What type of hot water system to have?

[Zebra]

I am wondering which type of boiler to have. There is currently no central heating or water heating system (I have a woodburner and a power shower currently).
It's a small house and my hot water requirements are minimal (fill the sink once a day, shower as required!).
My central heating radiator requirements are also small. I’ve done online BTU calculations, and it comes out at a total heat load of 15700 BTU, which is just under 5kW. On top of which, I’m frugal and wouldn’t run the central heating full blast.
There is nowhere to put a cylinder. So the seemingly obvious solution would be a combi boiler, as I have a gas supply. But there are a number of drawbacks, which lead me to wonder whether a gas boiler and radiators is indeed appropriate.
Firstly, combi boilers generally have a capacity of around 30-40kw, in order to heat hot water as quickly as most people find acceptable, and so when running the central heating with a 5kw load, are constantly switching on and off, running inefficiently and will wear out faster.
Secondly, gas prices will only rise in the future, whereas we may perhaps find new ways of generating electricity. As I intend to grow old here, I don’t want to install a system which could become obsolete.
Thirdly, siting the flue is tricky, and will require a plume management kit, all of which is ugly on the outside of the house.
The cost of installation along with buying the radiators would come to a few thousand. My electric bill for this winter came to £145, that’s for the whole of Nov, Dec, Jan and Feb, with heating all the hot water I need electrically, and using the woodburner for heat. (I was freezing most of the time this winter, but then I have no insulation in much of the house.) So at that rate it would take at least ten years to recoup the cost of installing the gas boiler and radiators.
An alternative for the hot water could be to install instantaneous undersink water heaters in the kitchen and the bathroom. They are about the size of a shoe-box, they fit under the sink, and they heat water instantly from the cold mains, like a power shower does. And I could fit them myself.
But what could the alternative be for the central heating? There is nowhere to put a cylinder, and I understand that all boilers except combis require a cylinder.
Electric panel heaters perhaps?
Has anybody else run into the same problem?
Many thanks

 

[DJH]

Night storage heaters suit us fine Zebra with and electric (a multipoint type in case I want to run the wash hand basin one day from it perhaps) under the sink. The reason we chose night storage heaters was that initially we thought we have the house as a holiday home and I didn't want a 'wet' heating system in case an oil fired boiler ( we don't have mains gas) went to lockout for some reason and pipes could have frozen and burst and run for a few months or more into the house with no one noticing. At least if the power is off for any reason it simply comes back on and resumes charging the storage heaters with no little red buttons to press!

We have the old fashioned 'lumpy' type heaters but I know there are now much more efficient and slimmer models with remote individual controls available. These we tend to have on a low setting just to keep the rooms slightly above ambient temperature and use stoves for raising the temperature to a more comfortable temperature.

We also have a mains pressure hot water storage cylinder which heats up using off peak electricity and it's so well insulated that it just drops about 5 to 10 degrees during the day if we don't draw hot water if say we are away all day.

Hope that helps but I really think you have an idea what you are going to do...

Doug

PS Hows the roof nowadays? Are your condensation problems sorted?

 

[Owain]

I am wondering which type of boiler to have. There is currently no central heating or water heating system (I have a woodburner and a power shower currently).
It's a small house and my hot water requirements are minimal (fill the sink once a day, shower as required!).

Do you mean an instant electric shower? They are usually considered pretty poor compared to either a combi or a mains pressure cylinder.

Firstly, combi boilers generally have a capacity of around 30-40kw, in order to heat hot water as quickly as most people find acceptable, and so when running the central heating with a 5kw load, are constantly switching on and off, running inefficiently and will wear out faster.

Modern gas boilers will modulate their output down and can usually be set to have a lower output for heating than HW. They are slightly less efficient as they run in non-condensing mode. The main criterion is the radiator load - if you have oversized radiators your house will warm up more quickly and the boiler will operate more efficiently.

Secondly, gas prices will only rise in the future, whereas we may perhaps find new ways of generating electricity. As I intend to grow old here, I don’t want to install a system which could become obsolete.

A cylinder based system has a lot more versatility as you can chose between gas, electric immersion, or solar thermal if you have a second heat exchange coil. You can also combine them with a solid fuel stove that has a backboiler, so heat your hot water and radiators by either/both solid fuel and gas. (Dunsley Neutraliser connects the pipes together.) It also gives you resiliency in that you're not dependent on one thing working; there's backup hot water from the immersion or the other heat source.

For energy cost, gas is about a third of the price of electricity per kWh and will probably remain so, as the electricity distribution network is only about 50% efficient. So as long as gas is used for electricity generation, which is likely for the foreseeable future (well, 10 years, which is how long you can expect a modern combi boiler to last) then using gas directly will be cheaper than using it in the form of generated electricity.

Thirdly, siting the flue is tricky, and will require a plume management kit, all of which is ugly on the outside of the house.

Depending on the house type and layout the boiler and unvented cylinder can go in the loft and flue through a gable end wall or a roof terminal.

But what could the alternative be for the central heating? There is nowhere to put a cylinder, and I understand that all boilers except combis require a cylinder.

They require a cylinder for the hot water; you can get a system boiler and just run radiators off it.

There is a considerably variety of cylinders available from tall and skinny to horizontal ones that can go under the eaves. They tend to be more expensive though. I've lived in lots of flats which had a small cylinder balanced on the wall between the bathroom and kitchen; it protrudes about 10" into the room each side of the wall at head height.

Although you intend to stay in the house a long time, installing gas ch is one of the few improvements that can cover its cost or add value to the property.

 

[worms]

I think this is an area where we all come with our own experiences and prejudices!

In my experience, modern gas combi boilers are unnecessarily short-lived, problematic and require expensive annual service-checks that you need to factor in to the equation. But perhaps I have been unlucky...

Electricity is convenient, normally reliable, but expensive.

I think if I was in your position, with a small house that has been brought up to modern insulation standards, I would be investigating what you can do around your wood-burner. You could probably get by with its output for normal or boost heating and have electric fall-back or background on the occasions when it is needed. If the heat really doesn't spread enough from where the wood-burner is to the rest of the house, you could get a stove with a back boiler to power a radiator or two. At your level of hot-water use, and space availability I'd be with you on the electric shower and instant hot water gadgets.

Bottom line is keep it simple and work with what you know!

 

[Flyfisher]

It's an interesting conundrum, especially when having a truly blank canvas rather than moving to a house that already has various systems installed. I also agree with worms that we all have our own experiences and prejudices - so I might as well state mine. :wink:

I wouldn't have a combi boiler because I know a number of people who have experienced the problems that worms described.

Also, if there really is no space for a hot water cylinder then options are limited. Electricity might be expensive when compared to gas/oil etc but if there is only a small amount being used then this can be less important than the capital costs of alternative systems. £145 for 4 months of winter DHW seems pretty good to me!

Space heating is a different matter. My very limited experience of electric storage heaters is their limited controllability. Again, if a cylinder really isn't possible then I'd think seriously about running some radiators off a stove boiler.

 

[Zebra]

Do you mean an instant electric shower? They are usually considered pretty poor compared to either a combi or a mains pressure cylinder.

Yeah but they're great compared to a bucket and a sponge.

if you have oversized radiators your house will warm up more quickly and the boiler will operate more efficiently.

Ah, now that's interesting. I hadn't realised that bigger radiators will mean that your boiler operates more efficiently. Thanks for that tip.

They require a cylinder for the hot water; you can get a system boiler and just run radiators off it.

Ah! Aha! Now this is the information I was after, I think. Yes, to be able to have a boiler which only does the central heating, and is sized appropriately to the heat load of the radiators. So you can do that with a system boiler?

you could get a stove with a back boiler to power a radiator or two. At your level of hot-water use, and space availability I'd be with you on the electric shower and instant hot water gadgets.

I understand that you can't run radiators directly off the back boiler: you have to store the water in a cylinder (which I don't have room for) otherwise the central heating would be very difficult to control, and could overheat.

 

[Flyfisher]

I understand that you can't run radiators directly off the back boiler: you have to store the water in a cylinder (which I don't have room for) otherwise the central heating would be very difficult to control, and could overheat.

I don't believe you have to have a hot water cylinder with a boiler stove, though you might be correct about the control issues.

I say this because we're having a boiler stove installed, albeit with a hot water cylinder. However the location of the cylinder is such that it can't be served by gravity and requires a pumped circuit. That's OK, but the system must be designed to failsafe in case of a power cut. In practice this means an adequately sized 'heat leak' radiator (or maybe more than one) that operates purely by gravity. This will prevent the heating system overheating in the event of a power failure.

So, control of the system, including any zoning you might want, is via fairly standard thermostats, electric valves and pumps with the addition of a 'default' mode where the stove boiler output will operate only by gravity. So you may only get heat to the upstairs radiators in the event of a power cut, which shouldn't be too much of a hardship.

Anyway, that's my understanding after talking to our heating system designer.

 

[Owain]

They require a cylinder for the hot water; you can get a system boiler and just run radiators off it.

Ah! Aha! Now this is the information I was after, I think. Yes, to be able to have a boiler which only does the central heating, and is sized appropriately to the heat load of the radiators. So you can do that with a system boiler?

Yes.

There are basically 3 types of boiler:

* Heat only - requires external circulating pump and, in a sealed system, expansion vessel. Can supply radiators, hot water cylinder, or both.
* System - has an internal circulating pump and usually an expansion vessel for use on a sealed system. (A sealed system is one that doesn't have a header tank for the radiators.) Can supply radiators, hot water cylinder, or both.
* Combi - usually has an internal circulating pump and expansion vessel for use on a sealed system for the radiators, and an instantaneous output for hot water. As an aside, combis can also run a hot water cylinder if the plumber plumbs and wires it in as a separate 'zone' in the same way as having separate zones for upstairs and downstairs radiators. It often makes sense to have a mains pressure but limited flow output from the combi for a shower, and a conventional low pressure high volume tank and cylinder for a bath.

Any of the above can be condensing, which is more efficient but needs a condensate drain pipe, or non-condensing, which are less efficient but probably less reliable (and usually not now permitted under Building Regulations).

Examples of system boilers:
http://www.discountedheating.co.uk/shop/acatalog/Gas_System_Boilers.html

Combis are a very competitive price-sensitive market and the difference between them and a heating-only boiler is small, eg a Glow-Worm Flexicom 24hx Heating Boiler is £559 but the combi version 24cx Combi Boiler is £591. (All plus VAT). £30 more, but your hot water will be 1/3rd the price of electricity, mixer showers are cheaper to buy than an electric shower, and don't need an additional circuit on the electrics. Yes, the combi models are more complicated and so may be less reliable. However, almost all the components will be common to both functions.

The 24cx, 30cx and 35cx combis can all have their central heating output adjusted as low as 9.3kW to match to the radiator load and reduce cycling. Other combis may be able to be 'range rated' to an even lower minimum.

For heating-only boilers as an example the Potterton Performa 12 HE System Boiler can be set down to 6.7kW condensing mode.

 

[Lime]

If you are using a solid fuel boiler to heat water you do need a hot water tank to act as a heat dump in case of a power failure.

Personally I heat my hot water using a large multipoint water heater (with our mains water pressure it knocks an ordinary power shower into a cocked hat) and use gas wall heaters and electric storage radiators to heat the house.

 

[Feltwell]

Do you mean an instant electric shower? They are usually considered pretty poor compared to either a combi or a mains pressure cylinder.

Yeah but they're great compared to a bucket and a sponge.

:lol:

A decent electric shower is fine - we've a 10.5kW model and it delivers a great shower. The problem tended to be with older, lower kW units where you could only get a warm trickle!

10.5kW needs a fairly hefty supply cable but Zebra will know all about that!

Apparently a common problem is people fitting uprated replacement showers but not changing the wiring - still, helps heat the room as well I suppose..... :shock:

 

[CliffordPope]

8Kw works fine for us. But I fitted a booster pump inline to the cold supply to the bathroom to increase the gravity pressure.
It also means it takes priority over anyone else in the house who may be trying to draw water. No more shrieks from the bathroom as the shower flicks from boiling hot to cold!

 

[Flyfisher]

If you are using a solid fuel boiler to heat water you do need a hot water tank to act as a heat dump in case of a power failure.

Are you sure?

I agree about the need for a 'heat dump' with solid fuel boilers, but a well-insulated hot water tank already full up with hot water is not going to be a very effective heat dump.

My understanding is that this requirement is usually implemented using 'heat leak' radiators, i.e. gravity fed radiators that can dissipate heat to the house.

 

[DJH]

Yes, I'll go along with that FF.

 

[Lime]

The heat leak radiator is installed to reduce overheating of the hot water cylinder under normal conditions when the central heating is calling for heat but the hot water store is not required/used.
Excessive heat is "leaked" out of the hot water cylinder via the radiator in circuit with the return pipe from the gravity fed hot water cylinder back to the boiler.
This leaked heat usually performs a useful function of heating in the bathroom.
The heat leak radiator cannot dissipate the amount of heat (unless of immense size) likely to be provided by a solid fuel boiler system running at full heat immediately following a power failure.
Unlike gas and oil, where the heating effect can be immediately curtailed, solid fuel takes a considerable time to reduce its heat output hence the need for a large heat dump.

 

[worms]

I guess regulations perhaps get in the way of simplicity here.

A simple coil in the wood-burner, piped to the upstairs radiator(s), throw in a pressure relief valve and an expansion vessel (or a vertical vent pipe that serves both purposes) and you would have a cheap and cheerful system that would meet most needs but break all the regs!

 

[Flyfisher]

The heat leak radiator is installed to reduce overheating of the hot water cylinder under normal conditions when the central heating is calling for heat but the hot water store is not required/used.
Excessive heat is "leaked" out of the hot water cylinder via the radiator in circuit with the return pipe from the gravity fed hot water cylinder back to the boiler.
This leaked heat usually performs a useful function of heating in the bathroom.
The heat leak radiator cannot dissipate the amount of heat (unless of immense size) likely to be provided by a solid fuel boiler system running at full heat immediately following a power failure.
Unlike gas and oil, where the heating effect can be immediately curtailed, solid fuel takes a considerable time to reduce its heat output hence the need for a large heat dump.

I may be using the wrong terminology regarding a "heat leak" radiator, though I think we're saying the same basic thing, i.e. any solid fuel heating system must be able to safely operate run purely by gravity in case of electricity failure (and thus any pumps in the system).

I don't understand how a hot water cylinder can help perform this function because Sod's Law will ensure that, at some point in time, the power will fail when the cylinder is already full of piping hot water - thus the continued heat from the fire/stove will have nowhere to go.

The only way to dissipate the heat from the fire/stove is to use gravity-fed radiators (and I agree it would likely require more than one). Typically, this will be the entire upstairs CH circuit and, if sized correctly, will be able to dissipate all the heat from a roaring fire/stove whent the power fails.

A DHW cylinder may be part of such a system but I can't understand why it's a necessary part - unless it's purely a regulatory issue.

In fact, worms' description seems elegant in its simplicity. A pump could be added for the operation of the downstairs radiators but the system should work safely without power when the upstairs radiator circuit would operate purely by gravity.

Would such a system really not meet current regulations?

 

[Lime]

I don't understand how a hot water cylinder can help perform this function because Sod's Law will ensure that, at some point in time, the power will fail when the cylinder is already full of piping hot water - thus the continued heat from the fire/stove will have nowhere to go.

There is a substantial heat dump capacity in a hot water cylinder even at relatively high water temperatures.
In the past when use of solid fuel and power cuts were more common and the solid fuel burn often less well controlled the secret of success was more well known.
When the boiler and hot water cylinder start to boil you run the hot water off.

 

[Flyfisher]

Running off the hot water will certainly do the trick, though it's not exactly a fail-safe system. Especially after a couple of glasses of wine in a comfy chair in front of a nice warm fire. :wink:

 

[Owain]

A pump could be added for the operation of the downstairs radiators but the system should work safely without power when the upstairs radiator circuit would operate purely by gravity.

Would such a system really not meet current regulations?

A heating-only gravity circulated system is certainly possible and I see no reason why it could not meet current regulations. That is not to say that all such systems do meet current regulations.

The big drawback, of course, is the radiators are on all the time whether wanted or not.

 

[DJH]

Usually in my experience domestic hot waters heated on solid fuel appliances rely on a gravity circulation system using the principle that hot water is lighter, or less dense, than cold water, so circulation works by the hot water rising and cold replacing it to be heated up repeating the cycle. It's usual to have a heat leak in this circulation system such as a towel rail or towel rail/radiator to make use of excess heat and also to help keep the temperature of the domestic hot water below scalding temperature.

The heating circuit which connects to all the other radiators is driven by a circulating pump switched on manually, by thermostat or an over heat switch which is either strapped onto the flow pipe near the appliance or by a thermostat fitted into the pipework. The theory behind this switch is that in runaway conditions when the water temperature in the appliance gets above a certain temperature the pump will switch on and dissipate hot water through the radiators. The drawback with this system is that pumps can and do fail. Electricity supplies can be either inadvertently switched off or suffer a power failure. Another drawback is that the radiators can achieve very high surface temperatures where scalding can occur if they are touched especially by young children.

Drawing water from the domestic hot water cylinder is not always the answer as indirect cylinders with a small primary coil inside will not be cooled quickly enough to reduce the temperature of the water in the appliance below from boiling. Drawing water from a direct cylinder,which is one that shares the same water that circulates inside the appliance, will have the required effect of lowering the water below boiling point quickly.

Sealed hot water systems rely on a spring loaded valve to open when water reaches a dangerously high temperature and the steam/boiling water is discharged through a pipe to the outside of the building. It's important that this discharge pipe does not in anyway endanger people in the vicinity of any discharge.

A gravity fed domestic hot water system relies on being fed by gravity from a tank or cistern in the roof and in boiling conditions hot water and steam will be discharged from the vent pipe into the cistern/tank. The same will be happening no doubt to the small feed and expansion tank which feeds the heating system with water.

Surprisingly not all large storage cisterns/tanks are made to accept hot water and can sometime buckle and swell out as a result of boiling water in them. An added risk is, if when the tank was installed, the installer did the old trick of heating up a length of copper pipe to push into the tank to make a hole for the tank connectors. This somehow changes the makeup of the material used to form the tank and it will split at this point. The effect of all this is that you now might have 50 gallons of near boiling water waiting to come through the ceiling.

Just a few observations from when I was carrying out plumbing work 25 years or more ago...

Doug

PS The time is an hour wrong on here...