Megaflo Prblem - Hot water running out quickly

[Bernard Poostchi]

Hello

I hope someone here can help me with an issue I seem to be having.

Just moved into a new house (5 bedroom/ 3 bathrooms) a few months ago. First thing we decided to do was replace the boiler as it was 20 years old and didnt want to risk anything over the winter months with 2 small children. At the same time, we also decided to replace the host water cylinder with a megaflo.

The problem we seem to be having is that we run out of hot water at night after my wife has given the kids and herself a shower. It seems to last approx 20 mins or so (and she usually turns off the shower to conserve water when soaping down the kids). I ran the shower one night as well and timed it to see how long it ran out of hot water and it was 20 mins continuous running. I normally take a shower in the morning and never run out of hot water although I am probably in there for 10 mins or so.

Now down to the technical bits.

The Boiler and Megaflo were plumbed in to the existing pipework (i.e. we did not run new pipe throughout the house). The old Cylinder was upstairs in the airing cupboard and the old Boiler was downstairs in the utility room next to the Garage. The new Boiler (Vaillant Ecotec Plus 637) was placed directly above where the old boiler was and the new Megaflo (Indirect

300L) was placed in a corner of the Garage. Length of 22mm pipework from boiler to megaflow for the flow is about 3 lengths of 22mm and the same for the return.

Please bear with me on this....

The flow from the boiler went into a 3 port zone valve. One side is for the CH and the other for the HW. The HW flow also has a 2 port zone valve (as required by Megaflo). I had vaillant come out to check the boiler and he confirmed that the boiler was OK. He remarked that he had never seen an installation with a 3 port valve. The installer confirmed that a 3 port valve is a valid combination, but agreed to change it to a 2 port valve to see if that will help clear the problem. Well, it didnt. The following week, Heatrae Sadia came out to check the Megaflo. He remarked that the flow and return of the pipe into the tank were joined with a bypass valve and this is not required;. In addition, the bypass valve was rated between 0.1 and 0.5 bar. As the sealed system is always running near 2 bar, the water would take the shortest route and most of it would go through the bypass valve and not into the tank. I contacted the Installer again and they came around and removed the bypass valve. In all fairness, he realised his mistake and said it should have been one between 1 and 5 bar. He had put this in as an additional safety measure, although one is not required as the boiler has one built in.. Now I thought this was going to cure my problems, but it didnt. I still run out of hot water after 20 mins. I have looked at the Megaflo installation book and the pipework seems to match whats in the book, so as far as I can tell, it seems OK.

Other points worth noting:

Pressure is very good - Dont have an issue with it. When the hot water runs out, the pipe at the top of the megaflo is cold. Other taps only produce cold water. I checked this to see if it was the thermostatic valve in the shower. Shower that is used most often and when the water runs out (not tested with the other one) is one of the old Aqualisa with a grey thermostaic cartridge (dont know what the flow rate is on these). Vaillant checked the 2 port valve and when he turned off and on, he could "feel" the water flowing (plus the slider on the valve moved). HW & CH is on constant on the timer. I let the thermostat control the CH and I want to have the HW heated and ready to go into action (not that it is capable of doing that at the moment). CH works like a treat. House is about 3500 sq ft. Other quotes I had when trying to choose an installer were suggesting a 100k BTU boiler due to the number of Rads I have (19 in total mixture of Towel racks and single/double/small radiators) I ended up going with a 126K BTU boiler. Installer is giving up as they cant find anything wrong. Was not impressed with the Heatrae Sadia Engineer that came out. He put a meter on the tank to see if he was getting a signal and that was it. Didnt do much else. Wife is giving me an ear bashing as the Megaflo was my idea.

Would appreciate any help anyone can give in trying to help resolve this problem.

Many Thanks in advance.

Bernard

[PeterK]

Bernard First I'm no expert. I considered a megaflow earlier this year and the sales brochure was to hand. What temperature is the cylinder stat set at? For a 300l tank Heatrae claim 39mins to heat up from 15deg to 60deg with

24.5kw input.

If the recovery time is a lot shorter then I think this is an unlikely suggestion but would account for your lack of hot water (ignoring the fact that the cylinder can take 25kw input and therefore should keep up with an intermittent use of a shower) - If the hot water outlet pipe inside the tank was too long then you would loose some capacity due to the increased air gap at the top of the tank but more importantly you would have a reservoir of hot water at the top of the cylinder that cannot be used.

PeterK

[Andy Hall]

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The run time for the HW depends on four things:

1) The cold water temperature 2) The hot water temperature 3) The rate of use

4) The amount of heat that can be put back into the cylinder while the shower is running

The fourth point affects the run time to some extent but also the recovery time from cold after the stored water is depleted.

You can make an initial determination based on factors 1 - 3.

Shower temperature is constant (you set it to around 40 degrees)

Hot water temperature is almost constant when the cylinder is replenished ( 60 degrees)

Cold water temperature varies during the year and at this time of year can be as low as 5 - 8 degrees. This is one reason why the effect is noticable now. More hot water vs. cold water is needed to produce a given volume of warm water.

You can measure cold water temperature and warm water flow rate - for the latter a measured vessel like a bucket, a stop watch and a calculator. Work out the rate in litres per minute.

The figures can be plugged into the following formula. This assumes no reheating during running.

Vc[Tf-Tc] = Vh[Th-Tf]

where

Vc = cold volume Vh = hot volume Tf = mixed water temperature Th = hot temperature Tc = cold temperature

Taking a simple example.....

Let's say the cold temperature is 8 degrees. We know that the hot is

60 degrees, there's 300 litres of it and the shower is at about 40.

Vc x (40-8) = 300 x (60-40)

32 x Vc = 6000

Vc = 187.5 litres

So adding this to 300 litres of hot gives us 487.5 litres.

If this is running out in 20 minutes, then it implies that the flow rate is just over 24 litres per minute.

All of that assumes that there is no replenishment of heat, and of course there is some.

However, you can plug in your own numbers and see what you get. Either way, they are in the expected area.

It might have made a small improvement if it resulted in more heat being delivered to the cylinder possibly that the valve allowed more flow and hence more heat transfer from the boiler.

That's somewhat nonsensical. The static pressure of the system would be at around 2 bar. The bypass valve works on dynamic pressure from the pump and is something entirely different.

I'm not surprised. Again it might affect how much heat is transferred to the cylinder.

Not the issue. Paradoxically, may be making the problem worse. See later.

You can measure it as described above.

The number of radiators has zero to do with the boiler size requirement. It is their total heat output that matters. Either way, 37kW ought to be enough to heat the house.

Apart from an inability to do basic arithmetic.

The example above explains the worst case with no heat replenishment of the cylinder during running of the shower.

One additional experiment you can do is to turn off the boiler and then run the shower as normal to empty the cylinder of HW. Then turn on the boiler and time how long it takes to fully reheat the cylinder.

While the relationship of temperature vs. time is not really linear, it will give you a very rough idea of how much hot water would be produced in 20 minutes. However, you can work that out from the formula above because you know all the figures apart from heat contribution from the boiler. Really that should result in a longer run time.

While it's running, read the gas meter at five minute intervals and check the rate of use of gas for each 5 minute interval.

I suspect that you have one or more of the following:

- Hot water temperature is less than 60 degrees

- Cold is less than 8 degrees

- Flow rate is greater than 24 litres/min

- Reheat from boiler is very slow.

Since you focussed on things to do with the rate that the boiler is delivering heat to the cylinder, it is worth checking that out to its conclusion.

First one has to look at what is theoretically possible.

If the boiler were instead a combination type, heating the water directly from mains cold to shower temperature, a 37kW model will only deliver about 15 litres/minute at 40 degrees with a 5 degree mains cold. Therefore, if you really are using water at around 24lpm, it could never hope to keep up - physically impossible. Of course, delivering the heat into a cylinder and then using it from that directly can't be any better once the stored water has run out.

You can check the rate of delivery of heat from the gas consumption side. At 37kW flat out, it should be using about 4 cubic metres per hour. You can measure that from the meter as described and from that work out the rate of heat production. If there is an inadequacy in the rate of transfer from boiler to cylinder, the boiler burn rate may reduce. It probably will do as the cylinder nears full temperature. The cylinder ought to be able to accept 25kW at least initially, so if the gas rate indicates low usage and recovery time is long, this may be an issue of boiler to cylinder pipe lengths or too low a pump setting. You may be able to make some difference to these things. However, unless the rate is down around 15kW or so, you won't make a lot of difference.

This really gives a few solutions left.

1) Reduce the flow rate to the shower. You can get shower heads with them fitted or add one in. Maybe you could get a shower head with different spray patterns. If you could reduce the flow rate to say 15lpm, you would get more than half an hour run time. 2) If the hot water temperature is low, then increase it to 60 or 65 degrees. However, do make sure that there is some kind of temperature limiter one the shower valve - most have this. 3) Fit a larger cylinder and perhaps a larger boiler.

There's no point in even suggesting (4), which is taking shorter showers......

[Jim Alexander]

(1) try to shower more succinctly

(2) you need to tell us the temperature the boiler is set to and the temperature the tank stat is set to (alternatively how hot is the water coming out of a normal hot tap)

Jim A

[Ron Lowe]

A 300l megaflo ought to be able to provide oodles and oodles of hot water.

If it's running out very quickly, it suggests to me that it never got properly up-to-temperature in the first place, and you are drawing off only the top strata of heated water before it's running cold.

Some observations/ comments in line...

Sounds OK so far.

Agreed, the 3-port in conjunction with the 2-port supplied with the megaflo is unusual. A lot would depend on how the wiring had been done.

OK, we now have a standard S-plan layout. That is good.

OK...

No. The bypass valve is sensitive to the differential across it, ( ie the difference between the flow and return lines ), not the absolute pressure the loop is running at. The whole primary loop runs at say 2 bar when hot. The flow side will be slightly higher than the return due to the effect of the circulating pump. The bypass is only seeing this differential caused by the circulating pump possibly pumping into a dead-end. It does not see the overall loop pressure. This is unlikely to be the issue.

The megaflo is a top-quality solution, and with 300L of stored hot water, it should last much longer than your previous stored-water cylinder. Only thing to be aware of is that since you move the showers to mains pressure, you will use significantly more litres / min than before, unless you were previously using a big pump. The length of time the hot water will last also depends on the temperature of the stored water, since the mixer will use less hot water volume if it is at a higher temperature. We use 2 showers here, on a 250l megaflo without problem. BTW, 20 mins seems quite a long shower. 2 showers at 20 mins each is 40 mins run time. Could you do that before?

You need to do a spot of troubleshooting yourself.

There is a lever on the honeywell zone valve that is supplied with the megaflo. This can be used to manually operate the zone valve, but it is also a usefull indicator of the current state of the zone valve. If the zone valve is energised, tle lever is 'floppy'. If the zone valve is not energised, the lever is stiffly sprung.

Turn both the CH and HW off at the timer. At this point, the Zone Valve should be closed. ( lever springy ). Turn on the HW only at the timer. Ensure the ZV is actually energising. Does the lever change from stiffly sprung to floppy? It needs to. Once the ZP fully actuates, the boiler and pump should fire up. Do they? At this point, the flow into the megaflow should become hot. Too hot to comfortably hold. This needs to be going in to the Right-hand port on the megaflo, marked 'Primary Flow'. The Primary Return coming back out of the megaflo Left-hand port should be somewhat cooler, as the heat is transferred to the stored water. Feel them both. Be sure the flow and return are hooked up the right way round.

Now, observe as the water heats. Does the zone valve remain open? If it closes prematurely, then you need to look at the thermostat setting on the megaflo. The stat ( and emergency cut-out ) are positioned at the bottom of the megaflo, so should only trip out when the whole cylinder is hot. It will take some time to fully heat 300l. I'd expect the zone valve to be open and the flow pipe to be very hot for half an hour at least, if we're starting from cold. Continue to monitor the zone valve position and temperature of the flow and return pipes. If the pipes are not too hot to hold, or the ZV is not open, then there is an issue here.

What sort of temperature does the hot water get to at a hot tap? It ought to be getting hotter than you can comfortably hold your hand under. If the temperature of the water is not high enough, it gets mixed with less cold water at the mixers and runs out sooner. It is possible that the stat needs tweeking up a little. It's under the grey cover on the front of the megaflo, at the bottom, between the primary flow and the cold feed.

As you draw off hot water ( eg fill a bath ), the zone valve should re-open and heating should re-start. Does it?

[Jim Alexander]

and (3) the gas rate and temperature data recorded on the Benchmark

[John Rumm]

Just to confirm, If your system has a programmer, you have it set so that hot water heating is on continously I take it? (i.e. demand for hot water heating is solely dependant on the cylinder stat).

Obviously to maximize hot water output from the cylinder you want the boiler to kick in replenishing it as soon as demand is detected.

[Bernard Poostchi]

All

Thank you all for your replies.

I am comparing this against my old 210L conventional cylinder, which had a pump attached to it. My wife could shower the kids and herself without any issues of running out of hot water (i.e. a single shower lasting 20 or so mins)

The zone Valves are operating corerctly and is letting water flow through as required. This has been checked several times by the installer and also the Vailliant engineer and by me (to the best of my ability).

I did a test tonight. I turned off the CH (and hour before I ran the shower) and only left the HW on constant. Shower went cold after 20 or so mins. Boiler was showing it was fired up and heating the HW all the time during the shower running.

Thermostat on the Megaflo is set at 72 Degrees. Boiler is on at MAX (which is almost 80 Degrees). When I ran out of hot water, Pipe on top of Megaflo was cold. Flow into tank from boiler was too hot to touch, but as I would expect the return was cooler. Over an hour and a half later and still the tank is not fully charged (i.e. no where the temperature it should be - I am basing this on running only the hot water tap at the closest point to the tank - It is mildly warm).

I have not done a flow rate check at the shower head. I might try this in the next day or two, but I doubt this is an issue here - I understand that I am now running at mains pressure so will be using more water, however this replaced a big pump before and I actually feel the pressure is slightly less on this in comparison to the pump I had.

I am going to call Heatre Sadia in again, as I suspect that there may be a fault with the tank itself (difficult to prove), but I suspect that the whole tank does not get up to optimum temp, but only a portion of it and thats why it gets cold so quickly. Heatre Sadia indicated that the recharge on this tank with a 37Kw boiler should take no more than 45min to 1 hour. It is clearly taking a lot longer than this.

Regards Bernard

[Andy Hall]

OK, Bernard,

Since the reheat rate is low, try measuring the gas use rate as I suggested in my earlier post.

That will tell you accurately what it is and how it changes during the reheat cycle. Since it's taking over an hour, you could measure every ten minutes.

Is it possible that the cylinder was not fully reheated when you started the shower? Obviously that would give a shorter running time.

It would also be worth checking the actual HW temperature from the cylinder - an inexpensive infrared thermometer will do that. The thermostat may say 72 degrees (which is actually pretty high for stored hot water), but is it really?

When you compared with your old system was that compared with what the old one was like in the summer? The cold water can be 10 degrees warmer and that makes a large difference to run time.

However, if you have a very slow cylinder recovery, basically you are using up all the stored heat in one go, so my more simple example applies. You can improve that part only by reducing flow rate. If you can improve heat transfer from the boiler, then that will help, but the difference won't amount to doubling it for example.

[Jim Alexander]

OK, so far so good, but what setting is the pump on?

interesting that, consistent with inadequate flow rate, incidently one of the Megaflo troubleshooting suggestions

why the reluctance when its one of the fundamental inputs to the problem?

well in the circumstances I would measure the gas rate over the heating cycle before pointing the finger. IMHO a hidden problem with the tank is the least likely system issue. The heat has to go somewhere, it very much sounds as if there insufficient energy into the tank.

Jim A

[Andy Hall]

It will always be cooler, as a result of heat transfer. If it's a lot cooler, then that can be because of inadequate flow rate.

Probably the main one.....

This is a modulating boiler, so if the heat transfer to the cylinder is inadequate, the burner will modulate down, hence measuring the gas usage rate.

However, even at full bore and 100% efficient transfer, this will not keep up with the usage rate suggested by the figures so far.

I agree though, that the cylinder itself is unlikely to be the culprit unless the coil is partly blocked or something.

Low flow rate through the boiler through any one or more of:

- Too low pump setting

- Too long pipe run in too small bore (may need upgrade to 28mm)

- Motorised valve not opening fully (consider replacement by revolving shoe rather than ball type)

- Valve to balance flow through cylinder coil vs. heating? Present but hidden from view, turned down low?

[Bernard Poostchi]

Jim & Andy - Thanks again for your replies.

The Vaillant boiler has its own integral pump. The default setting is Highspeed.

I am not reluctant in doing a flow rate check - Its just too late to do tonight, esp with 2 small kids that take up most of my time.

I will measure the gas rate again in the next day or so, whenever the kids will allow me.

I dont know if the cylinder was fully reheated as I dont have any direct way of telling. I will need to get an I/R Thermometer and check the HW temp at the tap. I will also check at the pipe coming out of the tank. During the day, we only probably ran the washing machine twice and the usual amount of washing up (i.e. not much as its piled up in the dishwasher). All in all, we have not used much water during the day, so I would expect the stored heat to be quite a bit, plsu the timer is on constant HW, so I would expect it to top up if necessary.

Comparison with my old system was from around Mid-Sept (when I moved in) to mid-Nov ( thats when I changed to Megaflo). I have had this Megaflo installed for just over a month now. So only had old system for 2 months.

I am not pointing fingers here, just getting really frustrated. Heatrae Sadia, Vailliant and the installer all agree that I should be getting more than 20mins of running hot water ( not sure what they are basing it on), but all say their bits working and the installer is saying he has installed to manufacturers' instructions, So I am left with trying to solve this puzzle after forking out a lot of money. Wife only sees she is running out of hot water, whereas before she was not.

Regards Bernard

[Andy Hall]

You're welcome.

I think that you have enough to look at now. At the end of the day, it's heat in and heat out. If you measure the various things, I think the issues will become apparent. Armed with the numbers it should be possible to figure out what's going on - I suspect a combination of things conspiring together.

[The Natural Philosopher]

Having read all through the answers so far..by people whose opinions I respect.. it seems that the heat exchanger from the boiler to the tank is not working well..the primary efflux should show maybe 10C drop against input .. not stony cold. My 250L mains pressure HW tank is hot after about an hour with a 15KW boiler..IIRC its set to around 60C..and

250L is enough for about 20-30 mins of shower or a bath and a shower..

I suspect you have a partly closed valve, or a blocked pipe in the installation somewhere.

[Jim Alexander]

A further piece of advice which may not be that helpful (now). I sympathise that you feel you are getting the runaround but I don't think that Heatrae Sadia or Vaillant have any contractual obligation to you directly, and can never take responsibility for any installation issue. This is 100% an installer issue (even if it turns out there is a HS fault) and by going direct to HS and Vaillant you are actually letting the installer off the hook. Unfortunately I don't have any good suggestions as to how to make a reluctant installer toe the line but compiling the technical evidence is certainly a pre-requisite. Work on the heat in heat out measurements and come back.

Jim A

[Jim Alexander]

Slightly OT but especially with kids around that is verging on dangerously hot hot hot (if it worked). 60deg flow ought to be sufficient.

Jim A

[cynic]

Have you considered the chance of a reduced flow rate through the primary coil of the cylinder? This may be down to a throttled valve in the loop, a bit of installation debris in the circuit somewhere (most likely in the entrance to the coil) or in the circulating pump impellor. Does the heating circuit perform correctly?

[Andy Hall]

Already covered.

Could be.

unlikely

Apparently, yes.

[Bernard Poostchi]

OK - Done a flow test and here are the results:

In the bathroom where most of the showering is done, I measured 5.5l in 9.6 secs so I make that approx 34l/min. This is very high and I'm not surprised that I run out of hot water now. I had turned the thermostat onto full hot, so 20 mins or so is about right (give or take a few mins or so).

I tried the same test in the 2nd bathroom and again turned the valve onto full hot and measured 6.75l in 39.7 secs, so I make this 10.2 l/min.

I will turn on the shower in the 2nd bathroom later on today and time how long it take to run out of hot water.

Looks like we will be using the 2nd bathroom a lot more now.

I need to find a way of reducing the flow through the first bathroom. This is a concealed unit (Aqualisa Classic) and I don't want the expense of ripping it all out and putting a new shower unit in (not just yet as I'm broke at the moment).

Anyway, many thanks for your help and support in trying to resolve this problem.

Regards Bernard

[Jim Alexander]

Not familiar with the shower but I assume it has a combined temperature/flow control (avoid in future). Try partially blocking the shower hose, anything that doesn't dissolve, corrode or swell should do.

Jim A