Boiler size - why get a big one

ToneControl

When we got this house, we were recommended to buy a Worcester Bosch 440CDi, because it's big and old and has 2 showers.

The boiler cost a fortune, and is proving expensive to maintain.
It heats the house up in less than an hour when needed.
Despite the hype, it can't support both showers at once, although water pressure is OK.

Looking at the spec of boilers, I can't see why the lower spec ones would not work for us when we replace it.
Beyond the small ones, they all seem to put out 35kW for heating.
The only difference seems to be the hot water flow rate, which basically means how long it takes to fill a bath.
AFAIK they can all supply one shower.

Why couldn't I use a Greenstar Style 35kW for example?

The manufacturer's website recommends the larger ones for large houses, but if you go through their questionnaire it recommends the 35kW one

Adey

Not going to be cheap to swap it. How long will it take to pay back that expense? Will you still be there?

Can you turn down the boiler temperature so you don't heat water to as high a temperature? You won't then add a load of cold water to cool it back down again afterwards.

exocet

Combi boilers in general are always more problematic / costly to maintain compared to non combi equivalent.

Is the 440 Highflow a combi with a built in small storage tank I.e. something like 10 litres of hot water available instantly? You pay more for that feature and it's probably not worth it.

Any combi will struggle to heat high volumes of cold water in winter for a shower but I agree that the Greenstar 35KW model would be as good as any other for a single shower. 

Real answer is a hot water tank and a standard boiler but space constraints commonly dictate otherwise.

Dominic

Ideally a pressurised Megaflow type system with a large steel pressurised tank but it's an expensive option.
Any combi will retard the hot flow ....so not great for a powerful shower but a conventional boiler witha good Pump like a Stuart Turner Monsoon will give you great showers (but you need a big tank or the pump will drain it quickly )

ToneControl

The other problem is that the boiler is in an old dedicated boiler room at the back of the house, where the gas supply comes in, whereas the main bathroom is near the front a long pipe run, and it takes 30 seconds to get hot water to the bath, but at least a minute to the shower or sink, since the flow rates are lower.

It's not the time required to get the boiler running, because there is that stored hot water at the ready, the kitchen sink 2m from the boiler takes 3 seconds to get scalding hot water.

Anyway the point being that you have to heat a lot of water that then just sits and cools down in the pipes after you wash/shave /whatever.

I visited a large modern house once with 2 normal size wall-mounted boilers in the garage, not sure why, but I have thought that ideally a dedicated boiler in the main bathroom would be more efficient, but I know that the pipe run for the gas supply would cost over £1k.
It would be nice to have a second boiler to heat some of the house, then if a boiler failed, we'd still have some warm rooms and hot water.

ToneControl

Dominic said:

Ideally a pressurised Megaflow type system with a large steel pressurised tank but it's an expensive option.
Any combi will retard the hot flow ....so not great for a powerful shower but a conventional boiler witha good Pump like a Stuart Turner Monsoon will give you great showers (but you need a big tank or the pump will drain it quickly )

If I was starting from scratch I would look into this yes.
So far the 440 CDi is 12 years old, so still working - we've had it on a manufacturer service contract, so many parts have been replaced, but they just bumped it up to ~£450 a year. I'm thinking that's way too high, so cancelled it.

It does make me wonder about the "better quality" that Worcester Bosch are supposed to offer, I fitted two £500 Vokera boilers in rentals 6 years ago that are still running with no repairs needed. In our last house I had a Worcester Bosch that also failed.

ToneControl

exocet said:

Combi boilers in general are always more problematic / costly to maintain compared to non combi equivalent.

Is the 440 Highflow a combi with a built in small storage tank I.e. something like 10 litres of hot water available instantly? You pay more for that feature and it's probably not worth it.

Any combi will struggle to heat high volumes of cold water in winter for a shower but I agree that the Greenstar 35KW model would be as good as any other for a single shower. 

Real answer is a hot water tank and a standard boiler but space constraints commonly dictate otherwise.

yes, it has a cache of hot water to improve flow for the first 10 mins.
That's the feature that is stopping the cheapest maintenance company we found from offering us cover. They "don't support" the highflow models

ToneControl

Adey said:

Not going to be cheap to swap it. How long will it take to pay back that expense? Will you still be there?

Can you turn down the boiler temperature so you don't heat water to as high a temperature? You won't then add a load of cold water to cool it back down again afterwards.

I'm just musing now, making plans for the future when it fails, I'm not going to just change it whilst it's working.

I'm not sure what you mean about the temperature

Adey

The temperature that the boiler heats water up to, is adjustable. If it is heating water up to a really high temperature, you are going to have to throw away a lot of that money you spent heating it up as you will have to add lots of cold water to cool it down again at the bath / shower / sink.

There should be a thermostat on the boiler to adjust this temperature.

ToneControl

Adey said:

The temperature that the boiler heats water up to, is adjustable. If it is heating water up to a really high temperature, you are going to have to throw away a lot of that money you spent heating it up as you will have to add lots of cold water to cool it down again at the bath / shower / sink.

I'm pretty sure that's not how it works. AFAIK the thermostatic mixer takes as much hot water as it can, and adds in enough cold to get to the target ~41C. With cooler hot water, surely you'd just limit the flow that is available to the shower, since less would be coming from the cold pipe? 

Anyway, you are advised to keep the hot water supply at 60C or above to avoid legionnaire's disease

Adey

It's been a while since we had a combi, so you could be right. I thought it had a thermostat for the primary circuit, but I may be wrong. 

I'm glad our current house has the trad system with hot water tank. Much more useable.

When boiler was replaced in previous house we put in a combi.  Because there is no hot tank I had to put a small heater in the airing cupboard instead to stop things getting damp /musty.

ToneControl

Adey said:

It's been a while since we had a combi, so you could be right. I thought it had a thermostat for the primary circuit, but I may be wrong. 

I'm glad our current house has the trad system with hot water tank. Much more useable.

When boiler was replaced in previous house we put in a combi.  Because there is no hot tank I had to put a small heater in the airing cupboard instead to stop things getting damp /musty.

The boiler has a temperature setting for hot water and another for the central heating circuit.

Was it possible to fit a small central heating radiator in the old airing cupboard?

strtdv

The servicing on that boiler is incredibly expensive, to the point where actually spending a fair bit of money on a new system could work out as cheap over the life of the replacement boiler. 
We've got a Greenstar 35kW system boiler with a pressurised hot water tank upstairs (a huge 300L one). It heats our 4 bed detached house no problem and we can run multiple taps/showers.
If you've got the space for the tank I'd highly recommend it.

No combi will be able to supply 2 showers at once.

The other advantage of a hot water tank is that if you ever fit solar panels to the house you can divert electricity to the immersion heater so you get free hot water all summer

exocet

Sounds like your current boiler is not a good match for your house.
The long pipe run from boiler to where HW is required is referred to as a "dead leg" - water that remains in that pipe after use, simply looses it's heat. This is normally more of an issue for tank storage because every litre of water drawn from tank is replenished by a litre of cold water. This becomes especially noticeable in winter months. 
Combi boilers negate this to a certain extent because water is not stored but in your case, you are drawing from the cached storage in your boiler which will then reheat more regularly to replenish.
So you are paying hefty maintenence on a more complex boiler that probably doesn't deliver the benefits of "quick start up delivery of HW" because of pipe run length. 

Adey

ToneControl said:

Adey said:

It's been a while since we had a combi, so you could be right. I thought it had a thermostat for the primary circuit, but I may be wrong. 

I'm glad our current house has the trad system with hot water tank. Much more useable.

When boiler was replaced in previous house we put in a combi.  Because there is no hot tank I had to put a small heater in the airing cupboard instead to stop things getting damp /musty.

The boiler has a temperature setting for hot water and another for the central heating circuit.

Was it possible to fit a small central heating radiator in the old airing cupboard?

I put a very small electric heater in there on a separate thermostat.

spark240

Viessmann with weather compensation control..