How much space required for this plant room?

[doitall]

The incoming gas service will be more than capable of supplying what you need, the meter may or may not need to be changed.

The other questions are best answered when you have figures, but expect the top end of your guestimate for a good one.

The break tank can be sized better when you have the demand and recovery rate figures.

Don't forget the kitchen cold should be raw water connected direct from the mains.

You may also wish to factor in a water softener if you have hard water, that would slow the recovery rate.

As we are saying you have all the ideas, what you need now is to thrash them out around a table with a couple of installers to get the best plan for you.

 

[indus]

Well I had somebody else over to have a look and give me a quote. Seemed to know what he was talking about and said he had done much bigger houses than mine with all manner of systems ie break tanks, double boilers etc

He said he wanted to come back in the week and measure the pressure/flow first thing in the morning so that the numbers reflected what was happening during 'peak hours' as it were.

All very encouraging I thought, very scientific/logical.

Anyway he does the measurements. The flow rate is actually 20l/min and the pressure approx 3.5bar static. He doesn't measure the dynamic pressure!

Says there are lots of options (as we all know) but on balance suggests

1) One 300l unvented cylinder, or one 400l max. In terms of two cylinders he says that my gas supply will only handle my current 30kW boiler and then an additional 24kW one. On this basis he says I won't have the boiler capacity to match the coil ratings of two cylinders, so no point. (Also standing losses would be greater)

2) A 450 litre accumulator. He says this actually contains 200 litres of water and will recharge in about 10 minutes (obviously if no other demand is put on the mains)

Keeping an open mind I've examined what he has said

1) In terms of hot water from the cylinder; I've revised the shower times down to 15 mins rather than 20 as tbh 20 mins is longer than most of us spend in the shower.

So the demand from the cylinder when two showers running is 2 x 12 x 15 x 0.6 = 216litres. So the 300 litre should easily cope with this. This should allow for a sink to be run as well or even another 15 minute shower straight after the first two.

2) In terms of cold water requirements. When the two showers are running this will be 2 x 12 x 0.4 =10l/min and I've got 20l/minute so no problem there? The accumulator is there as a back up.

The big question in my mind is whether there is enough pressure to make the whole thing work as planned. Any comments on this and any other flaws you may see in the logic gladly welcomed.

Thanks again

 

[doitall]

Yes sadly quite a few flaws.

300ltrs would be pushing the diversity to the limit, if I was spending this kind of money I would at least expect to run 4 shower at a time when needed.

Gas supply will only handle blah, blah is rubbish I'm sorry to say, you need a U16 or metric meter and you can have what is required.

When talking about Accumulators the Dynamic pressure is the most important, as it dictates how quickly the vessel will deplete.

One way to stretch the hot water is to have it hotter than normal lets say 70c instead of 60c, the problem now arises you need more cold water.

I wouldn't consider an accumulator unless you install the type with the pump, and even then I would look at the break tank and pump set as a first option.

 

[indus]

Thanks for your reply dia.

In terms of diversity, is it really worth me speccing a system for four showers working at the same time when this is hardly ever going to happen?

As mentioned the system needs to handle two showers and a sink or two with ease and without compromise to flow or water temp

Thanks

 

[indus]

Double post

 

[doitall]

Thanks for your reply dia.

In terms of diversity, is it really worth me speccing a system for four showers working at the same time when this is hardly ever going to happen?

As mentioned the system needs to handle two showers and a sink or two with ease and without compromise to flow or water temp

Thanks

Only you know the occupancy and likely use, one problem could arise when you sell, and you say your spec is no better than 2 showers and a kitchen sink. In my opinion there's no point in having baths if you can't use them, without consulting a timetable first.

If you chuck enough heat into a 300ltr cylinder, it would probably run indefinitely, however what you have to consider is the smaller the cylinder the greater the Δ t = will be as you draw hot water off, so the longer the recovery.

 

[Agile]

In my view two 300 li cylinders coupled with a single smaller boiler like 28 kW will provide longer showers and two baths at little cost.

This is based on the reheating being done after the water usage but taking up to two hours. Unlikely to be any problem as people dont usually take serial showers/baths!

Thats only about £950 more than a single 300 li cylinder which would be installed by most people to cater for two simultaneous baths/showers.

Of course the water supply also needs to be capable of supplying your chosen system.

I dont take much notice of the 4 Bar and 17 li/min which flow rate seems to be open pipe and unlikely with such a good static pressure and a new supply pipe.

Tony

 

[SimonH2]

2) In terms of cold water requirements. When the two showers are running this will be 2 x 12 x 0.4 =10l/min and I've got 20l/minute so no problem there?

You're out by a factor of 0.4 there. That would be the cold water consumption of the showers, but for what you'll be drawing from the mains, you need to include the hot water - because as you draw off hot water from the unvented cylinder, it is being replaced immediately with cold water from the mains.

So in fact your cold requirement is 24l/min for 2x12l/min showers.

The accumulator is there as a back up.

Well yes and no. Think of it more as a buffer - when there is good mains pressure and low flow, it will charge up; when there is high flow and the pressure drops, it will supplement the flow.
As you've already realised, as you draw of a higher flow rate from your mains, the pressure available drops - the higher the flow, the lower the pressure. That's a basic rule of pushing <stuff> though pipes, and there's also the issue of what other properties in the area are using.
What will happen is that if the mains pressure drops to the same as the pressure in the accumulator, the flow will split. Water will flow from the accumulator and the system will balance so that the flow from the mains results in the mains pressure matching the accumulator - assuming there is no pump. As you draw off water, the gas in the accumulator will expand (reducing the pressure) and the flow rate from the mains will increase so the pressures balance. If you reduce drawoff rate, the system will rebalance again - and if the mains can now supply the full drawoff rate while maintaining a pressure above what's in the accumulator, then the accumulator will start to recharge. With no drawoff, the accumulator will eventually charge up to the peak mains pressure*.

If the system has a pump then the dynamics change completely. If you have a break tank and a pump then the supply pressure is whatever the pump can supply. You are then using the accumulator to even out the pressure - and avoid running the pump all the time for even low drawoff rates.

* I assume that some systems will need a pressure reducing valve if the mains in the area can reach high pressures.

 

[doitall]

Don't need very high pressures SimonH2. You can quite easily get 30-40ltrs/min at 3bar if the pipes to and from the pump are large enough, 42mm feed and 35mm supply, splitting 28mm to the hot and 28mm to the cold.

 

[indus]

Thanks to all for their advice. Not sure if I'm much further forward to be honest. I think I'm getting myself confused.

The plumber who came around is saying one 300l, two boilers and an accumulator.

Dia says this may well not suffice and tbh I don't have the knowledge to figure out who is right.

A sectional break tank (of the size dia suggested) will cost £1600, pump will cost £2000 and extra cylinder £1000. So the extra hardware will be approx £5000 and then of course the extra labour.

I'll pay it if it's the only way to get the system to work but don't want to waste money.

Thanks again

 

[SimonH2]

The only guarantee is that whatever you fit, the next person along with do that "sucking through teeth" that tradesmen do best, before telling you it's a load of rubbish and you should have done it a different way

 

[doitall]

The only guarantee is that whatever you fit, the next person along with do that "sucking through teeth" that tradesmen do best, before telling you it's a load of rubbish and you should have done it a different way

To answer Simon since he want another pop at me as per usual.

One 300l unvented cylinder, or one 400l max. In terms of two cylinders he says that my gas supply will only handle my current 30kW boiler and then an additional 24kW one. On this basis he says I won't have the boiler capacity to match the coil ratings of two cylinders, so no point. (Also standing losses would be greater

What would be your reply if I told you this, that doesn't include the word rubbish. The OP will need a bigger meter that is or should be taken for granted assuming he has a U6 at the moment.

Modern cylinders have very little standing loss.

Finally I did this type of work for best part of 50 years and know what works and what is a compromise. You do not design a system on a maximum 25% load.

I have given an opinion for what it's worth and thats all it was. A final comment would be to go with Alex and mysteryman, over an accumulator.

 

[indus]

Ok final question please dia. Are you saying that all I need to do is uprate the meter and NOT the upgrade where the gas company has to dig up my drive and upgrade the supply?

Thanks again. I'm not doubting your experience and expertise but you.have to understand that for a layman like me things can get very confusing.

Thanks again

 

[gaswizzard]

Break tank is the way to go as DIA has suggested many times , if you have the funds then do it , if you have the funds and don't do it then you deserve all you get if the system doesn't deliver what is expected , it's not rocket science and cutting corners to save a few quid would be foolish , I suspect you know in your heart of hearts which would be the best way to go but as usual the £££££££s are standing in the way.

A stand alone 300 litre cylinder will not match your requirements period.

 

[indus]

Gaswhizz, its not the cost standing in the way. Its the fact that tge person who came to give me a quote said it was't needed and that my gas supply wouldn't handle it

All I can conclude from that then is that he isn't the right guy for the job. That is surprising as he came recommeded, has been in the game many years and does larger donestic installs ( and blocks of flats etc)

Thanks