UKH

Sorry to add another home heating question but such are the times and I couldn’t find any answers online…

we moved recently and the hot water set up is a boiler and a big hot water tank, it works great and makes hot water much quicker than our previous house’s combi boiler set up - but the thermostat is set to heat it up twice a day, I assume it gets it to a set temperature then stops. I can’t help thinking this is incredibly I efficient and costly because we’re paying to have a massive tank of water hot all the time. I always wince when I hear the boiler whirring away to make hot water when I know we aren’t having showers til the next day but I don’t know how long it takes to heat up and if you turn it off then you just have no hot water “on tap”…

anyone know if this set up does cost more to run or not? 

Depends where the lost heat from the tank goes and whether or not this makes the house a comfortable or uncomfortable temperature so you are opening window to cool it.  

If it's just warming the house in a way that you would normally be doing with the heating anyway then you lose nothing, if it's warming the loft space then that's not good.

How long's a piece of string? If you only occasionally have brief showers a combi will be cheaper based on it heating less water. If you use hot water frequently throughout the day it may end up very similar cost.

I think the efficiency and condition of the last individual boiler etc is also a factor here

Unless it's stopping the header tank and associated pipework freezing in the winter...

mmm never thought of it that way, it's an air cupboard upstairs next to the bedrooms. We've only moved in during mild weather, upstairs is always hotter but I assumed that's just because of hot air rising. During the heatwave upstairs was like a sauna but I don't think that's the water tank.

We have 1-3 showers per day, 1 shallow bath and a couple of sinks worth of washing up typically.

Maybe in winter it will make upstairs warmer so could be some benefit.

It's probably considerably less efficient than a combi in summer when the heating would normally be off so the heat from the tank is wasted. In winter it can warm the house and dry/air clothes.

You could add insulation to the tank, asociated pipes and or its cupboard and switch to one heating per day maybe or run it a bit cooler on the two cycles.

Or get solar panels and an immersion heater.

Jk

Modern hot water tanks are very well insulated, I suspect the wasted heat will be pretty small. If it was significant then your airing cupboard would be hot. Don't insulate the tank - you might actually make the overall insulation quality worse. The best place to insulate is most likely the loft directly above the airing cupboard.

Presumably all this is academic as any cost saving of running a combi boiler would be wiped out by ripping all this out and replacing it.

Your existing boiler is likely to be non-condensing and therefore much less efficient than a modern condensing combi boiler. The losses will add to those you are experiencing from the hot water tank. I would look at replacing the boiler with a modern condensing combi boiler. You will be able to free up storage space from removal of the tank, and also get rid of the header tank you probably have in your loft?

Don’t skimp on capacity for the replacement boiler, especially hot water delivery. A separate additional expansion vessel is also worth having

Thanks this is helpful, I haven't seen any tank in the loft though, and the people we bought the boiler off said it was only 2 years old so fairly new...

This is one of the plusses of keeping a hot water tank, you can more easily integrate systems like this which you can't with a combi. i suspect in a few years time the trend install combis will be reversed for more sustainable combinations of gas and solar heating. 

Don't insulate the tank - you might actually make the overall insulation quality worse.

How does that work, doesn't more layers = more insulation? 

Don’t understand how a boiler/tank combo heats water  faster than a combi? A combi boiler is instantaneous, hot water tank takes up to an hour?

Anyway, an interesting question, which leads to endless arguments in our house. What’s more efficient? Heat the the water up only when you need it, or just keep it running 24/7?

When I say it makes it faster - I mean, it comes out of the hot tap a lot faster than the old house (presumably because it's already hot!). At the old house with a combi boiler you were running the tap for 30seconds+ to get hot water out. This one it's hot after a few seconds. Maybe I'm saving on water!

Because the thermostat controller thing is set to twice a day heat the hot water, the tank has never been cold. Basically we don't really understand how to use it or how it works so we just leave it alone, we always have hot water but I'm just worried it's costing us a bomb in gas.

Gotcha. That’s obviously a plumbing issue. Our hot water takes ages from the tank in some parts of the house.

Do you have a Hive, or similar? We use that to control the hot water. We find it really straightforward. Although the debate between what’s cheaper, keeping it on or only heating when needed, is still up for debate.

As others have said, there's nothing inherently wrong with this system. Yes, the tank will be leaking heat but that should be fairly minimal - is the tank covered with a sprayed on layer of PU foam?  And is the airing cupboard noticeably warm when you open it? If so you might consider an extra layer of insulation.  You can get 'jackets' that slip over the top of the entire tank.

I can't see how adding more insulation can possibly be a bad thing, tlouth7, as long as you don't compromise the original insulation at all.

Lagging the pipework between the boiler and the tank will certainly help reduce losses there, if it's possible.

You can get a measure of how much energy you're using by looking at your gas meter daily for a few weeks, and working out how many kWh per day you are using.  If you do this before and after a weekend away you can then work out how many kWh are just being dissipated as opposed to usefully used. For six months of the year the heat loss will be useful to the house anyway. 

How good the boiler is is really a totally separate question, and if that's ancient then upgrading it might be a good idea.  But I certainly wouldn't be looking to rip the entire system out to install a combi just for the sake of it.

You need to look at it in terms of 'where is heat wasted?' With a tankless system you have the 'waiting period' where you let the water run to waste whilst it heats up the boiler and pipes on its way to you.  With a tanked system you have the heat lost from the tank whilst the water is sat in it.  I strongly suspect the tanked system loses more heat overall, but as the heat lost in winter heats your house up it's only a true 'loss' in summer when you can't make use of that waste heat. 

The main logic behind a tanked system is that it allows you to use a smaller, cheaper boiler with lower peak heating capacity - the boiler can spend an hour heating up a store of water you can use in ten minutes, whereas a tankless combi has to heat it as fast as you can use it.  If you wanted to run a hot shower, bath, and washing up in two sinks all at the same time it's possible a domestic combi simply cannot cope with the demand.  If the tank has an electric immersion you also have the option of using electricity to heat the water. Where I live, off-peak electricity is cheaper than gas, so it made sense to run the immersion off peak and gas 'on peak'.

FWIW, we're in a new-build and that has a hot water tank, so can't be that bad an idea. Can't be losing much heat from the tank as the cupboard is never that warm.

If there isn’t a supply tank in the attic then the system must be directly fed by the water main, and is an unvented system. It should look more complicated than a vented tank, if it is the correct type. I have a (vented) hot tank fed by my solid fuel stove at night when the heating is off. I’m in the process of fitting a new one but the old insulated one lost about 40W/hr. In your case I would try and change the thermostat so that it only heated the water at night when you aren’t using the heating and perhaps for a short time in the late afternoon for hot water during the evening. The reason I heat the tank at night is because if the tank is cold it significantly reduces the temperature of the radiators whilst it dumps the cold into the system and so that the heat from the stove has somewhere to go whilst the pump is off.

If you poke a meat thermometer through the insulation you could take a reading when it’s cold and then when it’s hot then knowing the time between the readings and the amount of water in the tank using a web based calculator will tell you how much power it used (or lost if done in reverse).

If you had a combi rather than a system boiler you wouldn't have a header tank.

And anyhow. Any tank should be suitably lagged to prevent freezing.

I've wondered if the combi boiler was less efficient at heating the same amount of water as traditional boiler due to the heating power being so much higher. Kinda like doing 40 miles at 80 mph versus 40 miles at 40 mph. Surely you can only transfer x amount of heat at a time into the water but no idea how close to the limit a combi gets.

Yes but the OP has a hot water tank, that requires a header tank and they're often in the loft, along with the mains riser and overflow. 

That would depend on the rate of flow of water through the system and the available area for heat exchange, not quite as straightforward as car speeds I think.

Just the opposite I'd have said.  If, as you say, your boiler is only two years old it's vanishingly unlikely not to be a condensing boiler, since it's been illegal to install non-condensing boilers since 2005, unless you get a special exemption (source: https://www.greenmatch.co.uk/blog/2015/10/condensing-vs-non-condensing-boil...).

Fairly obviously, given the above, it is perfectly possible for a non-combi boiler (called 'system' boiler I believe) to be also a condensing boiler.  There is no particular dependency between the two functions - one is to do with how hot water is delivered to the taps within the house, the other is to do with recovering heat from the exhaust gases (which is what makes them more efficient).  We had a new condensing gas boiler installed for our hot water tank based system ten or more years ago when we switched away from oil heating (previous owner's choice because they "didn't trust gas").

There are things you can do to try to optimise the gas usage for hot water.  The Energy Saving Trust suggests that the hot water cylinder should have at least 80mm of insulation (https://energysavingtrust.org.uk/advice/boilers/) so if yours is less than that then maybe consider enhancing it.  If you can find the thermostat that tells the boiler when it needs to heat the water in the tank (beware, this is not the same as the thermostat for the immersion heater) then you can check what temperature it's set to and maybe turn it down a bit.  But not too low: the HSE says that the temperature of the water in the cylinder should be at least 60°C to avoid legionella risk (https://www.hse.gov.uk/legionnaires/things-to-consider.htm).  Excessively hot water can also be a scalding risk - I've seen figures suggesting a risk of third degree burns after a two-second exposure to water at 65°C (https://www.cpsc.gov/s3fs-public/5098-Tap-Water-Scalds.pdf).  As a final consideration: the higher the temperature the water in the cylinder gets to, the quicker it loses heat, so having the set temperature lower reduces heat loss and therefore energy waste (at least in the summer months).  Even having an excessively large hot water tank can be a waste of energy (ours is quite dinky, but it's plenty for one bath and, being not over-large, it heats up again surprisingly quickly).

You should be able to adjust the timer that controls when the boiler heats the water in the tank.  For example, there's little point in having the tank kept hot during the day on weekdays if everyone in in the house is going to be at work or at school, so have it set to to come on just in the morning and in the evening.  It's also worth knowing how to override the timer - or where the immersion heater switch is - so that you can call up hot water at unusual times if needed.  Bear in mind that the hot water rises to the top of the cylinder, which is where it is drawn off from when you turn the tap on, so you don't actually have to wait for the whole cylinder to heat up before you get usable hot water.

as everyone else has said, depends how well insulated and where it's situated.

Ours is really well insulated and in it's own cupboard, so not much heat escapes.

It makes no sense at all.  More insulation = better, regardless of how good it is.

New ones tend to come with thick foam insulation on the outside, much better than the flimsy jacket that people used to put on them.

They're likely to become more rather than less common because heat pumps can't provide enough heat to run like a combi - they instead heat a tank slowly.

There are two options - unvented (mains pressure) and vented (header tank).  The advantage of the former is that you can get a better pressure shower, but the latter is cheaper and you can always fit a pump.  You also need to keep the latter maintained with frequent checks as the last thing you want is a big tank of scalding water going "bang".

We had this issue, so we decided to just heat the water for an hour every few days. If we want a bath we will put it on for an hour. It works really well, our gas bill dropped markedly. If you have a hot tank full of water it loses heat quickly to the environment, a cooler tank heated up less frequently loses heat more slowly.

I know that heat lost from a hot tank will help to warm the house, but certainly for us it heats up the wrong part of the building, I am not spending my time sitting in the upstairs hall.

Why can’t you get a combi with a hot tank? It must have been possible as I was offered a new hot water tank installation when I got my current combi boiler installed some five years ago. The heating engineer was talking about other forms heating of the hot tank integrating and the combi boiler could be set up not to fire up unless required and programmed to do so.

I just had the combi installed so don’t know the ins and outs of what would have been installed, but isolation valves and thermostat and programmer were mentioned.

There is often misconceptions that hot water tanks are old fashion and Combi's are the modern more efficient replacements. 

Generally speaking higher hot water usage households will be better off with a hot water tank.

Combi does not heat up water as efficiently due to its "instant" nature.

Using a system boiler in that way is just like a combi except you have to heat it before you use it.  Works fine if you don't have many uses for hot tap water, e.g. you have an electric shower and wash crockery in a dishwasher.

As you increase the thickness of the insulation around a cylinder you also increase the outer surface area over which heat transfer to the air occurs. Given that this convective heat transfer is a major part of the overall thermal resistance, increasing the area it acts over can be detrimental. This is especially true if the surface of your insulation is not as good quality (flat, low emissivity) as the tank itself. Secondly you need to support your insulation, probably off the floor. This creates conductive heat paths.

In any case it cannot be overstated just how good the insulation on a modern hot water tank is. Pipes are much more likely to be the major source of heat loss.

I agree, but I’m not a high usage household

I'm not sure I agree there, that just doesn't make sense. U- and R- value calculations are based purely on the thickness of the materials and 'internal' and 'external' surface resistances, the elevational surface areas don't come into it. 

I can half-appreciate why you'd say this but the benefit from increased thermal performance in the sectional build up (by adding more insulation) would be far greater than any notional loss of performance by a slightly increased external surface area. 

plus temperature differential is the driver for heat loss, so as more insulation reduces the differential it also reduces the heat loss. Of course, the more surface area you have the more heat is lost (because heat is lost per unit of surface area), but that's assuming the volume remains constant, which it isn't.

Agreed, as a simple analogy: surely a thick pair of mitts is warmer than a thin pair of the same material.