Inadequate hot water - could we use 2 coils in the cylinder?

[RigidRaider]

4 bed house with traditional open-vented system.

The old cast-iron core boiler was inefficient and the core was weeping but we used to have lashings of very hot water though the cylinder was small, only about 100 litres.

About 3 years ago we replaced the boiler with a Glow worm Flexicom 24hx running through a Honeywell Y plan valve. The major pipework to the rads is 22mm but to the cylinder is 15mm. The new 160 litre cylinder is a modern half-hour job with two coils; the upper for the HW circuit and the lower is finned for solar. Since I'm pre-heating the water by running my solar panel through a second 160 litre cylinder in tandem with the main house cylinder the lower finned coil isn't in use except as a dump for excess solar water on the rare occasions when my solar system achieves 60 c.

Since fitting the new boiler and cylinder the hot water has been consistently inadequate. This morning the HW was timed ON for 2 hours 30 so the cylinder should have been full of water at 60c. I went out on the bike, came back and had disappointingly tepid shower, then I turned on the HW and watched the temperature display on the front of the boiler: it started at 54 degrees, the burner fired at a low rate, the temperature climbed within about 2 minutes to 82 degrees and the burner cut out. This tells me the coil isn't extracting enough heat from the HW circuit. We have had the boiler checked by a Glow worm engineer and he thought the coil might have a blockage and the symptoms I describe above would seem to support this. For a week or two after the new cylinder was first fitted I could hear water trickling within the cylinder when the HW was running. I have recently fitted a new Y plan valve (original was clunking on changeover) and I'm sure the HW circuit is clear of air; the flow is quiet and there's an auto-bleed float thingy on a T right above where the HW circuit enters the coil.

The cylinder stat is set to 60c and is at roughly the same height as the bottom exit of the upper, main coil.

Could we re-jig the HW circuit so it runs thrugh both the regular coil AND the lower finned solar coil?

What tests can anybody suggest for a blocked coil?

My plumber is serious grown-up bloke, he has been back a couple of times and he thinks the HW is adequate and can't think of any reason why it should under-perform. Perhaps we are expecting too much from a modern condensing boiler?

Any thoughts?

 

[JohnD]

yes, it does sound like poor flow

what sort of pump have you got? Is it a modulating one like a Grundfos Alpha? What speed setting?

when you took out the old iron boiler, was the system powerflushed?

has it got a system filter?

I presume it is sealed and pressurised, with no feed and expansion tank in the loft?

is there a control valve on the primary flow or return to the cylinder?

Has it always been like this, or did it get worse recently?

 

[RigidRaider]

Thanks for the reply.

The pump is a Grundfos Selectric running on the middle, 2 setting.

The system was not powerflushed.

There's no filter.

It's an open-vented system, as I wrote. I though that meant not sealed and not pressurised? It has a small F&E tank.

There is no control valve anywhere on the HW circuit.

It has been inadequate since the day the new boiler and cylinder were installed.

 

[JohnD]

sorry, I missed about it being open vent

As it is, and used to be an iron boiler, and was not powerflushed, it is very likely to have sludge and sediment.

i am an amateur not a pro but have been through this.

I strongly suggest you give it a chemical clean, which is a simple DIY job. Very often it will cure the problem, but even if not, it will have done nothing but good.

Buy a litre of Sentinel X400, which is a mild, non-acidic sludge cleaner.

Go up to the loft, tie up the ball float to stop it filling, and bale out the water and any mud from the f&e tank, sponge it clean

Drain a couple of buckets of water out of the system at the lowest drain cock (put a bit in a glass jar for later reference)

Tip the X400 into the F&E and untie the float so the chemical will be drawn down as it refills

Run the system as normal for four weeks. Turn the pump to max to circulate more vigorously, but look in the loft while running to ensure it is not pumping out of the vent pipe that hangs over the f&e tank.

Turn off all but one of the radiators beforehand so that the chemical will be concentrated on flowing through the cylinder coil, for the first week at least, then open them and circulate to start cleaning them too. From time to time bleed the "on" radiator, or the cylinder pipework if you can. If the water goes jet black, that is old sediment being loosened and washed around by the cleaner. Compare it to your jar, and let it settle in another jar.

After 4 weeks, drain it out, flush with clean water, add corrosion inhibitor on final fill. If you can do simple plumbing and can afford £100, fit a System Filter such as the Fernox or Sentinel, preferably before you start the cleaning cycle, and it will trap any future circulating particles before they can build into a blockage. Empty it weekly during the cleaning and you will be amazed and delighted to see how much sediment it catches.

Also examine the pipe run and air bleed in case there is any chance of a trapped air bubble. Post some pics.

 

[Blagard]

It might be a good idea to check the Temperatures of the flow and return on the coil. You apply an external probe onto the pipe to get the readings.
It will also indicate if the flow is getting there at the correct higher temerature to start with.

If there is a substantail difference then it would indicate a poor flow through the coil as the cylinder water has plenty of time to cool the water in the coil. The temperature difference will be less with good circulation.

As to what the difference should be, I am not so sure. On a central heating circuit the difference could be 20 degrees at the boiler. At the coil, due it its short length I would suggest no more than 10 degrees.

One of the heating experts may be able to give you some better idea of the flow and return temperatures applicable across a cylinder coil.

Edit: I tried to find mine on the Web but couldn't. however even the one in the link should do on a short section of exposed pipe. look further as this took a minute to find!
http://www.oilybits.com/pipe-clamp-thermometer-0-120-celcius/prod_216.html

 

[RigidRaider]

Thanks for the helpful replies.

In fact I was already planning to drain and refill the system because when I fitted the new Y valve I wasn't able to get all the orange sludge out of the F&E tank and some went down into the system, meaning the system water is now cloudy. So I will take JohnD's advice and try running it for a month with the cleaner. I would love to fit a magnetic filter but can't think of a suitable place in the pipework as everything is under the floors or behind the kitchen units and inside the kitchen cupboard around the boiler, pump and Y valve is very tight. I wish I had a cellar, I could have had everything neatly laid out down there for easy access and servicing as well as a drain point good and low down.

 

[Slugbabydotcom]

The system was not powerflushed.

There's no filter.

Modern boilers do not get on with dirty systems!
The cleaner the system is the longer a modern boiler will last so don't be shy when it comes to spending money on cleaning your system.

If you can't afford a powerflush then consider taking each rad off and flushing them out with a hose pipe after the x400 as JohnD described above.

Then fit a magnetic cleaner and add an inhibitor

The crud in your system has coated the inside of your boiler heat exchanger and the coil in your cylinder [plus all the pipework in between]. This lining stops the transfer of heat. I've seen pipes reduced to less than half their diameter due to the build up of this residue.

 

[RigidRaider]

Agreed. The water that I drained out recently was clear but nonetheless the insides of the Y valve that I removed were coated with brown sludge so a flush certainly won't do any harm.

 

[JohnD]

you could perhaps fit a filter near the cylinder.

It will not be as good as on the main return to the boiler, but it will clean a proportion of the water each time it goes round and round the system.

 

[Slugbabydotcom]

you could perhaps fit a filter near the cylinder.

It will not be as good as on the main return to the boiler, but it will clean a proportion of the water each time it goes round and round the system.

A decent one on the return will suffice. I do use other filters but for this one I'd use a magnaclean pro.

 

[Slugbabydotcom]

The brown sludge indicates the presence of air probably drawn in through the vent but on rare occasions I've known it to be sucked in through a faulty pump [old grundfos pumps seem to be the most common for this]

Tends to draw air in to the system
1 When the system is badly designed ie the negative pressure from the pump pulls air down the vent or if the vent pipe is undersized.
2 When the pump is set too high and the system pumps over at the vent thus aerating the system water
3 When the feed pipe is blocked. This occurs just before the feed pipe connects into the system. You know its blocked because it takes ages to bleed the air out of the system on refilling and a magnet sticks to the copper pipework!

 

[CBF]

having the correct size pipework going to the cylinder might help

 

[JohnD]

yes, I agree. With a 22mm pipe you can always use a gate valve to throttle it back a bit if necessary.