choice of unvented cylinder and boiler combo

[patman209]

Hi All
We are looking at replacing our vaillant combi with an unvented setup. I had a couple of heating engineers round from ratedpeople and one suggested a conventional system with a water storage polytank and geldhill vented cylinder, vaillant 630 and 4 bar pump.
the other suggested a Main 300l unvented dcylinder and vaillant 630 or 637. The incoming flow is about 24 l/min at 2.5 bar.

We are leaning towards unvented incase we decide to extend into the loft, which is where the polytank would have gone. With the unvented we have a utlity room that can house the cylinder and boiler.

What are peoples thoughts on the Main unvented cylinder? I see they are quite a bit cheaper than likes of megaflo etc.

the house has 5 bedrooms and 3 bathrooms with 19 rads in total, most of which are proobably oversized for the rooms to be honest - mainly stelrad double panel single convector and double panel double convector.
I specofically asked for oversizing of the cylinder for future expansion of our family.

I did ask if 637 was too big a boiler and the chap who recommended the unvented setup did say that a 630 may suffice but there are a lot of rads in the house and they are quite high wattage.

Does anyone have a bad experience of the Main unvented cylinder? i have not had any exeprience of unvented systems so would liek some opinions.

Also what boiler controls do people on here like/dislike. I have read about the Vaillant weather comp ones - are they decent?

Thanks

 

[Agile]

I don't believe you have 24 li/min @ 2.5 bar.

The significant flow rate is the dynamic one where 22 li/min @ 1.0 bar is considered the minimum for proper performance. Even with that you would be limited by water flow rate if trying to use more then one bathroom at a time.

So my view is that you probably don't have an adequate dynamic mains flow rate.

Main is the same company as Megaflow I think. I have always used their other second brand Santon and been very happy with them. I would always suggest a minimum of 300 litres and would not be concerned by a 500 litre!

You need a whole house heat loss calculation. I would be surprised if that's over 24 kW. You only add 2 kW for water heating.

Tony Glazier

 

[patman209]

thank you Tony
The incoming flow and pressure is what was measured by affinity water at the stop tap. Does this seem unrealistic?

Thanks for the advice on the heat loss, I will ask the relevant questions to both chaps who came round.

 

[Terry lambert]

The unvented would give you a better performance than a combi. You could look to getting your cold water main upgraded or fit to your existing main and see what you think of the performance first. Alternatively you could fit accumulators or booster pumps

http://www.drain247.co.uk/water-price/

Megaflo Has a patented internal baffle to take up the hot water expansion where as others use a seperate vessel.

I fit RM Stelfllow, they are cheep, come with a 25 year warranty and I havnt had any issues

 

[patman209]

Hi Terry, the mains is 32mm poly. For us the main consideration is 'future proofing' for a bigger family.
I was initially leaning towards the idea of a vaillant 937 combi but if we need to use two showers or run a bath and hot tap together, then the combi may struggle.
Plus the vaillant 937 is quite an outlay at approx 1500 for the boiler which could be a compromise on flow for DHW.
Still erring towards the unvented setup. tony glazier in above suggests that 24KW for heat should be more than enough.
I left it up to the heating engineer who came out who said either vaillant 630 or 637.
I dont want to spend the extra if i dont need to so deliberating!

 

[mfarrow]

If you are happy with your existing boiler then just add an unvented cylinder to it and fit an S-plan on the boiler's 'radiator' piping.

I'm very happy with the addition of my Vaillant Unistor to my 15 year old Baxi 105e.

 

[patman209]

Hi mfarrow, we currently have a vaillant 28KW combi, which is probably about 12 years old. Its one of the turbomax models.
Nowt too wrong with it, I understand you can connect an unvented cylinder to a combi, but understand that new boilers are mor efficient etc so decided to upgrade the whole system.

 

[coppercylinder]

These unvented tanks aren't that sophisticated and as long as they're made of duplex stainless steel ,carry a lifetime guarantee and contains the complete fitting kit the I'd suggest just buying the best priced uk manufactured tank you can find on google or eBay etc. definitely avoid plumbers merchants as you'll find they're not competive on price.

 

[muggles]

Hi mfarrow, we currently have a vaillant 28KW combi, which is probably about 12 years old. Its one of the turbomax models.
Nowt too wrong with it, I understand you can connect an unvented cylinder to a combi, but understand that new boilers are mor efficient etc so decided to upgrade the whole system.

You'll never recoup the cost of replacing your boiler in efficiency savings. Keep the one you've got.

As for unvented cylinders, go for an OSO Super Coil. Coppercylinder is, I'm afraid, talking out of his arris

 

[patman209]

I have had a third heating engineer round now (from ratedpeople again). he has another solution.
he says to keep the vaillant turbomax combi (28KW) and install a second combi (main is what he recommends).
the existing combi would serve the downstairs DHW and CH, which includes a bathroom and kitchen.
The new combi would serve the upstairs for DHW and CH, which has 2 bathrooms and four bedrooms.
This could then act as zones, which he did say was also possible from the unvented.
has anyone got any opinions on having two combi boilers in one house?
One advantage i did think of was that if one fails there is some redundancy.
However I guess with the unvented you also have that with the immersion heater in the tank.

 

[muggles]

Sounds like a daft idea to me. You'll end up with two boilers both oversize for the system they're running, using huge amounts of gas, and you'll still end up with poor hot water performance of you try to use both bathrooms at once. Also, Main boilers aren't exactly the best ones out there

Your newest installer sounds like a complete muppet. You should keep the boiler you have and get an unvented cylinder

 

[patman209]

Thanks muggles. I may just approach some local heating engineers - I only used ratepeople as I don't know anyone competant.
Still leaning towards unvented cylinder setup but I guess it is useful hearing some suggestions.

 

[JohnHarrison]

I have had a third heating engineer round now (from ratedpeople again). he has another solution.
he says to keep the vaillant turbomax combi (28KW) and install a second combi (main is what he recommends).
the existing combi would serve the downstairs DHW and CH, which includes a bathroom and kitchen.
The new combi would serve the upstairs for DHW and CH, which has 2 bathrooms and four bedrooms.
This could then act as zones, which he did say was also possible from the unvented.
has anyone got any opinions on having two combi boilers in one house?
One advantage i did think of was that if one fails there is some redundancy.
However I guess with the unvented you also have that with the immersion heater in the tank.

I know someone who did just that. It was far cheaper and the zoning meant his gas bill dropped as the upstairs was off most of the time. It saved a lot of space and these cylinders can be very heavy, so you have to make sure the floor can cope. The system was much simpler as there was no zone valves, that look ugly with pipe all around them, to take up space and go wrong. He had two simple single stage timer clocks. For the baths only, he joined the outlets of the two combis using check vales to double the flow rate into the baths.

If going this way I would suggest an Intergas 36/30 which gives the highest DHW flow rate. It has only 4 moving parts. Make sure the two combis do not exceed the gas meter capacity.

The combis modulate down, so no problem there. Make sure both have wall thermostats. Servicing costs are not a problem as an unvented cylinder needs an annual service as well. When one combi did break down for a few days he still had heat and DHW in the house. Do not dismiss this way as he is very happy bunny all around in pocket, spaced saved and reliability.

 

[Dan Robinson]

And how low do you think a boiler will modulate Big Burner.


2 combis in a 5 bed house is mental.

You need to pipe twice the gas too.

 

[JohnHarrison]

2 combis in a 5 bed house is mental.

One is already there. The cheapest and best option all around in this case is another combi.

I saw two installed in a selfbuild. Similar to as patman209 described, but only two bathrooms. One did downstairs CH one upstairs CH. The two outlets were combined only for the hot bath taps using check valves. It worked brilliantly. It had two neat white boxes on the utility room wall taking up little space in the house. The garage was on the other side of the wall and the gas pipes came in from there. A great idea. It was far cheaper than one quality boiler, unvented cylinder and all the zone valves and the rest of it, which take up lots of space. It is best not to dismiss the obvious.