Hello - need help with 2 hot water cylinders

[Misfit]

Hi all,

Not sure if I'm posting in the right place but i have a problem with a new house with 2 hot water cylinders. It is a gravity fed oil fired system and the boiler, heating and one of the cylinders is fine, however the second newer(ish) cylinder installed when the previous owner extended the house is not heating up. I changed the thermostat which looked pretty old and didnt seem to be clicking at temperature properly but still no joy (although the new one does click properly as you change the target temperature.)

I think how the system seems to work is that the first tank calls for hot water and has a 3 way valve? (A,B,AB) that once the first tank calls for water allows the second tank to divert heating water to itself with the second thermostat. However I am a DIY guy and really have no idea what i am talking about so may have deduced this completely wrong.

Any ideas how a double tank system should work and what could be wrong with mine? I should add that it has worked fine for 18 months and I haven't touched anything before it went on the blink. In saying that i do remember we had Luke warm water in the second tank when i first moved in and i fiddles with the thermostats and eventually they worked fine but that was 18 months ago.

Thanks in advance!

 

[Last Plumber]

It depends what type of system you have, who installed it, what controls you have and how it is wired.
I have a few questions to help guide us in the right direction:-
I know you say it is gravity fed, do you mean it is an open vented system? (with a feed and expansion tank).
Is there only one motorised valve or is there one near each cylinder?
How many pipes leave the Boiler?
Just so we can start to visualise your set up.

 

[Misfit]

Thanks and your absolutely right it's a vented system.

There's a valve near the boiler that I think switches between hot water and heating and then one more valve near the first hot water tank. I think only one pipe leaves the boiler and goes to the first valve next to the boiler but I need to check again tonight. Does that help?

 

[Last Plumber]

It sounds like you have a fully pumped system with a 3 port valve for CH ( central heating) and DHW (domestic hot water), plus an additional 2 port zone valve for the other DHW cylinder.
Double check what you have. Take photo's if poss. If there is a valve next to the problem cylinder, it should have a manual lever on the head. See what position it is in and then turn the DHW on so there is a call for heat. You should be able to tell if the valve has opened. Once you have got to that stage we will know more.

Hope this helps.

 

[Misfit]

Thanks and will take a look tonight and report back. FWIW the boiler is in the garage and has one valve out there which seems to switch between DHW and CH (look at me using the abbreviations ), and the second valve is inside the house just above the first cylinder which is successfully heating water. The cylinder which isnt working only has a thermostat and no valve. Pics to follow later!

 

[Last Plumber]

So the 3 Port is the valve that feeds the problem cylinder?
There will be two pipes about 14" apart on one side of the cylinder. Feel them when the DHW is on and see if they get hot.
What does FWIW stand for then?

 

[Misfit]

Ha! For What It's Worth

Yes i think the 3 port feeds the problem cylinder and the pipes you describe were definitely not getting hot. Will post pics tonight of the set up.

 

[Misfit]

Another shot of the 3 way valve above the downstairs tank that gets hot water

The 3 way valve above the tank that is working (downstairs tank)

The downstairs tank that is working with a 3 way valve above it

The pump and valve next to the boiler in the garage (there are only a couple of pipes from the boiler itself)

The problem tank with the old thermostat

This is the problem tank with the new thermostat

Let me know if this helps. Couldn't work out how to post pics in the right order so probably should look at them in reverse order.

 

[Last Plumber]

Did you check if the lever on that zone valve (2 port) moved across or became slack when the DHW was called for?

 

[Misfit]

If you are talking about the valve above the tank, then that doesn't seem to move. when i push the lever across and lock it in position the pipe on the right of the picture heats up (the left side that goes to the tank below it is normally hot), whereas it is always cool whether DHW is being called for or not. I thought this might be the issue, but I left it locked in that position for 45 mins, the right hand pipe got really hot, but didn't see any signs of the tank upstairs starting to heat up, not any pipes around it?

 

[Last Plumber]

If you are talking about the valve above the tank, then that doesn't seem to move. when i push the lever across and lock it in position the pipe on the right of the picture heats up (the left side that goes to the tank below it is normally hot), whereas it is always cool whether DHW is being called for or not. I thought this might be the issue, but I left it locked in that position for 45 mins, the right hand pipe got really hot, but didn't see any signs of the tank upstairs starting to heat up, not any pipes around it?

No I mean the other valve near the pump. There is a lever on that too

 

[Misfit]

Aha!, that I don't know and will have to check it tonight and report back. I have always assumed that was working so have never really played with or watched it.

 

[Last Plumber]

Check that one tonight. With the DHW on and the cylinder stat calling for heat, the lever should be slack and move freely from one side to the other.
If it doesn't become slack, put it in the manual position and see if the water in that cylinder gets hot. You will need to fire the Boiler to test that so put the heating on or the other DHW cylinder.
The valve will be 240V. Just so you know.

 

[Misfit]

Got it and will update tonight, and yes I know everything is at mains voltage so I always shut down and test with my multimeter before I start to touch any wires! Thanks again.

 

[Last Plumber]

OK. If it is faulty, test the motor and feel the resistance of the valve itself.
You can buy the motor as a separate part for your valve.

 

[Misfit]

I'm none the wiser I'm afraid. The valve near the boiler seems to activate if either the DHW thermostat downstairs OR the CHthermostat is calling for heat. The lever is however loose all the time. Does this indicate anything?

 

[Last Plumber]

The lever on the other valve?
There are two, one near pump and one with three pipes

 

[Misfit]

The lever on near the pump (2way?) is slack whether the internal gubbins is triggered or not. I can see the internal mechanics moving (driven by the motor) but the lever remains slack at all times. doesn't seem right to me

 

[Last Plumber]

The lever on near the pump (2way?) is slack whether the internal gubbins is triggered or not. I can see the internal mechanics moving (driven by the motor) but the lever remains slack at all times. doesn't seem right to me

Doesn't sound right. That is a power open, spring return valve. When it opens it does so slowly (around 5 or 6 seconds? don't quote me but that's near as damn it), and when the power is cut it should close on a spring.

The trouble is, you don't know how it's wired or by whom.

You could kill the power to the Heating and see if it closes, or test (in box on wall) if it's brown and blue wire have L+N. 240v. They are supplying L+N to the motor itself.

You need to rule this out. If it is broken in the closed position your DHW in that Cylinder won't work obviously. If it is jammed open, I would expect your DHW to work when the boiler and pump are running for other things so that scenario would point to another problem.

It sounds like a Fairly simple job for a Heating Engineer. Unless of course you are competent in Plumbing and Heating and intend to address the issue yourself.

 

[Misfit]

So, I did actually put a meter across the feed to the motor, and when the heating is energised, the motor gets 240V and the springs get stretched by the motor. when i turn the controller off, the springs contract again (and 0 Volts is going to the motor). I wonder if somehow although the actuator still appears to be moving, something is still not right that the manual override lever is just slack all the time. on the question of competency I am reasonably handy but this isn't my gig either

From what you have said I think I will try the following. Swap out the actuator/motor and see if that does anything (looks like they arr only £30), if that doesn't work, I will then try and get a heating engineer. I will be back on here however for help on how I find one who doesnt want to rip me off! Thanks LastPlumber for your advice so far and I will let you know how I get on.

 

[Last Plumber]

So, I did actually put a meter across the feed to the motor, and when the heating is energised, the motor gets 240V and the springs get stretched by the motor. when i turn the controller off, the springs contract again (and 0 Volts is going to the motor). I wonder if somehow although the actuator still appears to be moving, something is still not right that the manual override lever is just slack all the time. on the question of competency I am reasonably handy but this isn't my gig either

From what you have said I think I will try the following. Swap out the actuator/motor and see if that does anything (looks like they arr only £30), if that doesn't work, I will then try and get a heating engineer. I will be back on here however for help on how I find one who doesnt want to rip me off! Thanks LastPlumber for your advice so far and I will let you know how I get on.

Thanks. Do come back and let me know how it went.

 

[Misfit]

So I changed the two way valve in the 4th pic above with no real effect other than the override lever now works as it should. However in changing it I think I figured out how the 2nd DHW cylinder (which i will call DHWC2) gets its feed and this valve is key. The best word I can use to describe the set up is "parasitic"!

So what I mean is the thermostat on the DHWC2 is connected to this 2 way valve, so when it calls for heat, the valve opens. However as there are two cylinders in the system this valve does not have any control whatsoever over the pump or boiler (i.e. It cannot control either). The pump only reacts to the 3 way valve in the house in pics 1,2 and 3 which reacts to the CH thermostat and DHWC1 thermostat to fire the boiler and divert water between CH and DHWC1

DHWC2 can only draw heat either the CH or DHWC1 is calling for heat and has fired the boiler and pump. In this case water flows straight up and through the red stopcock type tap in the 4th pic and toward the 3 way valve in the earlier pics. It also however takes a left turn and flows through the 2 way valve in the 4th pic (DHWC2 valve) if it is also calling for heat.

My problem seems to be that that stopcock needs to strangle the feed to the 3 way valve enough to force water through to DHWC2. I found that when I turned this off completely the pipe to DHWC2 finally got really hot (obvioulsy this is dangerous as if the CH/DHWC1 was calling for heat whilst DHWC2 was not calling for heat, the pump would be pumping against two closed routes so I didn't leave it like this for long). What I have done is fully opened that stopcock and slowly tightened it in half turns until DHWC2 was getting hot water to "balance" the system and it seems like it has resolved the problem with heat available to both DHW cylinders and the CH depending on which thermostat is calling for heat. The limitation still being the DHWC2 can only receive heated water if one of the other two thermostats are calling for heat at the same time.

Sorry for the long post, but interested if any one can make sense of it and agree / disagree with my understanding of how my system is configured and whether I have done the right thing!

Let me know

 

[Last Plumber]

It sounds like you need a rewire with a three channel programmer.
CH and DHW 1 + DHW 2
Yours is half Y plan and Half S plan but a Heating Engineer will be able to sort the wiring and advise on Plumbing side. My advice is to avoid Electricians. (unless of course it is someone who is very experienced with Heating Systems).
It may not be that difficult to sort it but you need someone who has actually seen the job.

 

[Misfit]

Got it and agree as the set up is always going to end up hit and miss. Do you know if Nest supports three channels? I was thinking of going down nest route anyway so would be ideal if it could handle three way control.