Replacement boiler: Combi or System?

[nickat]

Hello, I'm looking to replace a Worc/Bosch 440 Highflow. It has been suggested that I look at the Vaillant 937 or 637. I'm really looking for some general advice that would help me decide between them or an alternative. Could anyone say how the running costs compare between combi's and system boilers. We don't have a huge demand for lots of hot water at once (at least not yet), but in saying that running 2 showers at once would be a "nice to have". With a system boiler does it only feed the tank, so once it's empty you have to wait for more hot water. Would you keep the HW on constant with a S/B or time it. Any advice would be greatly appreciated. I tried Vaillant but at 60p a min for customer tech advice ....

Thanks, Nick.

 

[Chrisgy]

Combis only use gas when the tap is turned on or heating is on, system boilers like you said heat up a tank full of hot water, once it's used or cooled down the boiler will come back on, you could use a timer to interrupt this though.

Showers with a system boiler can be a lot better though have you looked at an unvented cylinder with a system boiler?

You could use a shower pump with a normal cylinder but not with a combi.

Depends if you really want to have two showers at a time. Also cost, with an unvented cylinder being most expensive option, but best showers.
2. Keep your old cylinder and have a shower pump fitted with a good output for two showers, still not sure on how good that would be running two at a time though. You might have to get a new cylinder if it's not big enough
3. Have a combi, use less gas if you don't use much hot water, but find it impossible to run two showers

 

[JamesC]

Also how good is your cold water flow rate?
If it's not any good get a system boiler.
Also how big's your house? I wouldn't really put a combi in anything bigger than a 2 bed personally.

 

[Spudhater]

2. Keep your old cylinder and have a shower pump fitted with a good output for two showers, still not sure on how good that would be running two at a time though. You might have to get a new cylinder if it's not big enough
3. Have a combi, use less gas if you don't use much hot water, but find it impossible to run two showers


To run two showers on a pump you will need to up the size of your CWST to at least 50 gallens and possibly your feeds to the cylinder to 28mm and then your cylinder will need dedicated feed in 22mm ( if the cylinder is large enough to start with) other wise you will pump it faster than it can recover, pumps dont like running dry !!!

 

[nickat]

Thanks for the replies. Our flow rate is fine. We can get 2.5 bar and above no probs, going by the gauge on the current combi. The house is big, 5 beds.

 

[Spudhater]

Unvented the way to go then.............

 

[tamz]

The OP has a Highflow COMBI to replace. These have an internal heatbank and a flowrate of just under 4 gallons/minute @ 35deg rise.

The Vaillants are both good boilers and the 937 will give you around the same output as you have.
A system boiler/unvented cylinder,although more expensive to install, will give you a better flow rate, probably above 50 litrs/min depending on incoming mains and would always be the better choice. They are a bit more expensive to run but not much when compared to your highflow. If piped up correctly the recovery rate of the unvented should be around 20 minutes.

 

[Spudhater]

Vaillant 937 20l/m for around 20min then 15l/m ( just to clarify) Bigger flow rate bigger price...

If u go UV you will at least have immersion back up, either way is a solution..

 

[nickat]

Thanks for the advice. It has been very helpful.

 

[doitmyself]

Hello, I'm looking to replace a Worc/Bosch 440 Highflow.

Why do you want to replace it??

 

[nickat]

The 400 is 10 years old now, keeps breaking down.

Would the 937 combi be able to run 2 showers at once ?

 

[Toddyplumb]

Plastic parts and O rings , dont you love em

 

[justlead1]

Hi You can not design a combi system to deliver any more than its max. If you go conventional, the time you save waiting for the bath to fill, at say £5-00 and hour will make you a couple of grand over ten years and a lot of frustration. They do have there benefits as i have never seen so many gas monkeys on the road as i have of late. A job for the boys if ever there was one. Good Luck

 

[Centralheatking]

Its a no brainer - dump the combi idea. The house is too big - combis are only 4 flats and small houses, unless you want to spend the nexy 10 years watchin the bath slowly fill up esp. in the summer when the utility drops the water pressure.

centralheatking

 

[jambo]

away to be fitting a 32/50 oil worcester 18 rads 3 bathrooms all electric showers do you think this will be ok as i surveyed it myself just need to know before i go ahead cheers

 

[tamz]

Thats a bit of a beast of a boiler. It weighs over 1/4 a ton. Unless the rads are BIG rads you have oversized it. What type/size house/castle is it. New system or boiler change? If boiler change what is fitted now?