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22-02-2020, 02:20   #1
Paul_Mc1988
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Cylinder heating but rads wont

Hi guys,

Bought a house recently and have done a whole home renovation but left the main heating system mainly untouched (due to budget). House is a three bed semi D

I added one rad onto the existing system. Upstairs has four rads. One in each bedroom and the bathroom. Downstairs has a rad in the hall, a rad in the living room and rad in the kitchen/dinner ( this is the new rad as there was previously gas fires in here). I removed all rads and fitted TRVs on them. I also installed two motorized valves at the boiler.

When I turn on the cylinder circuit the water gets up to the cylinder and the return pipe at the boiler gets warm quite quick ( 5minutes). When I turn on the radiator circuit some of the rads are getting luke warm others not at all. The inlet pipe to the rads is hot but the outlet either cold or luke warm. I have bled all the rads numerous times but no joy. Any ideas. The pump is a myson and probably there since the original install 20+ years. Boiler is a worchester condensing boilerThinking the pump may be struggling and on it's way out. Any ideas?
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22-02-2020, 02:32   #2
Turbohymac
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Possibly the water is taking the path of least resistance and simply coming in from your boiler and heading up those nice big 3/4 pipe to your hot cylinder and returning as easy back to the boiler..I had this same issue but I had to put an inline valve before the cylinder to cause less flow and more resistance which meant I had extra pressure and flow to force the water around the much smaller pipework and valves of my radiator circuit as i am only using 1 circulation pump..
Also rethink has this always been an issue or maybe you have caused extra restrictions on your rad circuit..
Good luck..
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22-02-2020, 05:56   #3
Paul_Mc1988
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Possibly the water is taking the path of least resistance and simply coming in from your boiler and heading up those nice big 3/4 pipe to your hot cylinder and returning as easy back to the boiler..I had this same issue but I had to put an inline valve before the cylinder to cause less flow and more resistance which meant I had extra pressure and flow to force the water around the much smaller pipework and valves of my radiator circuit as i am only using 1 circulation pump..
Also rethink has this always been an issue or maybe you have caused extra restrictions on your rad circuit..
Good luck..
Had also thought about that. The rad is a 3/4 inch too for the first 4 meters and it splits to 1/2 inch for the upstairs and downstairs. With the motorized valves in place only one direction opens so when I select heating that valve opens and then the boiler and pump come on so the only direction is the radiator direction. Thanks for the input .

Come to think of it when I have both motorized valves open the water only goes to the cylinder so it's looking more like a pump strength issue. Can you just put on a more powerful pump. Mine has two RPM settings and it's set to the higher setting already.

Last edited by Paul_Mc1988; 22-02-2020 at 06:04.
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22-02-2020, 10:21   #4
Turbohymac
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Ok.. like my earlier post
.forget about upgrading pump etc..a pump can only create a certain amount of flow but try to restrict the flow through your hot cylinder and your rads will then benefit...ideally you should actually be passing the water through the cylinder first ..to heat the water..then onwards to the radiators..but in your setup you need to restrict flow way down going to cylinder....it only needs a fraction of flow to heat the water. Not full flow.. put a lever valve inline to cylinder and shut it down half.. your rads should then be fine.. also you might even have to restrict the nearest rad to your boiler as the same principle applies..sometimes the furthest away rad can suffer from lack of adequate hot water flowthrough..
I think you're near sorted..dont bother replacing pumps etc ..just restrict..
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22-02-2020, 12:19   #5
John.G
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Hi guys,

Bought a house recently and have done a whole home renovation but left the main heating system mainly untouched (due to budget). House is a three bed semi D

I added one rad onto the existing system. Upstairs has four rads. One in each bedroom and the bathroom. Downstairs has a rad in the hall, a rad in the living room and rad in the kitchen/dinner ( this is the new rad as there was previously gas fires in here). I removed all rads and fitted TRVs on them. I also installed two motorized valves at the boiler.

When I turn on the cylinder circuit the water gets up to the cylinder and the return pipe at the boiler gets warm quite quick ( 5minutes). When I turn on the radiator circuit some of the rads are getting luke warm others not at all. The inlet pipe to the rads is hot but the outlet either cold or luke warm. I have bled all the rads numerous times but no joy. Any ideas. The pump is a myson and probably there since the original install 20+ years. Boiler is a worchester condensing boilerThinking the pump may be struggling and on it's way out. Any ideas?
Its not quite clear to me if one zone valve just feeds the cylinder coil circuit and the other zone valve feeds both the up and down stairs rads?. if so, or even not, it certainly won't be throwing away money in renewing the pump especially if you can install it yourself, I might suggest a DAB Evosta 4/7M, or (like my own) a Wilo Yonos Pico 1/6M. I have 8 double rads+2 singles and the pump on a 3M head (equivalent) can feed the 10 rads plus the hot water cylinder no problem, I do have a balancing valve on the cylinder coil but have never had to throttle it in, in > 40 years.
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22-02-2020, 14:03   #6
Paul_Mc1988
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Its not quite clear to me if one zone valve just feeds the cylinder coil circuit and the other zone valve feeds both the up and down stairs rads?. if so, or even not, it certainly won't be throwing away money in renewing the pump especially if you can install it yourself, I might suggest a DAB Evosta 4/7M, or (like my own) a Wilo Yonos Pico 1/6M. I have 8 double rads+2 singles and the pump on a 3M head (equivalent) can feed the 10 rads plus the hot water cylinder no problem, I do have a balancing valve on the cylinder coil but have never had to throttle it in, in > 40 years.
Yeah one zone feed the coil and the other zone the upstairs and downstairs rads.

I'll have a look at them pumps cheers for the help everyone. Be back on soon if I have any further issues
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22-02-2020, 14:31   #7
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Take off all the TRV heads. Make sure the pin on the valves moves freely. It only moves a few mm. If they are working, leave them off, turn cylinder zone off and Heating zone on. With this done, check that cylinder isn't heating and report back here on how the rads are heating.
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22-02-2020, 16:04   #8
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Take off all the TRV heads. Make sure the pin on the valves moves freely. It only moves a few mm. If they are working, leave them off, turn cylinder zone off and Heating zone on. With this done, check that cylinder isn't heating and report back here on how the rads are heating.
Will do this when I'm in the house tomorrow. Thanks Wearb.
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22-02-2020, 16:36   #9
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Also problem may have started with zone valves & TRVs installation, check that rads zone valve is pointing in right direction and is opening fully, its also possibly a pipework sludge problem but you would have seen this when the rads were removed. Air locks in system? maybe consider draining down system and refilling slowly with boiler switched off and both zone valves manually latched open to assist proper venting. (this is often overlooked).

Last edited by John.G; 22-02-2020 at 16:40.
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23-02-2020, 12:08   #10
Paul_Mc1988
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Also problem may have started with zone valves & TRVs installation, check that rads zone valve is pointing in right direction and is opening fully, its also possibly a pipework sludge problem but you would have seen this when the rads were removed. Air locks in system? maybe consider draining down system and refilling slowly with boiler switched off and both zone valves manually latched open to assist proper venting. (this is often overlooked).


What do you mean by the rad zone valve pointing in the right direction?
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23-02-2020, 12:20   #11
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There will be a arrow stamped on the body of the zone valve, ensure that the flow corresponds to this.
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24-02-2020, 13:42   #12
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Flushing the rads system did the trick here
Pipes were full of muck
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24-02-2020, 15:51   #13
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Got it sorted. Thanks everyone for the help. System just had a lot of air locks. Closed of all the rads valves. And started heating them one by one. Took about three hours to get it fully done. All rads piping hot.

One final query. The boiler is a worchester 15i condensing. How long after turning the heating off should it keep running the pump for? Is this an over run safety thing so hot water doesn't boil over in the boiler?
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24-02-2020, 19:53   #14
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Its to protect the boiler heat exchanger which is often made of aluminium, when you shut down the boiler then both zone valves close and the water will have no where to go except that (1) you have fitted a automatic bypass valve (ABV) or (2) you have plumbed in one rad with its flow taken upstream of the zone valve or (3) the boiler itself has a internal by pass which will eventually cool down the water but will take a long long time, your user manual will tell you if you have one. The pump overrun time would be ~ 2 to 5 minutes but some boilers keep the pump running until it sees the water temperature falling. I think some of the older boilers just kept running the fan for a certain period to achieve this cool down.
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25-02-2020, 19:36   #15
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Its to protect the boiler heat exchanger which is often made of aluminium, when you shut down the boiler then both zone valves close and the water will have no where to go except that (1) you have fitted a automatic bypass valve (ABV) or (2) you have plumbed in one rad with its flow taken upstream of the zone valve or (3) the boiler itself has a internal by pass which will eventually cool down the water but will take a long long time, your user manual will tell you if you have one. The pump overrun time would be ~ 2 to 5 minutes but some boilers keep the pump running until it sees the water temperature falling. I think some of the older boilers just kept running the fan for a certain period to achieve this cool down.
Hi John. Yeah the pump is running for a few minutes when both valves are closed. Will this damage the pump if the water has nowhere to go?. I dont have a rad upstream of the valves either!
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