unvented system with microbore?

We have been in our victorian house for about 4 years and have decided to replace the boiler. The current system is a conventional vented system, microbore pipes and an early 70s ideal standard boiler and some old and some new radiators. The house is fairly large 4 bedrooms, 3 bathrooms and 4 public rooms but the current system works fine apart from the very large fuel bills (about £200 per month).

We want to move the boiler from its current location so see this as the right time to replace it with something like the worcester CDI 40 HE condenser boiler (I reckon we need the power as the missus is planning on extending on the back). I have ruled out a combi as I had one in our last place and the performance was terrible (took 30 mins to fill a claw foot bath and had a terrible flow rate even though the mains pressure was good). My thinking is either a conventional system or a sealed system with something like a megaflo or the ariston equivalent. I dont plan to upgrade the radiators until we do up each room in turn and will then install a traditional old school style.

Ideally I like the sound of a sealed system and doing away with the filler an expansion tank getting mains pressure hot, etc., but:

- I have been told by a mate (who is a building surveyor) that I need to consider upgrading all my microbore pipework if I go for a sealed system. Is that the case?? I would have thought that the microbore would be irrelevant to either a vented or unvented system?

- I plan to put the boiler in a new utility area which would be around 40-50 ft (and several rooms away) from the storage tank. Would that be a problem in either a conventional or unvented pressurised system?

- I have a friend that has an unvented system in a new build and he has all kinds of problems as the house builder has not made the pressurised tank very accessible and I understand that they need annual maintenance to monitor the exansion bubble level. Is this a relatively simple check that would be done as part of a boiler maintenance contract?

Also I would be grateful if anyone could give me a ballpark view on the cost of this, i.e. supply and installing the worcester boiler, pressurised tank and the 40-50 feet of pipework to

I have British Gas coming round to look and quote for this in a couple of weeks so would be grateful for any info in advance of that. BTW I am not necessarily going to go with British Gas but thought that they would be a good place to start.

malcolmallan said:

We have been in our victorian house for about 4 years and have decided to replace the boiler. The current system is a conventional vented system, microbore pipes and an early 70s ideal standard boiler and some old and some new radiators. The house is fairly large 4 bedrooms, 3 bathrooms and 4 public rooms but the current system works fine apart from the very large fuel bills (about £200 per month).

Click to expand...

What you need is Heat Bank/thermal store that does CH and DHW. The CH circuit is taken off the cylinder. You can split the CH circuit to upstairs and down and have them independently timed and controlled. Upstairs can be off while down is on.
Look at:
http://www.rangecylinders.co.uk/products/flowmax/index.html
http://www.rangecylinders.co.uk/products/flowmax/double_pump/index.html

This will do CH and DHW and 3 bathrooms too. The makers will advise on size.

http://www.heatweb.com

You can have a secondary circulation pump to pump hot water around a loop that the taps tee off.

I have ruled out a combi as I had one in our last place and the performance was terrible (took 30 mins to fill a claw foot bath and had a terrible flow rate even though the mains pressure was good).

Click to expand...

There are high flow combis available, so don't judge them by that one. 3 bathrooms is beyond a wall mounted combi. Gledhill make a floor mounted 3 bathroom on-box solution job as do ACV. Both quality products.

- I have been told by a mate (who is a building surveyor) that I need to consider upgrading all my microbore pipework if I go for a sealed system. Is that the case??

Click to expand...

No. If it heats up the house now it is fine. Leave it. Have is desludged using "Fernox" cleaner.

Forget unvented cylinder, they can do this:
http://www.waterheaterblast.com

Now back to the real world!

Microbore is much better used in a pressurised system however if yours works OK on an OV system then there is no need to change with a new boiler. Some of your rads may not take the increased pressure and if you get s/h c.i. rads they certainly may not.

The system should be powerflushed or effectively chemically cleaned before any new boiler is fitted.

Your boiler size needs to be matched to the lower of the heat loss of the building OR the rad output. A typical double glazed three bed semi in England is about 10-12 kW with cavity walls. About 20% extra for scotland and again for solid granite/brick walls. 40kW sounds grossly oversized in your case.

Only a genuine Magaflow has to have the bubble restored however ALL unvented cylinders must have adequate access for annual maintenance!

Tony Glazier

malcolmallan said:

- I have been told by a mate (who is a building surveyor) that I need to consider upgrading all my microbore pipework if I go for a sealed system. Is that the case?? I would have thought that the microbore would be irrelevant to either a vented or unvented system?
.

Click to expand...

Yes as Tony says it will be ok. The key question is do all the radiators work ok at the moment. If so, don't rip them all out for the sake of it.

malcolmallan said:

- I plan to put the boiler in a new utility area which would be around 40-50 ft (and several rooms away) from the storage tank. Would that be a problem in either a conventional or unvented pressurised system?

Click to expand...

by the storage tank, do you mean the unvented cylinder? If so, the only negative will the slightly increased re-heat time caused by this section of pipework. Its likely to be insignificant in the scheme things, but obviously if the cylinder and boiler sit side by side its much better.

malcolmallan said:

- I have a friend that has an unvented system in a new build and he has all kinds of problems as the house builder has not made the pressurised tank very accessible and I understand that they need annual maintenance to monitor the exansion bubble level. Is this a relatively simple check that would be done as part of a boiler maintenance contract?
.

Click to expand...

Yes if you have the bubble type, then they are very easy to repressurise. If you don't want this hassle then you can have one with an external pressure vessel, such as Santon premierplus. Actually the Santon is made at the same factory as Megaflo and both have the same high recovery coil. I have started specifying and installing the Santon as a preference over Megaflo for the reason of cost and the issue of regenerating the bubble.

The coils inside the Santon/Megaflo are excellent and will reheat a 170litre tank in about 15 mins if the boiler has enough umph. The coil is right at the bottom of the tank for the fastest reheat.

'watersystems' solution gives unecessary complication. The control circuitry or the pump will go wrong at some point and you will have your missus screaming blue murder at you, while you have a tank of unusable hot water! Unvented cylinders have 3 safety devices for protection. WS will forget to mention this in his wild claims that they are unsafe. How many safety devices do heatbanks have?

I would also question your need for a 40kw boiler. In your case I would specify a 24kw boiler with the cylinder/rads on a W plan. With a high efficiency boiler this gives a usuable figure of around 21kw to work with. and thats a lot of rads!

Don't forget that most of the time the boiler won't be running anywhere near maximum! The key figure which a lot of people tend to forget is the lower end which will be somewhere around 4 - 8kw. The 4kw (eg the case of Atag boilers) means that the boiler will only start to cycle on/off when the load drops below that point. The 8 kw boiler will have to cycle a lot more than the 4kw which means more wear and tear.

Forget unvented cylinder, they can do this:
http://www.waterheaterblast.com

Click to expand...

I enjoyed the video at that site but to rule out an unvented cylinder because of what they can do when abused is like saying dont use a kitchen knife cos you might slip and stab yourself in the jam tart.

I wonder if the tank was filled with diet coke and mentos it would go 400 feet like that one did?

It is a requirement now to zone a house of that size anyhow. very good if you spend a lot of time at home.(downstairs.) I would also consider seperate zones downstairs to try to acheive an area for daytime living rather than the whole downstairs after the extension.I would give the best possible attention to cavity wall & loft insulation. There could be a short fall on the existing rads run on a condensing boiler at lower temp as i find that type of property is quite often under spec'd heat wise.the extra insulation could be enough to compensate. Oso cylinder also need to be regenerated annually. also if you do a large secondary circulation i found there isnt enough to cope with the added volume of expansion & an external extra vessel is required.

htgeng said:

'watersystems' solution gives unecessary complication.

Click to expand...

Is in actually simple indeed - if you understand it of course.

The control circuitry or the pump will go wrong at some point and you will have your missus screaming blue murder at you,

Click to expand...

I have seen them running for 10 years or so. You can also have a hybrid with an immersed coil and a plate heat exchanger, or immersed coil only if a highly reliable flow switch gives you the willies.

Unvented cylinders have 3 safety devices for protection. WS will forget to mention this in his wild claims that they are unsafe. How many safety devices do heat banks have?

Click to expand...

I gave an advantages/disadvantages of these. Search and read it. You fail to understand buffering as well - domestic plumbers haven't a clue on this. Heat banks are brilliant at this - boiler and CH buffering is given for free.

I would also question your need for a 40kw boiler. In your case I would specify a 24kw boiler with the cylinder/rads on a W plan. With a high efficiency boiler this gives a usuable figure of around 21kw to work with. and thats a lot of rads!

Don't forget that most of the time the boiler won't be running anywhere near maximum! The key figure which a lot of people tend to forget is the lower end which will be somewhere around 4 - 8kw. The 4kw (eg the case of Atag boilers) means that the boiler will only start to cycle on/off when the load drops below that point. The 8 kw boiler will have to cycle a lot more than the 4kw which means more wear and tear.

Click to expand...

A heat bank/thermal store using ant-cycle stats does not cycle the boiler. The boiler operates at full efficient in one long burn. You can even add plate geat exchanger if great capacity is needed. Just add another boiler to heat the cylinder, if the existing boiler doesn't cope - the two of them will still not cycle.

Please try to understand. Come back to me on points you are confused on.

Forget an unvented cylinder. A expensive potential bomb waiting to happen. IT COULD BE YOU

And that £60 to £100 annual service as well. Forget them, when a superior alternative is available. Free you mind up.

I need to up date my self as not clued up on W plan. (whats it stand for) is the thermal store u recomend an open system as not many MIs like o/vents on there boilers now.

w plan

similar to y plan exept water takes priority

Oh now i remember. the W derived from the only type of installer that would fit it. like the brake on an old ship.

Another potential problem with heatstores is the operating temperature. If (for effective working) the heatstore needs to be at (say) 80 C degrees, the most efficient condensing boiler in the world will be a waste of money! The water coming back to the boiler will MOST OF THE TIME be above 56 degrees. At that temperature, condensing is physically impossible and the boiler will be 'higher efficiency' due to having a large heat exchanger but NOT 'condensing'.

It's very tricky to design a heatstore/ condensing boiler setup with controls and multiple coils that DOES work efficiently and in condensing mode. Some makes (eg. ACV) do achieve good results. Most designs, especially those comprising components from different sources, don't.

Also, a heatstore effectively rules out use of weather compensation, which IMHO is the ONLY effective way of maximising boiler efficiency.

(I guess this will be the start of a long-ish debate.....)

All,

Thanks for your many detailed replies. I had not heard of or considered a thermal store and this definitely sounds worthy of consideration. I have been doing a bit of research and have a couple of further questions about this. Is it economical to keep a full tank of water permanently heated - as we are out during the day it is really the morning and kiddie bath time that we need the water? I read that they can produce 100Kw - is that enough to generate 20 litres per minute of hot (assuming mains pressure permits this). Finally the central heating and the hot water draw would often be on at the same time - as they are off the same circuit would that cause the heating to go cold?

The water pressure upstairs is poor (about 2 litres per minute) but downstairs is very good (between 10 and 20 litres per minute) depending on whether it is kitchen/bathrooms. We get by with the poor upstairs pressure by using a pump on our upstairs bathroom shower - I am assuming that this pump would become redundant under either an unvented or thermal store system? Downstairs we need the pressure as we have a v large bath (around 200 litres).

My plan is to go to a number of suppliers (starting with BG) and ask them to look at my set up and provide advice on a thermal store and unvented system. Ultimately I may go with my local kitchen/bathroom company as they did a good job on our upstairs bathroom and we are getting them to do a couple of bathrooms at the same time.

On the heating front, our house is not very thermally efficient as it already extended to the side so there are a few rooms with 2 or 3 external walls and at a single level. I plan to get the council out as they do a free energy review to see if there is anything I can do it improve that. The extension at the back that I mentioned the missus is looking at is basically an orangery, extending and replacing a poor existing extension and will include a fair bit of glass (room in total 32 x 18ft) although the first 15ft or so is stone built. This will need to be liveable in 365days a year as the kitchen will be going in there - so that should interesting!

Thanks again for the info and a happy new year to all when it comes.

malcolmallan said:

All,

Thanks for your many detailed replies. I had not heard of or considered a thermal store and this definitely sounds worthy of consideration. I have been doing a bit of research and have a couple of further questions about this. Is it economical to keep a full tank of water permanently heated - as we are out during the day it is really the morning and kiddie bath time that we need the water?

Click to expand...

They are "very" well insulated. They store hot water and in the morning the stored hot water is pumped immediately into the rads giving an instant warm up. Enough is left for DHW usage and the boiler comes in to reheat.

They have timers on them so it is not being heated up all day if you are out.

I read that they can produce 100Kw - is that enough to generate 20 litres per minute of hot (assuming mains pressure permits this).

Click to expand...

They will deliver 40 plus.

Finally the central heating and the hot water draw would often be on at the same time - as they are off the same circuit would that cause the heating to go cold?

Click to expand...

No. The recovery rate is fast, and the boiler decoupled from the heating and DHW systems, operating at maximum efficiency.

The water pressure upstairs is poor (about 2 litres per minute) but downstairs is very good (between 10 and 20 litres per minute) depending on whether it is kitchen/bathrooms. We get by with the poor upstairs pressure by using a pump on our upstairs bathroom shower - I am assuming that this pump would become redundant under either an unvented or thermal store system? Downstairs we need the pressure as we have a v large bath (around 200 litres).

Click to expand...

Yep, the pump goes. A thermal store will do the bath no problem as long as the mains delivers.

My plan is to go to a number of suppliers (starting with BG) and ask them to look at my set up and provide advice on a thermal store and unvented system.

Click to expand...

Avoid BG. A unvented is the worst choice as you require an annual service. A heat bank/thermal store gives buffering of the CH and boiler.

Ultimately I may go with my local kitchen/bathroom company as they did a good job on our upstairs bathroom and we are getting them to do a couple of bathrooms at the same time.

Click to expand...

I doubt they will know what a thermal store is. Those sort of companies are way behind. Bathroom companies tend to just do the bathrooms and the rest of the system is something they just connect onto.

Get the house up to scratch in insulation.

NB a modern water-heating coil can be a load of around 25kW on a boiler, something to bear in mind if heavy water usage is considered.

20 litres/minute of shower-temperature water, in the winter, is energy usage at a rate of 50kw or so.

ChrisR said:

NB a modern water-heating coil can be a load of around 25kW on a boiler, something to bear in mind if heavy water usage is considered.

20 litres/minute of shower-temperature water, in the winter, is energy usage at a rate of 50kw or so.

Click to expand...

Yep, and heat bank thermal stores give twice that.

The great thing about heat banks is that when heated directly by the boiler, with no coil, they can combine the outputs of the stored hot water of the heat bank and boiler. To the point in some cases that the heat bank supplements the boilers output, with virtually all the boilers output going into the plate heat exchanger.

This in effect makes the cylinder "larger". Giving more kW that what is stored in the cylinder.