Do I need a DBV?

Both situations are fine DH, and both will relieve the pressure and provide a circulation through the pump.
I wasn't suggesting otherwise ;)

I suppose the reason most instructions only show the valve in the pipe shorting flow to return is that many installations will already have this pipe in place, with a gate valve to limit the flow, so it's a simple matter of replacing the gate valve with an ABV.

In any case, auto-adjusting pumps, such as the Grundfos Alpha should remove the need for an ABV.
 
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Thanks for clarifying Doitall - I misunderstood. If I do need to put the DBV near the pump then its a separate job, and I might as well leave it and see how system performs once I've balanced rads. I think I could leave quite a few TRVs open anyway and let the room stat control them.

DH its interesting what you say about auto-adjusting pumps. Is this an alternative to a DBV?
 
DH its interesting what you say about auto-adjusting pumps. Is this an alternative to a DBV?
That's the theory.

Fixed speed pumps have the disadvantage that, when the flow rate decreases, the differential pressure increases. Auto pumps vary the speed according to requirements. Open Alpha2 then click the link under Download Instructions to read the leaflet - Page 5 explains what it does.
 
Thanks DH - I'll do some reading about the alpha 2.

If it's the same dimensions as my existing grundfos pump it would be a doddle to install - easier than chopping up 28mm pipes to insert Ts for a DBV. Wouldn't even need to drain down. Sounds more energy efficient than merrily pumping water round a bypass loop too.

I suppose the concern might be that the flowrate through the boiler might drop too low, if the pump speed reduces to keep the pressure constant as the rads close. I daresay they've thought of that?
 
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Thanks DH - I'll do some reading about the alpha 2.
Also look at the Alpha2 L, it's a cut down version - i.e. cheaper, without night setback and Autoadapt.

If it's the same dimensions as my existing grundfos pump it would be a doddle to install
Exactly he same

I suppose the concern might be that the flowrate through the boiler might drop too low, if the pump speed reduces to keep the pressure constant as the rads close. I daresay they've thought of that?
Flow rate is only important if the boiler mfr specifies a minimum rate. Which make/model boiler do you have?
 
Its a Potterton Prima F 100F.
Potterton's instructions say that some form of by-pass is necessary to dissipate the latent heat when the boiler shuts off.

If you have a mid-position (three pipes) motorized valve then you do not need a separate by-pass as at least one circuit will be open all the time. But if you have two 2-port motorized valves, you will need a bypass as both circuits could be closed.
 
I do have a mid-position valve.

My possible concern with the Alpha 2 is that with the MP valve in CH only position, if most or all of my TRVs are closed, the pump will run very slowly in order to keep the differential pressure to 0.2bar or whatever, this will result in very little water passing through the boiler. So it might have trouble dissipating the heat.

Perhaps I should contact Potterton and ask whether auto pumps are recommended with this boiler.
 
I do have a mid-position valve.
That saves an ABV.

My possible concern with the Alpha 2 is that with the MP valve in CH only position, if most or all of my TRVs are closed, the pump will run very slowly in order to keep the differential pressure to 0.2bar or whatever, this will result in very little water passing through the boiler. So it might have trouble dissipating the heat.

Perhaps I should contact Potterton and ask whether auto pumps are recommended with this boiler.
Shouldn't cause a problem, but there is no harm in asking.

One thing which has crossed my mind is the size of the pump required. You have a 30kW boiler, but it can be range-rated down to 24kW. This should be shown on the boiler's data plate. The reason I ask is that at full output you would probably need the 15-60 version of the Alpha2 as the 15-50 would not have sufficient head at the required max flow rate of 0.64 litres/sec.

Incidentally why do you need such a large boiler? With 12 radiators and 30kW, that's an average rad size of 2.5kW!!.

Have you checked what size boiler you do need? The EST online boiler calculator is a good place to start. Also, do you know the output of your radiators? Use the Stelrad Elite Catalogue as a guide.
 
Many thanks for all this helpful info DH.

The reason I have this boiler is that someone who once owned my house had it installed! Perhaps its far too big. I need to take a look at the boiler calculator.

Previous owner also installed 10 ludicrous cast-iron radiators which contain up to 40 litres of water each! They take ages to warm up and the output is pathetic. We do not have mains water and flow to header tank is a dribble, so filling this system after a drain-down takes days (and imagine the Fernox bill). I am gradually replacing the iron rads with modern ones - took two of them out this week and plan to replace at least two more before winter.
 
The reason I have this boiler is that someone who once owned my house had it installed! Perhaps its far too big.
Probably, unless you live in a huge Victorian pile with solid walls, cellar, sash windows, no loft insulation and draughts everywhere.

Previous owner also installed 10 ludicrous cast-iron radiators which contain up to 40 litres of water each!
The Stelrad catalogue won't be much help then.

I am gradually replacing the iron rads with modern ones - took two of them out this week and plan to replace at least two more before winter.
Good idea, but how do you determine what size radiators to install?

I would try and get them all replaced before next winter.
 

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