System Boiler / Unvented Cylinder Advice

SJBUCK · 19 May 2008

My first post here after many months of 'lurking'!

I need to order a 35-40Kw System Boiler and 210l Unvented Cylinder for a large house that I've recently bought and am in the process of renovating.

Would any of you have any advice on the best combination of the two? I've seen a Vaillant Unistor 210l cylinder (£958+vat) and EcotEC Plus637 (£892+vat). Is that a decent make? I'd originally looked at the 42Cdi but have been told they are overpriced as there are a lot of extras to buy that come 'free' with other brands of boiler. Is that the case?

Any advice much appreciated.

Cheers
Steve

SJBUCK · 19 May 2008

Replying to my own message, how sad

I forgot to mention that the house is an Edwardian detached over 3 floors. The 'kit' will be placed in the cellar driving approx

21 rads, 1 bathroom, 1 downstairs cloak, 2 ensuits (1 without shower).

There are only 2 people living in the house though. Does the above system sound sufficicient?

Cheers

Agile · 19 May 2008

I cannot see how two people will want 21 rads all on at the same time!

Have you done a heat loss calculation to work out the boiler power required?

My first thought was that 210 li was rather small if you have three bathrooms. There is no downside to putting in a 300 li cylinder which would be more appropriate for three bathrooms. I do accept that just two will find 210 li is fine though.

The boiler is a different matter. It greatly reduces the efficiency if you oversize it. I would expect that 18 kW or at the most 24 kW would be adequate if you have normal levels of insulation etc.

Tony

SJBUCK · 19 May 2008

So you'd go with a small boiler and a larger tank?

I've not done any calcs yet, I'll get the room sizes etc.. later today and see what it comes out at.

I doubt very much we'll have all 21 rads on at once! You never know though, that one time a year when you have guests over at xmas etc..etc

Cheers
Steve

SJBUCK · 19 May 2008

Looking at another thread a popular method of calculating required output was to take 5000 per room and add 15000 for hot water.
If I apply this to my house it's 15 rooms (plus hall/stairs/landing, does that count?) so 90,000Btu/h. Does that sound about right?

Looking at the Vaillant range of system boilers the ranges are...

EcoTec630 34,100 - 102,400
Ecotec637 40,900 - 126,000

I know I'll have to do a more accurate test but on the face of it wouldn't the 630 struggle or am I missing something? I'd like some overhead for any future extra rads/rooms etc...

Cheers

PS Is this a record for replying to your own thread?

gas4you · 19 May 2008

I think you should go for at least the 630. You can always have it range rated down upon commissioning to match your system exactly, say 26-27Kw.

Unistor are a very good cylinder, but to get the most out of this combination you will need to connect them through a VR65 control center.

As your property appears to be quite large I think you will find that you are required to fit multiple zones (at least 2 + HW for any property over 1500ft2.)

In this case the Vaillant system with the VR65 would not be suitable as it can only accept 1 zone + HW.

You would have to use the 630 and unistor fitted and wired to normal zone controls and stats.

Agile · 19 May 2008

SJBUCK said:
Looking at another thread a popular method of calculating required output was to take 5000 per room and add 15000 for hot water.
If I apply this to my house it's 15 rooms (plus hall/stairs/landing, does that count?) so 90,000Btu/h. Does that sound about right?

Thats rubbish! You cannot guess! The biggest difference is the wall construction!

You should also talk in kW not BTU nowadays.

Those boiler figures are the modulating range. You match the boiler's maximum power output to the central heating with an allowance of 2 kW for water heating.

The full calculated output is only required when the temperature is at -1° outside. ( but -2° if you live in Norfolk ).

Tony

SJBUCK · 19 May 2008

hmm OK, I'll do the calc tonight and get a true Kw figure

The specs I have on the boilers don't list a Max output for the system boilers, only the combi's.

The cost difference between the boilers is only about £50 between each range. I assume the running costs could be a lot higher though for the same required output though..

Thanks for responding so quickly

gas4you · 19 May 2008

SJBUCK said:
hmm OK, I'll do the calc tonight and get a true Kw figure

The specs I have on the boilers don't list a Max output for the system boilers, only the combi's.

The cost difference between the boilers is only about £50 between each range. I assume the running costs could be a lot higher though for the same required output though..

Thanks for responding so quickly

The max output for a 630 is somewhat surprisingly 30 Kw and a 637 is 37Kw. Wouldn't cost anymore to run. If your rads only require about 27-30 Kw then you will not get any extra heat from them because of a bigger boiler.

If your were worried about the size being sufficient you could put in a 637 and rate it down to 30Kw. Bear in mind though that the 637 is approx 3" deeper off the wall

Agile · 19 May 2008

SJBUCK said:
The specs I have on the boilers don't list a Max output for the system boilers, only the combi's.

The cost difference between the boilers is only about £50 between each range. I assume the running costs could be a lot higher though for the same required output though..

The heat only boilers maximum power is in their spec.

Its false economy to over rate the boiler power as it makes them less efficient and so you pay in running costs.

Its quite common for installers and owners to do that and in the extreme it can cause several other problems. I have seen three 32 kW boilers fitted to homes needing about 12 kW !

Tony

BigBurner · 19 May 2008

SJBUCK said:
My first post here after many months of 'lurking'!

I need to order a 35-40Kw System Boiler and 210l Unvented Cylinder for a large house that I've recently bought and am in the process of renovating.

If you insist on an unvented cylinder then go for the ACV tank-in-tank design. The recovery rate using at 28kW is excellent as the inner cylinder is totally immersed in the outer cylinder. Only the outer cylinder is heated by the boiler or an immersion. Approx' 210 litres will do two bathrooms as the boiler is working flat out to reheat the DHW when water is being drawn off. The ACV tank-in-tank also can be used as a part thermal store. The radiator heating loop can be taken off the outer cylinder where the primary water is. This reduces boiler cycling giving smoother operation. It also means the CH has full electric backup too. They are 100% stainless steel.

Here it is:

Rads or UFH can be run from it.
http://www.acv-uk.com/slme.htm

simond · 20 May 2008

Whilst we fit ACV cylinders and they are a nifty product, the uniSTOR is a good partner for the ecoTEC 6 series because of the controls.

BigBurner · 20 May 2008

simond said:
Whilst we fit ACV cylinders and they are a nifty product, the uniSTOR is a good partner for the ecoTEC 6 series because of the controls.

The UNistore is not a direct comparison to this ACV model, and it is not exactly cheap. I would say that an ACV and Glow Worm boiler would be cheaper than a Vailant/Unistore setup.

An ACV cylinder can be so much smaller as the reheat is so much more superior, especially when using a 28kW boiler.

This ACV gives a hell of lot more than just a basic unvented cylinder like the Unistore. The great thing about the ACV is that the CH can be run from the cylinder as well. A Wilo Smart pump can used on the CH loop and TRVs used on all rads with no wall stat needed. A cheaper, simpler boiler can be used like one of the Glow Worms which are made by Vaillant anyway. Don't underestimate CH & DHW full electric backup and reduction of boiler cycling giving a smoother boiler operation. A 9kW immersion can fitted for full backup.

The reheat is so fast you can have the boiler for a 5 or 6 minutes for just a sink full of water. There is no need to heat the whole outer cylinder. The reheat is approaching the speed of a directly heated heat bank thermal store using a DHW plate heat exchanger.

This ACV will last as all is stainless steel. And of course fit a Magnaclean on the rad loop return, as I do on all systems no matter what it is.

ACV have stopped some of their range that could have CH taken off the cylinder.

Another thing about the ACV cylinders is that they can also be used as vented cylinders using a cold water storage tank to feed it.

D_Hailsham · 20 May 2008

If you want a reasonably accurate ball-park figure for the size of boiler you need, the Recommended Boiler Size Calculator at Sedbuk is as good as any. It works on the "whole house" principle, i.e. it ignores the heat gains and losses between rooms and floors. This will tell you if your rad calculations for each room are near the mark.

The norm for heating up and intermittent use is about 10%. This is usually built into the calculator; so if you are at home all day you may not need this.

If you want a more accurate calculator, the Quinn Radiators (formerly Barlo) Heatloss Calculator can be downloaded for nothing.

BigBurner · 20 May 2008

Correction on my part. An 200 litre ACV with a 28kW boiler will do approx' three bathrooms. If a modern bath is approx' 100 litres, the 200 litres stored at a minimum of 65 degrees C, as the DHW is blended on on the ACV, will fill two baths without any reheat.

With the boiler supplying 28 kw the energy is being transfered to the cold water very quick indeed, reheating as water is being drawn off. I would with confidence fit a 200 litre ACV tank-in-tank in a three bathroom house. Bed and breakfasts are another matter as all baths may be on at the same time, which is rare in a domestic house.

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System Boiler / Unvented Cylinder Advice

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