Vaillant ecotec 637 and Megaflo controls

23 Feb 2012
Reaction score
United Kingdom
Hi all this is my first post.

Have been lurking through the threads on Vaillant ecotec and Megaflo controls to find out how I can improve my systems efficiency as Gas bills around £1250pa.

House is a detached five bedroom with two bathrooms both with showers and one also with a bath.

Two years ago the existing three bedroom house which is brick built with cavity walls and timber suspended floors was extended on two floors with pitched roof to modern building regs. The existing has recently had cavity wall rockwool insulation blown in.

A new unvented Vaillant ecotec plus 637 and Megaflo CL170HE was installed in loft space and wired up with a room stat, honeywell ST9400C programmer, two 2port valves all marshalled into a plastic junction box, also new water main installed.

The boiler was set to 75deg C output temp Megaflo to 60degC and all works fine apart from the gas bill.

I am looking for better efficiency from the system and try to get the boiler return temp down as low as practical so it can actually be a condensing boiler.

What I've done recently to tune system:
1. Discovered from the IDHEE "Whole house heating method" the losses are about 17kW including nominal 2kW for DHW.
2. Reset max boiler output to 25kW.
3. Set boiler flow temp to 60degC.

Just checked boiler temps and with outside 12degC and room stat hovering on 20degC the boiler flow is 57degC and return is 49degC with boiler firing.

The DHW was originally on constant but now timed to come on for three half hr sessions per day, which is a bit marginal with boiler set at 60degC for reheat.

Having read about the Vaillant VRC470 weather compensating programmer and VR65 I am trying to discover whether fitting these will enable the system to run more efficiently than present.

I spoke to Vaillant about this but its all a bit uphill with them and they have no technical manuals available.

One advantage apparently with vaillant controls the DHW reheat will improve as not be limited by the present 60degC flow temp.

Should I get the Vaillant controls or not bother?

Any comments gratefully received.
Sponsored Links
Hi all this is my first post.

Have been lurking through the threads on Vaillant ecotec and Megaflo controls to find out how I can improve my systems efficiency as Gas bills around £1250pa.

The boiler was set to 75deg C output temp Megaflo to 60degC and all works fine apart from the gas bill.

What I've done recently to tune system:
1. Discovered from the IDHEE "Whole house heating method" the losses are about 17kW including nominal 2kW for DHW.
2. Reset max boiler output to 25kW.

3. Set boiler flow temp to 60degC.

Any comments gratefully received.

If the heat loss is 17 kW then why did you set the max power to 25 kW?

If you dont set the boiler max flow temp to 70 C then you will not properly heat the water and cause the boiler to cycle.

You could set the water to 55 C and the boiler to 65 C.

In fact for your house thats a very reasonable gas bill.

Weather comp saves about 2%-12% depending on how you use it. Hardly worth it if you adjust the boiler according to the season manually.

Thick pullovers significantly reduce heating costs. As do wearing duffle coats indoors.

where i have replaced on off controls with vaillants weather compensation My clients have saved uoto 20% of there gas bill...
Don't know the accuracy of 17kW calc as it's a whole house calc and 25kW is a starting point for me, it's got to be better than 37kW.

If I can find the info I can do a room by room calc. for more accuracy.

I set the o/p temp to 60deg to see how the heating would perform.

Will the boiler modulate better with the vaillant controls on CH and DWH than the existing on/off controls?

When operating correctly with CH demand should boiler modulate down then run continuously at it's lowest temperature for max efficiency

Is it feasible to achieve a 20deg drop between flow and ret at boiler and will this only be when rooms are cold and rads losing heat faster, do you guys achieve this when installing new systems?

Does the boiler also modulate circ pump speed along with burner modulation?
Sponsored Links
yes with weather comp the boiler modulates much better in heating

and with the cylinder sensor the boioer aims to satisfy the demand as quickly as possible. You should match the cylinder coil to the hot water output on the boiler...

with compensation controls the flow temp drops to below 55c so the delta t becomes irrelevant...

the pump has two speed and below a certain threshold drops to low speed...
Many thanks ALEC1 this is the kind of info I need to know.

The local plumbers merchants sell the VR65 an VRC470 for £196.00 inc VAT for both, is that a good price? I'm sure the cost would soon be recovered with better efficiency.

One regret, in my ignorance, is the Megaflo as while it is apparently a good bit of kit and fitted by many with the Vaillant, the Unistore with it's NTC thermistor for analogue temp control appeals more to me. If it could be fitted to the Megaflo I'd give it a try.
How successful is it fitting the Vaillant VR10 thermistor in the Megaflo?

Can this be done with original thermostat in place or does this just pull out of dry pocket and VR10 fit straight in or is adaptor needed any probs with dissimilar metals?

Or does the method of creating a small pocket in the insulation so VR10 in contact with cylinder wall work better?

Or you could buy my Megaflo and I'll get Unistore :D

Would fitting VR10 be good for DHW control or should I forget about it.
the vaillant controls will be an improvement and you should be able to fit the cylinder sensor to the megaflo, but one of your issues is your massively oversized boiler, who on earth chose that?? the lower end modulation on that boiler i think is about 12kw which will give you cycling issues, which is not good for efficiency
it can be fitted to a megaflo, but the manufacturers won't don't bother ringing them...

just replace the current immersion stat with the supplied sensor...
Your boiler is massively oversized, so unfortunately it's never going to be as efficient as it could be, sounds like you'd have been better off two models lower with a 624, or even three models lower with a 618. The boiler you have simply can't modulate down to a low enough output. The Vaillant controls should help, but having an engineer swap the 637 for a 624 or 618 and flogging the old one on eBay might be well worth considering.
Haven't bought any new controls yet.

My builder's trade is a plumber, his electrician wired up the control system and all neatly done. The builder bought all the hardware req and I wasn't involved with checking the kW.

The condensing boiler and megaflo is all new technology to me and only discovered by accident as a local builder was revamping his house. I happened to walk past as his plumber was working in the garage on the final stages of installing a Vaillant and megaflo and he described the installation to me which was handy.

My builder had also installed similar equipment in other projects and I had seen examples of his work so had no reason to question things unduly. I can see now I should have been more involved as coming from an engineering background myself.

One thing I want to achieve is doing calcs for the kW required when the house is warmed up to say 20C so I can compare this figure with the lowest the boiler can modulate down to. I see from the spec with 60C flow and 40C return the boiler is 12.7-38.2kW. At a flow setting of 60C I get about 57C flow and 49/50 ret with boiler firing so I guess it is modulating.

I have surveyed all the rads and will estimate the total o/p although Stelrad tell me their figures are based on 80C flow and there are some tables somewhere in their rad bible which give o/p at lower temps.

I see 37kW equals about 127k BThu/hr so a big increase. The side extension to the house increased vol by about 50% so 80k +50% = 120k so I guess that was the builders thinking.

We did notice a big jump in our gas bills hence after a couple of yrs I've started to look into the heating system to see what can be done to improve things.

The first thing was get a new gas meter installed, which was free, so should be calibrated ok. I used to run the HW constantiy but now time it 3x per dy. We were on a direct debit gas account with quarterly readings but when it got to £650 in credit I cried "enough", got refunded and changed to a system where I read meters monthly. The BG online account is quite handy as you can look at the history and compare usage with last years.

One more thing. Our present timer/programmer has 1hr override for HW which is really useful, nothing worse than a cold shower it doesn't improve my temper, but I suspect I can only get that facility with the VR10 installed as opposed to the cyl stat, can anyone verify that?
Outputs of rads running at 60/40 F&R temps are roughly half the quoted output in the standard spec sheet. You should have a 20'C difference between flow and return temperatures at the boiler, a smaller difference means it's not condensing and therefore not running efficiently. Your smaller difference is another symptom of an oversized boiler, and possibly poor system design. It is, however, not an indication that the boiler is modulating, it will do that anyway, it's just an indication that your radiators are emitting heat into the rooms.
muggles, compensation controls force the flow temperature down as low as possible, thus ensuring both the flow and return temps are as low as possible without compromising comfort...

almost all the time the flow is below 55, an the reurn lower, thus the boiler is always condensing.

OP, vaillant controls cope well with an oversized boiler....don't fret about it!
Indeed they do, but they still aren't going to be as effective on an oversized boiler as they are on a correctly sized one. They'll cope, but the results could be better...

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links