC to 85 deg C you are probably going to get 80 deg water out of it after 14 hrs until you hit the boundary layer then you are going to get 5-20 Deg water - like a switch!!!

[marshian]

Quick bit of maths

 

300 L tank heated to 85 Deg C

 

You only use 250 Litres so 50 litres remain at 80 deg after 14 Hrs (Assumption is minimal losses)

 

That 250 Litres has been replaced by 250 litres at 10 Deg temp (It's winter average delivery temp)

 

So you've got 250 litres at 10 deg and 50 Litres at 80 deg

 

If you mixed them you get 300 Litres at 21.6 deg C

 

The mixing occurs when you turn an immersion on that's in the side or the bottom of the tank

[knobblycats]

6 minutes ago, marshian said:

 

 

Yes they do work differently - it's called stratification

 

The hot water stays at the top of the tank and loses a small amount of temp due to tank convection losses - it's replaced by incoming water at anywhere between 5 deg and 20 deg depending on the season - there is minimal mixing between the two

 

You heat a well insulated UVC to 85 deg C you are probably going to get 80 deg water out of it after 14 hrs until you hit the boundary layer then you are going to get 5-20 Deg water - like a switch!!!

 

That makes sense, thanks!

 

You've really helped me to get to the flaws of my thinking and I've adjusted my spreadsheet to account.

 

Looks like the payback time for an ASHP would be around 5 to 8 years depending on how it was used. Makes more sense now!

[knobblycats]

46 minutes ago, JohnMo said:

You can do this, but I would not.  I would do a normal UVC and external ASHP like @ProDave mentioned. I paid £800 (incl VAT) for my 210L HP cylinder. Plus £1300 for a 6kW heat pump.

Can you still get that heat pump for £1300?