Combi vs tank - which should I go for?

[b21playa]

Hi all

Currently have an open vent system with a 140L tank. This all needs renewing and wondering whether it was worth changing over to a high flow combi instead or to replace with an unvented cyclinder.

Currently only have 1 bathroom, but looking to add an ensuite at some point as well.

Thanks

 

[Deleted member 120897]

A lot depends on existing cold main pressure and flow rates, without these, it’s difficult to recommend one system over another.

 

[b21playa]

A lot depends on existing cold main pressure and flow rates, without these, it’s difficult to recommend one system over another.

Hi

I’m not sure about pressure but I have 22lpm measured by flow cup.

Looking at costs of each, there doesn’t seem to be much between a Worcester 550cdi and a system boiler & tank to buy. Understand there will be additional costs in installation.

I’ve had a few people come and quote for system boiler setup but varying between a 30kw and 37kw boiler.

Thanks in advance

 

[exedon2]

If there is any chance at all of bathroom and proposed ensuite been used at same time unvented cylinder ever time.
I'n my experience use of both will happen.

 

[b21playa]

If there is any chance at all of bathroom and proposed ensuite been used at same time unvented cylinder ever time.
I'n my experience use of both will happen.

Yes they will be used same time so at least that answers that one, thank you.

In terms of boiler sizing and type are you able to help as getting mixed responses?

 

[SJB060685]

Any firm quoting to do the job should have visited your property and spent at least an hour, maybe more taking room measurements and asking about insulation levels. Without that no one can accurately say what size boiler is needed.

 

[b21playa]

Any firm quoting to do the job should have visited your property and spent at least an hour, maybe more taking room measurements and asking about insulation levels. Without that no one can accurately say what size boiler is needed.

I’ve had a few over, they’ve not measured rooms though m. I have done though

 

[SJB060685]

If they’re not measuring room sizes and asking questions about insulation then I wouldn’t choose them to carry out the work. An accurate heat loss calculation needs to be carried first of all. Then you can discuss boiler/system options.

 

[b21playa]

If they’re not measuring room sizes and asking questions about insulation then I wouldn’t choose them to carry out the work. An accurate heat loss calculation needs to be carried first of all. Then you can discuss boiler/system options.

Thanks. Getting annoyed with people just quoting based on number of rooms

 

[SJB060685]

There are general rules of thumb but they are guesstimates and could be way off if different areas of the house have had any insulation improvements. If you want a decent installation certain things are mandatory, a proper heat loss calculation chief among them. If you have firms not even doing this and supplying you with a quote I wouldn’t even consider them to be honest.

 

[ShaunCorbs]

Unvented or stick an electric shower in the new e suit and combi then

 

[EvilDrPorkChop]

To be fair, at the quoting stage I wouldn't be measuring rooms and calculating room sizes for heat outputs. From experience you generally have a rough idea of what will be needed etc. If I spent every time calculating figures for each job I priced, then i'd not have any time to do any actual work, and I bet half the time i'd be wasting my time and doing the work for the plumber they actually go with cause no doubt someone will be cheaper.

Whilst I wouldn't expect it at quoting, I would like to think once someone has got the job that all the necessary calculations would be done as @SJB060685 says as I usually do. 9/10 i'm usually right with my original guesstimate.

 

[ShaunCorbs]

Sys / heat only I work the room sizes out or worst case I work out the existing rads

Combi cold water flow rate

 

[b21playa]

Our existing rads are rubbish, I’ve been trying to replace them along with upgrading pipework to accommodate. I get whole house (20ish rads over 3 floors) at around 120,000 btu based on online calculators and know very little insulation and solid brick walls.

 

[SJB060685]

If a thorough calculation is completed you will arrive at the most accurate result. Having said that there are often cases where the homeowner just doesn’t know what, if any insulation is present. Times like that you would use quick reference guides but as I said above these can be way off at times. Once you have an idea of your loads then you can start designing the system. Is the customer happy with a high temperature system? Or do they want to maximise efficiency as much as possible and have a low temperature system installed and so on and so on. Before any of this can be decided and priced up and quoted for you need those initial heat loss figures.

 

[b21playa]

If a thorough calculation is completed you will arrive at the most accurate result. Having said that there are often cases where the homeowner just doesn’t know what, if any insulation is present. Times like that you would use quick reference guides but as I said above these can be way off at times. Once you have an idea of your loads then you can start designing the system. Is the customer happy with a high temperature system? Or do they want to maximise efficiency as much as possible and have a low temperature system installed and so on and so on. Before any of this can be decided and priced up and quoted for you need those initial heat loss figures.

Hi

This is my own house, looking for get heat into it. It takes hours for it to heat up, our previous house was never like this.

 

[SJB060685]

Yes I know mate. My questions were generic ones not specifically asking you 👍. Excuse my ignorance but didn’t you create a post the other day about pipe sizes feeding radiators?

 

[b21playa]

Yes I know mate. My questions were generic ones not specifically asking you 👍. Excuse my ignorance but didn’t you create a post the other day about pipe sizes feeding radiators?

Yes I did pal. I’m trying to plan everything and changing radiators while I’m sorting a few rooms out downstairs. The boiler/tank will be changed before kitchen is completed as it’s currently in a cupboard in there. Hot water flow is not good and I’d like to remove tanks in the loft

 

[SJB060685]

Yes I remember that post and that your current system is a shambles. To be honest from what I remember of the other post is if any of those that have used your existing system as a reference to supply you with a quote so far then forget them. You need a good installer/firm who are going to do everything required and thoroughly.

 

[b21playa]

Yes I remember that post and that your current system is a shambles. To be honest from what I remember of the other post is if any of those that have used your existing system as a reference to supply you with a quote so far then forget them. You need a good installer/firm who are going to do everything required and thoroughly.

All figures are based off calculate I’ve used. You’re right the existing system is no good so I’ve disregarded everything in it, everything needs renewing, apart from pump and zone valves which are 3yrs old 🤣

 

[b21playa]

Just tested water pressure at washing machine fitting @ 4bar static

 

[ShaunCorbs]

And flow rate eg how long to fill a 1 or 2 l bottle at full open

 

[b21playa]

And flow rate eg how long to fill a 1 or 2 l bottle at full open

Flow rate was approx 22lpm measured with flow cup

 

[ShaunCorbs]

That’s perfect for an unvented

 

[b21playa]

That’s perfect for an unvented

Thanks for that, great to hear.

Only issue I have is I’ve read cold mains should go to cylinder first and then balanced off to taps. As my mains comes up in the kitchen and already connected to everything first, will this cause an issue?

Also what’s the recommended boilers these days. I have a valiant 438 which came with the house and heard they are not the best. Is this the same for a 637 and should I look at a different manufacturer

Thanks again