High Limit Stat tripping

THE ALM

Hey all,

Hope somebody can give me some pointers.

Simple enough setup I would think. Warmflow Whitebird Utility boiler feeding two circulating pumps, one feeding radiators and the other feeding hot water cyclinder.

Can have the heating or heating & hot water on together but when just running hot water the high limit stat will trip.

Have replaced the two circulating pumps in the last couple of weeks which has made no difference, old ones where starting to make a bit of a noise so needed replacing anyway (13yrs running at this stage).

Pipe into cyclinder would be warm and pipe leaving cyclinder also warm. Would find that the hot water would be warm but never hot enough that I would need to mix with cold water.

Would also find that I could bleed some air from closest radiator, also in utility room, regularly enough in case this has any bearing.

Any advice appreciated.

heating set up.jpeg

jimf

do you a problem with lime in your area

does your cylinder have a balancing valve

im kinda thinking the fact the water in the cylinder doesn't get really hot you may have a restriction through it

with both water and ch on the water has another circuit to flow so hl stat wont trip

THE ALM

jimf wrote: »

do you a problem with lime in your area

does your cylinder have a balancing valve

im kinda thinking the fact the water in the cylinder doesn't get really hot you may have a restriction through it

with both water and ch on the water has another circuit to flow so hl stat wont trip

no balancing valve, will double check to be sure.

Fairly high on the lime I would think. kettle get de-scaled regularly enough. I would imagine no easy way to check this with the cyclinder?

What would the normal course of action be then, new cylinder?

jimf

new cylinder might be a bit premature

get a plumber to check it over for you

it sounds to me as if the heat in the boiler cant dissipate fast enough through the cyl but im no plumber

the other lads will see this later and comment further im sure

Wearb

Did you tighten any of the pipes going into the cylinder?

THE ALM

Wearb wrote: »

Did you tighten any of the pipes going into the cylinder?

Nothing touched on the cylinder side. Only things changed were the pumps.

THE ALM

jimf wrote: »

new cylinder might be a bit premature

get a plumber to check it over for you

it sounds to me as if the heat in the boiler cant dissipate fast enough through the cyl but im no plumber

the other lads will see this later and comment further im sure

Seems that way so thought the new pump would have sorted it.

jimf

THE ALM wrote: »

Seems that way so thought the new pump would have sorted it.

pump is really just a circulator so wont be able to overcome any resistance

THE ALM

jimf wrote: »

pump is really just a circulator so wont be able to overcome any resistance

Trying to locate resistance will be the next course of action, no harm in having the new pumps and can rule those out as a cause. Trying to work out where resistance is mightn't be as easy.

John.G

THE ALM wrote: »

Trying to locate resistance will be the next course of action, no harm in having the new pumps and can rule those out as a cause. Trying to work out where resistance is mightn't be as easy.

The new pump is probably A rated with a number of operating modes, including constant head and variable head, you might see what operating mode its set on (and post the pump type/model) and even though as jimf states it wont overcome a huge resistance/blockage, I would ensure that its set to constant head with say a 4M setting to start with, the flow and return at the cylinder might then get red hot, you can worry about the heat transfer later.
I presume there is no motorized valve zoning as well as the circ pumps in your systems?.

THE ALM

John.G wrote: »

The new pump is probably A rated with a number of operating modes, including constant head and variable head, you might see what operating mode its set on (and post the pump type/model) and even though as jimf states it wont overcome a huge resistance/blockage, I would ensure that its set to constant head with say a 4M setting to start with, the flow and return at the cylinder might then get red hot, you can worry about the heat transfer later.
I presume there is no motorized valve zoning as well as the circ pumps in your systems?.

Pump is heat merchants own branded HM Pro 4-7 and is set at proportional pressure pp6, no zones valves on system.

John.G

THE ALM wrote: »

Pump is heat merchants own branded HM Pro 4-7 and is set at proportional pressure pp6, no zones valves on system.

If there is zero or very low flow then the pump will modulate down to 2/3 M. If you change to the max fixed speed it will run at full speed, it might tell you something even if you only leave it on this setting for a short period as you don't want the pump to overheat if complete blockage which looks probable as old pump gave same symptoms.

I would then drain down system and break the connections at the cylinder coil flow & return and prove with a hose or test pump that the coil has internal blockage, if not the coil then prove/disprove the flow & return lines clear. You can then make the right decision re changing the hot water cylinder.

agusta

Have you a non return valve on the hot water zone

THE ALM

John.G wrote: »

If there is zero or very low flow then the pump will modulate down to 2/3 M. If you change to the max fixed speed it will run at full speed, it might tell you something even if you only leave it on this setting for a short period as you don't want the pump to overheat if complete blockage which looks probable as old pump gave same symptoms.

I would then drain down system and break the connections at the cylinder coil flow & return and prove with a hose or test pump that the coil has internal blockage, if not the coil then prove/disprove the flow & return lines clear. You can then make the right decision re changing the hot water cylinder.

Will check that out with the pump and see how it goes although I fear a draining of the system will be in the offing...

THE ALM

agusta wrote: »

Have you a non return valve on the hot water zone

will need to check that to be sure, I am thinking no but will need to confirm.

John.G

THE ALM wrote: »

will need to check that to be sure, I am thinking no but will need to confirm.

That's a pretty good question as systems like "systemlink" who only use zone circ pumps as well have a poppet type NR valve fitted just after the circ pump to prevent thermosyphonyng when the zone pump is off, it might also be located on the return, this type of NR, IMO, can/could be prone to sludge build up so well worth checking it out.

THE ALM

John.G wrote: »

That's a pretty good question as systems like "systemlink" who only use zone circ pumps as well have a poppet type NR valve fitted just after the circ pump to prevent thermosyphonyng when the zone pump is off, it might also be located on the return, this type of NR, IMO, can/could be prone to sludge build up so well worth checking it out.

Nothing as complicated as a systemlink system and this is a picture before pumps changed and no NR valves on this end.circ_pumps.jpg

Edit: Actually is that the NR valve at the bottom of the pump attached to the bottom isolator valve?

John.G

THE ALM wrote: »

Nothing as complicated as a systemlink system and this is a picture before pumps changed and no NR valves on this end.circ_pumps.jpg

If the hot water cylinder is located well above the boiler and if no NR valve fitted then when your system was healthy one would think that there would still be have been some heating of the cylinder with the rad zone calling for heat but the hot water zone not.

Edit: Just saw your edited post there so check that out, I can't see it clearly enough.

THE ALM

John.G wrote: »

If the hot water cylinder is located well above the boiler and if no NR valve fitted then when your system was healthy one would think that there would still be have been some heating of the cylinder with the rad zone calling for heat but the hot water zone not.

Actually in a bungalow so boiler and cylinder would be pretty level to each other and I would say that there didn't appear to be any water heating when just calling for heat.

THE ALM

[QUOTE=Edit: Just saw your edited post there so check that out, I can't see it clearly enough.[/QUOTE]

will do now that I am sitting here thinking about it so will see this evening.

agusta

THE ALM wrote: »

Nothing as complicated as a systemlink system and this is a picture before pumps changed and no NR valves on this end.circ_pumps.jpg

Edit: Actually is that the NR valve at the bottom of the pump attached to the bottom isolator valve?

It does look like the two NR valves are there

THE ALM

Thanks agusta and John.G that has given me a couple of pointers to check out.

Thankfully a long weekend ahead so hopefully get a chance to have a look.

Blowheads

https://touch.boards.ie/thread/2057918084/11/#post108383444

Have a read of this, might help.
Since we re plumbed the piping exiting the boulder and new pump no high limit trip since, that's not to say it's fixed but might give you some ideas

THE ALM

Blowheads wrote: »

https://touch.boards.ie/thread/2057918084/11/#post108383444

Have a read of this, might help.
Since we re plumbed the piping exiting the boulder and new pump no high limit trip since, that's not to say it's fixed but might give you some ideas

Thanks Blowheads, some things to check there especially the flow and return temperatures.

Just having a, not very scientific, look at it it would seem that the temp before and after the pump is the same and would make me think that the flow through the pump is ok. Checking the temperature, again not scientifically and just by feel, it feels like the temperature entering and leaving the cylinder is constant.

I am wondering if the problem is that the heat flowing through the cyclinder coil is not dissipating quickly enough maybe just to a limescale covered coil?

Will have to get a thermometer to check and try to further narrow it down.

Blowheads

THE ALM wrote: »

Thanks Blowheads, some things to check there especially the flow and return temperatures.

Just having a, not very scientific, look at it it would seem that the temp before and after the pump is the same and would make me think that the flow through the pump is ok. Checking the temperature, again not scientifically and just by feel, it feels like the temperature entering and leaving the cylinder is constant.

I am wondering if the problem is that the heat flowing through the cyclinder coil is not dissipating quickly enough maybe just to a limescale covered coil?

Will have to get a thermometer to check and try to further narrow it down.

Is there air getting into the system? Bleeding rads regularly?

That also could be an issue if there is air getting into boiler..

Is there an automatic air valve on boiler? Mine didn't, does now

Lastly, what is the temperature setting on the boiler stat itself? I turned mine down to 65

John.G

Blowheads wrote: »

Is there air getting into the system? Bleeding rads regularly?

That also could be an issue if there is air getting into boiler..

Is there an automatic air valve on boiler? Mine didn't, does now

Lastly, what is the temperature setting on the boiler stat itself? I turned mine down to 65

Limescale could certainly be a problem especially since the hi limit stat is only operating when the boiler is feeding this circuit only but even with a clean coil the faster you circulate the water through the coil the quicker the water heats as the mean coil temperature is higher, the rapid recovery hot water cylinders achieve their target by circulating the water at tremendous flow rates...something like 40 LPM, IF the NR valve isn't causing a restriction (have you removed and inspected it?) then you should be circulating the water at a very high rate when you have a brand new pump installed. I can't remember what make/type of pump you fitted but its more than likely a A rated pump, I would read up the users manual on it and if not all ready configured this way I would put it on a fixed head setting of say 3M. Again I really think that you should also remove the NR valve and inspect it/renew it before spending too much money.

THE ALM

Blowheads wrote: »

Is there air getting into the system? Bleeding rads regularly?

That also could be an issue if there is air getting into boiler..

Is there an automatic air valve on boiler? Mine didn't, does now

Lastly, what is the temperature setting on the boiler stat itself? I turned mine down to 65

One rad I could bleed regularly enough. No automatic air vent on boiler just the prv at the end of the flow pipe feeding the pumps.

Boiler has a dual stat with settings 0-5 and is generally set at about 2.5.

THE ALM

John.G wrote: »

Limescale could certainly be a problem especially since the hi limit stat is only operating when the boiler is feeding this circuit only but even with a clean coil the faster you circulate the water through the coil the quicker the water heats as the mean coil temperature is higher, the rapid recovery hot water cylinders achieve their target by circulating the water at tremendous flow rates...something like 40 LPM, IF the NR valve isn't causing a restriction (have you removed and inspected it?) then you should be circulating the water at a very high rate when you have a brand new pump installed. I can't remember what make/type of pump you fitted but its more than likely a A rated pump, I would read up the users manual on it and if not all ready configured this way I would put it on a fixed head setting of say 3M. Again I really think that you should also remove the NR valve and inspect it/renew it before spending too much money.

that was another thought that I had in that the water was flowing at such a rate that it was flowing too quick and not having time to dissipate heat, will reduce the setting on the pump and see if it makes a difference. Cylinder is a standard 140L insulated copper cylinder.

The pump is Heat Merchants own HM Pro 4-7 which seems to be a re-badged DAB Evotron 4/7.

Will certainly check the NR valve before going too far.

Blowheads

THE ALM wrote: »

that was another thought that I had in that the water was flowing at such a rate that it was flowing too quick and not having time to dissipate heat, will reduce the setting on the pump and see if it makes a difference. Cylinder is a standard 140L insulated copper cylinder.

The pump is Heat Merchants own HM Pro 4-7 which seems to be a re-badged DAB Evotron 4/7.

Will certainly check the NR valve before going too far.

Would check having a AAV on the boiler, newer ones have them inbuilt.

You would need to know the return flow temperature to know the water is not dissipating the heat

Is there a bypass fitted? Just a loop on itself to allow the pump to circulate water if all valves closed

Is there a frost stat? What that set at?

For the boiler dual stat, few things to try, turn down number to 1.5 maybe, ( mine runs from 55 to 90) so 2.5 should be ~75. I turned mine back down to 65.
Maybe also consider to replace the dual stat, they can deteriorate
Check the hole for the stat is not dirty

John.G

THE ALM wrote: »

that was another thought that I had in that the water was flowing at such a rate that it was flowing too quick and not having time to dissipate heat, will reduce the setting on the pump and see if it makes a difference. Cylinder is a standard 140L insulated copper cylinder.

The pump is Heat Merchants own HM Pro 4-7 which seems to be a re-badged DAB Evotron 4/7.

Will certainly check the NR valve before going too far.

Yes you actually did say in your post No11 that its HM Pro 4-7 set @ PP6, I would change it to fixed pressure FP and try it at different settings starting as you suggest at a low setting and gradually increasing it but even if there is a restriction the pump should't have modulated below 2 to 3 M at minimum on PP6 which should still be quite OK to to give adequate circulation through the cylinder coil.
You also stated in post1 that the water in and out of the coil is warm, unless there is a total/huge restriction somewhere then the water in (whatever about out) to the coil should be be roasting hot....as hot as it leaves the boiler or pump almost, if its only warm where it enters the cylinder then there is only a trickle getting through and its no wonder the hi limit stat is operating.
http://www.heatmerchants.ie/v4/581ae913-e1fb-435e-a88b-ded312da5b38/uploads/HM%20PROflyer.pdf

THE ALM

Blowheads wrote: »

Would check having a AAV on the boiler, newer ones have them inbuilt.

You would need to know the return flow temperature to know the water is not dissipating the heat

Is there a bypass fitted? Just a loop on itself to allow the pump to circulate water if all valves closed

Is there a frost stat? What that set at?

For the boiler dual stat, few things to try, turn down number to 1.5 maybe, ( mine runs from 55 to 90) so 2.5 should be ~75. I turned mine back down to 65.
Maybe also consider to replace the dual stat, they can deteriorate
Check the hole for the stat is not dirty

No AAV fitted, it is warmflow whitebird utility model so no frost stat either. Will turn down dual stat and see how we go.