Setting boiler part load figure on installation.

[kayball]

Hi all. I have a Vaillant 418 ECO tech plus boiler with VRT360f and Vr65 running into an open vent system (S Plan) Now that the outside temperature is becoming warmer, I find that boiler is cycling quite often. Have now found that setting 'd0' alows boiler to be set at a lower 'part load' figure than the default 18kW. Is it advantageous to set the part load figure much lower if sufficient heat is available in house. Since only 4 radiators are in circuit at the moment (upstairs is closed off) and after rough calculation of requirement, I have set 'd0' down to 10kW which appears to be adequate for heating and DHW. Does this improve efficency or not? Cycling now seems much reduced. Have I done the right thing by setting part load figure to calculated requirement?. Should profesional installers set the part load figure on installation? There is no reference to this in the installation manual left to me by the registered installer.
Regards.
Kayball

 

[D_Hailsham]

Never heard of an X plan! C, S, W and Y plans are the only ones I know about.

If the boiler is cycling more now it is getting warmer, it's because the boiler cannot modulate low enough. The 418 has a modulation range from 5 - 18.3kW. The heat required is directly proportional to the temperature difference between inside and outside. So, if you need 16kW to raise temperature from -1C to 21C, you only need 8kW to raise it from 10C to 21C.

Setting d.0 only has an effect when the outside temperature is very low, it stops the boiler cycling if the max output is much higher than actually required; e.g a 30kW feeding a system with only 15kW of rads.

A d.0 of 10kW may be OK a the moment, but it should really be set to provide the required output when the temperature is coldest.

 

[Biggles..]

Because you have a 400 series vaillant you must rate according to load in D0,failure to do so will result in unwanted S53 codes.

 

[kayball]

Hi again. Thanks for your replies.

Sorry about the reference to 'X plan' Just a typo. Have edited to correct.
However, I still don't seem to have an answer to some of my actual questions.

(1) Should professional installers set the part load figure (d0) on installation?

(2) I have set 'd0' down to 10kW which appears to be adequate for heating and DHW. Does this improve efficiency or not?

(3) Have I done the right thing by setting part load figure to calculated requirement?.

Pump (Grundfos 15-50 Selectric)) is set to minimum flow and still only achieving 6 deg C differential across boiler with all rads almost shut off at each LS.
I have never experienced S53 problems with my 418
Regards.

 

[Agile]

It increases efficiency a little by enabling the boiler to condense more.

It also lessens cycling which lowers the efficiency.

Most installers dont even know how to set d0 and even those who do usually dont think its necessary, after all they are not paying your gas bill.

Even some people posting on here "dont think its necessary"! Maybe they think the greater cycling improves efficiency!

If it was mine I would set it lower to match the current rads plus 2 kW for water. You can reset it as often as you like to give the maximum efficiency.

Do you have an autobypass? Is this passing to lower the differential even when all four rads are on? Is it correctly located?

The boiler does expect a minimum flow rate through it and I am surprised that your differential is so low. Have you measured it across each rad?

Tony

 

[kayball]

Thanks for your reply Tony.
Yes I do have an autobypass installed but had not thought of checking it. It is a Honywell DU145 and it had been passing as I found when checking the heat of the by--pass pipe. This is connected correctly immediately after the pump between flow and return and before both MVs. I have reset the valve by increasing differential pressure and will have a play to establish best setting. I don't have any 'noise' problems so don't understand why it was open so much.
I will recheck differential temperature at boiler when autobypass has been set for optimum.
Thanks again for this last suggestion; its something I hadn't thought about and also for your recommendation regarding the setting of d0.
Regards
Peter.

 

[Agile]

Let the system become cool. Close the largest of the four rads still in use. Set bypass to highest setting.

Start heating and lower the bypass setting slowly until you just start to feel hot water passing to the return side.

If the three remaining rads are quite large then close off the largest of them as well so the bypass just starts to open with only two rads on.

I am considering a rad to be larger if over 1 kW. These are all totally approximately.

Tony