Combi boilers heating water tanks.

[westfire32]

I can only generalise on the layout surrounding this problem.
In my daughters house there are 2 combi boilers in the cellar (the house was at one time 2 houses) . these appear to connect to common pipework which then splits via divertor valves to provide Central heating via one divertor and a flow/return system which provides heat to TWO megaflow heaters in the attic (for bathrooms).through another divertor valve.
The flow/return pipework entering the megaflow tanks never seems to get very hot, and we are convinced that this aspect of the megaflow is hardly heating the water, and that the immersion heaters are doing most of the work.
I strongly suspect that the flow & return pipes into each megaflow is reversed, also as these pipes are the highest in the system they are fitted with automatic bleed valves (and one of these was definitely not at the highest point (which resulted in an air lock).
Further these tanks are probably 25 foot above the boilers, could this have any effect..
Balancing the system!!!!! is it possible that because most of the time BOTH heating and hot water heating is being called for, the hot water loop is losing out.
I am coming round to the idea of reconfiguring the pipework/ controls such that one boiler is dedicated to the heating& kitchen hot water, and the other exclusively to heating the megaflow tanks.
any comments, ideas, suggestions would be very welcome

 

[JohnD]

post some photos

the chances of anyone who can't see it (us) knowing better than someone who can (you) is very slight.

 

[westfire32]

Will do.. It may may take a couple of days though...

 

[simond]

In principle this system should work.

You haven't said whether it ever has? If it has, why change the design, if something has changed then someone has altered the balancing or a part has failed.

It is pointless re-engineering a system like this if you are not in the trade, and sending a few pictures to internet forums will never produce advice as good as you could get from a professional on site.

I doubt they would have used two megaflos off one combi, more likely one off each.

And if you don't know how much heat the immersions are contributing have you considered turning them off to find out?

 

[westfire32]

Simond, Thanks for your comments, much appreciated
Problem is that my daughter only recently moved into the house and we cannot be certain that the system did ever work, utilising the Combi boilers. It is more then likely that the immersion heater in each megaflow is the MAIN source of heating the water.
I can confirm for certain that there is only one flow & return pipe going into the attic, and it can be seen very easily that one flow & one return is teed off to supply each tank.
One of the areas which give us cause for concern is that before my daughter bought the house, it had originally been two houses, and whilst both the design and materials are of excellent quality, the workmanship leaves a lot to be desired, and certainly there are things within the heating/plumbing arrangements which leave a lot to be desired.
Please do not feel I am being rude when I say that I have had sufficient experience in my 50 yrs of working (related to the building industry) to know that something isn't quite right here. (for instance the automatic air release valve in the return line in the attic has been put on the flow pipe at a lower level and we did manage to cure an airlock in that tank feed system)
Regarding the switching on/off the immersion heaters, We are in the process of trying this, however as the only switches are located in the attic adjacent to the tanks, it is not easy for my daughter to do this, and I am based 80 miles away. We are also going to shut off the heating flow to each tank in turn to see how this contributes to the scenario.. long process

The reason for my post is to try and gain some additional information to add to our findings, and allow us to get the system working as it should do.

 

[Agile]

The flow and return to the Megaflow can be in either direction so thats not an issue.

You say the flow temperature to the cylinder is low. That would usually be as a result of boiler settings or a very severely reduced flow rate.

I can see no problem with your configuration and I do not recommend that you go changing it.

All that is required is someone competent to assess it and diagnose the problem!

It might just be a failed motor valve.

Tony

 

[westfire32]

Tony.

Thanks for your reply.
I appreciate that the reversed flow/return is not a major issue (however it does possibly serve to illustrate the expertise of the original fitter.)

Boiler setting is sufficient high to make the Radiators only just bearable to the touch.

Motorised valve has been checked by manually opening it after the outlet pipe has gone cold, and the rate at which the pipe from the this valve heats up is a reasonable indication that there is a substantial flow through it.
After considering the tests/checks that we have carried out, I am coming round to the point that you make regarding the flow rate being very reduced.
I feel that this is a result of
a) The system is feeding 10 radiators on the ground floor as well as 10 on the first floor. (I don't have the ratings of these Rads, but they are mostly large rads of unique designs (spirals, etc)
b) As i mentioned before the two megaflow tanks ( not sure of the capacity however they are at least 5 foot high) are in the attic approx 25 foot above the boilers.
c) after bleeding the highest point (next to the megaflow) the pipes to each megaflow only get lukewarm
d) The flow/return from the 2 boilers is in 22 mm pipe which then join to 1 22 mm pipe feeding to the motorised valves.

As I said in my last post, we need to carry out more isolating tests before coming to any firm conclusions.

 

[simond]

The first thing I would try is getting the rad MV turned off and seeing whether the flows to the Megaflos (each of which should be fitted with a separate MV) improves.

If it does not then as you say they are probably airlocked and you may need to cut in an airvent.

In principle the system will work.......

 

[Agile]

If you can keep your hands ont he rasds they are less than 50°C and thats far too low for water heating where at least 65°C is needed.

You have implied there is not a proper balance between the heating and the cylinders.

If this is the case then the cylinders will heat properly with the heating off. If thats so you need lockshield valves to balance the heating and hot water.

But you do also need to increase the flow temperature to 65-70°C to the cylinders.

The heat coils to the cylinders can be connected either way round but have slightly different characteristics in each case.

Tony

 

[westfire32]

Simon, Tony,
Thanks very much for your interest and feedback.
I am coming to the conclusion that, in principle there is not a great deal wrong with the system, but adjustments could be made to improve it.
My next task is to carry out the tests that I have already discussed, to build up a picture of what does what. Then possibly to make some minor adjustments to improve the system.

Tony,
I have noted your comment regarding the different temperatures at which the heating and hot water system should be fed. would you suggest raising the boiler output temperatures to 65/70 or would that be too hot for the radiators, bearing in mind there are two small children in the house. The layout of the pipework would make it fairly easy to dedicate one boiler to the heating system and the other boiler to the hot water circuit. Would this assist in maintaining the correct temperature for each system, but I suppose it would also depend very much on the ability of one boiler to provide sufficient central heating.
The boilers are rated at 24 KW & 25kw (for heating circuits).
Last point, regarding the megaflow tanks. the way in which these are piped indicates that when hot water is called for, each tank would probably supply the sytem at roughly the same flow rate, looking at the pipe layout.. The pattern of usage would indicate that one tank may be sufficient to supply the families bathroom hot water needs, If the other tank was isolated (via mv's) and left as standby, to be used at irregular intervals, do you think there would there be any "stagnant/stale water" problem

Thanks.

 

[Agile]

Traditional central heating worked on a flow of 80C.

With condensing boilers that has been reduced to 70 C.

I dont see any need to fiddle with the system.

All I can see that is wrong is the flow temp is set too low, turn it up to 65-70 C and it should work fine.

Tony