Hot Water problems - any plumbers out there?

[drgoatboy]

Don't know if anyone can advise or if its time to call the plumber in!
I moved house back in the autumn and have been enjoying lovely hot water until about 2 weeks ago.
Now it turns out that is the heating is on the hot water heats up a treat, no heating = no hot water. I didn't realise this until the warm spell we just had!

The boiler is a rather old Ideal mexico, big floor standing brute, with pumped central heating and gravity fed hot water feeding a HW vented cylinder 2 stories up.
If I just have the HW on the boiler fires and the pipes coming from the boiler heat up but only about a metre from the boiler and then they are cold. Seemingly the HW is just not circulating...

My first thought was air lock in the coil, but the fact the HW works with the heating on suggests its not that, there is no valves that I know of between where the pipes are getting hot and the HW cylinder. There is no thermostat on the cylinder for this particular boiler so I guess it can't be that either.

Anybody got any thoughts on what I could check?
I don't know for sure if the HW on its own has ever worked but the fact the boiler was installed in 1999 I can't believe the previous owners lived without HW in the summer for over 10 years...

[Harpo]

Have another look for a valve, electrically driven, adjacent to the hot water tank with Port A and Port B on it. This 'may' be the trouble.
Do you not have a cheap night rate on your electricity? Reduce your gas usage!

[Globs]

Harpo said:

Have another look for a valve, electrically driven, adjacent to the hot water tank with Port A and Port B on it. This 'may' be the trouble.
Do you not have a cheap night rate on your electricity? Reduce your gas usage!

This little three way valve actually controls your heating system, requests for water or/and room heating go into it, and it moves to the appropriate position and then switches on the boiler.

If it's getting the right signals it is probably faulty. Sometimes the little motors burn out in them and you can buy and replace them separately. Play with the roomstat and tankstat and you should see it move one way for water, the other way for heating and the middle for both.

[Mr Pointy]

^^^
But if it's gravity fed HW system there shouldn't be any valves or other devices in the loop?

[Globs]

Mr Pointy said:

^^^
But if it's gravity fed HW system there shouldn't be any valves or other devices in the loop?

Good point Mr Pointy.
I expect that gravity needs a hand then

[drgoatboy]

Correct Mr. Pointy, no external valves, entirely seperate feeds directly from the boiler for the hot water and central heating.
It would seem when the central heating is on and pumping that movement of water is enough to get the HW circulating.
Could I put a pump in line for the HW too?

No cheap rate electricity so immersion heater doesn't seem like a good plan, good thought though.

Thanks for all your thoughts!

[jagnet]

No water circulation on the gravity circuit could be anything from a build up of deposits in the pipework to the incorrect fitting of a hot water tank designed for pumped systems.

You can easily add a pump onto the hot water circuit, and I'd suggest including a cylinder stat and 2-port zone valve as well to give you full control over the dhw temperature. You'll find that doing so makes the system much more efficient as a result.

I'd also take the opportunity to give the system a good clean and then dose it with a good quality corrosion inhibitor.

[drgoatboy]

Thanks Jagnet, system has had cleaner in it for a few weeks before being drained down and refilled several times of late, inlcuding inhibitor. (in the middle of house renovation with some radiators being removed and some replaced) Its not been power flushed though.

I'd like to add the pump and stats myself but don't think I have the time to do it or the knowledge to wire it in to the boiler properly. Looks like another call to the plumber

[jagnet]

System cleaner and a good flush followed by inhibitor is perfectly adequate on an existing system.

Given the choice I'd rather do that and from the savings over a powerflush splash out on something like a Magnaclean, which is going to provide ongoing protection through the life of the system.

It wouldn't take a plumber long to fit those items and wire it up though, so if you're not feeling confident dealing with the wiring yourself it's probably best to get someone in. You're not going to be looking at a big bill after all.

[Deva Link]

drgoatboy said:

Correct Mr. Pointy, no external valves, entirely seperate feeds directly from the boiler for the hot water and central heating.
It would seem when the central heating is on and pumping that movement of water is enough to get the HW circulating.

If the pump and gravity circuits are separate then it's hard to imagine why running the pump would make any difference. If anything, you might expect the opposite problem - heating works ok but no HW as all the boiler output is being grabbed by the pump and circulated through the rads.

Does the boiler get a good run when it's on HW only, or is it cutting out quickly?

[drgoatboy]

Deva Link said:

If the pump and gravity circuits are separate then it's hard to imagine why running the pump would make any difference. If anything, you might expect the opposite problem - heating works ok but no HW as all the boiler output is being grabbed by the pump and circulated through the rads.

Does the boiler get a good run when it's on HW only, or is it cutting out quickly?

I can only assume the water being pumped around the heating system is enough movement in the boiler to get the thermo cycle in the HW working? The boiler itself is common (I assume), the circuits coming off it aren't...

If its just the HW the boiler runs only for a few minutes before cutting out, I assumed because there is no flow of water it was getting to temp very quickly and cutting the boiler off by the boiler thermostat

[Deva Link]

drgoatboy said:

If its just the HW the boiler runs only for a few minutes before cutting out, I assumed because there is no flow of water it was getting to temp very quickly and cutting the boiler off by the boiler thermostat

Once the water in the boiler gets hot, it should circulate (obviously), and then restart the boiler again. It'll cycle like this (known as short cycling). How is it controlled? A time clock, or are you just turning it on as required? It might take a very long time to heat a cylinder of water from cold though. You don't notice how long it takes in cold weather as the CH is on anyway.

[drgoatboy]

Its controlled on a timer and the HW is on for 2 hours ish, however it is obviously not circulating at all because the pipes to the cylinder are still cold. They are hot coming out of the boiler for about a meter and then cold...

Having done a little more research I may have found the potential problem....
We had the hot cylinder replaced recently, and as I understand it there is a difference between a gravity fed cylinder and a pumped cylinder...
Also as I understand it the gravity fed cylinders are now very hard to come by...
Furthermore apparently the coil in a pumped sytem is a different (smaller?) diameter so doesn't aid gravity fed circulation...
Therefore I'm begining to wonder if my plumber has put the wrong sort of cylinder in...

Wow what a lot of ...

Does that sound like it could be the possible cause...?


Thanks everyone for your help so far BTW...

[Deva Link]

drgoatboy said:

We had the hot cylinder replaced recently,..

I love the way information comes out a bit at a time...

[drgoatboy]

Deva Link said:

I love the way information comes out a bit at a time...

Gotta keep you guessing!
Sorry, I only just realised it might be relevant. It was installed by a qualified plumber so assumed (maybe wrongly) that everything was all right.
Do you think I have worked out the problem then?

[Deva Link]

drgoatboy said:

Gotta keep you guessing!
Sorry, I only just realised it might be relevant. It was installed by a qualified plumber so assumed (maybe wrongly) that everything was all right.
Do you think I have worked out the problem then?

I don't know, to be honest, but it seems a reasonable assumption. Did he change any of the pipework around the cylinder, or did the new connections line up OK with the old ones?

[drgoatboy]

Pretty sure everything all lined up, or was at least very close. Not at home right now to check but there isn't loads of new pipework or anything.

[Deva Link]

drgoatboy said:

Pretty sure everything all lined up, or was at least very close. Not at home right now to check but there isn't loads of new pipework or anything.

It wouldm't take much for the gravity effect not to work - the pipework has to be continually rising.

[drgoatboy]

arse... looks like this could be getting expensive

[Deva Link]

drgoatboy said:

arse... looks like this could be getting expensive

I woudl start by getting the guy back who changed the cylinder.