Can the flow temperature be too low?

[Nick Laslett]

On 19/12/2024 at 19:20, JohnnyB said:

I did discover last night that the DHW cycle was set so the immersion heater comes on if the cycle goes on longer than 20 minutes. As I have been running it so the water temperature drops by 10 deg before starting to heat I think it is taking longer than 20 minutes most of the time so that wouldn't have been helping the COP.

So many gotchas with ASHP configuration. At least this is something you can control.

 

Quote

There are 4 in our house and I had been getting complaints of cool showers buy the third shower if I have it switched off morning or evening.

This is the other topic that is a real challenge to solve. Lot of design thought needed if you want an efficient reasonable cost DHW supply. Traditional plumbing would just brute force this. I’m sure the oversizing of most HP is due to DHW reheat times. If you have showers happening in succession it is a good candidate for Waste Water Heat Recovery, but not always easy to incorporate into a build. 

 

Unfortunately I don’t have any answers for this, my simplified assumption is that my solar will contribute to the cost of DHW for 8 months of the year, and for the other 4 months, low cost tariff direct electricity. If you have the space, a bigger tank to take advantage of the cheaper rate electricity and just store more hot water.  Of course it would appear that things like solar, batteries and EVs are all need to get access to these better electricity rates. 

[JohnMo]

My 6kW ASHP (R32) states min 3,95 / nominal 6,08 / Max 6,99

 

As demonstrated above min output is coincidentally with max demand i.e. at lowest outside temp. I need house heating on, at about 10 to 12 degrees outside, which when looking at the performance chart ASHP is make about 6.7kW at 25 - 30 Deg flow temp. While at -10 its max output is closer to 5.7kW. So I am looking at nearer 40 to 50% turndown in reality. So my 6kW at minimal demand is making closer to 4 to 4.5kW at minimum demand temperature.

 

 

Edited Saturday at 11:16 by JohnMo

[Kelvin]

31 minutes ago, Nick Laslett said:

So many gotchas with ASHP configuration. At least this is something you can control.

 

This is the other topic that is a real challenge to solve. Lot of design thought needed if you want an efficient reasonable cost DHW supply. Traditional plumbing would just brute force this. I’m sure the oversizing of most HP is due to DHW reheat times. If you have showers happening in succession it is a good candidate for Waste Water Heat Recovery, but not always easy to incorporate into a build. 

 

Unfortunately I don’t have any answers for this, my simplified assumption is that my solar will contribute to the cost of DHW for 8 months of the year, and for the other 4 months, low cost tariff direct electricity. If you have the space, a bigger tank to take advantage of the cheaper rate electricity and just store more hot water.  Of course it would appear that things like solar, batteries and EVs are all need to get access to these better electricity rates. 

Rather than heating more water we’ve been trying to reduce how much we use. Our showers can report on length of shower and amount of water used etc.  My other half was averaging 10 min showers whereas I am less than half that. Since becoming aware of this she’s reduced her time in the shower by 3 minutes. Over a week it’s a huge amount of water. 

[JohnMo]

46 minutes ago, Kelvin said:

My other half

 

46 minutes ago, Kelvin said:

Since becoming aware of this she’s reduced her time

No chance of that here. I get life is for living, 'stop going on!'

[Kelvin]

28 minutes ago, JohnMo said:

 

No chance of that here. I get life is for living, 'stop going on!'

What’s prompted her awareness is more about her attitude that water is a precious commodity rather than necessarily heating it as we’re on a borehole. What’s prompted that change in attitude is a neighbour down the hill from us has lost her water supply due to Iron Oxidising bacteria problem with her water source. She was about to sell her house which she obviously can’t until this is resolved and even then it might be difficult. 
 

But, yes I get the ‘stop going on’ reply too. 😂 

Edited Saturday at 12:57 by Kelvin

[SteamyTea]

3 hours ago, Kelvin said:

water is a precious commodity

 

From my weekly comic back in 2022.

 

A recent report by the European Commission's Joint Research Center showed that almost half of the European Union's (EU) territory and Britain are at risk of drought in July. Drought conditions and water scarcity are affecting energy production and reducing crop yield.

 

TEMPORARY bans on using hosepipes to fill up paddling pools might seem like a parochial issue. But such modest efforts as most of England nears drought are a reminder that even countries perceived as perpetually rainy will need to get serious about saving water in a warming world.

 

Drought is part of many countries’ natural weather cycles, including the UK. But dry conditions are also expected to become more frequent and intense as Earth moves beyond the 1.2°C of climate change we have seen to date.

 

The worst is yet to come, which is bad news given how nations are faring today. Almost half of the European Union is in drought-warning conditions. France is experiencing its severest drought in history, with forecasts of corn harvests being down by almost a fifth and nuclear power output curbed by warm, low rivers. Half of the US is in drought too.

 

This scarcity of water adds to the pressure on the existing global food and energy crises, both linked in part to Russia’s invasion of Ukraine. Drought can also lead to more climate change by stressing and ultimately killing forests that act as vital carbon stores.

 

So what to do? Start taking this vital resource seriously, rather than for granted. That will require behaviour changes as well as technological fixes.

 

Mandatory water efficiency labels to help people pick better products like toilets and washing machines would be a start: the UK is readying these now. Household water meters are a must. UK water firms’ goal of halving pipe leaks by 2050 isn’t good enough. We need other big water infrastructure projects too, and we need to start now because they take time. A huge reservoir being built in the south of England won’t be ready until 2028. Better planning is needed: it is inexcusable that the UK’s only major desalination plant is currently offline.

 

As Christine Colvin at The Rivers Trust in the UK puts it: “This dry period needs to bite hard now, so we are ready for the future.” Inaction will result in worse than tears over empty pools.

[sharpener]

On 21/12/2024 at 15:57, SteamyTea said:

So what to do? Start taking this vital resource seriously, rather than for granted. That will require behaviour changes as well as technological fixes.

I agree. Rainwater harvesting has a part to play.

 

But it is easy to think it is the complete solution, however the reality is you need 100 sq m of roof even in the wet SW (>1000mm/yr) to meet the needs of two ppl (assuming 146 l/day pp. And not many houses have 50 sq m of roof pp so the scope is quite limited.

[SteamyTea]

5 minutes ago, sharpener said:

Rainwater harvesting has a part to play.

In some ways it does, but it is really highlighting a failure in our old Victorian sewage systems. We flush potable water down the bogs. Where I work we have automatic flushers on the urinals. This time of year we are only open 6 hours a day. So 18 hours of flushing nothing but clean water away.

8 minutes ago, sharpener said:

And not many houses have 50 sq m of roof pp so the scope is quite limited.

A borehole gets around that problem as the catchment area can me many thousands of m² per house (housing is about 3% if the UK's land area, all urbanisation is about 12%).

The real problem is waste, not portable, that is what we have to redesign (especially down here in Cornwall, have I mentioned that we have the most expensive water and waste in the country, which will make it one of the most expensive in the world).

[sharpener]

4 hours ago, SteamyTea said:

A borehole gets around that problem as the catchment area can me many thousands of m² per house

 

Yes. Ironically when they were replacing the 5000 litre rainwater tank with an underground one it kept floating out of position and it turned out we had an aquifer running through the site.

 

Wouldn't have thought to install it from scratch but previous ppl had done it (rather badly). Can't resist a challenge so have probably spent as much over the 17 yrs on re-engineering it as have saved in water/sewerage charges. And still haven't totally cracked getting the pH up above 6.5.

 

Grey water recycling offers other savings e.g. for flushing loos but the treatment you need to stop it going rank in the storage vessels is supposedly almost as complicated as making rw potable.