DIY central heating and TRVs etc

HugoSP

It may be worth your while doing a quick plumbing course at your local tech college if you can.

Unless you've got some practice, your project may seem a bit daunting. I know several people who have tried but failed, some have called me in to fix them. There are techniques that are important to know.

csj_2

I recently drained my pumped central heating / gravity hot water system to repair a minor leak in a joint. After refilliing the system (5 times!) and bleeding all the radiatiors, I am now only able to get hot water when the central heating pump operates and I am unable to get any hot water by gravity alone. The system worked fine until I drained and refilled it, and the central heating still works fine. Can anybody suggest a possible cure. I've read about antigravity valves sticking but my simple system does not seem to have one. Unless I can fix it I'll have to have my radiators heating up in the summer )only the upstairs ones have thermostatic valves on them.

BobProperty

csj wrote: »

I recently drained my pumped central heating / gravity hot water system to repair a minor leak in a joint. After refilliing the system (5 times!) and bleeding all the radiatiors, I am now only able to get hot water when the central heating pump operates and I am unable to get any hot water by gravity alone. The system worked fine until I drained and refilled it, and the central heating still works fine. Can anybody suggest a possible cure. I've read about antigravity valves sticking but my simple system does not seem to have one. Unless I can fix it I'll have to have my radiators heating up in the summer )only the upstairs ones have thermostatic valves on them.

This is why I don't like jobs like moving radiators. You have most likely either moved some crud around the system or introduced an air lock somewhere (or both). Getting it working again could take anywhere between 30 minutes and most of a day. How old is the system? Was there any inhibitor in the system? What was the water like that you drained out? It may be possible to back flush part of the system and see if that solves the problem. First though can you run the boiler while the CH is off so that just the gravity circuit is being used?

csj_2

Thanks for the reply.

I also suspected having dislodged some crud but I'd be surporised if that is the cause because the gravity pipes are much larger than the pumped ones which still work well. Unfortunately, my programmer is a very simple one and I can't run the hot water without the CH. I didn't have inhibitor in the system but the water wasn't too discoloured when it came out and it is now quite clean. The system is 9 years old which I would have thought is not that old.

How do i go about back flushing as you suggest.

BobProperty

Given that you have drained down the system to repair a joint, I assume you have some understanding of the system. I have to ask an obvious question though, for the benefit of anyone else with the same problem, but is there any sort of vent on the hot water circuit, like at the top of the heating pipe that heats the cylinder? I'm assuming not in your case, so what I am about to describe is how to get water in through the vent pipe and back around the circuit into the header tank. The consequences are that the water level in the header tank will rise so make sure the header tank will cope with being filled up (like it doesn't fall off the wall with the weight for one) and that the overflow works without leaking. What you do is connect the incoming feed to the header tank to the vent pipe and send the water back through the vent pipe, round the heating circuit and back into the header tank. This should push any crud or air back into the header tank. ( it can look like a "special effect" from a cheap B movie when it does it ). I recently did this by turning off the supply to the header (iso or mains it doesn't matter so long as you can do it) then connected an iso onto the connector to the header ball valve, attached to which is a series of bits of pipe and pushfit connectors ending in a 22mm pushfit which is attached to the vent pipe. With the iso off, you can now turn the supply back on as it will be held off by the iso. If you now carefully open the iso the water will run though the "temporary" pipework, into the vent pipe, and back round the system. You need to watch what is happening in the header tank as the water level rises, hopefully air and or crud will come back into the header tank and your problem is solved. However, the water level will be rising as you do this so watch that it is going out of the overflow and be prepared to turn the iso off if it is getting too full too quickly. (It shouldn't but you don't want find out it can't cope).
Hope you can follow what I've said, it's one of those things that takes less time to show you than to write it. Be careful out there.

csj_2

Thanks for the explanation. I might give it a try some day, eventful as it sounds. In the meantime, with this cold weather, my room stat calls for heat, drives the pump and we get hot water. I've also installed a timer for the immersion heater and I may have to live with that if I get scared of trying to back flush.

caleyles

BobProperty wrote: »

Given that you have drained down the system to repair a joint, I assume you have some understanding of the system. I have to ask an obvious question though, for the benefit of anyone else with the same problem, but is there any sort of vent on the hot water circuit, like at the top of the heating pipe that heats the cylinder? I'm assuming not in your case, so what I am about to describe is how to get water in through the vent pipe and back around the circuit into the header tank. The consequences are that the water level in the header tank will rise so make sure the header tank will cope with being filled up (like it doesn't fall off the wall with the weight for one) and that the overflow works without leaking. What you do is connect the incoming feed to the header tank to the vent pipe and send the water back through the vent pipe, round the heating circuit and back into the header tank. This should push any crud or air back into the header tank. ( it can look like a "special effect" from a cheap B movie when it does it ). I recently did this by turning off the supply to the header (iso or mains it doesn't matter so long as you can do it) then connected an iso onto the connector to the header ball valve, attached to which is a series of bits of pipe and pushfit connectors ending in a 22mm pushfit which is attached to the vent pipe. With the iso off, you can now turn the supply back on as it will be held off by the iso. If you now carefully open the iso the water will run though the "temporary" pipework, into the vent pipe, and back round the system. You need to watch what is happening in the header tank as the water level rises, hopefully air and or crud will come back into the header tank and your problem is solved. However, the water level will be rising as you do this so watch that it is going out of the overflow and be prepared to turn the iso off if it is getting too full too quickly. (It shouldn't but you don't want find out it can't cope).
Hope you can follow what I've said, it's one of those things that takes less time to show you than to write it. Be careful out there.

In the above post it states that there is no vent at the cylinder, By my reading of your problems you have a basic pumped central heating and a gravity primary circulation i.e. flow and return to cylinder, if this is the case then there should always be a vent and expansion pipe coming off at the highest point which is normally the flow to the cylinder and most normally a cold feed going into the return(although occasionally this can be a combined cold feed and vent or a cold feed at the boiler). If it is definetely gravity to the cylinder and there is no vent then possibly you have a primatic cylinder and not an indirect cylinder. Either way it makes the issue of the water getting hot with the pump on a bit of a mystery as most gravity systems have an A.T. stat fitted on the return from the cylinder, the idea of which is that when the cylinder reaches the required temperature it brings on the pump which takes away heat from the cylinder and prevents it from boiling or venting over into the tank.
Assuming it is gravity then the chances of the pipes to cylinder choking are fairly remote as they should be 28mm or at the very worst 22mm If everything was working before you fixed this joint could it be something you may have not reconnected right? As Bobproperty says a lack of water can sometimes be the problem although if it works with the pump on then obviously there is water in the pipework.I would go back to the beginning where you started and double check your work etc, As regards flushing if a system has never had an inhibitor fitted then even a system much newer than your nine years could be blocked. If all else fails it would be worth getting someone in to have a look
Hope I haven't confused you and good luck.:wall:

BobProperty

caleyles wrote: »

In the above post it states that there is no vent at the cylinder, .....

It's so difficult explaining this in writing, maybe I should have said air vent. I was hoping the poster had something like this at the top of the water heating pipework:
http://www.screwfix.com/app/sfd/cat/pro.jsp?id=19155&ts=66826

but, as I think I said, I assume he hasn't. He has got a vent over into the header tank. (I think and that's what I was assuming).

csj_2

The two kind folk trying to explain my problem have just sent me rushing up to the attic to try to find this vent you're talking about. All I find there are 2 tanks - a large cold water tank (to presumably feed cold water for the upstairs taps) and a small cold water tank (which I assume feeds my central heating boiler). Both have their own stopcocks expansion overflow pipes. Unfortunately, the small central heating tank overflow is set to discharge into the large cold water tank. I presume that this would mean that if I backflushed the system the overflow. with all the crud, would flow into my main drinlking water tank.

I am unable to find any kind of air vent anywhere other than the expansion pipe coming off at the highest point back to the central heating tank. No sign of a vent near the hot water cylinder in the bathroom airing cupboard either.

I assume therefore that it is definitely gravity to the cylinder , but no vent and that it presumably is not a primatic cylinder.

My pump is definitely controlled by the only one room thermostat that we have (the pump works with a cold system when I turn the thermostat up or when the pilot blows out on of our stormy south westerly gales.

I'm not sure what is meant by an A.T. stat fitted on the return from the cylinder, which you say would bring on the pump when the cylinder reaches the required temperature to take away heat from the cylinder and prevents it from boiling or venting over into the tank.

I agree that with 28 and 22mm pipes the chnaces of them choking are unlikely. The only thing that I disconnected was a dripping valve on one side of the pump. This was reconnected as it was and water flows freely through it (as felt by the heat when the boiler comes on).

I have a terrible gut instict that the plumber didn't set the system up quite right some years ago (when he left he couldn't get it to stay alight and I found from the manual that he should have reconnected an override wire on my Baxi differently for a gravity system).

Looks like one day I'll have to cough up and get a different plumber in to check it all out. Perhaps loosening one of the pipes from the boiler into the hot water cylinder might release some air but i'm afraid of trying that myself. I do remember when the system was first installed pover 30 years ago the plumbers then couldn't get the hot water to flow. Since then, at different times, I've had a replacement cylinder (to cure a leak in my original one), a new boiler 9 years ago (also because of a leak in the main heat exchanger of the original) and some additionasl radiators installed. there have been 4 different plumbers involved at various times (plus myself installing 2 extra radiators) and no one has commented on an incorrect setup. But then, I didn't ask them to look because the system was behaving fine.

Will keep you informed if the system suddenly starts to work properly. At the moment we're still getting hot water when the room temperature drops and the thermostat calls for the pump (which then drives hot water to the cylinder).
I noticed that in my daughter's house the system is set up so that whnen the thermostat clicks on the boiler fires up as well as the pump and her plumber told her to keep the boiler thermostat at maximum. That didn't look right to me either but both her system and ours delivered warm rooms and hot water. Wish I knew which, if either, was the correct setup.

Thanks for all your comments and suggestions.