Why loft vents for boiler and immersion cylinders?

[Doctor Drivel]

I have seen that happen a few times, evn being open vented and not blocked

[Dave Fawthrop]

|> Tony Williams wrote: |>

|>> BTW: My old plumbing book has a gruesome tale about |>> a plumber who blocked a vent pipe with a cork, to try |>> and reduce the amount of water lost during a drain |>> down. The resultant vaccuum caused parts of the |>> plumbing to collapse inwards. Blocking a vent pipe |>> may not be good news. |>

|> Good point sir! Probably the copper hot water storage cylinder, which |> will be crushed by atmospheric pressure if you were to drain it without |> relieving the vacuum (hose out bathroom window, no hot taps open). | |I have seen that happen a few times, evn being open vented and not blocked | |> On unvented hot water storage cylinders |> the mandatory T&P relief valve |> also acts as a vacuum breaker valve |> (I think, goes off to get book to check).

Done that myself :-( Should have known better. :-(

[Doctor Drivel]

I never mosunderstodd at all and I agree with you. I was looking into what he had. If the boiler is a sealed compatible boiler (which it "appears" to be) then there is no immediate chance of explosion, and as I said get it sorted ASAP.

If a sealed compatible boiler then the high limit cuts in. Another protection level.

That is why on combined feed & expansion pipes it has to be 22mm minimum. The one pipe has system water move up and down the pipe, so water with inhibitor will be passing the tee reducing the liklihood of only fresh water being there leaving behind scale. In very hard water areas it is best to fit a 28mm tee and a small length of 28mm pipe. An aerator can be fitted instead of a tee. Have compression joints for disconnection purposes and the unit can have the odd clear out in case. The aerators have a greater volume inside, so les lilely to scaleup, and are recommended by the likes of Baxi and Potterton on combined feed and expansion pipes.

Yep. You can also get water circulation in one pipe. I have come across that before.

[Aidan]

You can't have read it then. Re the pressure relief valve, he said;

He has dismantled the safety valve.

The ones I've seen in the past 10 or 15 years were factory pre-set, no user adjustable parts. It's probably a spring-loaded antique and the setting has been lost in dismantling. The boiler is probably a similar vintage. Almost certainly not suitable for sealed system operation and no manual-reset secondary thermostat.

Now it hasn't got an open vent or a working safety valve either.

[Doctor Drivel]

..and cleaned it.

Some of them you could split and clean, he has done that. That does not mean it is now unoperatational.

"every" boiler I have seen with an integral pressure relief valve has been suitable for a sealed system (why is it there?), that is going back to boilers made 30 years ago (a few still around). In ye olden dayes, a presure relief valve was fitted to the flow pipe of open vented boilers. This would blow off in the kitchen. It was made illegal and required a pipe to outside. Then they never required one at all on open vented systems.

I told him to check the suitability of the boiler for sealed systems. I would be confident the valve would work.

[David Hansen]

On Sun, 5 Mar 2006 11:04:58 -0000 someone who may be "Doctor Drivel" wrote this:-

"A traditional boiler, loft tank, vented cylinder type affair."

Correct. However, that does not answer the three questions I asked about it.

[David Hansen]

On Sun, 5 Mar 2006 13:58:44 -0000 someone who may be "Doctor Drivel" wrote this:-

Here you appear to be mixing up two pipes. The first pipe is the feed and expansion pipe, the second pipe is the vent pipe. This is strange as you got it right earlier in the thread.

The name feed and expansion pipe is something of a clue. In such a pipe water will/should indeed move up and down, as the system is warmed up or cools down. As it happens water will also move up and down the vent pipe, because the level of water in the vent pipe will/should be the same as the level of water in the F & E tank.

[Doctor Drivel]

Nope. A combined feed and expansion pipe is one pipe. Legal on systems which sealed system compatible boilers may be fitted. And no need to have a blow-off valve either.

Yes it is. I have the clue and also explained. You should understand what it is.

[David Hansen]

On Sun, 5 Mar 2006 16:01:37 -0000 someone who may be "Doctor Drivel" wrote this:-

Yet you then go on to make precisely the same mistake again.

A feed and expansion pipe is indeed one pipe. However, as I said and you snipped, you appear to be mixing up this one pipe with the vent pipe, which is another pipe. As I said, this is strange as you got it right earlier.

Bluff and bluster on this again, if you wish.

[Doctor Drivel]

It is the vent pipe as air works its way up this pipe. Air doesn't care. It will go up any pipe.

[Christian McArdle]

At the risk of agreeing with Drivel, a combined feed and expansion pipe also handles the venting. There is only one 22mm pipe from the system to the loft. This pipe handles feeding of the system with water, expansion from the system into the header tank and venting of air or steam to the loft space. It splits into 2 at the bottom of the tank and a vent only pipe then continues above the tank.

Christian.

[Doctor Drivel]

As I know more about it than anyone else just read and take heed.

It doesn't split at all. It is one 22mm pipe to the bottom of the F&E tank. Been standard for years. As 95% of all boilers are sealed system compatible, doing it this way saves on pipe, time and no pump over or drawing of air in.

Potterton/Baxi specify a air ejector when the combined Feed and expansion pipe meets the system. Works very well. With this method extremely low heads can be attained (200mm from top of boiler).

[legin]

I went to the recent Homebuilding show at the Nec. The reason was that having googled and followed recent threads I was thinking that the best probable solution would indeed be to fit a heatbank in my self build. I went with the specific question in mind. How do you keep the condednsing boiler condensing if a temperature of circa 75 degrees is required for the heatbank. I even suggested mixing the return from the heatbank with the return from the underfloor heating to get the return temp down enough to get lull the boiler into condensing mode. Apart from some exhibitors being unsure of how a system could be piped, the overall conclusion was to fit an unvented cylinder. This would take better advantage of a condensing boiler. Anyone any better advice to offer. Regards Legin

[Andy Hall]

One thing to bear in mind is that a boiler does not reach Nirvana or orgasm when it "goes into condensing mode".

What actually happens for boilers designed for condensing is that there is an efficient heat exchanger able to work with a temperature differential between return and flow of 20 degrees and possible more.

The lower the return temperature, the more efficiently the boiler is running. This happens regardless of whether it is condensing or not.

What happens at the dew point - around 54 degrees - below which condensing takes place, is that the *rate of change* of efficiency with return temperature increases below this point.

This is as a result of the contribution of the latent heat that is released from the phase change of water from gas (i.e. steam from combustion) to liquid. Note that I am talking about steam in the true and invisible sense here and not "steam" that is visible and is in fact, water vapour.

There isn't a step change of efficiency at the dew point - just a knee in the curve.

In terms of optimising overall efficiency, anything that can be done to reduce the temperature of the return water to the boiler will help. UFH is possibly one aspect for this. Another, if you are going to use radiators, is to make sure that these give the heat required when you run them at 70 degree flow and 50 return. Conventionally, radiators are sized based on 82 and 70. To achieve 70/50 requires them to be approximately a third higher nominal output when choosing from the manufacturer datasheet. The effect of doing this is to run the boiler at a lower temperature overall for more of the heating season.

Where a heatbank is used for water heating, normally the objective is to maintain it at as high a temperature as is reasonably practicable - i.e. 82 degrees. THe implication is that the return would be at around 60 degrees and you are out of condensing range.

This is not really a problem for several reasons:

- The cylinder will cool as HW is run with cool water replacing hot at the bottom. The plate heat exchangers used are able to trasnfer heat very efficently to the cold water (100-200kW equivalent is typical), so quite a drop in temperature in water returned to the cylinder.

- The point at which the boiler fires can be arranged to be when the water is quite a bit cooler such that in any case there is a low return temperature to the boiler. It is only going to reach the 62 degree point relatively late in the reheat cycle if adjusted correctly.

There is not really a lot of point, therefore, in striving to achieve

55 return by reducing the flow to 75 max. because also you will lose heat storage capacity in the heatbank

[Doctor Drivel]

A plate heat exchanger is "highly" efficient. It can take in 80C primary water from the store and have 25C coming out of the plate, transferring all the heat to the incoming cold mains. This 25C water is pumped directly into the bottom of the store, where the return pipe to the condensing boiler is. So, very cool water at the bottom. Water baffles prevent de-stratification.

Look at:

the solar and gravity circulation. Dedicated UFH heat banks have a cooler lower section for UFH. Look at the return from the plate heat ex, it goes into the bottom of the store. Also the UFH blend cool water the bottom with warmer water a bout 1/4 to 1/3 of the way up. The boiler return is right at the bottom where all the cool water returns promoting condensing.

A store may be 75C at the top, but stratification will ensure that the lower section may be around 25 to 30C. All looking good. With two cylinder stats to eliminate inefficient boiler cycling, the boiler is only called in to reheat the whole store with one long efficient burn. The return water is a very low temperature and promotes condensing efficiency. The ignorant salesmen who spoke to you think, oh run temp of 75C, dew-point 54C, so no condensing. Total tripe. You need to understand the operation. Look at

Look at the ACV HeatMaster, a combined boiler/thermal store. It is one of the most efficient boilers there.

"Direct" boiler heated heat banks (using a plate) are very efficient. Thermal stores use an inefficient (to the plate) coil for DHW take off. These do not promote a lower return temp as much.

That is tripe.

an annual service charge for one too. Say it lasts 20 years that is £1600 minimum just to service it at today's rates; that is to have a cylinder of hot water standing there. In reality it will be much, much, more. At that price you could afford to replace a thermal store every 10 years and still ahead of the game. Many stores are guaranteed 20 years for the cylinder anyway.

It would not. Most unvented cylinders do not have a quick recovery coil. Better than Part L, not quick recovery.

[Doctor Drivel]

Wrong. Modern banks are 75C

Wrong. See my explanation.

You size to suit.

[David Hansen]

On Thu, 9 Mar 2006 17:00:52 -0000 someone who may be "Christian McArdle" wrote this:-

You appear to be describing a particular system. If so then it is far more dangerous than one with separate feed & expansion and vent pipes. Unless, of course, other precautions are taken that are roughly equivalent.

[Doctor Drivel]

It is not more dangerous at all, as long as the boiler is sealed system compatible. The problem is having on pipe over the F&E tank, teeing off this and entering the F&E tank at the bottom is that air can be drawn on the vent section. The vent section is totally surplus to requirements, as air will vent via the pipe to the bottom of the F&E tank.

A problem with a 22mm combined feed and expansion is that the water in the pipe/F&E tank can move up and down. If the tank is full of fresh water then scaling may occur at the tee causing a blockage which can be dangerous. One of the reasons the tee and pipe should be 22mm.

Because of this some makers stipulate that the 22mm combined feed and expansion should be minimum length to minimise fresh water scaling at the tee. Others stipulate a 3 port air ejector, which is a largish cylinder about 1.5 inches in dia, so reduces the risk. Many engineers don't like air ejectors as they are a point that could collect sludge, again causing a blockage. The fitting of a Magnaclean filter solves that problem.

Many boiler are now being fitted in lofts. The problem with this is getting the pressure relief valve to the outside (this could be ugly), so open vented boilers are used. This creates a situation of low head. Combined feed and expansion pipes solves this. Baxi say 200mm from top of boiler to underside of F&E tank, using an air ejector and one 22mm pipe. It works brilliantly. One problem with a combined feed and expansion is that they are at times a pig to fill up, with air locks about the place. In truth it is usually because no auto air vents are used at key points. The ejector makes sure easy filling and no air problems.

[Christian McArdle]

It is the return temperature that determines condensing efficiency, not the flow temperature. The temperature at the bottom of the heatbank, provided the thermostatic control is good, should actually be quite low.

Christian.

[legin]

If that is the case, what will be the flow temp. Assuming that the boiler will lift the temp say 20 degrees. If the return temp is say 45 then the flow temp will be 65. Going through a heat bank previously heated to say 75, then it will have a cooling effect on the heatbank. Thanks to Andy for the detailed response. I understand what you are saying about the knee in the curve. However at this stage it would be nice to get it right and have the most efficient system. That to me means making sure that the condensing boiler actually condenses. The heat bank technology is completely new to me. Is there not a boiler out there that could supply heating to the heatbank at say 75 degrees until it is totally satisfied then reduce its temp down to say 65, to ensure that the return is well within the condensing range. I know that this would mean using a S plan to supply the heatbank and not actually use it to feed the rad circuits and underfloor circuits. Or could it be plumbed as a heatbank supplying the rad/ ufh circuits, with a low return as Christian suggests. Would the boiler then be capable of lifting the temp say 25 degrees. Also dare i say thanks to drivel. I know his ramblings have been the source of a lot of banter, but I have got to admit that it does amuse me. More importantly he has raised my awareness of heatbanks to the point where I am now seriously considering using one. Legin