Combi vs Standard Boiler

[marty_p]

I am considering replacing my old standard boiler for a new high effeciency combi, but am concerned about drop in HW pressure / flow rate that can be experienced whilst showering if a tap is run elsewhere. The overall benefits of a combi are very tempting but the flow rate problem is a concern. Anyone have any experience of combi showering? Am I worrying unduly or should I stick with the existing system and just install a pump to get a decent shower? Small 4 bed house with single bathroom and one kitchen. Thanks.

 

[gas4you]

All normal combis run the risk of the problem you are concerned about.

I always tell my prospective customers to NEVER expect to be able to run two hot outlets at a time.

 

[easyflow]

Combi advantage is you dont need tanks or acylinder, and hot water is instant.
Combi disadvantage is, takes longer to fill a bath and pressure drops if somone else uses a mains outlet, and theres no water storage.
But unless you have a very tight schedule no problems.

 

[heatmanuk]

try this boiler just look

 

[WDIK]

Combi advantage is you dont need tanks or acylinder, and hot water is instant.
Combi disadvantage is, takes longer to fill a bath and pressure drops if somone else uses a mains outlet, and theres no water storage.
But unless you have a very tight schedule no problems.

nearly correct, problem is you need to over size dhw and have heating modulation to be able to satisfy the criteria

 

[marty_p]

OK, thanks everyone. I think I will stick with installing a pump and hope that I don't keep running out of hot water. I am not bothered about the bath filling duration. It's the showering experience that is important. The risk of interrupted flow when another tap is run I think is too great and it does seem to be a real problem for a combi. If I fit a pump and it runs the HW supply down too quick, then I can always switch to a combi at a later date. I guess going for a combi now and then wanting to switch back later will be very expensive. Cheers.

 

[assured heating]

Have you thought about having a convential boiler and an unvented cylinder these give a more balanced hot and cold when showering providing your mains pressure is sufficient and you have the back up hot water from the imersion if your boiler breaks down ,and you can do away with your Mcws and f&e tanks in loft

 

[marty_p]

Thanks. I had briefly considered it but not given too much consideration to be honest, but I will look at it some more. It's a good idea, thank you. The mains pressure is pretty good. Downside I assume is that you are still limited to the quantity of stored HW.

I guess what I am really after is hearing from people who have experience of using a shower run from a combi boiler.

 

[ub7rm]

I recently had a combi installed and put in a mixer shower. I can tell you that compared to an instantaneous electric shower its a million times better! However if you have a pumped shower at the moment I think you'll be a bit dissapointed.

The boiler can chuck out about 10 l/s of hot but I've never measured what comes out of the spray head, I guess there will be some cold water in there as well at normal showering temp.

I live in a 1 bed flat with my g/f so we've just got a 24kw job which is more than adequate for a decent shower but the more powerful the boiler the better the shower flowrate (assuming pressure isn't a limitation).

As has been said before it is a bit slower at filling a bath than the old tank and immersion heater but if you can manage thinking ahead by about 10 minutes this shouldn't be a major problem.

I would say as a DIYer that if there is nothing wrong with your system other than you want a decent shower go for a pump as this should give you better performance than a combi. If I read you right you have a gravity mixer at the moment? If you were to go for a combi I suspect you would see an improvement.

HTH

 

[marty_p]

Thanks for the reply and info. I do have a gravity fed shower at the mo, mixed via a basic bath tap mixer. I did use a combi shower at a friends house and the pressure and flow rate was very good. I was quite impressed and surprised. It would be totally suitable for a shower, but he did say it goes cold when someone else runs a tap in the kitchen. This then put me off a bit. To me it seems a toss up between the problem of having to run the risk of an interputed flow during a shower with a combi boiler versus the risk with a pump of running out of hot water.

 

[Soggy_weetabix]

Have you thought about having a convential boiler and an unvented cylinder these give a more balanced hot and cold when showering providing your mains pressure is sufficient and you have the back up hot water from the imersion if your boiler breaks down ,and you can do away with your Mcws and f&e tanks in loft

A perfect solution to your problem!

DAVE

 

[tonybhoy]

At the moment I'm having two showers a day from a 35 KW combi and it is fantastic. I get about 2.5Bar of incoming cold mains pressure,which is what most unvented cylinder's pressure is on the hot water outlet , this gives me around 14 liters per/min at the hot bath tap.

Yes of course the flow rate drops if someone runs another tap/outlet but in my house that is rarely a problem whilst your in the shower and in anyevent your only talking about a minute or so (filling a cistern for example)before you are back to full showering,no big deal really.

BTW,a 35 KW combi boiler's hot water output is roughly equivalent to 4 X 9KW electric showers' output.

BWT 2,gravity showers are approx 0.4 bar or less at the shower head ,depending of course on the available head of pressure in the house.

 

[BigBurner]

Combi advantage is you dont need tanks or acylinder, and hot water is instant.
Combi disadvantage is, takes longer to fill a bath and pressure drops if somone else uses a mains outlet, and theres no water storage.
But unless you have a very tight schedule no problems.

This is incorrect. High flow combi can cope with three outlets at once. if the mains flow is enough of course.

 

[BigBurner]

Have you thought about having a convential boiler and an unvented cylinder these give a more balanced hot and cold when showering

They do not as they are off the mains like a combi. Pipe up a combi properly and they give little trouble.

providing your mains pressure is sufficient and you have the back up hot water from the imersion if your boiler breaks down ,and you can do away with your Mcws and f&e tanks in loft

An in-line instant electric water heater can be in the combi outlet for backup - price about £100.

 

[BigBurner]

Have you thought about having a convential boiler and an unvented cylinder these give a more balanced hot and cold when showering providing your mains pressure is sufficient and you have the back up hot water from the imersion if your boiler breaks down ,and you can do away with your Mcws and f&e tanks in loft

A perfect solution to your problem!

DAVE

Apart from the increased cost, increased space it takes up and the annual service charge. The only unvented cylinder I would fit is a an ACV tank in tank. It is also a thermal store that the CH can be run off.