Efficiency of Heat Banks

[rmwilford]

I'm replacing the whole heating system in a 4 bedroom, 2 bathroom house and am seriously considering a heat bank. What I really want to know is how efficient they are. Fuel is getting ever more expensive and I'm sure most of us will be trying to be as economical as possible in the near future.

So, can anyone give me an idea of how Heat Banks compare to other systems when it comes to the amount of fuel used to run that system?

Basically, I want to use as little gas as possible to heat and provide hot water to my 4 bedroom house.

Many thanks in advance for any help.

 

[Agile]

With heating you get what you pay for and the mimimum is achieved with a combi boiler.

The cheapest way to heat hot water is with a non storage solution using a combi boiler!

A heat store has continual heat loss all the time it is kept hot! Its not always the most efficient way to use a boiler either.

Some people say that it allows a condensing boiler to condense because of stratification. Closer inspection shows thats not the case unless the boiler can operate with a flow return differential of 60°C which is never the case.

Tony

 

[rmwilford]

Hi Tony

So are you saying that a combi would be the most efficient solution? Should I consider using 2 x combi boilers as the house will have 1 x bath, 2 x showers plus kitchen, utility and downstairs loo? It seems to me that a single combi could suffer from reduced flow rates if demand was high.

Regards
Richard

 

[mickyg]

it will be very difficult to make direct comparisons between heat banks and other systems in terms of gas consumption etc.. and you'll always get a different opinion from every diffferent person. best bet is to do some research and make your own mind up. Personally I would rather stick to a normal system with a high recovery unvented cylinder. If you have a well insulated home then you could use weather compensation to help out with the bills. Have a look at Vaillant website for info as they do the lot.
I wouldnt recommend the use of a combi in a multiple outlet system with more than 2 occupants.

 

[Agile]

A combi is always the most energy efficient way of heating hot water as there are no standing losses!

How effective combis are at delivering your hot water requirements are another question.

Although less energy efficient, an unvented cyklinder would be the best source for HW but only IF the mains water supply is capable of the maximum flow rate you would ever require and that might be say 40 li/min! But the same flow rate requirement would apply to combis.

If you really must run a bath and two showers at the same time then a vented cylinder is likely to be the best solution although less energy efficient.

Tony

 

[BigBurner]

A heat store has continual heat loss all the time it is kept hot! Its not always the most efficient way to use a boiler either.

Some people say that it allows a condensing boiler to condense because of stratification. Closer inspection shows thats not the case unless the boiler can operate with a flow return differential of 60°C which is never the case.

Tony

This is really totally and utterly incorrect. I advise the OP not to take any notice of it.

1. It is the most efficient way to use a boiler. It is at optimum operation for most of run time with full flow through the heat exchanger, which is important in a small tubed boiler. Inefficient and harmful boiler cycling is eliminated.

2. Even when the heat bank setpoint is set to 80C the boiler will be condensing at least 80% of re-heat. More when set to 70 to 76C.

3. Thermal layering which heat banks provide in DHW and boiler re-heat, provide a cool spot at the bottom of the cylinder. A DHW plate heat exchanger will put 20C into the bottom of the cylinder. The boiler only re-heats when a large proportion of the cylinder is cooled and the bottom section will be very cool. The boiler re-heats in one long efficient burn.

30C and below is very efficient when using condensing boilers.

Take no notice of this, "closer inspection".

4. A heat bank also acts as a CH buffer which has many great advantages.

A normal cylinder also has continual heat loss when it is hot too. Heat banks tend to be better insulated than DHW cylinders.

The worst scenario is directly heated radiators and a Part L cylinder heated via a coil. Boiler wear is maximised and efficiency lowered in this setup. If TRVs are fitted all around and they all close up, the auto by-pass opens giving a direct short cut from flow to return dropping efficiency like a stone.

 

[BigBurner]

it will be very difficult to make direct comparisons between heat banks and other systems in terms of gas consumption etc..

Cranfield University tested the less efficient thermal stores and concluded they save 15% in energy bills, besides your boiler lasting. A heat Bank would better this figure using a condensing boiler.

I wouldnt recommend the use of a combi in a multiple outlet system with more than 2 occupants.

There are high flow combis available. I would recommend one of these

 

[Agile]

Oh dear!

A combi boiler in an empty house all year will spring into life as required on any day but otherwise use no energy at all.

A thermal store sitting hot all year will lose several kWh per day! That has to be paid for.

 

[BigBurner]

Oh dear!

Oh Dear indeed!

A combi boiler in an empty house all year will spring into life as required on any day but otherwise use no energy at all.

That is true.

A thermal store sitting hot all year will lose several kWh per day! That has to be paid for.

kWh per day? More like a few watts. The Gledhills are so well insulated they stay hot for many, many days.

If you are not there you turn it off. It can be on a time clock.

The great thing about a heat bank is that DHW is available within minutes as the heat of the boiler goes directly to the top of the cylinder, or twin cylinders in many cases of Gledhills, and out to the DHW plate heat exchanger.

The overall annual performance is superior to a directly heated rad and coiled cylinders system and most combis too. The expensive combis that modulate down low, with weather compensation, can be quite efficient.

 

[rmwilford]

Thanks everyone; replies generating good debate. I'm leaning a little towards a heat bank at the moment for the additional reason that solar panels can be connected. That may or may not be an economical option at the moment but if the technology becomes cheaper (mass production) and more efficient as gas becomes more expensive, then I can see the pay-back times dropping.

Although...... will solar work well with a heat bank if the thermal store needs to be at a relatively high temperature? I always thought that solar could only heat a store to a much lower temperature. Or is it a case of "any heat added is a bonus"?

 

[BigBurner]

Although...... will solar work well with a heat bank if the thermal store needs to be at a relatively high temperature? I always thought that solar could only heat a store to a much lower temperature. Or is it a case of "any heat added is a bonus"?

Solar can give high temperatures, however not often in the UK. Best to have a high limit stat on the cylinder set to 95C. A heat bank can operate on 95C as the DHW temp is blended down and the heating circuits are at lower temps. Solar contributes to the DHW by raising the temperature of the store. It also assists towards CH as CH and DHW use the same water in the store. CH and DHW may have electric backup as well. Many advantages here.

Most heat banks, using plate heat exchangers, operate between 70 and 75C. They can successfully operate at 65C, however the store needs to be much larger.

UFH works very well with a store too. It is the only option for you. Look at:
http://www.advanceappliances.co.uk/

 

[BigBurner]

Hi Tony

So are you saying that a combi would be the most efficient solution? Should I consider using 2 x combi boilers as the house will have 1 x bath, 2 x showers plus kitchen, utility and downstairs loo? It seems to me that a single combi could suffer from reduced flow rates if demand was high.

Regards
Richard

Two combis is an effective cheap way to do it. Split the DHW loading between bathrooms and join the two outlet using check valves and a shock arrestor for the bath.

One can serve upstairs CH and one downstairs CH. No solar though.

Make sure the two of them together do not exceed the meter capacity. Make sure each one has its own gas pipe back to the meter.

The Mikrofill Ethos combi is 53 kW and has a very high DHW flowrate. This is worth looking at.

 

[rmwilford]

BigBurner

Thanks for all the replies. Very informative and balanced.

Regards
Richard