Vaillant 831 or 837 - advice needed!

[vaderag]

I'm replacing my boiler and I wanted some advice.

Basically, i'm going for one of the above boilers - i know the 837 is bigger in size and power, and that's not an issue.
The issue is that it's £140 more, and potentially another £350+ if the pipes need to be rerun and resized to fit it.

I also may need the pipes for the 831, but there is less of a chance...

2 of us live in a 3 bed victorian semi, 2 bathrooms and a loft conversion. I'd like to be able to run both showers at the same time, as the flow rate is acceptable...

Can anyone advise if the 831 would be fit for this purpose, or should i sump for the 837...

Thanks

 

[muggles]

If you think you're going to be able to run two showers simultaneously off a combi you'll be sorely disappointed. The 837 is the only real option of the two if you really must try it, I'd be advising a hot water cylinder though

 

[vaderag]

I had been advised a hot water cylinder, however the cost is too high for me...

The water pressure in the house is very good tho and I tried both showers on a cold tap last night and the pressure was there.

If its a case of not being able to do it with either tho, I may as well just go for the 831???

Advice?

 

[vaderag]

Incidentally, just another point/thought... It suggest here http://au.answers.yahoo.com/question/index?qid=20100114070625AA7hA34 that i'mm need between 3.5L and 7L per minute for a shower...

One of mine is tap pressure, the other is power (i think), so running both would be 10.5L / min

According to the Vaillant brochure here (http://www.vaillant.co.uk/stepone2/data/downloads/2b/43/00/ecotec-brochure-2010-final.pdf) the 831 gives a DHW flow rate @ 35°C of 12.7 where as the 837 gives 15.2 - both of these should therefore be sufficient, no?

Or am i missing something entirely!? Our mains pressure on the cold tap is just over 15 i believe...

Thanks

 

[Agile]

You seem to have misunderstood all the basics.

A good shower takes 10 li/min. Two take 20 li/min. Normal combis dont give this.

An unvented cylinder will supply this if your dynamic flow rate is adequate.

A vented cylinder will also supply this but need a good sized storage tank.

Tony

 

[vaderag]

You seem to have misunderstood all the basics.

Which bit exactly? Am i misunderstanding the numbers?

A good shower takes 10 li/min. Two take 20 li/min. Normal combis dont give this.

Okay - but those figures are double that in the link above - why so different? Also, surely you have to bear in mind that not ALL the water coming out the shower will be hot - will be a mix of hot and cold to make the right temp no?


Fact of the matter is, I'm replacing my combi with a combi - i can't afford an extra grand to swap to a completely different system... I've just bought the house and it has a duff boiler with a busted heat exchanger - I'm only replacing because I have to - we'd been told the boiler was fixed!!

Since I have to do it tho, I want to make sure I do it as right as possible...

 

[Tipper]

I run a Vaillant 831 in a 3 bed flat with 2 showers.

It works fine.

What's also important, and nobody on here ever mentions it, is the shower head flow capacity. If you go for the ones with lots of large holes cascading down lots of water (you know, 'dustbins lids!) then one will rob the other. You need to keep the pressure up at the shower head, but if you are sensible with the shower head flow capacity, number and size of holes essentially, and have both the same then it will be OK.

You are right about HW + CW flow rates being combined at the shower head.

(I also have a Vaillant 937 at home with two showers but that ain't so good because my wife refuses to let me reduce the head on one of the showers!)

My two showers (in the flat) both have the same shower head and mixer valve and they both work fine and together. The piping layout/pipe sizes were also carefully laid out to maintain system pressure both to the combi and from the combi to balance the HW output to each bathroom.

 

[vaderag]

I run a Vaillant 831 in a 3 bed flat with 2 showers.

It works fine.

What's also important, and nobody on here ever mentions it, is the shower head flow capacity. If you go for the ones with lots of large holes cascading down lots of water (you know, 'dustbins lids!) then one will rob the other. You need to keep the pressure up at the shower head, but if you are sensible with the shower head flow capacity, number and size of holes essentially, and have both the same then it will be OK.

You are right about HW + CW flow rates being combined at the shower head.

(I also have a Vaillant 937 at home with two showers but that ain't so good because my wife refuses to let me reduce the head on one of the showers!)

My two showers (in the flat) both have the same shower head and mixer valve and they both work fine and together. The piping layout/pipe sizes were also carefully laid out to maintain system pressure both to the combi and from the combi to balance the HW output to each bathroom.

Thanks Tipper - that's really useful information...

I am erring towards the 831 because of the potential savings. It may still end up costing me for the extra pipework, but if it doesnt it's a significant saving, and if it does what I need, then great

 

[Agile]

A good shower takes 10 li/min. Two take 20 li/min. Normal combis dont give this.

Okay - but those figures are double that in the link above - why so different? Also, surely you have to bear in mind that not ALL the water coming out the shower will be hot - will be a mix of hot and cold to make the right temp no?

Most people including your wife would expect a good shower which is 10 li/min. The big heads need 14 li/min.

Of course you can be very mean and put in low flow heads but thats not the proper way to do it.

When the water comes in at 5 C during the winter, the boiler has to increase it to 45 C to send to the shower. Boilers are rated at a flow rate for a 35 C temperature increase so that immediately means a lower flow rate. Virtually no cold water needs to be added to give a typical 43 C shower temperature.

Tony

 

[vaderag]

Okay, that makes a lot of sense...
Also might explain why currently my shower need to be turned fully round to hot in order to get a warm shower... or could that just be that it was fitted wrong!?

 

[D_Hailsham]

Okay - but those figures are double that in the link above - why so different?

I would query why the flow rates in the link you gave are so low.

you have to bear in mind that not ALL the water coming out the shower will be hot - will be a mix of hot and cold to make the right temp no?

But all water, both hot and cold, has to be supplied from the same incoming mains. So, if you have a flow of 10lpm and want a shower at 40C (typical shower temperature) you can either mix 5lpm of hot at 60C with 5lpm of cold at 20C; or you can just heat the water to 40C. Which ever way you do it, the flow rate will be 10lpm.

You only seem to be concerned about the hot water aspect. What about the heating?

The 831 has a minimum output of 8.7Kw and the 837 is 12kW. As your heat requirement is likely to be about 14kW, the 831 would be more suitable as the lower limit is further below your requirement.

You can check the heating requirement by using the Boiler Size Calculator

 

[vaderag]

Okay - but those figures are double that in the link above - why so different?

You only seem to be concerned about the hot water aspect. What about the heating?

The 831 has a minimum output of 8.7Kw and the 837 is 12kW. As your heat requirement is likely to be about 14kW, the 831 would be more suitable as the lower limit is further below your requirement.

True - i am mostly concerned with the water, as that's the reason I'm replacing the boiler. I've been told that either would be able to heat the house sufficiently by all the people who have quoted me...

I'm a bit confused why the lower limit being further below the requirement is a good thing?

 

[D_Hailsham]

I'm a bit confused why the lower limit being further below the requirement is a good thing?

It's because the requirement is not constant.

Boiler's are sized for the worst case, usually an outside temperature of -1C, but the number of days that the temperature is that low may only be 10% of the year. So for most of the year the boiler is running at less than the calculated output. If you need 22kw to keep the house at 21C when it is -1C outside, you will only need 11kW when is is 10C outside.

Having the minimum output as low as possible means that the boiler is able to modulate down as the outside temperature rises. Once it gets too warm outside, the boiler will not be able to modulate low enough, so it runs in on-off mode, which is not so efficient.

 

[Beerlover]

Put Vaillants Modulating controls on it - VRT392 (room compensation) or VRT430 (room &/or weather compensation) This will alter the boiler flow temperature and optimise the range rating.

Go with the Ecotec 837 You can add the controller later.

 

[vaderag]

Put Vaillants Modulating controls on it - VRT392 (room compensation) or VRT430 (room &/or weather compensation) This will alter the boiler flow temperature and optimise the range rating.

How is that different to any other thermostat? I'm getting a honeywell fitted with the boiler...

Go with the Ecotec 837

Any particular reason?