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

 

[ShaunCorbs]

Just fit the combo valve eg prv at your mains stop tap or a secondary pressure reducer to keep the whole system at the same pressure

First thing to do is get a heat loss done then you know the load of the new boiler

 

[b21playa]

Just fit the combo valve eg prv at your mains stop tap or a secondary pressure reducer to keep the whole system at the same pressure

First thing to do is get a heat loss done then you know the load of the new boiler

So I do have an option, although looking to replace the lead mains with mdpe and have the stopcock relocated. All depends on cost as it’s around 30m run to the street from the front of the garage.

Can I do the heat loss calcs myself? Just trying to save a bit of cash as everything is mounting up. I can then get someone to come and fit & commission boiler and tank

 

[ShaunCorbs]

What’s your budget for the whole ?

 

[b21playa]

What’s your budget for the whole ?

Probably not enough but will be done slowly. Was hoping £5k for everything

 

[ShaunCorbs]

As your system is working atm I would get the water main done first as it’s lead you might be able to get a deduction from the water board

 

[b21playa]

As your system is working atm I would get the water main done first as it’s lead you might be able to get a deduction from the water board

Yes I’m going to phone around tomorrow for this. Then will try and find some deals on tanks etc once I’ve worked out which one.

 

[b21playa]

Managed to find a heat loss calculator online. I’ve input all the rooms with details of sizes and windows/wall type/insulation levels and got an output of 92,000 btu or 27kw. This is for a 3-storey house with solid brick walls and some decent sized rooms.

Looking at tank specs, a coil may need 20kw itself to heat tank but guess as this isn’t all the time, don’t need to have 20kw additional from a boiler. Maybe a 30kw would do

 

[EvilDrPorkChop]

Managed to find a heat loss calculator online. I’ve input all the rooms with details of sizes and windows/wall type/insulation levels and got an output of 92,000 btu or 27kw. This is for a 3-storey house with solid brick walls and some decent sized rooms.

Looking at tank specs, a coil may need 20kw itself to heat tank but guess as this isn’t all the time, don’t need to have 20kw additional from a boiler. Maybe a 30kw would do

30kw and do HW priority.

 

[SJB060685]

As above go for a PDHW setup. Now that you have arrived at a figure of 27kw can you see the problems you would experience if you let the engineer who recommended 37kw install one that size?

 

[b21playa]

30kw and do HW priority.

Thank you. I did think 37kw was excessive. I think final question is really regarding which boiler and system or heating only.

 

[EvilDrPorkChop]

Thank you. I did think 37kw was excessive. I think final question is really regarding which boiler and system or heating only.

System.

If it was me, i'd go for the Vaillant EcoTec Plus 630. In doing so will allow you to use the Vaillant controls which will do HW priority.

You will probably want to allow for some extra expansion too with the volume of water.

37Kw would of been fine tbh, as you've got to remember they have a 1:6 (or the newer models 1:10) modulation range. It isn't running at 37kw constantly.

 

[hometech]

continuous water heater is the way forward. Then add a small combi for the heating. Would be much cheaper option at around £1200-1400 for the 2

 

[SJB060685]

I’d consider the Viessmann 200 system boiler and controls. 17-1 modulation range and 19-1 on the newer models I believe, full stainless steel hex and advanced weather compensation as standard.

 

[b21playa]

System.

If it was me, i'd go for the Vaillant EcoTec Plus 630. In doing so will allow you to use the Vaillant controls which will do HW priority.

You will probably want to allow for some extra expansion too with the volume of water.

37Kw would of been fine tbh, as you've got to remember they have a 1:6 (or the newer models 1:10) modulation range. It isn't running at 37kw constantly.

So even if I got a good enough deal on a new 637 that would be fine? In terms of extra expansion do you mean by an additional expansion vessel?

The 637 was actually recommended to me by the first installer I had over, he said it would suit whole needs.

Would the valiant controls work with any cylinder and how would that affect the current 3 channel Drayton wiser system I have .

Many thanks

 

[b21playa]

I’d consider the Viessmann 200 system boiler and controls. 17-1 modulation range and 19-1 on the newer models I believe, full stainless steel hex and advanced weather compensation as standard.

Are these more efficient with greater modulation? Guessing price wise they are all around the £1500 mark. Tank wise I was suggest a tempest or gledhill 250L

 

[SJB060685]

Good modulation is paramount and no other gas boiler manufacturer on the market comes close to Viessmann modulation. Poor modulation means a lot more cycles at the boiler when the outside temperature is not design temperature. This puts a lot of wear and tear on burner components and obviously not as efficient. A Viessmann cylinder will come with everything needed to plug into the boiler a run along nicely but you could choose another cylinder brand if you wanted to.
The Viessmann is already built for a 4 pipe set up with an internal diverter valve so PDHW is easily achieved. That valve can function as a one port outlet though if running new pipes to the cylinder is not possible and you have use external valves.
Vaillant are good boilers but in my opinion nothing comes close to Viessmann. That is just my opinion though.

 

[b21playa]

Good modulation is paramount and no other gas boiler manufacturer on the market comes close to Viessmann modulation. Poor modulation means a lot more cycles at the boiler when the outside temperature is not design temperature. This puts a lot of wear and tear on burner components and obviously not as efficient. A Viessmann cylinder will come with everything needed to plug into the boiler a run along nicely but you could choose another cylinder brand if you wanted to.
The Viessmann is already built for a 4 pipe set up with an internal diverter valve so PDHW is easily achieved. That valve can function as a one port outlet though if running new pipes to the cylinder is not possible and you have use external valves.
Vaillant are good boilers but in my opinion nothing comes close to Viessmann. That is just my opinion though.

Thanks, I’ll look into these.

Wonder why their standard warranty is only 3yrs, would need their trained installer to get the 5/10/12years

 

[SJB060685]

I believe non Viessmann trained installers only qualify you for a 3 year warranty. A Viessmann trained installer qualifies you for 5 years I think and I think there’s a chargeable upgrade to 10-12 years warranty if installed by a Viessmann installer depending on your boiler model.

 

[hometech]

Good modulation is paramount and no other gas boiler manufacturer on the market comes close to Viessmann modulation. Poor modulation means a lot more cycles at the boiler when the outside temperature is not design temperature. This puts a lot of wear and tear on burner components and obviously not as efficient. A Viessmann cylinder will come with everything needed to plug into the boiler a run along nicely but you could choose another cylinder brand if you wanted to.
The Viessmann is already built for a 4 pipe set up with an internal diverter valve so PDHW is easily achieved. That valve can function as a one port outlet though if running new pipes to the cylinder is not possible and you have use external valves.
Vaillant are good boilers but in my opinion nothing comes close to Viessmann. That is just my opinion though.

If my heat demand is average 15kw and max is 22 kw and I have a boiler that is 24kw, how much modulation do I need?
just out of interest, if viesman is so perfect, why does it represent such an insignificant percentage of the sales in the UK?
Combis heat water or they heat the circuit, they can't do both simultaneously. A system boiler 4 pipe system works the same way as the combi, the heat is directed between circuits.
Having separate devices for specific jobs is the most efficient and cost effective way forward.
Storing water that you have heated to then keep hot and then mix down because its too hot to use directly..that's just silly....how on earth can that ever be considered good practice? its just wasteful..
We live in the 21st now, not the 19th.....

 

[SJB060685]

I never said Viessmann were perfect, I simply pointed out a few things as to why they’re considered superior by many and I did state that it was just my opinion, although I have many associates who feel the same.
I have my views and so do you and whether you agree or disagree with me I respect yours. I’m not going to get into a debate with you as I’ve seen some of your contributions in the past cause confusion and ultimately the conversation goes nowhere.
I will keep my mouth shut on this thread now.

 

[b21playa]

Just fit the combo valve eg prv at your mains stop tap or a secondary pressure reducer to keep the whole system at the same pressure

First thing to do is get a heat loss done then you know the load of the new boiler

Hi Shaun

I’ve got a new mains being fitted beginning of March. I’ve asked for 32mm which will terminate in the garage. I’ll do a temp connection to the existing in 22mm pex which I already have but ultimately it needs to feed to cupboard where new tank will be. This will need to run under the floorboards, can this be done in 28mm pex/pb pipe?

Many thanks

 

[ShaunCorbs]

Yes as your running plastic it’s best to upsize like you’ve done eg 22mm copper 28mm plastic / pex

 

[b21playa]

Yes as your running plastic it’s best to upsize like you’ve done eg 22mm copper 28mm plastic / pex

That’s great.

Water main guy recommended 25mm so as not to reduce pressure but as prv is 3bar I thought it was ok to move to 32mm
I had read 28mm has the same internal diameter as 32mm mdpe so thought best to run this all the way to the prv location. Just need to decide on pex or polybutylene pipe.

The 28mm plastic will terminate to a stopcock and then run internally for a short distance in 22mm copper to the prv

 

[ShaunCorbs]

Yes fine to move upto 32mm

Yes 28mm copper is same dia internally as 32mm alk