Vaillant boiler cycling when using Underfloor Heating

[windsortg]

Apologies if this has already been answered before but I recently had installed a wet Underfloor Heating (UFH) system to a newly built orangery 6m x 4m). The rest of the house is a wet central heating (CH) system containing 21 radiators. I have a Vaillant ecoTec Plus 438 condensing boiler (around 13 years old) that powers the CH system, Hot Water (HW) and UFH.

The boiler is in the garage, the individal zone valves (one for CH, one for HW, one for UFH - shown in attached photo & labelled 1, 2, 3) together with a Grundfos UPS2 25-80 pump, and Megaflo cylinder are all positioned in a landing cupboard. I have attached another photo showing the zone valves but also the position of an automatic bypass valve (highlighted by the yellow arrow).

When I have both, the CH and UFH systems turned on, the boiler works happily away and both, CH and UFH heat up nicely the radiators and orangery respectively. However, the problem I have is that if I just want to turn on the UFH only, the boiler is continually cycling. From what I have researched, this is due to demand of the UFH being less than the boiler minimum output. The Vaillant boiler is not modulating properly. The boiler comes on for around 2 mins but then turns off for 5-6 mins.

The pipework around the UFH manifold is not really getting hot and I just feel that with the continuous cycling of the boiler, this is both, causing wear & tear on the boiler and using excess energy.

Is there a resolution this? I am thinking that when the cold winter sets in and I wish to initally turn on the UFH in the orangery (since UFH takes longer to heat up compared to radiators - the orangery comprises of screed flooring with porcelain tiles), the orangery is going to remain cold as the UFH is not heating up properly due to the continuous bioler cycling.

Since my existing Vaillant boiler is not as efficient as the latest boilers, I am not against having a new boiler installed that can modulate down effectively but I do not want to find that by changing the boiler, I still have the same problem.

 

[jeff the gasman]

Your boiler is very oversized for anything but a castle!
Get a local engineer in to down rate your boiler to suit the property.
Biggest isn’t best with boilers.

 

[windsortg]

thanks @jeff the gasman. The boiler is currently set to a maximum heat output of 20kW (by setting d.00 on the boiler to 20). Even with this setting, the boiler is continuously cycling when only the UFH is on. If the boiler is 'down rated' any further, will this not be insufficient to cater for 21 rads and UFH at the same time?

 

[JonathanM]

I don't know what the following setting actually means. I'm just flagging this up in the hope that somebody else can advise if it helps. Please don't make any changes as I have no experience of a system like yours!

d.17 = Heating flow/return regulation change over - 0 = flow, 1 = return (factory setting: 0). When connecting underfloor heating or wall heating to the appliance which does not have its own temperature regulation, the temperature regulation mode can be changed from flow temperature regulation (factory setting) to return temperature regulation

The other thing is, have you done some basic diagnostics to see why it might be cutting out? Have you monitored the flow temperature reading (or perhaps return temperature, depending on d.17) on the boiler display to see what temperature it is at when the boiler cuts out. And how that relates to the set flow temperature.

There have been some discussions on here about whether some Vaillant boilers have a particularly long burn at full power, after ignition, before they modulate down. But I think one poster said that only applied to open vent models and was something to do with making sure the pressure was OK. I think yours is an open vent boiler?

It looks like your boiler should modulate down to 6.3KW?

 

[JonathanM]

It looks like your boiler has a large number of status codes which display its current status. I don't know if they shed any light on things?

 

[jeff the gasman]

thanks @jeff the gasman. The boiler is currently set to a maximum heat output of 20kW (by setting d.00 on the boiler to 20). Even with this setting, the boiler is continuously cycling when only the UFH is on. If the boiler is 'down rated' any further, will this not be insufficient to cater for 21 rads and UFH at the same time?

How many bedrooms and living rooms do you have?
Old or modern house.
Terrace or detached?
Wall construction?
Ceiling height?
Window area and construction?
Floor construction?
Insulation?
All these factors are taken into account when sizing!
You could fit 21 radiators in 1 room and still only need 3 KW to heat it!

 

[roguetrader]

A known problem on those 438's.
Only sure fire answer is a header to give the boiler a chance.

 

[windsortg]

Thanks everyone. I have a system boiler and does not have a d.17 setting.

@jeff the gasman - I have a 1970s 4-bed detached house with 3 living rooms plus the orangery (which is heated through UFH). The house is brick construction. Ceiling height is 2.3m with large, double glazed UPVC windows throughout the house. All windows were changed earlier this year giving good heat efficiency.
The majority of the ground floor is solid, concrete flooring with little/no insulation as this is how it was constructed when the house was built. Most of the exterior walls are cavity wall insulation but the first floor of the original part of the house is tile hung, block construction with inadequate insulation.

@roguetrader - where you say this is a known problem for 438s, do you mean it’s a problem of the boiler continuously recycling when the UFH system is switched on without the CH system being on?

 

[roguetrader]

Big boiler, small demand.
A lot of these were fitted without proper sizing.
Not the boilers fault, just overboard specification.

 

[Johntheo5]

Some gas fired boilers seem to be able to cycle without any major problems, I often see my relation's vokera's supplying a max at times 3kw load with no particular problems with a boiler minimum output of ~ 5.6kw, not much different to the above boiler, which of course is a Vaillant noted for their 60 secs delay to releasing the ignition settings to modulation after fire up, probably the big problem IMO. (Maybe this has since been "rectified")

 

[Harry Bloomfield]

There have been some discussions on here about whether some Vaillant boilers have a particularly long burn at full power, after ignition, before they modulate down. But I think one poster said that only applied to open vent models and was something to do with making sure the pressure was OK. I think yours is an open vent boiler?

418 open vented, fitted with Vaillant 407f, with outdoor sensor controls - yet mine does not exhibit a long burn. It just runs the fan briefly at full speed, then runs very quietly at what ever burn it needs to match the demand.

Prior to fitting the Vaillant controls, then it did appear to run flat out until the stat clicked off.

Could the OP's problem be that he is just using basic on/off controls? If so the boiler will not have much idea what is being demanded from it, it the way of heat output.

 

[jeff the gasman]

No more than 24kw then at the most.
Do heat loss calculations for your house but don’t include the orangery.
Something like this will help.

Heating Calculator - BTU Calculator

Our heating calculator / BTU calculator can help work out the appropriate heat output for each room of your home.

www.plumbnation.co.uk

 

[JonathanM]

Thanks everyone. I have a system boiler and does not have a d.17 setting.

Are you sure there's no d.17? I think I have the right installation manual now for your boiler, which says there is. (I can upload, or provide a link if you want to check it is the right one.)

I've also now found this thread from 2011, from somebody who seems to have a similar problem with the same series boiler, and they fixed it by changing d.17 - post #82.

https://www.diynot.com/diy/threads/does-balancing-increase-efficiency.266773/page-6

I seem to have now resolved the 'short-cycling' issue I had by changing d.17 to 1 - This changes from flow 'regulation' to return regulation according to the instructions.

EDIT: d.17 seems to be in three different versions of the installation manual, spanning 2006 to 2012

 

[jackthom]

418 open vented, fitted with Vaillant 407f, with outdoor sensor controls - yet mine does not exhibit a long burn. It just runs the fan briefly at full speed, then runs very quietly at what ever burn it needs to match the demand.

Prior to fitting the Vaillant controls, then it did appear to run flat out until the stat clicked off.

Could the OP's problem be that he is just using basic on/off controls? If so the boiler will not have much idea what is being demanded from it, it the way of heat output.

That’s useful information. When I started using a Vaillant ebus controller to handle the DHW side of things it did get rid of the startup burn which used to be at about two thirds of full power and sometimes caused the boiler to trip.
I hadn’t realised that using the official Vaillant room thermostat might also calm down the boiler run-up for heating.

 

[JonathanM]

418 open vented, fitted with Vaillant 407f, with outdoor sensor controls - yet mine does not exhibit a long burn. It just runs the fan briefly at full speed, then runs very quietly at what ever burn it needs to match the demand.

Prior to fitting the Vaillant controls, then it did appear to run flat out until the stat clicked off.

Thanks. That might answer the conundrum why some people do and some don't with these boilers.