Oversized boiler - efficiency issues?

[Steelmasons]

Hi Steelmasons thanks but could you elaborate?

Had a Gas certified gentleman service the boiler recently, all measurements within specs,

I get the impression that you would like to squeeze as much efficiency as possible out of this 637 EcoTEC , post the combustion analysis results and possibly we can squeeze a tad more..

 

[Steelmasons]

setting the boiler to 40c and using hot only would save a lot.

Really??...40c for stored water?

 

[Waldovr]

@Steelmasons

No not exactly - I know the boiler is well up to the task. Problem is that it's too big for what I need and I am seriously concerned this makes it run very inefficiently (from a cost perspective...)

Even though it can run at lower power this is only the burner, never the pump and in my situation this results in a bypass increasing the return temperature.

Does anybody have any thoughts on efficiency when running a big boiler like 637 with a two speed pump in the lowest ranges it can do (around 6kW?).
And any options to improve this?

Thanks

Waldo

 

[Steelmasons]

Have you thought about a LLH?..
Possibly I'm not helping you with regard to what you want to hear?
How much would you be willing to spend in order to gain a tad more efficiency?

 

[John D v2.0]

Really??...40c for stored water?

Ah didn't spot the cylinder, it'd work for a combi.

 

[Agile]

How are measuring the flow and return heating temps?

 

[The831Bunny]

I'd say in this case weather compensation would raise your efficiency. You would need a vSMART, or a VRC470 or VRC700, a VR66 or VR70, a VR10 sensor and someone to rewire the controls. The system bypass needs adjusting correctly to maintain the return temperature correctly too. It would also open up the possibility of adjusting the cylinder load independently of the heating. Would the outlay be worth it in fuel costs saved? In the short to medium term with no onward maintenance costs on the new equipment... only just.

 

[Waldovr]

Thanks @The831Bunny that confirms what I was thinking.

A shame it would only just justify the expense. But there's a feel good factor for using less fossil fuel too obviously. And I've got the suspicion it might be more comfortable to have the radiators on gently all the time. Currently you notice the boiler switching off when it's colder outside.

And I like that the system controls the temperature exactly and not via some on/of thermostat bolted on to the cylinder in the loft.

From what I gather it's basically rewiring the motorised valves from the old Honeywell control centre into a new box and wiring that box to the boiler; powering up the box and it's done.
Which qualifications are required to do this work?


Thanks

Waldo

 

[Waldovr]

Have you thought about a LLH?..

Well I have come across Low loss headers and as I understand it would decouple the boiler flow from the radiator flow... But I don't see how this would improve the situation, as the radiator flow will still be less than the boiler pumps around; basically it would mix colder return with some of the flow just the same as the current bypass in the boiler does, won't it?

Correct me if I'm wrong please!

Thanks

Waldo

 

[Waldovr]

How are measuring the flow and return heating temps?

The boiler reports what it measures inside just before and after the heat exchanger - d40 and d41.
I have bought a temp sensor thingy just for this with two contacts to click on the pipes and measure the temperatures just before and after the boiler.

There's a big difference in return values as the bypass is inside the boiler and is opening up. Which is explained by the wattage calculations in my op I believe.
Flow temps are almost identical.

Waldo

 

[D_Hailsham]

It sounds as if the bypass was not correctly set at installation.

The boiler is designed to run with a maximum differential of 20C. The 11C differential only applies to non-condensing boilers

The same pump is used for all versions.

 

[Waldovr]

It sounds as if the bypass was not correctly set at installation.

The boiler is designed to run with a maximum differential of 20C. The 11C differential only applies to non-condensing boilers

The same pump is used for all versions.

Thanks for your reply @D_Hailsham;

Same pump, are you sure? I've found documents for ecotecs and they have different pump graphs for different models, and my models' documentation lists different flows for different boiler versions. Latest versions seem to have a smarter pump that has a larger range of settings but that's not my model, I'm afraid. (It's a 2012 model).

I have had the central heating engineer check the bypass valve and he called Vailliant and they said he could adjust it to max. It did reduce the bypassing but not by much.

So either there is something wrong with the system (blockage? Bypass broken? 22 mm pipes not big enough?) or the pump is oversized for this system. Can't think of anything else but suggestions welcome.

I don't think it's the balancing as they all get around the same temp diff and furthest radiator lock shield is fully opened. So currently it is maximum flow with around 10 degrees differential.

20 degree differential would be best for condensing but trying to get a higher differential would only mean lower flow rates I believe as I would have to close lockshields to get bigger differential?

And as I understand things so far that would still mean the pump would work harder, put higher pressure on the water to shift it and ... bypass valve opens up more ... mixing hot water with my 20 degrees cooler return?

Please correct me where I go wrong in this logic!



Waldo

 

[Agile]

The boiler will be giving the flow and return temps after the bypass has allowed flow to the return.

The actual return temperature from the heating system may well be a lot lower and so the differential more than the boiler reported 10 C you are quoting.

Tony

 

[D_Hailsham]

Same pump, are you sure? I've found documents for ecotecs and they have different pump graphs for different models, and my models' documentation lists different flows for different boiler versions. Latest versions seem to have a smarter pump that has a larger range of settings but that's not my model, I'm afraid. (It's a 2012 model).

Sorry for the mistake about the pump; the 637 & 837 use a different pump from other models. Unfortunately the current manual does not contain any pump graphs.

The tech data table lists different flow rates for the various models. Is this what you are referring to?

You say you have a 2012 model. What is the complete VU number and what is the edition of the Installation manual (on back of manual)?

I have had the central heating engineer check the bypass valve and he called Vailliant and they said he could adjust it to max. It did reduce the bypassing but not by much.

The only way you will reduce the bypassing is by reducing the flow rate through the boiler, which means balancing for a higher differential. How far open are the lockshield valves on your rads (you said the last one was full open, so I guess the others are similar). Don't worry about the LS valves being closed to much, they only need to be open about half a turn to achieve the required flow rate.

22mm pipe is OK for 18kW but not for 37kW, unless it is very short.

And as I understand things so far that would still mean the pump would work harder, put higher pressure on the water to shift it

The lower the flow rate, the less the friction in the pipes, so the lower the pressure required. The pump therefore has less work to do, which means the point at which the bypass opens will be lower.

 

[Waldovr]

@D_Hailsham:

Pipe 22mm and quite long pipes; but boiler now range rated at 18 kW so that should be ok I guess. One more question to the person that put this oversized boiler there...!

Boiler says VU GB 376/5-5 R1. Manual is 0020116700_02 GBIE 062012. Serial number indicates 2012 (xx12xxxxxx)

This manual does not have the graphs but it does mention different flow rates for different types of boiler; yes that is the data from the list. That would the flow required at full power and 20 degrees differential (numbers concur with my math). I think I was wrong assuming this is the minimum allowed flow for the pump...


The previous model had documentation with a graph for the pump:
(True, older model, but I have a suspicion it's still the same pump and abv, so for the sake of argument, let's assume it is correct?)

So the question basically boils down to "How can I get this boiler to pump around only about 600-700 litres per hour and still keep that bypass closed??"
Restricting flow through radiators would move the flow/lift point on the above chart to the left and up, correct?

Until the ABV kicks in; I assume that is where the flat parts at 350, 250 and 175 mbar are. (That would correspond to min, default and max settings for ABV as in the manual).

Currently pump is in lowest settings (on auto and power has been range rated to 18kw). I think that's the grey line .-.

I've calculated flow through boiler by range rating it at several limits of kW's and checking what temperature differential is reported (d41-d40), ie. 12kw gives me 8 degrees; 15kW gave me 11 degrees, 22 kW gave 16 degrees. That translates to at or around 1200 litres/hour, give or take a few. (degrees are rounded to the nearest so there's some error here).
(This all with pump at low setting and ABV max at 350 mbar).

Assuming the chart is correct and I read it right this shouldn't be the case with the ABV at 350 mbar?

So, why? Blockage in the pipes somewhere so the pressure is way higher? Muck in pipes or bypass? Bypass spring broken and always open? Or am I reading the chart wrong?

Thanks again for all help in getting me to understand this system!

Would really want to be certain what's up before forking out money on WC and finding out the system was full of debris or the boiler is just way too big...

Waldo