Electric combi boiler comparisons

[mkzx77*AT*googlemail.com]

Hi, this appears to be the right group to post on, and it doesn't look like this has been answered in previous posts or your FAQ, but apologies if I've made a mistake...

I'm trying to find a little info about the different models of electric combi boilers that are available, and there doesn't seem to be an awful lot of information online. I was hoping that someone on this group might have experience with one of more of these manufacturers, with regard to their electric combi range:

Thermaflow Dimplex Sunvic Trianco Heatrae Sadia

A list I came across on:

other manufacturers I should be looking at? Any I should avoid? Any information at all really, is gratefully received.

Background: I want to install an electric combi in a small, well insulated studio flat where gas is unavailable. It's to heat 3 small radiators, with water flow from kitchen taps and a bathroom (with bath and shower).

Thanks,

Mike T

[Brian Sharrock]

You might consider rewriting your requirement to ' I want to heat three small radiators and provide hot water .. using electricity as the heat source.' This obviates the 'boiler' manufacturer/ source entirely. My 'Heatweb' heat bank is equipped with an immersion heater which would achieve all that you've given in your reqiurement

{There are other manufacturers}.

[John Stumbles]

I know it's not what you're asking but have you considered options with cheaper running costs, since what you're proposing will be mega expensive to run??? I'd be thinking along the lines of a thermal store/heat bank[1] heated by off-peak electricity and topped up if necessary by peak-rate. That would do CH rads and mains pressure DHW. You could get one (possibly custom-made) in a rectangular format to go in some suitable space.

If you could fit wet underfloor heating you'd make better use of your precious expensive electric heat than with radiators, too.

[1] see

[Dave Plowman (News)]

I can't see any reason to use a 'wet' CH system heated by an electric combi boiler. Perhaps you'd say why you want to?

[Owain]

AFAIK there are no electric *combi* (instantaneous water heating) boilers - electricity simply can't supply enough instantaneous heat. Electric peak-rate wet central heating systems usually use a small electric boiler (10kW) for the radiators, combined with a stored hot water cylinder, which might be on off-peak electricity.

Having an electric boiler seems to have all the disadvantages of wet central heating negating the advantages of electric heating (no leaks, low maintenance, etc).

For a small well-insulated flat then storage heaters are likely to be effective; if the occupants are at work during the day and often out in the evenings then peak-rate electric heaters may be more economical to run as required rather than having storage heaters on every night. Storage heaters are quite expensive to buy compared to a £30 convector heater.

All fixed forms of heating would need individual raduial circuits back to the CU, especially as in a studio flat the kitchen is unlikely to be on a separate circuit.

Owain

[mkzx77*AT*googlemail.com]

Thanks for your reply John / Brian - I hadn't considered a heat bank, so I'll look into it....

I've had a quick look at that wiki page, and my main concern with these systems is that they don't seem to offer instantaneous hot water on demand (am I correct in thinking that?). Both the Thermal store / heat bank look like they store water - does that mean hot water could run out with over use?

Thanks,

Mike

[mkzx77*AT*googlemail.com]

Hi Dave, its mostly convienience - I've had gas combi boilers in the past and they've worked very well for me - in my current place gas is unavailable so intuitively I thought an electric combi boiler would be the way to go. Are you saying I should be going for a dry CH system as well as some kind of heater specifically for hot water?

Mike

[mkzx77*AT*googlemail.com]

Hi Owain - is that link I posted in the OP not for electric combi boilers?

My main concern is for instantaneous hot water really, rather than central heating. A combi boiler seemed a good way to kill two birds with one stone.

Mike

[Owain]

I was offline so couldn't check specifically. But if you were looking at the Dimplex Ascari then that is for heating radiators only.

You will not get, in the UK, electric instantaneous water heaters >~10kW (40 Amp) for domestic use. That's an electric shower. It will not fill a bath in any reasonable time. Many people say it will not provide a reasonable shower, especially in winter.

If you want greater instantaneous water heating then you will need a

3-phase electricity supply installed and a commercial water heater.

If you do not want a full sized HW cylinder then you can get under-sink storage water heaters for washing-up and the like, and use an electric shower in the bathroom. You will not be able to run a bath off these.

Owain

[Dave Plowman (News)]

Problem is a gas combi boiler will have an input of approx 30kW to supply constant warm water. The average domestic electrical supply is only capable of giving around 20 kW - and you'd not like having to switch off everything else when drawing water.

Individual electric heaters are pretty simple to install. Not so with gas fires as they require a flue each. So with gas it makes sense to use a central boiler etc, but not with electricity.

To make use of cheap electricity you'll need some form of storage system for both heating and hot water. And like all storage systems it won't be ideal. But it could provide the majority of the heat etc needed and be topped up if required with full price electricity.

[John Stumbles]

Yes. Ideally you'd have a sufficiently large thermal store that that would never/rarely happen. You could design the system to re-heat using peak-rate eleccy either automatically or manually, or even have an instantaneous heater for peak-rate DHW when/if the stored supply runs out.

Obviously there's a trade-off between installed costs and running costs.

[Brian Sharrock]

How instantaneous is instantaneous? On my HeatWeb system _HEAT_ is stored/banked in a insulated cyclinder as a heated working fluid *; mains pressure water (cold) is fed into a 160Kw heat-transfer system (a sort of small car radiator gizmo; heated working fluid contraflows down the other- 'way' of the heat transfer gizmo and the mains water has become 55 deg C _instantaneously_ (or at leasr as instantaneous as makes no difference). The water might have to displace 'hot' water that's cooled down in what;s the 'dead leg' leading to the tap(s) in kitchen/bathroom.

*'working fluid' is actually water ; but I find it confusing to talk about _this_ hot water heats up _that_ water to make it into hot water - differntiating between the 'working fluid' and the 'hot' water; works for me!

You've already given a demonstration of why I 've used the term 'working fluid'. What's stored in a Thermal Store is Therms- or if you prefer Joules. This energy is in the form of H2O raised above ambient temperature -by some form of energy input. The store itseld does not run out of water -it may run out of energy.

There's owt for nowt. In other words - your output demand may exceed your input.

Hot stores however exploit thermal stratification and almot _instantaneously_ ( there's that word again) the heated stratified (top) layer of the working fluid is availbe to the heat exchanger gizmo to raise your mains input water to the blended /demand temperature. The Cold water > DHW and the 'working fluid' do not mix -they are in completely different circuits.

[The Natural Philosopher]

Any storage system will run out with use.

Sizing it is part of the design specs.

with a combi, it simply runs out instantly if you draw more than a thin tepid trickle.

Unless your combi is the same size as a small boiler and pressurized water tank put together, and costs even more... ;-)

[The Natural Philosopher]

If you want instant hot water, use a pressurized tank with a pair of 3KW elements in it.

That has a pretty rapid rate of heat rise.

And hot water costs a lot less than a warm house.

[The Natural Philosopher]

I never can see why any of this is any better than heating a tankful of hot water and letting THAT be the store.

[mkzx77*AT*googlemail.com]

Many thanks for all your replies guys,

I guess I was quite wrong in assuming an electric combi was a perfect alternative to a gas combi.

Heat banks look like a good way to go (thanks for the explanation Brian / John - makes a lot of sense)

cheers,

Mike

• I may be back after more research, when I realise again how little I know :/

[mkzx77*AT*googlemail.com]

Jesus - I was actually considering a 9kW combi :/