Mega Flo vs Thermal store

[SimonH2]

Rather than ripping out this system and spending money on a Mega Flo wouldn't a more practical solution be to just install good quality electrical 10 kW power showers in my two new loft rooms and the new first floor en suite?(pp pending)

Possibly, possibly not. Your 10kW shower can take 40 amps, and two of them can take 80A. What size is your main fuse ? What other loads do you have - such as electric oven and/or hob ?
It might not seem a good idea if, while preparing Sunday lunch, two people decide to shower and you lose the electric supply to the whole house and have to wait for the supply utility to come out and change the fuse
And even a 10kW electric shower isn't going to be the geyser you could expect from a stored hot water system - heat bank PHE are often rated at 100kW or more.

Such questions would be best asked over in the Uk Electrics section - but be aware that there are some equally obstructive trolls in there as well, so get your flameproof virtual underwear on first

The beauty of thermal storage (in whatever form) is that it's relatively easy and can be designed to cope with short term peaks in demand. Anything instantaneous is limited to the supply capacity at that time - whether that's electricity supply capacity restricting the size of shower, DHW coil restricting draw off rate/temp from a thermal store, or boiler capacity for a combi.

What you would need to do is work out available flow rate - which has already been mentioned several times. Hopefully Agile can tell you the best way, I think I know what I'd do but it may be wrong - I'd be disconnecting the house plumbing from the supply stopcock, and attaching a temporary pipe that I could direct into a bucket for a few seconds to see what comes out with no restrictive pipework between stopcock and taps etc.

You then need to work out what realistically you want to support. If showers, then what flow rate, how many at a time, and for how long. If baths, how much water, how many for a given space of time.

Also, consider if it would be acceptable to lower flow rates/pressures to limit demand. Eg, when a third shower is turned on, would it be acceptable for the other two to reduce in flow rate ? This can be achieved and would go some way to reducing your problem.

Given that supporting the theoretical maximum demand could be complicated, bulky, expensive, or all of these - do you want to scale back on anything ?

Only when you know all this can you work out :
If it's even possible with instantaneous heating - note that even if you put in a large combi boiler just to supply the extra showers, you'd probably be restricted in gas supply.
Given that you almost certainly won't be able to do it with instantaneous heating, how much storage you will need.
And of course, decide on what type of storage to use.

My preference would be for a thermal store as you have but larger and with higher capacity DHW coil, possibly with a PHE as per my earlier drawing to ensure that this isn't the limiting factor in the system. An unvented cylinder would work as well, but I believe it would give a less graceful degradation when you hit the limit and a longer recovery time before the hot water was usable again.
A larger unvented cylinder would give you a longer "run time", but for a given heat input a corresponding longer recovery time - and of course higher standing losses from the larger surface area.


Again, I think you need to find yourself a proper engineer who fully understand the issues - rather than a "general plumber" (for want of a better term) who may only be able to guess and provide a system suitable for a smaller house.

 

[SimonH2]

As per my last post, this is an idea I have for restricting the flow if you exceed available heating capacity - either because you've exceeded the heat exchanger capacity, or just because the store is getting depleted and the temperature differential is dropping.

Addition of flow limiter

• SimonH2
• 16 Oct 2011
• 1

As per "Thermal store + PHE", add a flow limiter to restrict flow when draw off exceeds...

The theory is that the bypass valve sets a minimum flow rate, but under normal circumstances the thermostatically controlled valve will be fully open (hot water is hot). If hot water temperature starts to drop, then the thermostatically controlled valve will start to close and reduce the flow to compensate. Note that the cold water draw off (at least for showers etc) comes from after this restrictions so the pressure balance would be maintained.

Note that this is just an idea I've knocked up, and stability could be an issue. It would be possible to get into a situation where the valve closes because the temperature is dropping, shuts too much, temperature goes up again, valve opens, and so the system hunts. I suspect the answer to that would be to ensure that the response rate of the valve is large compares to the transit time of water through the heat exchanger(s).

Really I'm posting to show that the problem is solvable given the will, and cash, to solve it - and of course the knowledge and skills to design a suitable system.

BTW - there is something commercial designed to do something similar for different reasons. Don't take this as any sort of endorsement, but the Combisave does something very similar - but the idea is to restrict flow rate so the combi can get up to temperature much more quickly from a cold start. If you've had the "benefit" of living with a combi, you'll know that when you turn on a hot tap, you have to wait while the boiler fires up, heats up, and eventually gets the DHW flow up to a reasonable temperature - and the delay is longer the higher the flow rate. I can see the point of it, but I agree with other that question the payback period in pure cash terms - you can achieve better for no expenditure by learning how to use the combi effectively and manually controlling the flow rate, but IME you will struggle to persuade others to do it.

 

[indus]

Thanks Simon, you've taken a lot of time to help me, it is very much appreciated.

Does anybody know how I would locate somebody in London/Essex who really has the know how to sort this issue out? ie somebody who really understands thermal stores.

Thanks

 

[transam]

Combis thermal store set up !

Take a look at the the combistore.

Zero , at times running cost's ?? Just a thought .

 

[JonasX]

The primary difference is this.

In your thermal store, it's heated top-down at up to the full capacity of the boiler. The design of the DHW coil sets the limit at which you can remove heat from the store, and the outlet will start to lower in temperature as the overall thermal content of the store reduces. If set up right, the boiler will continue to keep the top of the store hot, but at reducing flow rate as the bottom of the store cools down - the DHW temperature is a function of temperature difference and flow rate - so at the reducing differentials lower in the store, the overall heat transfer will reduce but there will be no abrupt change. If you were to draw off heat for long enough, the system would stabilise with heat output matching heat input from the boiler. Reducing the draw off rate will result in an almost immediate increase in draw off temperature since the top of the store is still going to be hot and the transit time of the DHW through the heating coil would be increased and allow for more heat transfer.

In a hot water cylinder (whether vented or unvented), reheat is by a coil near the bottom of the store. This heats the water around it, and this hot water convects around the cylinder. In still water this will result in some top-down heating, but largely it is "bulk heating" where the whole store is heating at once. Once you've drawn off all the hot water, you will then have cold or lukewarm water - and again if you keep drawing off water the system would stabilise at a temperature where the heat drawn off in hot water would balance the heat put in by the heating coil. Reducing draw off rate will not result in an immediate increase in draw off temperature as the whole cylinder has to heat up.

Many of us remember the days when reheat times could be hours and so if someone used up the water for a bath, then you would have to wait a while until there was enough water for the next bath. Modern cylinders are now much better, with high efficiency coils and hence faster reheat times. However, having a (very quick) look around, the capacity of the reheat coil doesn't seem to be mentioned much - even those offering a high capacity coil for quick reheat don't seem to give any figures.

Simon, very good outline of the sluggishness of bottom mounted reheat coils. They reheat slowly. The boiler does not dump all its heat into the cylinder as much is pumped back to the boiler. It reheats bottom up, not top down as the heat bank does.

A heat bank/TS has boiler's heat dumped right at the top of the cylinder; all of it. This is useful heat at good temperatures. This can be pumped into a DHW plate heat exchanger giving useful hot water. The heat bank reheats quicker.

Plate heat exchangers are far more efficient than coils. If a normal DHW "direct" unvented cylinder is reheated via a combi's water section using a bronze pump switched via the cylinder stat, it would re-heat far faster and use less fuel, dumping the heat at the top of the cylinder as per a thermal store. Some Continental makers state this can be done with their combis. It re-heats the cylinder very quick and acts a priority system always heating the cylinder when DHW calls, then reverting to CH as per a normal combi.

A neighbors ATAG combi can have the DHW temp, user selectable, to to around 70C. The model with the GasSaver incorporated. The Gas saver only works on DHW. It hit me that if the CH water was pumped through the DHW part with a filter before of course, ignoring the normal CH side, the whole system would go through the GasSaver. This would only work for low temperature UFH only, not high temperature rads as his return temperature would need to be low. The DHW cylinder would need to be heated via a large coil of course. Why Gas Savers are only for DHW is beyond me as they can be made to be used for both UFH & DHW. But that would mean a special UFH only boiler.

It just hit me, re: the Legionella debate. This ATAG can run DHW temperatures way above 60C. It can be wound up once every few months by the user, and all hot taps slightly opened for two minutes, then set back to 52C. Any slight trace of Legionella is then killed in the hot pipes. Simple.

 

[indus]

Just an update.

I had the 'plumber' who changed my boiler 2 years ago around today. He is the guy who I trust, it took him a bit of time to figure out the TS back then and he admitted that it is not what he usually fits.

Unlike the others though he did eventually understand it and never told me to just rip it out and fit a Mega Flo.

Anyway, he thinks the HE within the TS is buggered (or perhaps was never really up the job and now is completely buggered)

So I asked whether I should replace it with another TS and he said no replace it with a Mega Flo. He also said if for no other reason than that trying to get anybody to fix the thing when there is a problem is a nightmare.

He sat down and did some calculations based on the size of the property and number of bathrooms should all the planned extensions go ahead. He doesn't think one Mega Flo and one domestic boiler will do the trick to achieve what I want.

He suggests something along the lines of two 300litre Mega Flos and either two domestic or one commercial boiler. This will mean moving the existing TS from the loft and having some sort of 'plant room' downstairs.

In my proposed extension was a 10ft x 12ft room that I was going to use as an office/kids study room, looks like that will be the plant room now!

So nothing to do for now, lots of calcs to do and I need to notify him when the extension starts downstairs. This is going to cost a pretty penny, but hopefully at the end I'll get the result I'm after.

Thanks to all for the help provided, in particular to Simon for taking so much time.

 

[Norcon]

Did he check your flowrates/pressure as Agile has suggested?
Very important. Next time you see him be sure to tell him.
Though you shouldn't need too as he should be telling you that he will perform some flow tests.
600l sounds way ott.

 

[indus]

Did he check your flowrates/pressure as Agile has suggested?
Very important. Next time you see him be sure to tell him.
Though you shouldn't need too as he should be telling you that he will perform some flow tests.
600l sounds way ott.

This was a preliminary visit to see if anything needed changing right now ie whilst the loft is still in a mess. His take was that though he would suggest a Mega Flo, just changing it over now won't give me what I'm after.

We did some rough calcs of the size of the house but he has asked me to take some time and measure the house properly and the give him the numbers. He will then do the pressure and flow measurements and put together a couple of options.

Norcon, why do you say 600l is ott? If all the plans are passed the house will have 5 showers and 1 or 2 baths. A 300 litre MF will only do 2 X 15 minute showers before it starts to run cool.

My system needs to be able to cope with AT LEAST two 15 minute showers at about the same time on a daily basis as well as a couple of hot water taps filling sinks. This is before you factor in guests staying from time to time and when my kids are bit older and they start showering rather than bathing.

Thanks for your input, its appreciated.

 

[Dan Robinson]

I agree it is OTT. a 330 OSO with 28mm tappings would probably do. You can even flood the top 50 litres and get a 380 if you find you need a little extra hot water.

We have houses with similar requirements to your running this setup. The OSO also has the advantage of being easily over pressurised. Our last one it sitting at 4.5 bar IIRC.

Alternatively look at cylinders that are recharged through a plate. Specflue and Mikrofill do them.

 

[SimonH2]

I know this might sound like a radical idea ...

... but how about just fitting showers with a much lower flow rate ?

I have to say that your planned water consumption does sound rather excessive. It's a lot of water - didn't I hear there's a drought order in the SE ? And it's going to take a lot of heating, that's an incredible amount of hot water to be flushing down the drain.

And on that subject, I was browsing on the internet and I came across this ...

http://wiki.diyfaq.org.uk/index.php?title=Drain_Heat_Exchanger

BTW - at home the main shower works just fine with about 6 to 8 feet head from the header tank in the loft. I haven't measured how much water it uses, but it's a lot less than the figures you are bandying about.

 

[indus]

I agree it is OTT. a 330 OSO with 28mm tappings would probably do. You can even flood the top 50 litres and get a 380 if you find you need a little extra hot water.

We have houses with similar requirements to your running this setup. The OSO also has the advantage of being easily over pressurised. Our last one it sitting at 4.5 bar IIRC.

Alternatively look at cylinders that are recharged through a plate. Specflue and Mikrofill do them.

Dan, please could you briefly explain what a OSO is? And then could you tell me how long this system would run 3 showers at the same time?

Many thanks

 

[indus]

I know this might sound like a radical idea ...

... but how about just fitting showers with a much lower flow rate ?

I have to say that your planned water consumption does sound rather excessive. It's a lot of water - didn't I hear there's a drought order in the SE ? And it's going to take a lot of heating, that's an incredible amount of hot water to be flushing down the drain.

And on that subject, I was browsing on the internet and I came across this ...

http://wiki.diyfaq.org.uk/index.php?title=Drain_Heat_Exchanger

BTW - at home the main shower works just fine with about 6 to 8 feet head from the header tank in the loft. I haven't measured how much water it uses, but it's a lot less than the figures you are bandying about.

Simon, I'm not sure what you mean by my planned water consumption being excessive? My total water consumption will not be excessive but most of it will be required within a short space of time. I presume this is actually no different to most households?

 

[Dan Robinson]

OSO is a brand of cylinder like the Megaflo (only better and much more versatile.

Capacity of the system will depend on far more variables than you have posted information though.

 

[JonasX]

I agree it is OTT. a 330 OSO with 28mm tappings would probably do. You can even flood the top 50 litres and get a 380 if you find you need a little extra hot water.

We have houses with similar requirements to your running this setup. The OSO also has the advantage of being easily over pressurised. Our last one it sitting at 4.5 bar IIRC.

Alternatively look at cylinders that are recharged through a plate. Specflue and Mikrofill do them.

600 litres? Get rid of the plumber! The Mikrofill:
http://mikrofill.com/index.php?wcId=3&prId=5

The cylinder is heated via a plate heat exchanger, so rapid reheat. The tech figures are impressive.

Or, have a cheapish 300 litre "direct" cylinder and have the water section of combi to heat it at 60C - all is needed is a bronze pump on the cold water feed to the combi. The pump gives flow into the combi from the cylinder activating the flow switch starting he burner. The combi needs to be capable of taking hot water into the cold inlet - a number can. A smallish 24 kilowatt combi will reheat a 300 litre cylinder very quickly. 24 kilowatt boiler is very large for the the average CH requirements, but will re-heat a cylinder super quickly and keep hot water at the top of the cylinder at all times, hence reducing the cylinder size.

http://mikrofill.com/index.php?wcId=18&


A 24 kilowatt boiler will reheat from cold a 300 litre cylinder in about 1/2 hour. The reality is that in 10 minutes about 400 litres of DHW will be delivered when the combi comes kicks in. The Mikrofill delivers over 500 litres in 10 mins from a 300 litres cylinder.

 

[Onetap]

And on that subject, I was browsing on the internet and I came across this ...

http://wiki.diyfaq.org.uk/index.php?title=Drain_Heat_Exchanger[/QUOTE]

I'd recommend you ignore that.

I've had a few internet rows with the guy who wrote it (not you was it?). He tends to give expert-sounding advice but, if you happen to have any knowledge of what he is giving out on, there are huge technical flaws in all of it. He seems to believe he's an expert in everything, rather like Dr. Drivel.

The problem with that (it was rowed about) is that it has a huge fouling factor due to the accumulation of hair (long, short and curly) soap scum, limescale and other hideous things that go down the drain. It would need regular cleaning and the heat excahnger proposed was plastic!