Booster Pump into Combi Boiler? DIY solution?

[GCarnegi]

I have around 1 bar pressure (tested this morning at the mains stopcock) and my main bathroom is upstairs which gets minimal shower flow, and the combi for the whole house is in the loft. The bath which is also upstairs also takes more than 20 minutes to fill to anything like a decent level.

So I'm trying to work out the most appropriate (and cost effective) boosting solution.

I've been looking at installing a Draper BP1 55l/min booster pump in my garage, and taking the inlet from a pair of 50gal tanks in my garage attic which is at ground floor ceiling level, with the output going to the domestic cold water, which in turn feeds the combi in the loft. I was also going to put mains water to the pump output with a pair of non-return valves (from pump and from mains) so the only direction of flow can be towards the domestic cold water and onwards to the combi. Obviously, normal operation would be cold water via the pump, but could still be direct from mains (with poorer pressure) in the event of loss of power to the pump.

Reason I'm putting it in the garage is because I read that noise may be an issue (pump spec says 85dbA) - and also that's the only place I have room for the tanks as I've just converted the loft to an office space. Lastly, there's already a feed direct from the incoming 28mm plastic pipe to a tap in the garage just in the right place.

Is it okay to feed a pumped supply to the combi? For that matter are the draper pumps designed with domestic supplies in mind? Anyone done anything like this before?

Should I be shooting for around 50l/min AT THE SHOWER? The pump manual (downloaded) says max 55l/min, but of course, that's assuming zero head.

 

[doitall]

I think you should be aiming for 12-15ltr/m which is all the Combi will produce.

 

[BigBurner]

Contact the water company as I think the minimum is 9 or 10 litres/min they should provide. Check. You can't put a pump on the main pipe.

Anyhow you have real carp flow, but all is not lost. What is the static pressure in bar? If over 1 bar then go for a cold water accumulator and a combi - well you have a combi, but a hood combi.

Accumulator:
http://www.mikrofill.co.uk/products_vessels.php
Explanation:
http://www.heatweb.com/products/accumulators/accumulators.html

They store cold water at the static pressure of the mains. Then they give superb water flow on all hot and cold taps. They work in a water outage. 300 litres will do. Half the size for the water storage. When a bath is filled it charges up again. At 8 litres/min that will be around 18 minutes for a full charge. The bigger the less time you wait as more cold water storage. Even when exhausted it will still give 8 litres/min, so the combi will never run out of hot water.

If you need a good combi. Look at the Broag 39C. Quality, low cost & great price/performance and buy the outside weather temperature sensor for economy. It will fill the bath fast enough for you - about 7 to 8 minutes.
http://www.avantarange.com/39c.html

So you end up with cold water cylinder and combi and no open cold water tanks in the loft....and great pressure and flow to what you had. A win, win.

 

[doitall]

Dr Drivel strikes again, rip the lot out mate and start again

He has 1bar static Drivel, do tell how an accumulator the size of his car will improve matters.

If you read his post he is installing a tank in the loft of the garage to supply the pump, perfectly acceptable provided he uses a negative head pump, or better still a vertical pump set.

 

[GCarnegi]

So will it work then?

One reply says that an accumulator alone will do the job. I'm no expert here, but I'm thinking if you put 1.0 bar into an accumulator, then you'll get 1.0 bar out of it again, right? pumping the main? - didn't consider it for a minute. thanks. Okay, maybe for a minute but thought better of it.

Another reply says that I can only get 12-15l/min from the combi - I thought I would get from the combi whatever I put into it, and if I have 3-bar static pressure going in than that would produce more flow than if I have 0.5 bar going in. Is this more an indication of how much flow the combi can heat? Sorry I don't know much about plumbing domestic water pumps, but I do know about sizing 1200+hp pumps set a mile under the north sea at 400bar or so discharge ressure to get a specific flow out at the wellheads - I'm guessing the principles are the same though I understand that these don''t tend to feed combi boilers!

Sorry, I should have also mentioned I was going to put the pump in the garage too (at floor level), so the pump is still gravity fed (maybe 0.2 bar at intake) from the tanks in the garage ceiling,, so it wouldn't be pulling on a negative intake (is that what you meant?). It would then produce enough head to provide around 2.8bar in my loft, where the combi is...ie 3.5 bar at pump discharge minus 7m (0.7bar) head = 2.8 bar at combi.

In theory, I think it's sound. I just wanted to know if there were any gotchas I should expect. eg, what if I do try to pull 50l/min through the combi? Will it have enough power to heat that sort of flow? It's a 28kW unit. Should I expect to find leaks in my central heating / DHW circuits affter jumping from 0.5 bar to 2.5 bar?

Would there be any advantage to putting some sort of small accumulator vessel nearer the combi? For that matter, isn't there usually a small accumulator built in to these things?

 

[swbjackson]

A 28Kw boiler will give you 11 to 12 l/min of hot water at a 35C temp rise.

 

[BigBurner]

1 bar static will work on an accumulator.

 

[doitall]

1 bar static will work on an accumulator.

yes Drivel it will, but the OP will never get more than 1bar, which kind of defeats the exercise, when he's looking for 2.5bar.

 

[doitall]

So will it work then?

Another reply says that I can only get 12-15l/min from the combi - I thought I would get from the combi whatever I put into it, and if I have 3-bar static pressure going in than that would produce more flow than if I have 0.5 bar going in. Is this more an indication of how much flow the combi can heat? Sorry I don't know much about plumbing domestic water pumps, but I do know about sizing 1200+hp pumps set a mile under the north sea at 400bar or so discharge ressure to get a specific flow out at the wellheads - I'm guessing the principles are the same though I understand that these don''t tend to feed combi boilers!


In theory, I think it's sound. I just wanted to know if there were any gotchas I should expect. eg, what if I do try to pull 50l/min through the combi? Will it have enough power to heat that sort of flow? It's a 28kW unit. Should I expect to find leaks in my central heating / DHW circuits affter jumping from 0.5 bar to 2.5 bar?

No it will not work with a combi boiler if you want more than 12L/min ish

A pump set and storage tank set-up is fine and we install them all the time, what you will have to consider is an alternative hot water supply.

You could for example install an unvented cylinder or dare I say it a thermal store, and then utilize the pump, to keep the existing combi you could split the heating into a Y or S plan

 

[GCarnegi]

Thanks for the input BigBurner, but my problem is nothing to do with restrictive incoming pipework, which I think is the main reason for installing an accumulator. There is 1.0 bar in the street, so no-matter what I do I'm never going to get more than that in the house unless I install a tank and pump.

I measured my shower yesterday and it took about 75 sec to fill a 12litre basin, meaning I'm running around 9l/m.

My boiler documentation states a capacity of 11.7l/min which isn't a huge increase, but I also have to remember that not all the shower water is coming from the combi - some is simply cold mains water.

So if I sized my pump to give me enough head to generate 11l/min hot water through the combi, plus around 3l/min cold, that would give me 14l/min to the shower / bath etc - an improvement of over 50%.

I worked out that if I am drawing 15l/min from the tank(s), and the mains is refilling them at 10l/min (worst case), then a 200l tank would give me 40 minutes supply (drawing down at 5l/min). Two tanks would give me twice that time. Is that how it works?

This would also give me much better flow for my garden, where the hosepipe presently gives out no more than a limp squirt!

 

[mogget]

BigBurner has a habit of recommending an accumulator and/or a high flow combi for everyone, regardless of the circumstances.

A little bit about the negative head issue - negative head simply means that the tap being fed is at a higher level than the water level in the storage tank. Once the pump is running it's no problem, the question is, how to get the pump started!

In a positive head setup, a flow switch activates the pump, which then boosts the pressure of the water. If the tap is above the tank level, there will be no flow when it's opened and the pump will not spin. I'm not sure of the exact method used to solve this problem, but if memory serves, it involves check valves, an extra flow switch and a small expansion vessel.

One thing I don't recall being mentioned is that you may want to keep the kitchen cold tap on the existing main, so you're not drinking tank-stored water.

 

[GCarnegi]

Makes sense. So the circuit would look like this?

http://guycarnegie.com/resources/images/water.jpg

Also, the pump I was looking at has an integral (bolt-on) expansion vessel and flow switch, so all I'd really need to do would be to separate the two of them, put the pump below the tanks, and the expansion vessel and flow switch high up in the attic - connected electrically back to the pump in the garage. Right?

 

[mogget]

Like I say, I don't know too much about pumps, so don't take my word for any of this!

Looking at your diagram, I think it would work if a second flow switch was placed on the inlet to the expansion vessel, and electrically connected in parallel with the first. Flow switch no.1 would activate the pump upon a demand for water, and flow switch no.2 would control the recharging of the vessel when the demand ends.

In reality the vessel will have only one water connection, so it will be teed into the pipe.

I think that putting it above the boiler is a red herring, and that it would work just as well next to the pump.

If the charge pressure was lost - say someone opened a tap in a power cut - the system would not self-start. This could be solved with a water pressure switch (if there is such a thing!) fitted on the outlet of the expansion vessel. Again this would be connected in parallel with the flow switches and would need to make on low pressure(below 1 bar say). That way the vessel would be kept charged at all times.

 

[BigBurner]

Thanks for the input BigBurner, but my problem is nothing to do with restrictive incoming pipework, which I think is the main reason for installing an accumulator. There is 1.0 bar in the street, so no-matter what I do I'm never going to get more than that in the house unless I install a tank and pump.

I measured my shower yesterday and it took about 75 sec to fill a 12litre basin, meaning I'm running around 9l/m.

Your mains pipe is poor. It is 1 bar and giving a flowrate of about 10 litres/min - so you indicate. A accumulator will store cold water at 1 bar and deliver it at around 30 litres/min at 1 bar. Get it? It will deliver that to your combi as well, giving the maximum the combi can handle. A 1 bar shower is fine -few are disappointed. Power shower pumps are mainly rated around 1 bar.

An open vented tanks and pumps that will give any more than 1 bar is expensive and a poor solution.

 

[BigBurner]

BigBurner has a habit of recommending an accumulator and/or a high flow combi for everyone, regardless of the circumstances.

Another bathroom changer. You don't know what an accumulator is.