Condensing Gas Boilers

[Mark Siddall]

The boiler in my house is on its last legs. Unfortunately I'll have to replace it before I manage to undertake a low energy retrofit. As a consequence I'm going to want a new boiler that allows me a great deal of flexibility in the future.

From the perspective of reliability, maintainance, efficiency and longevity (if info is available) which are the three best makes/models of condensing gas boiler that are currently on the market?

Also, knowing nothing about the finer issues of specifying a boiler, what other key decision making critieria should one be considering when reviewing condensing gas boilers? (Say, if/when down rating the boiler which boilers offer the best capacity of modulation?)

Thoughts and responses eagerly awaited.

Cheers,
Mark

[Alan Clarke]

Mark
hopefully we'll see a variety of opinions here! All coloured by which particular one went wrong.
For what it's worth, my list is:
Vaillant – well known for installation and service, but offering good controls eg weather comp, if you want it, and you like Germanic complexity. Offer different temperatures for heating and hot water
Rehema Avanta – seems well behaved at low loads, and simple integral controls for weather comp, or just low rad temp and quick water reheat, whilst using whatever time/room controls you like rather than over-complicated manufacturer's types (vaillant and Viessmann)
Worcester Bosch – combis were most reliable when I did EST condensing boiler surveys – still good as far as I know.
(I haven't include Viessmann since despite being technically fab, I just can't understand the control systems, and nor can anyone else!)

Note I usually specify system boilers, but maybe you're thinking of a combi – same basics, but afraid I don't have much experience of combi-specific issues (ie controllability of “keep-warm” facilities, time/energy to heat up from cold, etc)

Alan

[Anonymous]

I would agree with Alan on this : generally I would specify Broag first and Worcester second. I think (as with cars!) with any boiler you can sometimes get a “Friday afternoon” model. I have been impressed in the past with the customer service from Ideal (they came out on a Boxing Day to fix my parents boilera few years back).

I have specified Worcester as they provide a small 10kW boiler which will modulate down to around 3kW. Alot of other boilers have a turn down of around 5kW as a minimum output. I found an Italian manufacturer which went down below 3kW but would not specifically recommend them.

Nathan

[Anonymous]

As a consequence I'm going to want a new boiler that allows me a great deal of flexibility in the future

You're probably right, but don't forget the possibility that it may be simpler to specify for current requirements now and then replace with something better suited to the situation after the retrofit. Yes, I know, yet more complication and angst!

From the perspective of reliability, maintainance, efficiency and longevity (if info is available) which are the three best makes/models of condensing gas boiler that are currently on the market?

Also, knowing nothing about the finer issues of specifying a boiler, what other key decision making critieria should one be considering when reviewing condensing gas boilers?

Especially if you're considering how well the product will meet the specification after the retro-fit, I think you need to consider the whole system, not just the boiler. Will the DHW be vented, pressurised or use a thermal store, for example? How will the boiler and solar system interact?

I don't profess any expertise on boilers (I bought a WB a few years ago that seems to be OK) and I think you've had two good answers here. If you want to broaden your search, I'd suggest the Navitron forum, where there seem to be quite a number of installers with their heads screwed on.

[Mark Siddall]

Dave,
Thanks – May head to Navitron (never really got stuck into that forum….so far I've tended to remain focussed on fabric rather than widgets).

Mark

[Alan Clarke]

Mark
some Worcester Bosch combis can have weather comp (eg 27CDi) others can't, I think.

Note Broag=rehema, and they all have weather comp built in so you don't need expensive/complicated add on controls, which is a winning feature imo – but your installer probably won't know how to set up and may not even believe you. (Have even had Broag commissioning agent deny its existence!)

Alan

[Mark Siddall]

I've been speaking to a local plumber he is currently installing quite a few boilers by Main http://www.mainheating.co.uk/combieco.htm (Main is owned by Baxi). He says that they are realitively cheap and that one of the reasons for them being cheap is that they are using siliconised aluminium as the heat exchanger. Has anyone heard about siliconised aluminium? Is it considered to be as robust as stainless steel? (I know standard alumunium is no good as the PH of the condensate corrodes it.)

Mark

[David Olivier]

Alec

Is it true that one or more Viessmann boilers can operate with variable flow and fixed temperatures? I think I skimmed a doc. that stated this recently. It does have the advantage that less water need be pumped.

Many district heating systems on the continent operate this way, but in the past boiler manufacturers tended to say that they didn't want low flows through their product. Hence individual heating systems used a variable flow temperature (well they didn't in the UK but the UK is distinctly unusual compared to the rest of Europe).

David

[Anonymous]

The viessmann vitodens 300 (and a few others such as atag q series) does have a modulating pump which is linked to the burner so is tolerant of variable flows. The heat exchanger in the vitodens 200 and above is like a spring so it is indifferent to the delta t across it. modulating pumps

however modulating pumps can be “conflictive” where pipe sizing and balancing isn't perfect…and modulating pumps come in two types constant pressure and those linked to the burner…

certainly opening and closing zones with two port valves isn't the type of variable flow that viessmann had in mind when they designed the boiler..

[David Olivier]

I'm suspicious of complicated controls. I feel they're not needed and in the wrong hands they can consume more gas (or oil/LPG) than they save. More of a priority is installers who understand the concept of weather compensation, adjusting the rise in flow temperature with falling outside temperature and adjusting the outside temperature at which no heat is needed. Varying these two temperature settings caters well for most buildings and further adjustments probably save much less.

One weakness of WB seems to be that a WB recommendation typically comes from an installer who will pressure you to use a combi (maybe needing a new gas pipe, without instaneous DHW and with compensation control most UK buildings can manage with 10-15 kW, not 50 kW) and insist on same controls for condensing boilers as applied to conventional boilers; i.e. wrong controls for the condensing type and wasting 15-20% of the fuel, which we can ill afford especially if 80% of gas will be imported by 2020.

[Anonymous]

Mark, not sure if you have changed boielr yet? and Alec/others, I would welcome your comments on where to locate the condense pipe. I am sure everyone is aware of the issues with the recent sub-zero weather freezing condense pipes, (and subsequently tripping boilers and leaving homes not heated!) especially in boilers which drip condense where the pipe is located somewhere which may freeze (i.e. outside walls etc).

Is it preferable to have a boiler which purges the condense, with the view that less will freeze in the condense pipe. I know that ideally the condense whould be run inside the house, but stories appear that even accredited installers for WB have not done this.

My interest is like Marks in that I will have to change a boiler in the futrue and I have heard of horror stories through family members, where BG and WB both blame the cold weather for boiler faults. Also should teh condense pipe to into ground water drains, waste water or a soakaway? Not always an easy option in a flat??
Regards
Chris
(interested surveyor, Cumbria)

[Mark Siddall]

Chris,
Provided that your water utility com will allow it the condensate could be drained into an internal SVP or a waste pipe (needs some thought about where you put the boiler). The advantage of using the SVP is that the condensate will not freeze (it exits below ground), the waste pipe has a larger diameter so is less likely to freeze.

Mark

[Anonymous]

If exterior condensates can be avoided they should is the answer..

David, people find controls complex and involved, because they use them infrequently. Correctly set up controls need minimal intervention, and many just require remembering which knob to turn. Even simple controls can be conflictive for some..

[Mark Siddall]

An update:

1) I ended up with a Broag Avanta. Install went fairly smootly, esp considering the plumber hadn't installed that make or weather comp before. Plumber considered that it was tougher to get in than the Baxi units that he was used too.
2) The plumber was suprised at how quiet the boiler was compared to his normal kit.
3) The plumber simply showed me the basics for operation – I had to figure out how to opperate the iSense controller for the weather compensation (thanks to Alan C for giving me a very useful steer on this one time when he was up for a site visit. (Finding the correct settings for the iSense proved difficult as the relevant info was not in in the User Guide; it was only in an installer guide that was found on the internet by Alan.)
4) Once over the initial setting up hurdle the iSense has been found to be quite straight forward. A plumber that installs this regularly could easily get it comissioned so that less IT/heating litterate type could simply focus upon setting the timer/temps.

During the winter I intend to try out full weather comp (run off TRVs only) and the weather comp with room stat (uses the stat in the iSense).
…..I understand that Weather comp with room stat is appropriate for PHs, whereas full weather comp is not (it is more likely to lead to into on/off cycling.)

Mark

Also see https://aecb.net/forum/index.php/topic,3395.0.html

[Mark Siddall]

The Broag Rehema Avanta combi is going through its first winter quite successfully. The winter has been fairly mild but over all the weather compensation is working well and the peaks and troughs in temperature are no longer quite so extreme; so the temperature curve seems to be about right. The iSense controller is a doddle. Very happy with the usability as it is easy to adjust to run on for a short period or to programme for any day of the week.

I recently learned that Alpha now do boilers with weather compensation: http://www.alpha-innovation.co.uk/custom/upload/instructions/InTec/intec%20S%20User%20Inst%2023-5-11.pdf

Mark

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