Energy Calculators Needed.

[SoreTween]

The heavily insulated pipe can only be a win, so seems like a good idea regardless.

Kim raises a good point here though it may be a tad academic as you've bought the pump. Also I'm deliberately playing devils wot sit.
You are comparing an uncirculated, uninsulated system with a well insulated circulated system. Two changes. What would be the saving just insulating the pipe as you describe?
I really don't intend to criticise, our house would likely benefit from the same system so being a stats freak my interest is somewhat piqued.

[jsabine]

Are you sure abut the 16.5 waste? It seems an awful lot.

If *my* maths is right, that's only about 45 litres a day.

At a flow rate of 10l a secondminute  - which isn't great for mains, though no doubt AM doesn't have his taps on full every time - and 30s for the water to run hot, that's only opening the tap 9 times a day. Halve the time and the flow rate, and it's still only 35 or 40 tap openings.

The £200 energy cost is what seemed high to me: that would be just under 20% of our (heating+hot water+cooking) gas bill.

Granted, I'm comparing UK mains gas prices for two people in an urban Victorian semi to a bill with (almost) none of those factors in common, but I'd probably want a sanity check of waste not being more than, say, 25% of my hot water consumption, at least, not without some measurements to back up the maths.

[Aunt Maud]

The current pipe from the boiler has been "insulated" by the plumber who moved the boilers' monkey. It has not been done very well and the hot pipe runs against the heating loop for the rest of the run to the kitchen tap, so it's warm for about 3 meters. All the other pipe and pipes to the taps and shower are nearly impossible to get at to insulate.

The 16.5 cubic meters of waste water is based on running the kitchen tap till it gets warm and is at least 9.5 litres of water down the drain each time and that's what I've based the water waste on. There are 4 people, including two water hog teenage girls who like to run taps and take showers. They can only have 65 litres worth of shower at a time, as that's the capacity of the water storage tank, on purpose 

As I've said we live in Denmark, which is known for charging extortionate rates for gas, water and electricity and has chilly winters.

The house is virtually uninsulated (slowly changing to highly insulated) 16th century timber frame with a thatched roof and is 30m long and 7m wide with the condensing boiler in the highly insulated new bike shed/workshop extension........got to keep them warm.

Bathrooms at each end although the furthest on is a building site and we've never used it anyway, kitchen in the middle, two wood burners, half a decent central heating system and we currently don't use the upstairs, although that will change in a year.

I'm going to make a drain back solar system on the workshops green roof to pre heat the water flow to the DHW tank, summer months only.

[rr]

Legionella is an issue in highly insulated or circulating hot water systems. Water needs to be hot or cold, never lukewarm, you need eliminate dead legs and keep any spurs short. Good practice to run all the taps at least weekly as well.

[Tod28]

" At 10l per seconds!!!!" you wish - that's 600L per minute!  Standard showers run at 8-10l per minute with 'eco' showers at 6Lpm.  Flow rates above 2 m/sec in pipes begins to get noisy from cavitation.

With regards to energy saving, the circulating pipe is acting as cental heating and will continuosly loose heat to the building as it is maintained at the hot water temperature.  A non circulating system will only loose the heat from water remaining in the pipe when the flow stops.  1m of 15mm pipe contains about 0.12L so only 3.6L for the 30m run.  Running your hot water system at 55C would save far more energy over the year.

[Kim]

Running your hot water system at 55C would save far more energy over the year breed Legionella.

FTFY

[Aunt Maud]

Legionella is a very important thing to guard against, so I'm going to run it at the same temperature as now, I like the hot to be hot for the washing up. I've managed to get the kitchen spur down to 600mm, with a bit of child labour, and all the others should be similar.

The internal diameter of the pipe is 10mm, but with the sweeping bends and lack of extra fittings, I would imagine the friction to be fairly similar to a 15mm copper system with T's and soldered joints. I may have to run the pump on its second setting to gain adequate water pressure all over the system if it falls too much.

[jsabine]

" At 10l per seconds!!!!" you wish

I meant, of course, per minute ...

[Feanor]

Legionella is a very important thing to guard against, so I'm going to run it at the same temperature as now, I like the hot to be hot for the washing up. I've managed to get the kitchen spur down to 600mm, with a bit of child labour, and all the others should be similar.

The internal diameter of the pipe is 10mm, but with the sweeping bends and lack of extra fittings, I would imagine the friction to be fairly similar to a 15mm copper system with T's and soldered joints. I may have to run the pump on its second setting to gain adequate water pressure all over the system if it falls too much.

I don't think the secondary circulation pump should have much effect on the flowing pressure at the taps.
It passes a modest current; sufficient only to maintain hot water in a well-insulated pipe.

Increasing the pump rate will only slightly reduce the pressure at the taps.
Reducing it will only slightly increase the pressure at the taps.

It will act as an slightly open tap further downstream, so increasing the flow slightly over what the flowing tap would have produced anyway.
This will cause a marginally higher pressure drop in the pipework to the tap, depending on the resistance of the pipework.

I'd quite like to install secondary circulation here, but it would require too much ripping-up of stuffs to get a sensible DHW flow-loop.

[Aunt Maud]

Luckily I've doubled up as designer, demolition, carpentry and plastering contractor from the start on this, as I'd hate to do it as a single fitting job in someone else's house.

I'm insulating all the brick and timber external walls by constructing a stud wall inside with lath and plaster, so that there is a 300mm void which is then filled with the marvellous Leca® and the roof space could be considered empty, except all our stuff has been piled on the side I want to install the CH and DHW loops.

When it's all done we should have a dry and well insulated old house which uses as much energy as a modern one, but without all the plastic, formaldehyde and adhesives found in modern buildings. That's the goal anyway.

[ElyDave]

If you are turning up the pump, make sure you have enough positive head at the inlet.  If it's too low, that's where you can get cavitation, essentially bubbles forming and collapsing in the low pressure side of the pump.  This is very destructive to pump impellers and can get very expensive. Always something that was checked assiduously when I was a young chemical engineer.

As for cavitation in pipes, that's not the case.  As flow increases you'll obviously see a higher back pressure build up, and increased flow noise, but that's not cavitation.

[Aunt Maud]

Flow rate, resistance, head and such is where I get my knickers in a twist. ( Simple head of pressure I'm OK with, but when the loop drops down and rises again, hmmmm?)

If the pipe drops down as much as it rises from one side of the pump spindle CL to the other, in a closed loop, will there be an increase/decrease head of pressure or is it just friction that I need to overcome ?

[Kim]

I'll probably be defenestrated for using electricity as an metaphor to help understand plumbing, but I'd expect Kirchhoff's laws apply.  Ie. in a closed loop, the sum of the heads will be zero, and all you have to worry about is the friction.

[ElyDave]

If it drops down and then rises again, there's no pressure drop / increase due to "head", but you still need to account for losses due to turbulence, fittings and friction.   Remember that old experiment in physics of diferent shaped glassware, all with the liquids the same level.

Make an allowance for scaling as well.

Been there, done that, confused the hell out me at the time as a 23 year old graduate engineer.

[Aunt Maud]

Woooooo!!!!!

Hot water circulation pump just fitted and working at all taps. I bought a new Stainless steel Grundfoss pump off Ebay for £70 £120 and had a revelation this morning about the pipework, after thinking it over for a year and a half.

Pump running lovely and I reckon to save 3 litres of hot water every time we run either the kitchen hot tap, bathroom tap or shower.

All I need to do is figure out the pump as there were no instructions, but it does have some kind of auto adapt control thingumajig.

[Aunt Maud]

I'm trying to get an idea of the potential saving in wasted hot water and gas and convert it to Dkk.

The digital display on the pump shows the current power input as 9 watts, I can get it to run as low as 5 watts, but to me this means absolutely nothing.

I want to try and change it to kw/h and then I can work out how much electricity it uses a year.
How much gas it uses keeping the water in the loop hot by flow and temperature difference between the flow and return at the boiler
And how much cooled down hot water we throw away every time the hot tap runs compared to not having the pump.

At the moment, with the pump, it's 2 litres waste at the kitchen tap, which is furthest from the circulation loop.


Then I can work out exactly if I'm saving any money having it or not.

Currently my backoffagpacket calculations say I'm saving 18 cubic meters of hot water from going down the drain per year.

[Quisling]

Running your hot water system at 55C would save far more energy over the year breed Legionella.

FTFY

Pumped secondary return loops are standard practice on larger properties, and definitely for commercial properties.
The approved code of practice on Legionella requires a weekly sterilisation cycle, so as long as the system is controlled to raise the water temperature (normally overnight) once a week then you should be okay.  Obviously, if you're into legionella management you will also need to flush any outlets thoroughly for the prescribed duration at the prescribed temperature.  This is especially relevant for showers since legionella predominantly a risk via airborne droplets.

Circulating water at 55C is less risk than having water heating up and cooling down slowly in insulated pipes, where it will spend a good part of the day around 30-40C which is ideal temperature for legionella to thrive.

Unless you're in a commercial building with heavy commercial use like a hotel, I'd suggest a time controller on the pump to switch off overnight.  You'll lose the "instant" hot water at the outlets but if the pipes are well insulated it should take a while to cool off and you'll save perhaps 7 hours of pump power demand a day.

[Aunt Maud]

I'm sure the DHW output is higher than 55ºC, as I can barely keep my hand under the hot tap when it's going.

Pumped DHW loops are very common in Denmark, as water, electricity and gas are very expensive, unlike Norway where the national hobby is "Leaving the lights on".

[Aunt Maud]

A measuring jug and a little running water proves that there is a saving of 4 litres of water per tap/shower each time they are run to hot. We're currently running the hot water 10 times per day, so that's a saving of 14600 litres of waste hot water over the year.

I'm quite happy with that, even though we use some gas keeping the water warm and £8 of electricity/year running the new pump.

I'm going to run a little more copper pipe and see if I can get the current 2 litres of waste with the pump down to a half a litre at each tap.

Beats buying an energy saving lightbulb hands down.