Conventional boiler system question

[taz777]

A friend of mine suggests I should replace a conventional boiler with a combi boiler. The property is a typical 3-bed semi. It's just for one or two people at the most and we're unlikely to have more than one tap on at a time.

What he's saying is that i'd be heating up a full tank of hot water unnecessarily with a conventional system.

Would using timer controls to operate the DHW and CH system a couple of times a day, for a couple of hours at a time, negate this argument?

Also, how does a conventional boiler copy with the hot water running out in the tank? Would it fire up and heat water on demand?

 

[echoes]

I had to make this decision a few months ago, and opted to keep the HW cylinder and not have a combi.

Pros for combi:
Unlimited HW on demand
Gives a lovely shower (provided no other hot taps are used at same time)
No space taken up by a HW cylinder

Cons:
No HW at all when it breaks down (all eggs in one basket), no immersion backup.
Won't give good supply to more than 1 DHW outlet at a time
Wastes water and gas while the combi heats up the DHW.
Airing cupboard might get cold.

What he's saying is that i'd be heating up a full tank of hot water unnecessarily with a conventional system.

If your existing cylinder is well lagged, then it will stay hot for days. Sometimes my HW doesn't need to come on in the morning at all before showers. Provided that the HW is timed and pumped, you won't be heating a cylinder unnecessarily.

Also, how does a conventional boiler copy with the hot water running out in the tank? Would it fire up and heat water on demand?

If the HW is timed to come on when you are draining the tank's HW, then it will re-heat it, although probably not as fast as you can use it. Most programmers have a 'boost' feature to get an hour or two's extra HW heating outside of the the normal times.

 

[bengasman]

By the time you need a new boiler, simply choose a top quality engineer, and let him sort it out; that's what you pay him for.

 

[mickyg]

Just to add to the Pro's for keeping a conventional system:

Combi boilers are more unreliable than heat only boilers
Combi boiler spares are becoming increasingly expensive to a point of stupidity
If a system part goes down on a conventional system you can get another one down the local merchant for a fraction of the equivalent combi spare.

 

[SimonH2]

What he's saying is that i'd be heating up a full tank of hot water unnecessarily with a conventional system.

This is a common 'misconception'

If you have (say) 100 gallons of hot water, and you use 10 gallons, then you only have to heat the 10 gallons of cold water that replaced what you used. I've had 'discussions' with people who will swear blind that if you had a smaller tank (say 50 gallons) then it would take less energy to reheat it. In practice of course, the cold will mix with the hot and so you'll heat more water, but starting from fairly hot.

It is true, however, that when you have a tank of hot water standing there, it will lose heat all the time. Good insulation will reduce that, and if you put it in a closed cupboard then you will have a warm cupboard which is good for keeping cloths dry - I believe people call them "airing cupboards" - and further reduce the heat loss.

Tank size and boiler capacity are a tradeoff. As you draw off hot water, the tank will cool from the bottom up and at some point the boiler will fire up. For a typical DHW cylinder, the heating coil capacity is limited and you won't be able to draw off hot water indefinitely without it cooling somewhat - and beyond matching the capacity of the coil, having a larger boiler won't help.

For the central heating, there is really next to no difference between a combi and a conventional system - the boiler will be short cycling all the time you have the heating on and there is a room stat not satisfied. However, a combi typically has to be grossly oversized (to give the DHW flow rate) compared to the heating load. I reckon my flat has an average heating load of around 2kW, maybe a bit more. Because it's a combi, the boiler is rated for almost 30kW (but still can't give a decent DHW flow rate) and can range down to about 9kW (ie it's minimum output is getting on for four or five times the typical load on it).

While expensive, and with certain downsides of their own, you can get the best of both worlds (stored heat with electric backup by immersion heater AND mains pressure DHW*) with a thermal store/heat bank. With a thermal store your DHW flow rate is restricted by the capacity of the heat exchanger coil inside the tank, but a heat bank can have a huge plate heat exchanger with a capacity of 100kW or more. If you run the entire system open vented, then the reheat capacity is only restricted by the boiler size - and so you can run the boiler at it full output in shorter spells, and it should be in condensing mode most of the time.

The store will also separate the boiler flow from the central heating flow - so you can have a system with TRVs on all radiators and use a modulating pump (like the Grundfos Alpha II) which leads to a quiet system.

My brother has fitted a thermal store in his house (but then he has solar and wood burning stove as well), and I've just fitted one in my flat.

* Without having a pressurised cylinder.[/i]

 

[bengasman]

What he's saying is that i'd be heating up a full tank of hot water unnecessarily with a conventional system.

This is a common 'misconception'

If you have (say) 100 gallons of hot water, and you use 10 gallons, then you only have to heat the 10 gallons of cold water that replaced what you used. I've had 'discussions' with people who will swear blind that if you had a smaller tank (say 50 gallons) then it would take less energy to reheat it. In practice of course, the cold will mix with the hot and so you'll heat more water, but starting from fairly hot.

The energy needed to heat up 20 litres by 30 degrees is exactly the same as the amount needed for 40 litres by 15 degrees.

It can not be denied that a cylinder that heats up water that is not being used will waste energy, as any hot body will lose heat.

 

[SimonH2]

The energy needed to heat up 20 litres by 30 degrees is exactly the same as the amount needed for 40 litres by 15 degrees.

Have you ever had to try and explain that to someone who doesn't believe it <where's the :banghead: smily ?>

It can not be denied that a cylinder that heats up water that is not being used will waste energy, as any hot body will lose heat.

Indeed. But the energy loss can be limited by careful insulation, and arranging that what heat is lost goes into the property and contributes to the heating you'd be doing anyway.

 

[ajrobb]

Fit multiple jackets to the HWC and lag the hot pipes. You can reduce the heat loss to the point that the cupboard is useless for airing. You can fit a combi-boiler to a Y-plan or W-plan system, so you still get the powerful shower from a combi OR run a bath quickly from the HWC. Yes, the HWC normally has an immersion heater as a backup. It can be a pain waiting for water to run hot from a combi.

 

[GeorgeBainbridge]

While expensive, and with certain downsides of their own, you can get the best of both worlds (stored heat with electric backup by immersion heater AND mains pressure DHW*) with a thermal store/heat bank. With a thermal store your DHW flow rate is restricted by the capacity of the heat exchanger coil inside the tank, but a heat bank can have a huge plate heat exchanger with a capacity of 100kW or more.

Oversize the plate and water returning the cylinder can be just 5C above the main water temp. Great for condensing efficiencies.

If you run the entire system open vented, then the reheat capacity is only restricted by the boiler size - and so you can run the boiler at it full output in shorter spells, and it should be in condensing mode most of the time.

I think you mean direct,without a coil, not open vented. True, the store cylinder does not care how big the cylinder is. A very large boiler can be fitted to say a 100 litre cylinder. The boiler will run flat out and reheat very quickly. Oversizing a boiler on a heat bank or store is not an issue. If you get a good deal on a big boiler buy it it and fit it.

The store will also separate the boiler flow from the central heating flow - so you can have a system with TRVs on all radiators and use a modulating pump (like the Grundfos Alpha II) which leads to a quiet system.

Yep. The heat source is isolated from the rad circuit operating independently from it. The boiler runs in the ideal hydraulic environment. The CH circuit runs wonderfully with Smart pump.

 

[Tibbot]

Combi boiler spares are becoming increasingly expensive to a point of stupidity

There is no escaping this valid point. However the same could be said for all modern gas boiler spares, just that a combi has more of them.

 

[ianniann]

i'd be heating up a full tank of hot water unnecessarily with a conventional system.

f you use water several times a day then there is essentially no waste. Modern cylinders take days to cool down. So if you use hot water maybe only at the weekends then you might be wasting a cylinder every week. You should also think carefully if the cylinder is in a loft or other unheated area. A cylinder inside the main heated shell of you home is just "wasting" heat into your home, which nearly always needs some form of heating. A bigger source of waste for most people is the length of the pipe run from a combi boiler or cylinder to the tap. The water is wasted directly and then that run of pipes is left full of hot water which will cool down before you use the tap again.

Would using timer controls to operate the DHW and CH system a couple of times a day, for a couple of hours at a time, negate this argument?

Not really. If you knew you were going away for a week or two then you could switch of the water heating before and get your last shower "for free" Most combis also have features to maintain the heat exchanger (and occasionally a small tank) hot to provide quicker hot water. This is another source of heat slowly leaking away 24/7.

how does a conventional boiler copy with the hot water running out in the tank? Would it fire up and heat water on demand?

No, your shower will go cold. You will have to wait a few minutes, or ideally an hour or so, to get a good dose of hot water back into the cylinder. Combis are designed differently to pass heated water directly to the taps, and are also usually a lot more powerful to heat water enough in real time. For the same reason, they are usually much more powerful than necessary for heating your home.