Pumps for large heating system

[G&M]

We have a five bedroom farmhouse with about 15 radiators. The boiler is in an outbuilding whulst the pump is in the house. There is about thirty-five metres (times two) of pipe between the two. It isn't possible to move the boiler nearer.

The old pump and LPG boiler were having problems so I have installed an

90,000 BTU oil boiler with a Grundfos Alpha pump. However I'm still not sure the water is being circulated fast enough due to the resistance of the piping. The oil boiler isn't commisioned yet but the LPG certainly struggled big time.

Is there any definition of what the minimum acceptable water velocity is ?

And if it is too low, is it possible to install a second pump by the boiler just to pump water through the long legs, presumably with an automatic bypass valve in the house, and then use the Alpha just to pump the water around the house ?

[Set Square]

thirty-five

If the water velocity is too high, you'll get noise in the pipes. If it's too low, you'll have too big a temperature drop across your radiators.

Start by getting yourself a decent non-contact thermometer (like the 30 quid IR one from CPC) and balance your system to get an equal drop across all radiators. Ideally, this should be about 11 degC. If it's substantially more than this, water isn't getting to your radiators fast enough to balance the heat loss to the room - and you need to increase the velocity. If your pump is running flat out, but is still not enough, I'd be tempted to install another one very close to the existing one but in the other pipe - so that one is in the flow and the other in the return. [Make sure they're the right way way round so as not to fight each other!] Connect them together electrically so that they both go on and off at the same time. In effect, one will be circulating water between the house and the boiler and the other will be circulating it round the radiators - but they will just be working in series, so there's no need for any additional by-pass circuits.

[I hope that the long pipes between the house and out-house are well lagged!]

HTH.

Cheers, Set Square

[G&M]

Thanks - that's even easier to do.

In the process of doing so. This farm building had the original warm roof concept - hot water pipes right under the tiles.

[Set Square]

One thing you'll have to watch - if it's a vented system - is that the addition of a second pump doesn't cause it to pump over. If it does, you may either have to change where the feed and expansion pipes are connected or turn both pumps round so that the circulation is in the opposite direction. If you *do* reverse the flow, make sure that any TRVs fitted to radiators are bi-directional.

Cheers, Set Square

[IMM]

thirty-five

When calculating the rad/pipe sizes of a system the pump size is also determined. You may have is far too many elbows giving too much resistance fooling the Alpha which then drops its speed.

Two practical ways around this:

1. Fix a high head fixed speed pump at least the same head as the replaced pump.

1. Instead of installing loops with one pump to assist the other, just split the system into two zones with a time clock on each. Then you could have say the upstairs off during mid day. Divide and rule and save fuel too, which is a good thing with a oil boiler in price and storage longevity.

[G&M]

Now the second IS a good idea. Saves on zone valves which cost about the same as pumps anyway. Is there a minimum distance feed to each pump from the T point, or can they both be connected closely together ? I'm thinking of the case where both come on and if one is sucking stronger than the other the other gives up and sulks.

[IMM]

Flow from boiler -> then a tee -> then a non-return valve for each pump and then the pump for each zone. The DHW can be regarded as a zone too, so another tee and non-return valve for the DHW pump.

[look]

I'm usng an Alpha on a 20 rad system, 5 beds mostly at ground floor level.

It has no problem even at the normal setting. The system is pressurised and TRV's on most rads.

Robert

royall at which dot net

royall at which net

[Bob Mannix]

35m -yikes! As a "quick" trial you could try getting a second pump, and adding it in parallel to the existing one (wherever it is), with an extra bit of pipe. For the purposes of the test you could turn it on and off by hand. This "should" double the pressure. Should it prove useful a permanent installation might require a relay box, as the boiler/controller might not like the starting current of two pumps, and isolation valves etc. , for removal and in case one fails

[Set Square]

You'll have to find a way of controlling the boiler so that it only comes on when one or more zones are calling for heat. Zone valves may be easiest option, because they have volt-free contacts which close when the valve opens and which can be used to switch the pump. You won't need non-return valves if you use zone valves. Otherwise you'll have to keep the non-return valves *and* add relays to do the boiler switching.

Does your boiler need to keep the pump running for a bit after it stops firing? If so, that will be an interesting problem to solve - if you have 3 pumps, all controlled independently!!

Cheers, Set Square