WB 37cdi vs Vaillent 832 flow rate

[Bzoow]

I have been quoted on a replacement for my old boiler with HW tank and the suggestions are a Worcester Bosch 37cdi from one installer and a Vaillent 832 from another.
I am a bit concerned about the change to a combi, mainly because of potential leaks and slow bath fill. I don't really have too many concerns about my current working system but the boiler is ancient (Gloworm Spacesaver 75).
My other concern is the choice between the two boilers. I hear WB are hard to install (not my problem) and maybe not quite as good as the Vaillent but it's smaller and has a DHW flow of15.1l/min compared to the Vaillents 12.9l/min. My house is a large 4 bed 30's semi with downstairs shower, kitchen and utility and upstairs bath.
Is the DHW flow rate difference something I should be concerned about?
I did a rough cold water flow check from the tap next to the boiler and got a 2l bottle to fill in about 8.5 secs giving approx. 14l/min which sounds low although I did not think my cold flow was particularly low when compared to other houses.
Thanks for any advice!

 

[Agile]

I think you are about to be ripped off over the water flow.

You need a dynamic flow rate of at least 15 li/min @ 1.0 Bar pressure.

Look at FAQs and search other discussions as its discussed every week!

In any larger house a stored water system is better.

Tony

 

[ianniann]

If you are happy with your system then why change? I assume you get sufficient hot water at the times you need it and don't have issues with the space taken by the tanks? Lots of work (ie. time and money) to change over to a combi and you could end up with issues supplying hot water to your multiple bathrooms. If it ain't broke ...

The flow rates sound right to me. 14l/min isn't exactly record setting but it's normal enough. You may find you can open up your stopcock fully and get a little more, or maybe not. The flow rates quoted for the boilers are strictly the potential flow rates for a given temperature rise. The flow rates you will actually achieve are entirely driven by your mains pressure, less a little bit for passing through the boiler. You will usually get a better show from a combi because the water is supplied at higher pressure than most gravity fed systems, but not usually so powerful as a good power shower. Of course the nanny state would like you to use less water than any of these setups, offering low-flow showers well below 10l/min to save water.

The main issue with combis is that the higher the flow rate, the cooler the water will be. Both the boilers you quoted will be able to heat plenty of water for a shower using your system. I've never had an issue with bath filling time using even a very small combi, but some people don't like it. The 37kW boiler will heat more hot water to the same temperature than the 32kW, or the same water flow to a hotter temperature, which will help when someone else demands hot water while you're in the shower. Of course Vaillant can supply a 37kW boiler also.

Probably not relevant, but you should consider the heating requirements of your house. This could be anything between about 5kW and 20kW depending on the size and level of insulation, so well below the combi that most people would fit. Note that the 37CDi actually only provides 31kW of heating power and the EcoTec Exclusive 832 only 27kW (30 from the Exclusive 838).

 

[Bzoow]

Many thanks for the useful info. I am agree about the if it ain't broke philosophy but I guess you are referring to keeping with stored HW rather than not updating my Gloworm for a condensing boiler. I must admit even doing that is going against my better judgement somewhat as it's working OK and had been for the 12 yrs I have lived here with only a pilot clean out in the whole time.

Can you clarify your comment please ianniann

"Probably not relevant, but you should consider the heating requirements of your house. This could be anything between about 5kW and 20kW depending on the size and level of insulation, so well below the combi that most people would fit. Note that the 37CDi actually only provides 31kW of heating power and the EcoTec Exclusive 832 only 27kW (30 from the Exclusive 838)."

If the boiler outputs 27-30kw surely that's enough for a 20kw house or does that not take the DHW kw demand into account?
I think the house would tend toward the higher demand as it's 30's and no cavity walls three of which are exposed to wind and weather.

 

[mickyg]

did you ask for a combi? Given your flow rate and likely water usage it sounds like a bad idea.

 

[Agile]

If the boiler outputs 27-30kw surely that's enough for a 20kw house or does that not take the DHW kw demand into account?

I think the house would tend toward the higher demand as it's 30's and no cavity walls three of which are exposed to wind and weather.

Its most unlikely you would need over 20 kW. If you do then you gas bill is going to be about £2000-£5000 pa.

An oversized boiler like a large combi can only modulate down to about 30% of the max and so has to resort to on/off modulation which is inefficient.

Tony

 

[Bzoow]

I did not ask for a combi - it was suggested as a good option to put all the controls in one place and get rid of the HW tank. Also avoided having to run a new cable for the pump overrun as my pump is upstairs and boiler downstairs.
Is my crude method of timing the fill of a 2l bottle accurate enough to make a good estimate of CW flow?

 

[mickyg]

I did not ask for a combi - it was suggested as a good option to put all the controls in one place and get rid of the HW tank. Also avoided having to run a new cable for the pump overrun as my pump is upstairs and boiler downstairs.
Is my crude method of timing the fill of a 2l bottle accurate enough to make a good estimate of CW flow?

its crude but not unreasonably inaccurate. you may get better results with a bucket.

 

[D_Hailsham]

If you want to know how much heating you require, use the Boiler Size Calculator.

 

[greemyamochop]

http://www.worcester-bosch.co.uk/installer/products/gas-boilers/greenstar-18ri

is what id offer you if you were my customer depending on heat requirement.

Running an extra supply for pump over run is noting compared to the work involved in changing from your current system to a combi and if youre already unsure of its relevant merits then id move towards the ri
.
As said before, unless you want to free up more room in your property and youre happy with your system dont change it, i wouldnt.

I tend to fit Worcesters in 95% of installations due to their back up (which hopefully isnt needed!) and they have engineers in our area where most others manufacturers dont and they are certainly not difficult to install.

I changed one in a similar property 3 weeks ago and it was finished easily in a day although your circumstances might be different it would take considerably longer and therefor more expensive to opt for the combi

 

[ianniann]

I agree. Lots of plumbing work to switch to a combi. One wire run to stay with the cylinder? And do you really want to replace an old boiler and stick with an old pump anyway? This all sounds like the recommendations are being made with the convenience of the installer in mind more than what you want or need. I'll say again, you need to decide which setup is the best for you and tell the installer that's what they will be installing. And in your case, it sounds like you have to think about whether anything at all needs changing right now. The boiler is ancient and inefficient, but there is not a massive cost benefit to replacing even an ancient working boiler. Of course it will have to be done one day and gas isn't likely to get much cheaper, but don't be panicked into spending thousands of pounds you don't have to knock 10-15 quid off your monthly bill.

It is simply good practice to run a heating calculation before you put in a boiler. Maybe your fitter knows at a glance the ballpark figure for your house, but more likely they are just specifying a massive combi because it saves having to think. If you only need perhaps 5kW then you need to look very carefully at whether a massively over-sized 37kW combi makes sense. A combi is nearly always over-sized for heating, but while double isn't the end of the world, five times what you need is getting out of hand. There isn't a massive difference in cost between a tiny boiler and a big one, but there is a difference. At the other extreme, if you have a big old leaky house and the sizing comes to nearer 20kW then you don't want to be fitting a tiny system boiler. In your case you have a handy guide to what size boiler is sufficient, assuming it is sufficient.

 

[Biggles..]

Regardless of flow rate,i wouldn't recommend an exclusive to my worst enemy.too many teething problems for what is effectively a repackaged eco max.
Like for like would be the 837,which IMHO beats the Worcester hands down.

 

[Bzoow]

Many thanks for all the good advice. I have decided to stick with the current working system for now and pass on the scrappage voucher. Of course, sod's law say's the day it expires my old Gloworm will develop a terminal leak or something but I will take that chance.
Thanks to this forum I will certainly be better prepared for the inevitable change when it comes, hopefully in a few years time!

 

[ianniann]

You didn't mention the scrappage voucher Still, I assume you factored that in to your financial calculations and they still weren't compelling.

 

[Agile]

Many thanks for all the good advice. I have decided to stick with the current working system for now and pass on the scrappage voucher. Of course, s**'s law say's the day it expires my old Gloworm will develop a terminal leak or something but I will take that chance.

The scrappage scheme is not specific to any particular boiler type. Yours can still be used for an 18 kW or 24 kW heat only boiler and to retain your existing cylinder.

The voucher cannot be passed on to anyone else and is also specific to the named installer.

Tony