How do you calculate hot water requirement for new boiler?

[Doryzurich]

First post, so please bear with me.

We need a new boiler in our house and have had a lovely plumber calculate our Heat Calcs (heat loss?) for the house to be 34.87 kw. We have 3 bathrooms, but only use 2 of them unless we have guests (rarely!!) - my question is how do we calculate the hot water need in order to calculate the size of the boiler? We have a Tribune HE 210 litres unvented cylinder which has always been plenty big enough for our needs.

Or have I got the wrong end of the stick and you can't add the two together to get a rough idea of the KW you need from the boiler? We currently have a vented 'regular' boiler with an unvented HW cylinder - and are looking to move to a sealed system throughout (not sure whether that makes a difference to the calculation).

Many thanks - and apologies to the professionals out there who are tired of answering obvious numpty questions from desperate housewives!!

 

[Dan Robinson]

You used to make a small allowance for hot water in addition to the heating.

Most of us now prefer to size for heating and set the controls accordingly.

 

[ALEC1]

a decent new boiler and controls will have hot water priority giving a relatively quick reheat time. The cylinder will have a kw rating. Boiler diagnostics enable the engineer to match the two...

coincidence of use is a useful concept to understand, rarely do you run two baths at the same time...

Cylinder manufacturers can advise on the necesary volume...

 

[Doryzurich]

So i only need a 35kw boiler.....? That's great - but why wouldn't you take into account the hot water need? If heating takes almost 35 kw, where does it get the power from to heat the HW? Sorry if this is a stupid question, just trying to understand....

 

[Doryzurich]

Sorry ALEC1, only just seen your response. So the Tribune HE 210 says it has Kw/24hrs standing heat loss of 1.85, but from your comment about 'boiler diagnostics enable the engineer to match the two' - are some cylinders better matched with some boilers? I.e. if we moved to a Viessman 200 System 35kw, it would be adviseable to change the cylinder also to a Viessman - or should the Tribune HE be equally has efficient?

Bit of a loaded question as we've been advised to move up to a Viessman 250 litre even though the 210 litre seems fine for us - just needing to understand what I'm missing....

 

[ALEC1]

the thing to look at is the coil size kw output which the manufactures can help you with.

The bigger the coil the quicker the reheat time and more efficient the boiler

There will be issues with somethng called a two port valve, but as your cylinder is out of guarantee it should not matter. The installer should be happy to use the boilers internal valve and will have to use a viessmann cylinder sensor and controller

A newer cylinder may have better levels of insulation...

 

[Doryzurich]

Thanks for this - starting to make a bit more sense!

So I've checked the coil rating for the Tribune and it's 23.08 kw - however, if I've understood correctly, if the boiler is set up as 'hot water priority', then I only have to put in a boiler big enough to cover the heating (34.7kw), rather than heating plus hot water, which would give me a whopping 57.78kw moving into the semi-commercial boiler size.....correct??

Second question - if the Tribune cylinder takes 22mins to reheat, this should be the maximum amount of time that a 35kw boiler diverts 23.08kw of energy away from heating the rads....is my theory right?

 

[CBF]

if you are happy with your existing cylinder why are you even thinking of changing it, just replace the boiler, what is the size of your existing boiler

 

[Doryzurich]

Hi CBF - existing boiler is Worcester Greenstar 40cdi. We don't want to change the cylinder, but the heating chap doing the calcs is saying that a move from 210 to 250 "would help the load of the cylinder making it more efficient". I'm assuming I'm being a bit thick not understanding why. Do you know the reason? i thought perhaps Viessman boilers run more efficiently with Viessman cylinders....?

Tbh - i just need to understand how this all translates into running costs. The boiler needs replacing - the cylinder seems absolutely fine and big enough for us - but if replacing with a slightly larger one so that the boiler can run more efficiently reduces down my gas bill, so all good. But I just can't get this info anywhere!!! (Or maybe i'm not asking the right questions).

Any advice?

 

[Dan Robinson]

Your existing cylinder will be perfectly fine (lime scale notwithstanding).

Why are you changing the boiler? A Greenstar 40CDi should be at the stage in its life where it needs changing.

 

[Doryzurich]

We inherited the boiler 5mths ago when we moved into the house. Has never worked properly and no-one seems to be able to fix it. Found some old receipts at the back of a cupboard and looks like the fan was replaced twice whilst under warranty and the electrics have been dodgy. We've decided to upgrade the pipework (22mm to 28mm) and move the boiler due to a pending extension and I'm loathe to spend a few hundred quid of fixing up what might be a 'Friday boiler' when for a bit more I can get a new boiler with a 5 year guarantee. Question is whether to replace like for like (another Worcester 40kw) or to actually reduce kw to 35 (Viessman 200) given my heating guys calcs. Hence all my original questions surrounding calcs for hot water.

Hope this is all making sense - really helps to write it down!

 

[Dan Robinson]

In that case...

Intergas HRE SB40 (maybe even a 30)
Atag Q38S
Vaillant Ecotec Plus

Would be the order I would start looking at replacements.

Followed then by:

Worcester
Veissman.

 

[Doryzurich]

Thanks Dan - brands that haven't been mentioned yet so well worth a look.

Have a good evening.

 

[Dan Robinson]

Likewise

 

[CBF]

another 40 ltrs of water to heat up that you have said yourself you don't need, so can't really see how this is more efficient