Combi or unvented direct cylinder?

[RickyLee53]

Hello,

I posted some time ago about which boiler would be best. I have since upgraded the water coming into the house to 32mm mdpe and my flow rate has improved a lot.

I have a static pressure of 3.6 bar. I have a flow rate of approx 30lpm. (this was tested by running 2 taps (bathroom) at the same time. For 30 seconds, I had 15l of water total. If running just the bath for 30 seconds I got 10 litres)

I've been looking at this boiler
http://www.bhl.co.uk/product/VAILLANT_ECOTEC_EXCLUSIVE_838_HE_COMBI

My plan was to use this for the CH (direct cylinder). It would also provide hot water for the downstairs taps and possibly 1 of the ensuite bathrooms. The cylinder would provide the rest of the hot water for bath/showers etc.

My questions are,

What is a good unvented cylinder?
Is the boiler a good choice?
What size pipe should I run to the cylinder/hot water. I was going to take 1 28mm pipe to the bathroom, then branch off 22mm for bath/shower cold, then another 22mm to the cylinder. Is this bad practice? Should I run 2 seperate 22mm pipes.

 

[dcawkwell]

I would go for a glow worm ultra power. A cross between combi and unvented cylinder all in the one nice box.
Will work as an unvented cylinder then as the cylinder runs out switches
to combi delivery rates.
You will never run out of water.
If not many in the house you can switch the cylinder off and just have combi mode.
You also have the backup of an immersion heater should the boiler fail
and 5 year warranty.

Because you have mains pressure you can probably run 15mm to bath and other fittings. 10mm is quite ok for kitchen and basin taps.
22 mm from the cylinder itself.

 

[petertheplumber12]

If you want an unvented cylinder why don't you have a vaillant system boiler with an unvented cylinder, that will give you good heating and better hot water flow rate than any combi. And vaillant are excellent boilers in my opinion, Glow worm are also owned by vaillant and are very good too. vaillant thermo compact 628E is a system boiler, look that one up it is probably cheaper than the combi too.

 

[Mark Br]

I would go for a Vaillant system boiler and unvented cylinder too. Thermocompacts haven't been made since god knows how long though !!! ( or am I wrong?)

Why go for a direct cylinder rather than indirect?

 

[RickyLee53]

I would go for a glow worm ultra power. A cross between combi and unvented cylinder all in the one nice box.
Will work as an unvented cylinder then as the cylinder runs out switches
to combi delivery rates.
You will never run out of water.
If not many in the house you can switch the cylinder off and just have combi mode.
You also have the backup of an immersion heater should the boiler fail
and 5 year warranty.

Because you have mains pressure you can probably run 15mm to bath and other fittings. 10mm is quite ok for kitchen and basin taps.
22 mm from the cylinder itself.

Very good suggestion, I'd not seen those, however I see 2 issues. The first is their capacity, (I have a very large bath which will require close to 250l hot water.)

Also the physical size, the boiler is situated in the ultility room inside a "tall unit" so the depth is too large to fit in the cupboard.

I will run immersion heaters on the unvented in case of a problem.

I'm wanting the best possible flow/pressure. So i'd rather "overspec" the pipes, even if it costs a little more.

 

[RickyLee53]

I would go for a Vaillant system boiler and unvented cylinder too. Thermocompacts haven't been made since god knows how long though !!! ( or am I wrong?)

Why go for a direct cylinder rather than indirect?

I meant indirect! With the backup of immersion.

I was going to go for Combi so there is always hot water at the taps, and 1 bathroom. Then I can pick and chose when to warm the stored water. I'm not sure of the warm up times, and efficiency of doing it this way but at least I have the options.

It could be a bad idea, and I'm open to ears. I'm not a plumber and plan on doing the work I can myself. Hence asking on here before I start/proceed.

 

[dcawkwell]

I would go for a glow worm ultra power. A cross between combi and unvented cylinder all in the one nice box.
Will work as an unvented cylinder then as the cylinder runs out switches
to combi delivery rates.
You will never run out of water.
If not many in the house you can switch the cylinder off and just have combi mode.
You also have the backup of an immersion heater should the boiler fail
and 5 year warranty.

Because you have mains pressure you can probably run 15mm to bath and other fittings. 10mm is quite ok for kitchen and basin taps.
22 mm from the cylinder itself.

Very good suggestion, I'd not seen those, however I see 2 issues. The first is their capacity, (I have a very large bath which will require close to 250l hot water.)

Also the physical size, the boiler is situated in the ultility room inside a "tall unit" so the depth is too large to fit in the cupboard.

I will run immersion heaters on the unvented in case of a problem.

I'm wanting the best possible flow/pressure. So i'd rather "overspec" the pipes, even if it costs a little more.

Not a problem the boiler will supply infinite hot water the size of the cylinder is irrelevant it is just how fast you can heat it.
The unit will exceed the performance of a 300 litre unvented tank
with only 170 litres of storage and therefore has a smaller foot print.
You would install the ultrapower where the cylinder would have gone.
It will be smaller foot print. The ultrapower can supply up to 36litres a minute of hot water.

 

[RickyLee53]

I would go for a glow worm ultra power. A cross between combi and unvented cylinder all in the one nice box.
Will work as an unvented cylinder then as the cylinder runs out switches
to combi delivery rates.
You will never run out of water.
If not many in the house you can switch the cylinder off and just have combi mode.
You also have the backup of an immersion heater should the boiler fail
and 5 year warranty.

Because you have mains pressure you can probably run 15mm to bath and other fittings. 10mm is quite ok for kitchen and basin taps.
22 mm from the cylinder itself.

Very good suggestion, I'd not seen those, however I see 2 issues. The first is their capacity, (I have a very large bath which will require close to 250l hot water.)

Also the physical size, the boiler is situated in the ultility room inside a "tall unit" so the depth is too large to fit in the cupboard.

I will run immersion heaters on the unvented in case of a problem.

I'm wanting the best possible flow/pressure. So i'd rather "overspec" the pipes, even if it costs a little more.

Not a problem the boiler will supply infinite hot water the size of the cylinder is irrelevant it is just how fast you can heat it.
The unit will exceed the performance of a 300 litre unvented tank
with only 170 litres of storage and therefore has a smaller foot print.
You would install the ultrapower where the cylinder would have gone.
It will be smaller foot print. The ultrapower can supply up to 36litres a minute of hot water.

Sorry,

I wasn't clear with what I said. The combi is going downstairs in a unit. But the cylinder is going in the loft. There isn't a gas supply to the loft. And its a fairly long run. So that would still be a slight issue.

Also, if it stores 170 litres. And flows 36l a minute, It would be "empty" in 5 minutes approx. The "combi" aspect flows 12.5 l's per minute. I'm not sure if it uses this to refill the tank, if it does that would give another 70 litres of hot water. The other 23.5l per minute i'm assuming comes from the cold so the water would be pretty much cold. Unless when the water is up it just switches to combi.

Obviously, this is an issue with the normal unvented cylinder as well I guess? After 200l's of hot water have gone the temp will have dropped by 2/3's approx.

It's certainly worth thinking about though, it'd save a LOT of time effort/money. Its just a shame they don't do a larger unit.

 

[petertheplumber12]

I still think your better having a system boiler and unvented cylinder, they have a rapid recovery facility and will reheat in 20 mins with boiler on all the time. they are also insulated well enough to be 95% efficient at keeping the heat in.

 

[dcawkwell]

No just run a gas pipe into the loft.

The boiler delivers 36litres of pure hot if your cold main can supply it
and your piping take it. I very much doubt it.

The 170litre model will have no trouble filling your 250 litre bath
and reheat faster than your unvented cylinder.
A 300 litre tank will take 30minutes to reheat before it can supply any more hot water. The ultrapower is ready to go
supplying hot water at 12 litres a minute until the cylinder can reheat.

 

[RickyLee53]

No just run a gas pipe into the loft.

The boiler delivers 36litres of pure hot if your cold main can supply it
and your piping take it. I very much doubt it.

The 170litre model will have no trouble filling your 250 litre bath
and reheat faster than your unvented cylinder.
A 300 litre tank will take 30minutes to reheat before it can supply any more hot water. The ultrapower is ready to go
supplying hot water at 12 litres a minute until the cylinder can reheat.

Ok, just thinking outloud here.

I've just done another test and get 30-32 lpm from the mains cold, this, I could upgrade should I require. (I ran a 32mm MDPE line to the road, however when calling the water board they said they couldn't come out for ages. So I just connected to the old pipe, because it was such a short length my flow was massively increased)

The unvented cylinder I read about can do 60lpm. However I cannot quantify this, but I imagine 30lpm is probably enough, and will fill my bath 9 minutes anyway. I doubt even running the biggest taps I can flow more than 30lpm.

Thats the thing with the pipes, I'm changing them all and overspec'ing everything. I have a 28mm gas supply, 32mm mains in, 28mm cold feed. I'll run 22mm to bath tap's. then step down at the last minute.
Since upgrading my mains supply my current taps 15mm pipes, make a squeling noise. I only get 15 lpm approx out of each tap. But IF I run an open pipe 22mm I get the 30+.

The Ultrapower does sound very tempting. Especially been about half the price. Here's another suggesting which could be stupid I don't know.

Could I run a Combi and Unvented to the same pipes. To basically have the Ultrapower but with more power. But I'd need some kind of valve to turn off the cylinder when it dropped below a certain temp.

I'm just worried the 170litre cylinder won't be enough. Lets say I have a shower. Flow approx 25lpm from the hot. (large 450mm square head) That would only give me 7 minutes till I'm out of hot water.
While on this subject though. If I have a 300litre cylinder. How much useable water do you get out. Obviously as the cold start to enter it will cool.

Do you slowly have to increase the shower temp for the temp to remain stable?

Sorry for going on, i'm now more undecided than I was before.

 

[doitall]

I think the OP is forgetting what a combi can actually heat @ 42c, it could take 15mins plus to fill a 250Ltr bath

A good boiler and a 210Ltr unvented will be miles better than a combi.

No good having 30 ltrs/min coming out of the bath tap at 20c unless you love cold baths.

 

[ALEC1]

People miss the point about storage combi's even the 19kw vitodens 222 supplies 170 litres of hot water in the first 10 minutes...from a tank of 100.

the 35kw does a whopping 280l in the first 10 minutes... more than the the time it takes to fill a bath.

then they both go on to supply hot water at a reduced rate.. for ever, or until gas runs out...! Add a recirculation loop for reduced delivery times at the taps...

thats because of the way we use hot water.. a bath is by far the biggest draw off, but showers and other activities are not so onerous and can easily be accomodated for with a storage combi...

a big issue with unvented cylinders is their poor recovery rate...if you must go that route install twin coil...and the use of a combi to heat a hot water cylinder precludes the use of compensation controls which will help to run heating more efficiently, not such a big deal with gas as cheap as it is now...but that choice may come back to haunt you...

hot water priority is a must with big draw offs...and if you opt for vaillant the vrc65 gives this automatically...

 

[RickyLee53]

I think the OP is forgetting what a combi can actually heat @ 42c, it could take 15mins plus to fill a 250Ltr bath

A good boiler and a 210Ltr unvented will be miles better than a combi.

No good having 30 ltrs/min coming out of the bath tap at 20c unless you love cold baths.

I'm not 100% sure what you mean, or if you read correctly? A combi on its own is/was never an option. I'm going for a 300l unvented, with a combi for the "normal" taps. I.e. downstairs, ensuites. The unvented will be purely for the master bath/shower.

 

[RickyLee53]

Hello Alec, thanks for your input.

170 litres in 10 minutes isn't what I want though. Ideally i'd like to fill the bath in around 8 minutes (250 liters). The second option is pretty viable.

I'm not sure what you mean about recirculation loop, do you mean run it like the CH and having the hot water running all the time? Thats a pretty neat idea, but surely very inefficient.

The recovery rate is something i'm a little worried about. Hence the reason for going combi and unvented. Realistically there will be 2-3 people bathing in 1 evening at the most. If a 300 liter tank can recover in 30-40 minutes that would be plenty enough time. If its longer however we might struggle.
Twin coil, I assume means twin coils in the cylinder? I've not seen that mentioned. I looked at this cylinder, purely for the sake of staying with the same brand.

http://www.discountedheating.co.uk/...Unistor_310_L_Unvented_Indirect_Cylinder.html

The last bit totally lost me, i've just searched the VRC65 and it looks like a zone controller. I did plan to run something like this. I.e. Heating and cylinder on seperate circuits. Are you suggesting I wouldn't be able to do this with a Combi? I thought I could.

I haven't dismissed the Glow Worm Ultrapower, but I think I might regret it if it doesn't have enough power/storage. Its probably a similar amount of hassle getting the gas to the loft area as it is running a larger cylinder and Combi/boiler.

People miss the point about storage combi's even the 19kw vitodens 222 supplies 170 litres of hot water in the first 10 minutes...from a tank of 100.

the 35kw does a whopping 280l in the first 10 minutes... more than the the time it takes to fill a bath.

then they both go on to supply hot water at a reduced rate.. for ever, or until gas runs out...! Add a recirculation loop for reduced delivery times at the taps...

thats because of the way we use hot water.. a bath is by far the biggest draw off, but showers and other activities are not so onerous and can easily be accomodated for with a storage combi...

a big issue with unvented cylinders is their poor recovery rate...if you must go that route install twin coil...and the use of a combi to heat a hot water cylinder precludes the use of compensation controls which will help to run heating more efficiently, not such a big deal with gas as cheap as it is now...but that choice may come back to haunt you...

hot water priority is a must with big draw offs...and if you opt for vaillant the vrc65 gives this automatically...