CM-Zone and boiler cycling

[flipper321]

A little advice please.

We have a cm-zone wireless thermostatic valve system coupled with a Vaillant EcoTec 837 Plus boiler.

In the morning the only room calling for heat is our bedroom, the boiler quickly heats the rads up to the required temp and then cycles regularly between heating (for say 30 seconds) and then the "anti-cycling" pump over-run.

Depending on outdoors temps it can do this cycling for anything up to a couple of hours (optimum start)

Is this healthy/ok for the boiler?

Should we install higher output/volume rads?

Should we have some "always on" rads elsewhere in the house?

Any other thoughts?

Thanks

 

[flipper321]

A quick additional question.

Can anyone explain the anti-cycling feature of my boiler as it seems it was intended to counter the exact problem I'm encountering?

I should have said in my above post that the time between "burns" is no more than about 3 mins.

 

[D_Hailsham]

Do you have a big house?

Use either Sedbuk Boiler Calculator or EST online boiler calculator to calculate your heating requirement then post the answer.

 

[flipper321]

It's a pretty big 3 storey 4/5 bed house.

That calculation comes out at about 20kW.

I've noticed that a 2 minute anti-cycle time for a boiler set to 70 degrees is per spec but the anti-cycle time can be increased to about 6.5 minutes. Also these times are increased if I turn down the radiator temp on the boiler?

I should note it's pretty rare that we have the heating on in the whole house due to the zoning, and as described in the morning it's possible only one small rad may be on. We do need the capacity to heat our downstairs area though which is fairly large and open plan and incorporates a couple of pretty big rads and some wet undefloor heating.

 

[Biggles..]

Your boiler setup is currently too great for the heating load. You need to downrate your boiler in d0 ,normally you would set it to the figure dh had you calculate but due to your cm zoning
you may find this still to be too high for correct modulation.

Well worth considering adjusting the auto bypass value and anticycle time in the installer levels.

Lee

 

[flipper321]

Thanks Lee -

I'll turn down the heating output in d0 (assume this doesn't affect hot water heating) - and try out a higher anticycle value.

Can you explain the bypass bit - I think the bypass valve on my boiler is a manual affair (no settings from what I can tell) but don't really understand what it does or how I should change it to improve my issue.

Many thanks

 

[D_Hailsham]

Can you explain the bypass bit - I think the bypass valve on my boiler is a manual affair (no settings from what I can tell) but don't really understand what it does or how I should change it to improve my issue.

A bypass is a short cut between the flow and return. It is connected in the flow after the pump and in the return between boiler and rads. The purpose of a bypass is to maintain the correct flow through the boiler when TRVs are shut. The bypass contains a valve which automatically opens and closes as TRVs close and open to maintain the correct flow rate.

The bypass on your boiler is automatic. Instructions for adjusting it are in the Installation Manual.

I agree with Lee about down-rating your boiler in d.0. That's why I asked the question about house size and pointed you to the sizers.

 

[flipper321]

Appreciate the help guys.

Will downrate the boiler and see how I get on. After that I'll consider changing the anti-cycle timings.

We do seem to get a lot of "whooshing" at the moment, particularly when a lot of the TRVs are off - is this likely to be rectified by the down-rating or is this an issue with regards to getting the bypass valve set correctly?

To slow down the flow I assume you need a lower bar setting of the valve? (It's range is .175 to .35 and is currently set around .25)

 

[flipper321]

I've adjusted the boiler heating setting a increased the anti-cycling time but in the single rad scenario we still see a very high cycling rate.

Is there any secret to adjusting the bypass valve down or is it a case of suck it and see?

 

[flipper321]

This is still a problem.

Despite down rating the boiler to a minimum the demand of a morning is so low that it's is still cycling too much - increasing the anti-cycle timer increases the amount of time it takes to heat the roomby too much.

Will amending the bypass valve assist?

 

[Biggles..]

The problem you have is that 1 radiator only is too small for the minimum you can set your boiler output too(12kw from memory) but the 12 kw setting will not be enough to heat your house at peak demand.

Opening more radiators will help, the thought of adjusting the bypass was to let flow back into the boiler trying to trickthe boiler to modulate down sooner.

This is the only trouble with cm, good idea but unless linked direct to the boilers modulation it will struggle on small loads.

A more expensive solution would be fit a header between boiler and heating,new pump on heating and let the cm control this pump rather than the boiler,boiler would run under control if the boiler stat. Loss of Efficency is the outcome of this but improves the cycling
issues.

 

[flipper321]

Thanks Lee,

last few questions (promise!).

The pump has two settings - would it help using the lower speed (default is higher speed) ?

On the rads in question would it make any difference opening the lock shields up - as they are the first rads in the system they are locked down reasonably low for full system balancing?

Are you saying I should increase the "bar" on the bypass valve to try to fool the boiler, would reducing the "bar" not reduce flow to the boiler and cause the system longer the heat up?

Cheers

 

[flipper321]

Bump

 

[Biggles..]

Hi sorry for the delay.

Thisll be quick but please don't take it the wrong way,

opening the lockshield on the one radiator you hving calling first thing will make no difference to the cycling,but will probably effect the overall balancing and function.

Turning the pump down may result in the boiler firing for
longer but may not then be sufficient when all the system is calling.but then you will still have a cycling issue.

The overall problem is that the designer,installer,specified has not competently designed your system to suit your needs.in a nut shell when only 1 radiator is calling at say 2kw max your boiler will trying to shift 12kw minimum so cycling is going to occur.

In short to work round your problem,the only working solution appears to be have more zones calling at the same time or leave a bypass radiator open to try and disapait some of the heat.

Without some alteration to your design or adjusting your settings,you'll have to live with the cycling.

Lee

 

[Delboytwentyone]

Hi,
I have a similar problem with my Vallient echotech plus 428 when driving only the conservatory radiator which is controlled by its own roomstat and motorised valve. The boiler comes on the flow temperature rises very quickly to 80 Deg plus, boiler shuts down and then repeats the cycle. When the boiler is supplying whole house all works well. Boiler output set to 18kw, conservatory rad size 2.2kw. Please can you go into more detail regarding fitting header between boiler and heating as this might provide a solution. Thanks