Vaillant ecotec plus 825 - water going on internal bypass

[ArturL]

Dear all;
I feel I need some ideas as to where the problem might be.
I have a Vaillant Combi ecotec plus 825, 1 zone 9 rads, 6.7kW heat loss CH system.
This is controlled by a WC SensoHome.

The issue is that the boiler registers incorrect delta on Flow & Return (ca. 8-9C drop) due to water constantly circulating on the internal bypass.
Radiators are balanced properly and each one returns a drop of 18-20degC.
When the return is coming to the boiler it is mixed with hot water from the Flow going on internal bypass.
Subsequently the NCT sensor for the return which is located after the bypass is reading much higher temp than the whole circuit drop actually is.
I have a differential thermometer and measuring the return right under the boiler is showing a drop of ca. 18deg. With higher flow temps it is closer to ideal 20deg. Yet when this is mixed with the flow on the internal bypass - this is then raised to only 8-9deg drop as per the temp sensor (at D.041 installers menu).

The bypass valve in the diverter has been fully screwed IN to force the flow to the RADIATORS rather than go on the internal bypass and this helped a lot (with the factory default settings it caused the flow going on the bypass so much that the drop was ca. 4-5 deg). So with this screwed all the way IN, it is much better and given the WC adjusts the flow to max 55C - the boiler is constantly in condensing mode, so we're kind of good here.
But still I believe the efficiency would be better if the actual Delta achieved in the circuit could be registered at the boiler return.

What can be done to remedy this issue of internal bypass letting the water circulate almost at its own whim....

 

[Johntheo5]

Here's what you can achieve just by reducing the boiler flowtemperature, the dT decreases but the return temperature reduces to give very high boiler efficiencies, remember condensing only begins at a return temp of 55C.

 

[ArturL]

Thank you Johntheo5 for your reply.
I understand this is some of your own measurements put in the table.
I know very well when boilers start condensing but thx for reminding me that
I am not sure if I understand correctly what you're trying to put across here.
Are you saying that it is correct that the bypass valve is opening freely letting the heated "flow" water mix with the return and the delta of 9deg is correct although the whole circuit drop measured right before the return pipe hits the boiler (so before the internal bypass mixing) is 18-20deg and this is OK and normal?
If the bypass stayed shut as it'd expect it, as there seems no reason for the ABV to open, the return temp would be at dT ca. 18-20deg.
Am I missing sth here?

 

[Johntheo5]

These calculations are based on a dT of 20C and a assu.ption that you require a high rad output on occasions, ,(87% of max), and based on that the rads will perform or should exactly as I've show by reducing the flowtemp, having the bypass closed gives the best results but it should open or a external ABV should open on pump overrun.
As shown above the dT reduces "automatically" due to the nature of hear transfer in the rad.
If you want to give me your actual flow/return temps then I can re do the above calcs if you wish
.

 

[ScottishGasMan]

The issue is that the boiler registers incorrect delta on Flow & Return (ca. 8-9C drop) due to water constantly circulating on the internal bypass.
Radiators are balanced properly and each one returns a drop of 18-20degC.

Unfortunately your system is set up wrong.

You do NOT strangle radiator valves to achieve 20dt across the system. You size the heat source and radiators to give 20dt. Since your boiler is not sized for that, you cant get it without causing other issues.

You should have opened up all the radiators so the water was only bypassing minimally or not at all then balanced to whatever was right for your install. ie, let system run with all rads fully open, once room hits about 20 degrees, check DT accross boiler F&R, then check the same is true over each radiator. The ones that are lower DT you restrict slightly to get them to match the DT at the boiler.

This ensures a good flow rate at the system which reduces the water being bypassed at the boiler naturally, allows the pump to auto speed up and slow down to suit the boiler, and most importantly you wont trash the main heat exchanger if theres ever a fault, which you risk by shutting the bypass down.

DT20 is a theoretical difference in most cases, and its certainly not achivable correctly using combis that are far too big for the heating load of the house.

In my own case, Heat loss is around 6-7kW, 18kW boiler, DT typically 5-6 degrees, but with the higher MWT of the radiators I can run 30-40 degrees flow temp most of the year, with an absoloute max of 45.

If I had a boiler that was 7kW at full flow rate, then I'd have DT20 accross the system.

 

[ArturL]

Thank you ScottishGasMan for your time to respond.

Unfortunately your system is set up wrong.

You do NOT strangle radiator valves to achieve 20dt across the system. You size the heat source and radiators to give 20dt. Since your boiler is not sized for that, you cant get it without causing other issues.

My system is designed and set-up to give 20deg delta and the required flow rates for the rads to give the desired output are achieved with AutoBalancing valves from IMI Heimeier. As with those types of valves (be it the Drayton ones, Danfoss, Heimeier or Oventrop or Honeywell or whoever else manufactures them too) they are dynamic and do not allow the rads to be overflown with water keeping the right amount of flow / pressure that is required for the system to achieve designed values.
Set-up for the worst case scenario currently, yet the flow rates at lower flow temps (due to higher outside temps) should still give out ca. dT15 on each of the emitters. And this values can be obtained when measuring each radiator with differential thermometer as well as on the return pipe underneath the boiler.

You should have opened up all the radiators so the water was only bypassing minimally or not at all then balanced to whatever was right for your install. ie, let system run with all rads fully open, once room hits about 20 degrees, check DT accross boiler F&R, then check the same is true over each radiator. The ones that are lower DT you restrict slightly to get them to match the DT at the boiler.

As I said in my post - the whole circuit returns proper drop of almost ideal 18-20degC BEFORE it is mixed internally with the flow heated water on the internal bypass in the boiler. So with how it was set-up the F/R drop is correct at the rads as well as for the whole circuit.

This ensures a good flow rate at the system which reduces the water being bypassed at the boiler naturally, allows the pump to auto speed up and slow down to suit the boiler, and most importantly you wont trash the main heat exchanger if theres ever a fault, which you risk by shutting the bypass down.

So is it considered shut down when the ABV is fully screwed IN?? If this was shut I would expect no water to go that way, yet it is passing the water through.

DT20 is a theoretical difference in most cases, and its certainly not achivable correctly using combis that are far too big for the heating load of the house.

I get you. I agree this combi is oversized, yet still the smallest in this product line. Inherited by the previous owners
It is 19kW, however the low end is 3.9kW. which is lower than the max heat-loss of the property being ca. 6.7kW.

In my own case, Heat loss is around 6-7kW, 18kW boiler, DT typically 5-6 degrees, but with the higher MWT of the radiators I can run 30-40 degrees flow temp most of the year, with an absoloute max of 45.

The thing I am finding odd is that regardless of the flow temp (we have the controller set-up to go in a range between 45 and 60degC) which effectively gets 45-53 depending on the outside temp and auto-adjustable curve as calculated by the controller is best - the dT does not change and is always 8-9degC (according to d.041)

The pump is the modulating version which was effectively installed with the latest versions of the Ecotec Plus boilers after 2016 I guess and it is set to run with auto adjustable speed (in the installer menu). So I understand it knows what it is doing and at what speed it is required to go at.

So where might be the issue with this set-up given the above clarifications ?

 

[Johntheo5]

Thank you ScottishGasMan for your time to respond.



My system is designed and set-up to give 20deg delta and the required flow rates for the rads to give the desired output are achieved with AutoBalancing valves from IMI Heimeier. As with those types of valves (be it the Drayton ones, Danfoss, Heimeier or Oventrop or Honeywell or whoever else manufactures them too) they are dynamic and do not allow the rads to be overflown with water keeping the right amount of flow / pressure that is required for the system to achieve designed values.
Set-up for the worst case scenario currently, yet the flow rates at lower flow temps (due to higher outside temps) should still give out ca. dT15 on each of the emitters. And this values can be obtained when measuring each radiator with differential thermometer as well as on the return pipe underneath the boiler.



As I said in my post - the whole circuit returns proper drop of almost ideal 18-20degC BEFORE it is mixed internally with the flow heated water on the internal bypass in the boiler. So with how it was set-up the F/R drop is correct at the rads as well as for the whole circuit.



So is it considered shut down when the ABV is fully screwed IN?? If this was shut I would expect no water to go that way, yet it is passing the water through.


I get you. I agree this combi is oversized, yet still the smallest in this product line. Inherited by the previous owners
It is 19kW, however the low end is 3.9kW. which is lower than the max heat-loss of the property being ca. 6.7kW.



The thing I am finding odd is that regardless of the flow temp (we have the controller set-up to go in a range between 45 and 60degC) which effectively gets 45-53 depending on the outside temp and auto-adjustable curve as calculated by the controller is best - the dT does not change and is always 8-9degC (according to d.041)

The pump is the modulating version which was effectively installed with the latest versions of the Ecotec Plus boilers after 2016 I guess and it is set to run with auto adjustable speed (in the installer menu). So I understand it knows what it is doing and at what speed it is required to go at.

So where might be the issue with this set-up given the above clarifications ?

If you have set up the system to give a dT of 20C, then for any given change in flowtemperature, ie OT compensation, then the flowrate MUST change to achieve that 20C dT, your TRVs certainly won't do that, they control based on the room temperature only and some like yours will try and maintain a minmum flow through the rad based on the dP across the TRV, auto speed control will also help to achieve a set dT but except the boiler logic can control the flowrate by using the dT to do so then its a pretty tall order to achive this.

Example again based on the radiator heat "laws"
If the rad has been set up as following, flowtemp 75C flowrate 0.62LPM/kw then the returntemp will be 55C, dT 20C. output (87.2% of a T50 rad)
If the flowrate remains the same and if the OT curve determines that the flowtemp required is 43.5C then he return temp will be 36.5C, dT 7C. output (30.4% of a T50 rad). If a dT of 20C is required to give the same output (30.4%) then a flowtemp of 50C will give this (returntemp 30C) BUT the flowrate must be reduced to 0.22LPM, so not a easy task and if the minimum allowable flowrate is set too high on your TRVs then the dT will just keep decreasing with decreasing rad heat demand.

 

[ScottishGasMan]

The boiler is part of the system, so the system is NOT designed for 20DT, as the boiler would have to have a maximum output that matched load.

There's a world of difference between book setup of a system and real life.

The bypass screwed fully shut shouldn't be "closed" there should still be some water flow through it, although the fact you have left the pump on auto means there is material risk of catastrophic damage to the appliance, if you read the boiler manual it will tell you to set the pump speed on 100% if your closing down the bypass, as its purpose is not what your using it for. (Which by the way, if your not a heating engineer you shouldn't be in there anyway as you disturb the sealed chamber of the boiler which should only be done by someone competent who has the correct tools to ensure safe functioning afterwards)

If the boiler was sized for the system (which I get cant be achieved with a combi) Then the flow rate of the system at DT20 would be the same as the boilers nominal flow rate, so the bypass would not open in the first instance.

The boiler is designed to achieve DT20 across the flow and return at Full load and nominal flow rate, Your emmitor circuit cannot take either of those, so the bypass must be able to open and protect the boiler. Yes it modulates down, but given your calculations you will be on the boilers minimum load at approx. 8 degrees C and above outside temp when the property heat loss is about 3.9kW or less.

The pumps in the boilers are designed for the nominal flow, so if at maximum on a 25kW boiler it need about 1000 litres per hour of circulation, then this is what its designed to do. Modulating pumps can slow down, but not by much, ie 50% pump signalled speed is not then 500litres per hour, more like 800 or above.

Your emitter circuit at full output needs about 287 litres per hour at max, on a boiler that gives 1000+ litres per hour on max, so there is a significant discrepancy there between a "full system" designed for DT20 vs an "emitter circuit" designed at DT20.

If you want maximum condensing and a happy appliance, leave the bypass as set, alter the rad valves to a DT of 5 or 10, and let more water flow through the system, the fact you have Weather compensating control is the most important factor on efficiency, the lower flow temps you run the less the DT matters.

I've used the IMI valves and In my opinion autobalancing valves are nice, they do what they say on the tin, but there a gimmick that doesn't promote best efficiency on most UK systems (COMBIS).

I removed them from my system in favour of lockshields, manually balanced system, I get between 4-6DT accross the rads and the boiler produces more condensate than ever. If the mean water temperature of the radiator is higher (which you get with narrower DT) then the flow temperature can be lower, meaning more of the year you have condensate forming on more of the heat exchanger, VS running higher temp with wider DT which can achieve less condensing (assuming flow temp is higher)

 

[ScottishGasMan]

Also forgot to mention, there is a hysterisis level when the boiler cant modulate low enough to the load. What that means is if your targeting say 40 degrees, if on minimum load, the water moving through the boiler is going too slow or the water returning is too warm for it to maintain 40 degrees and it starts to overshoot since it cant lower the heat input any more, then it will allow an over heat of about 5 degrees usually before it cuts the burner off. so even set at 40 you will often see 45 when your outside the modulation range of the boiler, and higher if the flow rate is significantly reduced.

 

[ArturL]

very lengthy responses, so will need some more time to properly digest the content.
Due to the weekend and fatherhood responsibilities - I shall leave that for the evening read In the meantime thank you both for the replies.