Boiler firing, but not when it should

[vala]

Got an external Worcester oil combi.

A buffer vessel is connected to the flow and return from the boiler and this is used to supply heated water to the UFH manifold.

The buffer temp is monitored by 2 thermostats which are connected to a Grant GSX1 controller, and an easy relay fires the boiler a) when there is a call for heat AND providing the temp in the buffer is below 45°.

Today I noticed the boiler was firing every 45-60mins or so, and on the boiler control panel the heating temp was at max.

A quick call to Worcester technical and it would appear the boiler stat is what is being satisfied rather then the stats on the buffer vessel.

 

Anyone have any suggestions as to how to get the boiler to fir based on the stats in the buffer vessel?

[ProDave]

There are multiple "control loops" in operation here.

 

1) the buffer temp and it's 2 thermostats and this easy relay "calling for heat"

 

All that means is the buffer tank needs some heat input to keep the UFH working, so it calls for heat from the boiler.

 

2) upon getting the call for heat the boiler will turn on the pump and fire up to heat the water.  The boiler has an internal thermostat to set the water temperature (this is the temperature of the water that leaves the boiler on the flow pipe not the buffer tank temperature)  Once the water leaving the boiler reaches the set temperature, the burner shuts off.

 

1) and 2) are asynchronous meaning they operate completely independently.

 

So all that is happening is the boiler can heat the water quicker than the buffer tank can disperse it, so the boiler will not be on all the time, it will turn on and off to maintain the temperature it needs.

 

All sounds pretty normal, apart from I doubt the boiler temperature needs to be anything like near maximum.

[vala]

13 minutes ago, ProDave said:

There are multiple "control loops" in operation here.

 

1) the buffer temp and it's 2 thermostats and this easy relay "calling for heat"

 

All that means is the buffer tank needs some heat input to keep the UFH working, so it calls for heat from the boiler.

 

2) upon getting the call for heat the boiler will turn on the pump and fire up to heat the water.  The boiler has an internal thermostat to set the water temperature (this is the temperature of the water that leaves the boiler on the flow pipe not the buffer tank temperature)  Once the water leaving the boiler reaches the set temperature, the burner shuts off.

 

1) and 2) are asynchronous meaning they operate completely independently.

 

So all that is happening is the boiler can heat the water quicker than the buffer tank can disperse it, so the boiler will not be on all the time, it will turn on and off to maintain the temperature it needs.

 

All sounds pretty normal, apart from I doubt the boiler temperature needs to be anything like near maximum.

@ProDave

thanks for the reply.

The thing is, I’m currently at the house (not living here at the moment as got some other bits to sort first before moving back in), and there is a call for heat from the UFH. The controller is currently reading 39° and 41°. It’s set on that controller to turn on the boiler if below 45° and when there’s a call for heat.

But the boiler isn’t firing.

The boiler temp control is now on 50° as recommended by Worcester technical.

Edited February 19, 2022 by vala

[ProDave]

But what are the actual as measured flow and return temperatures measured on the flow and return pipes in and out of the boiler?  And what is the measured temperature of the water in the buffer tank?

 

To me is sounds like the boiler can heat the water a lot quicker than the UFH can use that heat, so you would not expect the boiler to be firing all the time would you?

[vala]

@ProDave
The temp in the buffer vessel according to the controller is 40° at the flow and return. The minimum temp set on the controller is 45°, so as there is a call for heat at the moment and the temp in the vessel is below 45°, the boiler should be firing.

And yes, you are right that the boiler shouldn’t be firing all the time, yet during today there has been a call for heat all day and the UFH pump has been on. The temp in the vessel earlier was 63°, yet the boiler still fired, when it shouldn’t have as there was sufficient heated water in the vessel.

This is when I noticed it and called up Worcester technical who suggested the boiler was probably firing up when ever the boiler stat was x amount of degrees below the temp set on the boiler.

They suggested to lower the boiler temp to 50°, which I have done. And since then the boiler hasn’t fired and the UFH has used the heat stored in the vessel.

But now the heat is lower then the ‘on’ temp set on the controller, there is a call for heat, but the boiler is not firing.

[ProDave]

Okay the obvious question, is this a new system and has it EVER worked properly?

 

If new and no it has not, then the installer has not wired it properly or not understood a control box that is outside his normal experience and done something wrong, call him back.

[vala]

@ProDave

The boiler was installed prior to us owning the house, however essentially the heating system is new.

And strangely it was all working as expected during the testing phase of the UFH.

Then it was all turned off and only turned back on again 3 or so weeks ago.

I'm now only working at the house as and when on some weekends hence not ever noticing this issue earlier.

Wiring I'm pretty sure is all correct as it's been fine before, just weirdly it's started acting up

And for reference here is the schematics for the wiring,

[dpmiller]

Surely the boiler stat has the veto here?

[ProDave]

On the drawing, what are "H/S", "TOC" (TDC?), and "B"

[vala]

@dpmiller

do you mean the boiler stat is what is being satisfied rather then the buffer stat's?

 

@ProDave

H/S = hubswitch. This is the Wunda hubswitch which allows connection of the smart controller and thermostats to the internet.

TDC - temperature difference controller. This is the Grant temperature difference controller which monitors the 2 thermostats in the buffer. The thermostats both connect to this controller.

B = boiler.

[dpmiller]

Just now, vala said:

dpmiller

do you mean the boiler stat is what is being satisfied rather then the buffer stat's?

 

yes. boiler stats tend to have quite a wide hysteresis so if it's clicked off, the water could need to drop 5-15C before the boiler will cut in again

[ProDave]

Ok H/S might play some part in smart controls but for this control system it only takes power, so plays no direct part, so I will ignore it.

 

TDC is the key to this, the thing that senses a temperature difference and "calls for heat"

 

So the simple logic there is the relay is performing a logical AND function, so When the TCD "calls for heat" AND the WUNDA UFH controller calls for heat as well, it sends presumably a call for heat to the boiler on the grey wire from terminal 10.

 

All looks okay there so to diagnose it you need to start doing some diagnosis.  Do you have any test equipment like a multimeter and know how to use it?  Does the realay have any visible indication so show when it is energised or not?

[vala]

@dpmiller

is there anyway of getting round the boiler stat and relying on just the buffer stat's?

 

@ProDave

before I left the house this evening I did check the relay with a multimeter to see if it is indeed energising and can confirm it is, only when there is a call for heat. I did a test and turned the UFH off, and tested the relay and can confirm no power sent to the switch live of the boiler.

There's 240v on the COM of the relay but I forgot to test the TDC.

I hope this shows the relay is ok and is energising when it should, but I do need to test the TDC and the switch live at the boiler.

[dpmiller]

5 minutes ago, vala said:

@dpmiller

is there anyway of getting round the boiler stat and relying on just the buffer stat's?

 

turn it up a bit.

[ProDave]

Just connect your multimeter to terminal 10 of the wiring centre.  When the UFH wants heat ANS the TDC says there is a temperature difference you should see 230V on terminal 10.  If either the UFH controller is not wanting heat, or the TDC is satisfied you should see nothing (there will always be a ghost voltage with a digital multimeter so don't expect to see 0V)

 

That is all there is to it.

 

Another thought, a lot of oil fired combi boilers (I don't know this one specifically) maintain a store of hot water.  This is usually done because an oil burner takes longer to fire up than a gas burner so would not otherwise be able to deliver hot water on demand quick enough.  Could it be the boiler firing from time to time to maintain this hot water store that is making you think it is firing for heating when it should not be?

[vala]

@dpmiller

if I was to up the boiler control to 60° and see where that got me on at the buffer.

but doesn't this then just eliminate the need for the buffer stats? And before when the boiler would cycle every 45mins or so (with it set at 75°), with the boiler stat at 50° at present, it took a good few hours for the temp in the buffer to come down to 45°. So to me that says much less cycling if I could use the buffer stats.

 

@ProDave

i'll get this done in the morning when I'm back at the house and report back.

thanks.

[dpmiller]

@vala no, the boiler stat is a final protective really, the buffer controls should be doing the work.

 

If the house is being heated well in all situations you might consider having a smaller jet fitted to the boiler to reduce it's capacity a bit?

[vala]

@ProDave

Tested terminal 10 with the UFH off, reading is 0v. Turned on the UFH, there is a call for heat AND the buffer is below 45° and we have 240v.

 

And yes there is a heat store within the boiler housing. But I don't believe it is firing due to that as the HW controls are off and hot water is not used in the house. I believe it will still maintain a set temp but this is dictated by the HW control knob on the boiler (which is set to 4 or 5.

Still doesn't explain why the boiler is not firing when there is a call for heat AND the buffer is below the on temp of 45°.

 

@dpmiller

when the boiler was set to 75° and was firing every 45-60mins or so, it did keep the house heated well, during the night we lost 1-1.5°.

One thing to note is that the buffer is set to turn off at 75° yet when the boiler was firing up (boiler control set to max), it never did reach 75°, stopping at around 68-69°. I now assume this is because the boiler stat is satisfied. And as the boiler  turns on when the boiler stat is low I can't see how it's using the stats on the buffer to dictate when to turn on and then to switch off.

[vala]

And just for reference last night before I left the house I turned off the UFH.

This morning the temp in the buffer had dropped to 28° and 30°.

When I turned the UFH back on to test terminal 10, the boiler did indeed fire up, but has not stayed on to satisfy the stat on the buffer. It has turned off and the buffer reads 40° and 41°.

So does this confirm that the boiler is firing when there's a call for heat, but is turning off when the boiler stat is satisfied?

[ProDave]

9 minutes ago, vala said:

@ProDave

Tested terminal 10 with the UFH off, reading is 0v. Turned on the UFH, there is a call for heat AND the buffer is below 45° and we have 240v.

 

And yes there is a heat store within the boiler housing. But I don't believe it is firing due to that as the HW controls are off and hot water is not used in the house. I believe it will still maintain a set temp but this is dictated by the HW control knob on the boiler (which is set to 4 or 5.

Still doesn't explain why the boiler is not firing when there is a call for heat AND the buffer is below the on temp of 45°.

 

@dpmiller

when the boiler was set to 75° and was firing every 45-60mins or so, it did keep the house heated well, during the night we lost 1-1.5°.

One thing to note is that the buffer is set to turn off at 75° yet when the boiler was firing up (boiler control set to max), it never did reach 75°, stopping at around 68-69°. I now assume this is because the boiler stat is satisfied. And as the boiler  turns on when the boiler stat is low I can't see how it's using the stats on the buffer to dictate when to turn on and then to switch off.

I tried to explain how 2 independant control loops were working in an earlier post.

 

The boiler will shut off (stop firing the burner) when the water leaving it reaches the set temperature. That is normal. to do otherwise would boil the water. So the boiler will NOT always be firing when you get 230V on terminal 10.

 

If the buffer tank thermostat is set to turn off at 75 degrees and the boiler temperature is less than 75 degrees, then the buffer thermostat will NEVER turn off.  Why has the thermostat for a buffer feeding UFH been set so high?

[vala]

@ProDave

apologies for not understanding this sooner.

first off I apologies, I should have noted it's 70° the buffer is set to. I kept writing 75° as that's what it is on the boiler. 
If the buffer off temp stays at 70°, and the burner has stopped will the boiler still circulate water till the buffer stat is satisfied?

Or does it all just come to a halt?

Also what I'm failing to understand is why when there is a call for heat at the buffer is below 45°, the boiler was not firing?

As for the 70° off temp, this is what was noted when I asked the query in a thread I started a while back. I assumed to allow a greater store of heated water for the UFH and to hopefully further reduce cycling of the boiler.

Edited February 20, 2022 by vala

[ProDave]

39 minutes ago, vala said:

If the buffer off temp stays at 70°, and the burner has stopped will the boiler still circulate water till the buffer stat is satisfied?

Or does it all just come to a halt?

If say the buffer stat is set to switch off at 70 and the boiler stat was set to 75, the buffer would continue heating up.  But with only 5 degrees difference the rate of heat actually going into the buffer would be slow, so it might take a long time to get there.  And during that time, the boiler would fire occasionally to keep it's flow temperature to it's set point.

 

39 minutes ago, vala said:

lAso what I'm failing to understand is why when there is a call for heat at the buffer is below 45°, the boiler was not firing?

That is a case where under that situation you would have to check if the 230V was present on terminal 10, i.e. is the temperature difference controller operating as expected.

[vala]

@ProDave

ok, I'm now understanding the boiler stat.

Still puzzled as to why when there's a call for heat and the buffer is under 45° it's still not firing up.

And I've just checked the terminal 10 and there is 230v present.

The controller solely monitors the temp via the 2 thermostat probes, the relay is what is connected to the switch live of at the boiler.

Checked through all the settings and it literally is just what temp you want on and what temp to turn off. Nothing else to it.

[Temp]

The stat on an oil boiler is like a limiter. The flame runs flat out until the flow temperature reaches the limit set by the boiler stat.  With a small load the flow temperature can hit the limit very quickly  causing cycling. The buffer tank is meant to stop this by acting as an extra load so the flow temperature stays below the boiler stat limit, or at least helps it stay below the limit for more of the time.

 

For this to work a lot of things have to be right... The boiler stat must be set high enough or it gets in the way and causes cycling. The pump speed must also be fast enough to suck the heat out of the boiler and put it in the buffer. The buffer must also be big enough and the stats on the buffer must be set up to so that the capacity is used (having a big fuel tank on a car is a waste if you always fill it up when it drops below 90% full).  

 

It took me awhile to tune up our system. When first installed it cycled so much the average output was too low to heat the house when it was very cold outside. I ended up fitting a bigger pump and smaller jets in the boiler. That pretty much eliminated cycling.

[Temp]

15 hours ago, vala said:

Still puzzled as to why when there's a call for heat and the buffer is under 45° it's still not firing up.

 

When it's in that state try turning up the boiler stat. If it fires up you know that the boiler stat is limiting.