What size boiler needed

[sherylryan]

When we are discussing the size of a boiler, we aren’t referring to its physical dimensions. Instead, we are referring to the output in kilowatts (kW), the unit that is used to measure how much energy is output by the boiler in the form of heat. It is important to choose the right size boiler for your home because it will mean you are not wasting energy and money you don’t need.

Generally speaking, the more heat and hot water you require, the higher the output you will need, although other factors also play a part in choosing the right size.

Hi there, yes I understand we are referring to the output in kw. As I mentioned we need 23 kw/h for ufh, 15 kw/h for radiators. 3 kw/h for DHW and 3.5 kw/h for trench, total 44.5 kw/h. If all of these were calling for heat at once a would boiler with 35kw/h output would struggle?

 

[John.g]

How are the heat loss calculations reconciled with that 44.5kw, presuming they were carried out before sizing rads etc?.

 

[sherylryan]

How are the heat loss calculations reconciled with that 44.5kw, presuming they were carried out before sizing rads etc?.

Yes all heat loss calculations were carried out before sizing Rads etc. we’ve taken out a 24kw gas boiler, 16kw & 9 kw heat pump and replaced with 35 kw boiler ‍♀

Also on another note Re expansion vessel. I have today calculated 214 litres of water in the whole of the central heating system, not counting the 310 litres of DHW. (Total water content of system with DHW Is 524 litres)

Also the PRV is lifting in the boiler releasing water into the condensate pipe.

 

[John.g]

With precharge & filling pressures of 1.0bar/1.5bar a 12L EV = final pressure of 2.92bar when system hot. (too close to the bone of 3.0 bar safety valve lift).
A 18L EV will give a final pressure of 2.29bar under the same conditions.

Check EV size/volume, precharge/filling pressures should then be checked, requires system drain down but quick test of diaphragm can be done by pressing the EV air end schrader valve with finger nail or whatever, if water issues, diaphragm failed, if air pressure (or none), requires pre/filling pressures checked.

 

[sherylryan]

With precharge & filling pressures of 1.0bar/1.5bar a 12L EV = final pressure of 2.92bar when system hot. (too close to the bone of 3.0 bar safety valve lift).
A 18L EV will give a final pressure of 2.29bar under the same conditions.

Check EV size/volume, precharge/filling pressures should then be checked, requires system drain down but quick test of diaphragm can be done by pressing the EV air end schrader valve with finger nail or whatever, if water issues, diaphragm failed, if air pressure (or none), requires pre/filling pressures checked.

This is in my firebird instruction manual, our EV is set to 1 bar

 

[ShaunCorbs]

You need a 25 l expansion vessel or the next size up eg 35l depending on what you have you can add to it

 

[John.g]

What size EV is installed??.

 

[sherylryan]

What size EV is installed??.

12 Litre

 

[sherylryan]

You need a 25 l expansion vessel or the next size up eg 35l depending on what you have you can add to it

Hi Shaun, yes that's what i was thinking.

Tonight I found a document in our plumbers manual and it states that they are using the DHW expansion vessel, I can't see how this will work as it is 10m away from the boiler on the discharge side of the pump and on the return pipe at the bottom of the cylinder.

 

[John.g]

Is this EV connected in to the COIL return at the bottom of the cylinder??.

 

[sherylryan]

Is this EV connected in to the COIL return at the bottom of the cylinder??.

Yes it is. (sorry EV just out of shot in photo)

 

[ShaunCorbs]

And what is the capacity of this one ?

 

[sherylryan]

And what is the capacity of this one ?

It’s 10 litres. I’m not entirely sure how this would work as the flow for bother ufh manifold goes through the those valves you can see in pic, but there return pipes are 5 metres away from that. The return pipe in the pic (at the top) is for Dhw only.

 

[sherylryan]

Is this EV connected in to the COIL return at the bottom of the cylinder??.

Yes it is John.

 

[ShaunCorbs]

It’s 10 litres. I’m not entirely sure how this would work as the flow for bother ufh manifold goes through the those valves you can see in pic, but there return pipes are 5 metres away from that. The return pipe in the pic (at the top) is for Dhw only.

But all of the returns are connected tbh I would say your on the limit of expansion

Are there service valves on them so you could drain and check the pressure one of them might be low / flat

 

[John.g]

Is the 12L installed in the boiler or outside it and if so is it connected to the boiler circ pump suction side or discharge side?.
Even the 12L on its own if properly charged, per firebird, precharge and fill pressures of 1bar then that alone would only mean a final pressure of 1.97bar with system contents of 214L and 2.23bar with 250L.

 

[sherylryan]

But all of the returns are connected tbh I would say your on the limit of expansion

Are there service valves on them so you could drain and check the pressure one of them might be low / flat

Sorry just checked it and it is 18L pre charged to 1.5 bar.
How does this work being situated here when I thought any additional vessel had to be close to the boiler.
Also how would this work with the radiators when there flow is in the completely opposite direction .

 

[sherylryan]

Is the 12L installed in the boiler or outside it and if so is it connected to the boiler circ pump suction side or discharge side?.
Even the 12L on its own if properly charged, per firebird, precharge and fill pressures of 1bar then that alone would only mean a final pressure of 1.97bar with system contents of 214L and 2.23bar with 250L.

It’s on the inside of boiler, but pump is on the outside. It has a hose running from vessel to firebox. Hope that makes sense.

 

[John.g]

Does the pressure gauge show 3bar with PRV lifting and what pressure is it showing when system up to temperature?.
You will see a plastic cap (like your car tyre valve cap) just in front of the EV flexible hose, just unscrew/remove (the plastic cap) and depress the needle valve briefly with your finger nail or if not with a small screwdriver and see what comes out.

 

[sherylryan]

Does the pressure gauge show 3bar with PRV lifting and what pressure is it showing when system up to temperature?.
You will see a plastic cap (like your car tyre valve cap) just in front of the EV flexible hose, just unscrew/remove (the plastic cap) and depress the needle valve briefly with your finger nail or if not with a small screwdriver and see what comes out.

System is on now, showing 1.3 bar on boiler, air comes out of vessel when needle is depressed .

Can on vessel be pre charged to 1 bar and the other to 1.5 bar, do they not have to be the same?

 

[John.g]

That other 18L EV has got nothing to do with the heating system, you have a unvented HW cylinder and that 18L EV is to take its expansion, are you sure its not a PRV (expansion valve or main TPRV) from this system that's lifting?.
Its unlikely that the boiler PRV is lifting if the boiler pressure is not exceeding say 2.8/3.0bar.

Even though you have established that the 12L EVs diaphragm hasn't failed the precharge pressure should still be checked as the boiler PRV will lift if this precharge pressure has fallen from 1.0bar to 0.31bar, filling pressure 1.0bar. OR (will lift) if the precharge pressure is correct at 1.0bar but the filling pressure is 1.5bar. So requires system drain down, also the boiler pressure gauge may be faulty, if not and PRV is lifting at < 2.8bar then PRV needs renewing.

 

[sherylryan]

That other 18L EV has got nothing to do with the heating system, you have a unvented HW cylinder and that 18L EV is to take its expansion, are you sure its not a PRV (expansion valve or main TPRV) from this system that's lifting?.
Its unlikely that the boiler PRV is lifting if the boiler pressure is not exceeding say 2.8/3.0bar.

Even though you have established that the 12L EVs diaphragm hasn't failed the precharge pressure should still be checked as the boiler PRV will lift if this precharge pressure has fallen from 1.0bar to 0.31bar, filling pressure 1.0bar. OR (will lift) if the precharge pressure is correct at 1.0bar but the filling pressure is 1.5bar. So requires system drain down, also the boiler pressure gauge may be faulty, if not and PRV is lifting at < 2.8bar then PRV needs renewing.

That makes John, I didn’t think the 18L EV had anything to do with the heating system. No the PRV valve on this system goes to a completely different discharge drain.

Would the new pump running at 1.35 bar but was at 2.1 bar cause problem, in the below clip out of instruction manual they talk at static head.

Also attached you will see systems description and the plumber gave us he clearly makes reference to the 18L EV being part of the heating system.

 

[John.g]

Can you point out exactly where this PRV (that's lifting ) is located? Also it appears that there is one EV on the CH system, the 12L one?.

 

[sherylryan]

Can you point out exactly where this PRV (that's lifting ) is located? Also it appears that there is one EV on the CH system, the 12L one?.

Yes there is only the one 12L EV on system . Should this be connected to the pump?

 

[John.g]

OK, thanks, I see that's a TPRV, normally, what I've seen is just the 3 bar PRV with the over temp protection done with the boiler thermostat at 110 to 113C.
So if this TPRV valve lifts with boiler pressure < a indicated 2.8bar, either a faulty TPRV or improperly charged/filled EV and the pressure is exceeding 2.8/3bar. Presume pressure indication is changing with system cold/hot?.

 

[sherylryan]

OK, thanks, I see that's a TPRV, normally, what I've seen is just the 3 bar PRV with the over temp protection done with the boiler thermostat at 110 to 113C.
So if this TPRV valve lifts with boiler pressure < a indicated 2.8bar, either a faulty TPRV or improperly charged/filled EV and the pressure is exceeding 2.8/3bar. Presume pressure indication is changing with system cold/hot?.

Yes the pressure is changing with cold/hot system only slightly though.

 

[John.g]

OK then, suggest noting that pressure the next time the PRV lifts.

 

[sherylryan]

OK then, suggest noting that pressure the next time the PRV lifts.

Ok will do thanks John. You said in a previous post the boiler pump is normally around .4 bar, 4m head, if this external pump is set to 1.35 bar 13.5m head is this an issue with the expansion vessel, as the expansion vessel charged to one bar allows the system a static head of 5m.

 

[sherylryan]

Ok will do thanks John. You said in a previous post the boiler pump is normally around .4 bar, 4m head, if this external pump is set to 1.35 bar 13.5m head is this an issue with the expansion vessel, as the expansion vessel charged to one bar allows the system a static head of 5m. View attachment 76721

Another question, I have watched the pump running on both the kitchen (troubled) and laundry manifolds, individually. Correct me if I’m wrong but should the kitchen manifold not run at less RPM than the laundry manifold that had 50m flow and return pipe? The kitchen manifold runs in 32mm cooper from boiler (5m) until it tees off for manifold then it runs in composite 26mm for 2 metres till it joins manifold.

Doesn’t the boiler pump only control the flow and return from the manifold and the manifold pump controls the flow and return in the manifold circuits?

 

[ShaunCorbs]

That other 18L EV has got nothing to do with the heating system, you have a unvented HW cylinder and that 18L EV is to take its expansion, are you sure its not a PRV (expansion valve or main TPRV) from this system that's lifting?.
Its unlikely that the boiler PRV is lifting if the boiler pressure is not exceeding say 2.8/3.0bar.

Even though you have established that the 12L EVs diaphragm hasn't failed the precharge pressure should still be checked as the boiler PRV will lift if this precharge pressure has fallen from 1.0bar to 0.31bar, filling pressure 1.0bar. OR (will lift) if the precharge pressure is correct at 1.0bar but the filling pressure is 1.5bar. So requires system drain down, also the boiler pressure gauge may be faulty, if not and PRV is lifting at < 2.8bar then PRV needs renewing.

It’s connected to the return ?

 

[John.g]

If you are referring to the 18L (white) EV, it appears to be connected to the Cylinder cold water inlet? (with blue isol valve), post #87.

 

[John.g]

Another question, I have watched the pump running on both the kitchen (troubled) and laundry manifolds, individually. Correct me if I’m wrong but should the kitchen manifold not run at less RPM than the laundry manifold that had 50m flow and return pipe? The kitchen manifold runs in 32mm cooper from boiler (5m) until it tees off for manifold then it runs in composite 26mm for 2 metres till it joins manifold.

Doesn’t the boiler pump only control the flow and return from the manifold and the manifold pump controls the flow and return in the manifold circuits?

Ok will do thanks John. You said in a previous post the boiler pump is normally around .4 bar, 4m head, if this external pump is set to 1.35 bar 13.5m head is this an issue with the expansion vessel, as the expansion vessel charged to one bar allows the system a static head of 5m. View attachment 76721

Will reply to manifold pump later.

Re boiler external pump, if the EV is connected into the suction side of the pump, the pump discharge pressure should be the EV pressure+the pump head, in your case, 1.3bar+1.35bar=2.65bar, if the EV is connected at the pump discharge then the pump discharge pressure will be the same as the EV pressure, in your case. 1.3bar, the pump suction should then be the EV pressure-the pump head, 1.3bar-1.35bar=minus 0.05bar or minus 0.5M which means part of the system can be running under negative pressure, not desirable as air can be drawn in, in practice boilers (especially Gas) often have their EVs connected into the discharge side but with a EV pressure of 1.3bar and a pump head of say 0.7bar will still result in a positive pressure at the suction side, 1.3bar-0.7bar=0.7bar or 7M.

 

[ShaunCorbs]

If you are referring to the 18L (white) EV, it appears to be connected to the Cylinder cold water inlet? (with blue isol valve), post #87.

Yes not cold inlet coil, cold is below

 

[sherylryan]

Will reply to manifold pump later.

Re boiler external pump, if the EV is connected into the suction side of the pump, the pump discharge pressure should be the EV pressure+the pump head, in your case, 1.3bar+1.35bar=2.65bar, if the EV is connected at the pump discharge then the pump discharge pressure will be the same as the EV pressure, in your case. 1.3bar, the pump suction should then be the EV pressure-the pump head, 1.3bar-1.35bar=minus 0.05bar or minus 0.5M which means part of the system can be running under negative pressure, not desirable as air can be drawn in, in practice boilers (especially Gas) often have their EVs connected into the discharge side but with a EV pressure of 1.3bar and a pump head of say 0.7bar will still result in a positive pressure at the suction side, 1.3bar-0.7bar=0.7bar or 7M.

The EV connects into the firebox, I guess you would say its on the suction side the pump? is 2.65 bar discharge not to high?

 

[John.g]

Its may cause erratic operation of the UFH manifold TMV because you have hot water at high pressure on one side of the TMV and much lower pressure on the other side coming from the cold manifold return.