Condensing bolier flow and return temps - effect on rad sizing?

[stevetheplumber]

i remember when we cubed the room in ft and times by 5 to get the rad size then choose the rad that fitted the window

 

[doitmyself]

i remember when we cubed the room in ft and times by 5 to get the rad size then choose the rad that fitted the window

I think some installers still use that method.

 

[kay-jay]

i remember when we cubed the room in ft and times by 5 to get the rad size then choose the rad that fitted the window

If you do that and compare it to results from a mears calculator its always there or thereabouts [emoji89]

 

[wetdrip]

Ok so hear goes I work my room heat losses out @ delta 50 1 degree = 2% i up size by 20% which gives me a delta 40 heat figure for each room. I then work out the system requirement and add 10% for pipework heat loss. I range rate the boiler through the perameters at a Max of this figure. I set flow temp to 65 allowing room temp to be 15...... Delta 40! Completely balance the system for equal heat loss per rad and fit an external weather compensator. Fit a fully modulating boiler with adjusting pump speed and system learning capabilities and there you have it the most condensing system you can get. This will cost you mind and most are not willing to pay for this level of service!

That's the one quirky thing that gets me, not just on here, but youtube videos as well.
You state 10% heat loss for pipework; and here's the thing: a massive percentage of the pictures we see on here and youtube, the installers never bother to insulate the pipework.

P.S. And as someone who still has his WB GREENSTAR 28i j ( Which magazines best buy) still in its box. This thread is a tad painful:-(

 

[doitmyself]

i remember when we cubed the room in ft and times by 5 to get the rad size then choose the rad that fitted the window

I applied that formula to my living and dining rooms; and got the following results:

Living - 7485; Dining - 3903.

I then realized that, if the formula uses cubic feet, it probably calculates heat in BTU. So I applied the necessary conversion and got:

Living - 2.19kW; Dining - 1.14kW

Compare this to the current (original) rads: Living - 2.07kW; Dining - 1.13kW

Coincidence or what?

Problem is: the house has been fully insulated since it was built about 25t years ago, so the current requirements are: Living - 1.09kW; Dining - 1.01kW.

The rads are now oversized by an average of 75%, which means that, even in the coldest weather the flow temp is about 65C and the boiler condenses most of the time.

 

[newcastle phill]

Flow temp isnt the most important factor.
Return temp is the more so.
Whats your return temp

 

[wattsie1987]

That's the one quirky thing that gets me, not just on here, but youtube videos as well.
You state 10% heat loss for pipework; and here's the thing: a massive percentage of the pictures we see on here and youtube, the installers never bother to insulate the pipework.

P.S. And as someone who still has his WB GREENSTAR 28i j ( Which magazines best buy) still in its box. This thread is a tad painful:-(

I just use the 10% as a rule of thumb but it is surprising how much heat can be lost through pipework. Why do you say quirky?

 

[wattsie1987]

I can just imagine the conversation between the proud owner of a brand new £350K house and the site's sales rep when he is advised not to move in to the house as the heating system is not powerful enough to bring the house up to an acceptable temperature.

As for the second suggestion, is that always possible? Turning up the water temperature will increase the rad output, but you will still be limited by the boiler output. If you need 10kW for a 10C rise, you will need 20kW for a 20C rise. Who installs a boiler with 50% spare capacity?


But isn't that how systems are supposed to be designed, i.e. for an outside temp of -3C?

I always size the boiler to the system at Max output so there will be more for the boiler to give if needed. I think you've missed the point where I said I oversize the rads by 20% then lower the flow temp. If the flow temp was at Max the system would be 20% more powerful than needed and have that chilly -3 house up to temp in no time!

 

[wetdrip]

I just use the 10% as a rule of thumb but it is surprising how much heat can be lost through pipework. Why do you say quirky?

Just weird, but not in a good way.

 

[Rory]

Well, as the OP I should report that the system is in now - 3-4 days work for 2 guys was done in 2 days, complete new CH system with WB 28i Junior, the WB magnetic thing, and Wave controller.

As requested, the installer has tended on the large size for the rads and the large living room has a couple of double rads with single fins, rather than the singles with single fins originally offered. Although it's a smallish house, there's actually quite a bit of radiator acreage now.

Chilly here tonight but arriving at the house it was certainly very warm.

As 'promised' by the installer, flow and return seemed very similar temperatures. I think it's going to need a bit of playing with over the next few days. A query about condensing mode was responded to with an explanation of modulating.

Wave seems like a bit of fun - got the app and linked in remotely with no issues. Turned the temp down a bit as we're not there! Setting the heating parameters for the weather compensation has me completely baffled - it says to ask your installer.

 

[doitmyself]

As 'promised' by the installer, flow and return seemed very similar temperatures.

What are the flow and return temperatures?

 

[Rory]

What are the flow and return temperatures?

I can't say with any precision at the moment, but by touch both very hot with little difference between them. In fact it wasn't obvious which was the flow and return and for a moment I thought they'd put the magnetic filter on the flow, but I looked at the installation book and it's on the return.

 

[doitmyself]

I can't say with any precision at the moment, but by touch both very hot with little difference between them.

That doesn't sound right. There should be a noticeable difference between the flow and return - about 20C - despite what the installer told you. If you can, borrow an infrared thermometer and measure the flow and return pipes near the boiler. Do the same for a few of the radiators. As the pipes are all brand new and probably unpainted, you will need to wrap some insulating tape round the pipe and measure the temperature off the tape, holding the thermometer almost in contact with the pipe.

 

[Rory]

That doesn't sound right.

I know! I'm just a bit dismayed that the installer doesn't get it.

I've got an IR thermometer so no problem to set it all up when I get time.

What I don't know (and much searching hasn't helped) is what effect the Worcester Wave unit has on the flow temp when using weather compensation mode. I'll have to communicate with WB.

 

[newcastle phill]

Try opening the rad valves more to help the system give up more heat and turn the boiler temp down (sorry can't remember boiler type and too lazy to read back haha)

 

[Rory]

Try opening the rad valves more to help the system give up more heat and turn the boiler temp down (sorry can't remember boiler type and too lazy to read back haha)

Oh - that's the opposite of what I'd have thought. My instinct would be to close them down to slow the flow. Worcester 28i Junior, by the way.

 

[chris watkins]

Oh - that's the opposite of what I'd have thought. My instinct would be to close them down to slow the flow. Worcester 28i Junior, by the way.

And you would be right Rory.
You need to balance them yourself, closing them down allows the rad to loose more of the heat energy in the water resulting in a lower return temp back to the boiler.
The boiler electronic controls are looking to maintain a 20deg drop across the system by modulating but if the return is close to the flow temp it thinks the house is up to temp & shuts down often not coming back on for 10 - 15mins.

The trouble with balancing is that to do it correctly you need a steady design flow temp (of say 70) with the design room air temp (say 21) with a design outside air temp (at say -3) you could then adjust the flow rate to give that 20 deg drop.
As you can now see it is not an exact science on domestic system but that doesn't mean you should not try, play around when the system is warm & the outside is cold.

 

[newcastle phill]

Sorry my bad.
Misunderstood what he said and typed back to front.

But yes needs balancing.
Still think hes gonna struggle tho by sounds of it.
Have a good play and let us know

 

[Worcester]

Heat given up (energy) = flow rate (litres /min) x temp drop (Δt) x time x SpecificHeatCapacity of Water(Constant)

We design ASHP systems with only a 5° Δt
So need 2 x the flow rate as a 10° Δt to achieve the same power / energy during a given period

ASHP's have minimum design flow rates and also a target Δt.
As heating engineers we design our systems accordingly. To high a Δt and the HP's don't like it, same with too low a flow rate, and we put devices in to measure both during both installation and in use.
- Surely everyone designs systems like this don't they?
( I guess that's the difference between a heating engineer and a plumber / gas safe fitter... you may be competent to install a boiler, it doesn't make you competent to design and install a heating system)

It's a case of design and balance.. what is the optimum Δt specified for your boiler?

Use a decent electronic thermometer to set that across each of your radiators by adjusting the lockshield, (assuming the radiators are correctly sized of course) and then you'll end up with a properly balanced system.

 

[Ermintrude]

K type thermocouple on anton

 

[Ollieg]

Delta T on a Worcester should be 20C??

OP did you have to set up your heat curve using the app?

 

[tamz]

Delta T on any condenser should be 20º but i've yet to find one even on my own.
Closest i've got is 18

 

[Rory]

OP did you have to set up your heat curve using the app?

Well, Worcester Bosch said "The curve set points adjust the heating curve used to calculate the flow temperature depending on the temperature outside, unless advised this should not be adjusted after selecting the heating system type."

The default settings are Start Point Curve 20C, End Point Curve 75C.

Snag is that I've no idea what this means! I've looked at various explanations on the web and there's usually ranges of curves with factors of 1.2, 1.3 etc up to 2.0 or so. There appears to be no way of moving the curve in the Wave app.

You can also set the min and max flow temps, and the heating system type, and room influence.


I did start a new thread on Weather Compensation: http://www.ukplumbersforums.co.uk/c...eather-compensation-pump-runs-all-time-2.html