2 Quotes for new heating - what option should I choose - advice appreciated

[hztm2008]

As the original poster I just wanted to say thanks for all the info. To be honest, the subject matter has now turned to things way over my head. A few years ago we wanted to change our warm air heating but ended up keeping it due to the difference in opinions across the various companies/engineers we asked to quote. We find our ourselves in the same position now. We have spoken to 3 companies all of which tell us different things. One favours the combi, one tells us the combi is no good and we need the unvented cylinder and the other suggests we go with the old fashioned arrangement with tanks in the loft. We asked all 3 to provide quotes for combi and unvented cylinder options. As far as combos go, each has recommended a different size boiler with different reasons. None of them have checked our incoming water supply apart from turning on the tap and saying that it will be fine. I appreciate that the combi/cylinder decision is down to us but I struggle with the boiler sizing and operation. I showed all three the water coming out the hot tap and asked how this would compare; it should be similar but cannot guarantee it - well surely different size boilers will produce different results? Showed all 3 the pressure from the shower, one said the pressure would be about the same, one said they could not be sure (and that was the largest boiler!) and one said much better. I need to research this further but expect we will just keep the warm air again. Thanks again for the all the info.

 

[Roger928]

27.2 kW coil ? Does that imply it will transfer 27.2 kW into the water in the cylinder ? At what delta T is that measured ?. ( delta T being the difference in temperature between the water in the cylinder and the water circulating through the coil ). Quoting the value to 1% ( 0.2 in 27 ) requires similar accuracy in the details of when that performance is achieved

The overall heat transfer coefficient (h) for coils in water is 0.4 - 0.57 kW/m2.k. Source - Perry's Chemical Engineering Handbook

If you have a 22mm coil, 10metres long, thats a heat transfer area of 0.69m2.

The heat transferred will be given by

Q = h * A * DeltaT.

Delta T is a bit tricky, as you need to a) average it, and b) iterate it.
To heat say 20l/min by 20C needs 27kW.

Doing it the other way round, A delta T of 30C on this coil, will transfer about 10kW, So that would heat 20 l/min by about 7.5C
Which isn't that much.

 

[Doggit]

Are you sure electric shower produces 9l per minute or do you think it does. Is that seasonal or consistent throughout the year

It was only a suggestion to compare the existing electric shower to what could be achieved by a combi boiler, nothing more. Common sense should tell you that I can't determine the "actual" flow rate any better than you can as neither of us are on site. The figure was taken from my mothers 10.5KW shower in the summer, as we've just had to repair one of the heaters in it.

 

[Dan Robinson]

Think you might be surprised

4.3 l/min is the real figure at DT 35.
In the summer with a DT of say 23.... you might get 6.5 l/min

To be getting 9 l/min, you're only getting a DT of 16.5 - a pretty crappy shower.

 

[Doggit]

She lives in a sheltered accommodation complex, and by the time the water gets to her, it's already pretty warm, but I'll recheck the rate.

 

[Dan Robinson]

The rate will depend on the position of the temperature dial which is just changing the size of the orifice the water comes through.

 

[Doggit]

Now this is the problem with this site; people lose site of the original problem, and end up not only going off topic, but endlessly round the houses.

My original comment was a suggestion for the OP to look at his electrical shower, and use this as a starting point to consider the size of the Combi boiler that he needed - and now look where we're going. Completely off track on a hypothetical discussion.

 

[Dan Robinson]

Not really off topic... You're asking the OP to compare apples with oranges, we're explaining why.

 

[Mikefromlondon]

My 2.4Kw Kettle takes roughly 2 minutes to boil a liter of water to 100C from about 12c, from this you can work out how many kilo watts boiler you need. (This was based on pure guess.)

So i did a precise experiment using house hold stuff, a kettle with element reading 24.6ohms, which at 230V ac supply equates to 2,150 watts, and exact measure 1 liter cold water at 20c, washed kettel in cold water to equalise its internal temperature to the same as the incoming water temp i.e. 20c,

It took 2min and 55 seconds for the water to reach 100c
Ignoring any losses from the bottom of the kettle as my kettle does not have the submerged coiled element but new style element that is underneath the flat bottom.

Timed as follow:
20 to 40c it took 1 min exact
to 50 c 1min 19s
to 60c 1 min 35s
to 70c 1min 53s
to 80c 2min 15s
to 90c 2 min 30s
to 100c 2min 55s.

These figures may be pretty close to actual theoretical worked out figures using lab gear and conditions and heat loss reduced to almost zero.

So that tells me that if I want 5 liters per minute shower flow at 40c (from 20c to 40c) I will need something like 2.15Kw x 5 = 10.75Kw electric heater

Or this can also mean that to get10 liters per min hot water at 40c (from 20c to 40c) rise of 20 c in temperature I will need 21Kw of heat energy and even more if the ambient water temperature was say 10c.

I know of someone who had a 10.5Kw electric shower installed, he was not happy with the overall flow rate and temperature of the shower, he was probably comparing it to gas powered shower via a boiler rates at around 24 - 28Kw.

 

[hztm2008]

I truly appreciate the advice and information offered on this thread but I have to say that I think the industry is really missing a trick....Six of us along my road are considering changing our warm air systems but all of us, despite consulting with a number of professional companies / engineers, have a clue what to do. I fully appreciate that we need to decide between a combination boiler and water storage cylinder, but surely a professional can correctly size the boiler required through calculations and experience. As the end customer, I am not really interested in flow rates, heat loss, how much heat the boiler needs to apply to the cylinder coils, etc, etc. All I need to know is will the water flow coming out of my taps and shower be the same, better or worst than it is now. If this is not possible to answer then surely there must be some standard calculations or methods for sizing the system to ensure adequate operation. When you look at the forums, a lot of posts tend to result in professional engineers arguing among themselves. Earlier this year a BG engineer recommend a combination boiler, half hour later his supervisor turned up and said this was rubbish, we needed an un-vented cylinder. After a 20 minute debate they agreed that a combination boiler would be fine. The engineer suggested we would need a 28 or 32kW, the supervisor did not agree and stated that we would need a 37kw, and so it went on. If the professionals cannot agree then what chance does the customer have.

Please do not take this the wrong way, I am not putting down heating engineers and I truly value the information provided in the forums but it really is so difficult to work out what to do when everybody is saying different things. Maybe we have just been talking to the wrong companies

 

[Steelmasons]

My 2.4Kw Kettle takes roughly 2 minutes to boil a liter of water to 100C from about 12c, from this you can work out how many kilo watts boiler you need. (This was based on pure guess.)

So i did a precise experiment using house hold stuff, a kettle with element reading 24.6ohms, which at 230V ac supply equates to 2,150 watts, and exact measure 1 liter cold water at 20c, washed kettel in cold water to equalise its internal temperature to the same as the incoming water temp i.e. 20c,

It took 2min and 55 seconds for the water to reach 100c
Ignoring any losses from the bottom of the kettle as my kettle does not have the submerged coiled element but new style element that is underneath the flat bottom.

Timed as follow:
20 to 40c it took 1 min exact
to 50 c 1min 19s
to 60c 1 min 35s
to 70c 1min 53s
to 80c 2min 15s
to 90c 2 min 30s
to 100c 2min 55s.

These figures may be pretty close to actual theoretical worked out figures using lab gear and conditions and heat loss reduced to almost zero.

So that tells me that if I want 5 liters per minute shower flow at 40c (from 20c to 40c) I will need something like 2.15Kw x 5 = 10.75Kw electric heater

Or this can also mean that to get10 liters per min hot water at 40c (from 20c to 40c) rise of 20 c in temperature I will need 21Kw of heat energy and even more if the ambient water temperature was say 10c.

I know of someone who had a 10.5Kw electric shower installed, he was not happy with the overall flow rate and temperature of the shower, he was probably comparing it to gas powered shower via a boiler rates at around 24 - 28Kw.

A simple calculation would of taken all of 5 seconds had you known but instead you decide to fook around boiling your kettle FFS...

 

[AGAS]

My 2.4Kw Kettle takes roughly 2 minutes to boil a liter of water to 100C from about 12c, from this you can work out how many kilo watts boiler you need. (This was based on pure guess.)

So i did a precise experiment using house hold stuff, a kettle with element reading 24.6ohms, which at 230V ac supply equates to 2,150 watts, and exact measure 1 liter cold water at 20c, washed kettel in cold water to equalise its internal temperature to the same as the incoming water temp i.e. 20c,

It took 2min and 55 seconds for the water to reach 100c
Ignoring any losses from the bottom of the kettle as my kettle does not have the submerged coiled element but new style element that is underneath the flat bottom.

Timed as follow:
20 to 40c it took 1 min exact
to 50 c 1min 19s
to 60c 1 min 35s
to 70c 1min 53s
to 80c 2min 15s
to 90c 2 min 30s
to 100c 2min 55s.

These figures may be pretty close to actual theoretical worked out figures using lab gear and conditions and heat loss reduced to almost zero.

So that tells me that if I want 5 liters per minute shower flow at 40c (from 20c to 40c) I will need something like 2.15Kw x 5 = 10.75Kw electric heater

Or this can also mean that to get10 liters per min hot water at 40c (from 20c to 40c) rise of 20 c in temperature I will need 21Kw of heat energy and even more if the ambient water temperature was say 10c.

I know of someone who had a 10.5Kw electric shower installed, he was not happy with the overall flow rate and temperature of the shower, he was probably comparing it to gas powered shower via a boiler rates at around 24 - 28Kw.

hope you made yourself a cup of tea after all that ( and no I'm not taking the p!ss just in case your offended )

 

[Dan Robinson]

When you look at the forums, a lot of posts tend to result in professional engineers arguing among themselves

Because there are multiple ways of shaving the same cat.

All I need to know is will the water flow coming out of my taps and shower be the same, better or worst than it is now

. As the end customer, I am not really interested in flow rates, heat loss, how much heat the boiler needs to apply to the cylinder coils,

Eerrrmmm you contradicted yourself in consecutive sentences.

Maybe we have just been talking to the wrong companies

Indeed.

What most of us decent installers would do is present the options and ask you to make an informed decision. We're not the ones living with the decision.

 

[hztm2008]

My 2.4Kw Kettle takes roughly 2 minutes to boil a liter of water to 100C from about 12c, from this you can work out how many kilo watts boiler you need. (This was based on pure guess.)

So i did a precise experiment using house hold stuff, a kettle with element reading 24.6ohms, which at 230V ac supply equates to 2,150 watts, and exact measure 1 liter cold water at 20c, washed kettel in cold water to equalise its internal temperature to the same as the incoming water temp i.e. 20c,

It took 2min and 55 seconds for the water to reach 100c
Ignoring any losses from the bottom of the kettle as my kettle does not have the submerged coiled element but new style element that is underneath the flat bottom.

Timed as follow:
20 to 40c it took 1 min exact
to 50 c 1min 19s
to 60c 1 min 35s
to 70c 1min 53s
to 80c 2min 15s
to 90c 2 min 30s
to 100c 2min 55s.

These figures may be pretty close to actual theoretical worked out figures using lab gear and conditions and heat loss reduced to almost zero.

So that tells me that if I want 5 liters per minute shower flow at 40c (from 20c to 40c) I will need something like 2.15Kw x 5 = 10.75Kw electric heater

Or this can also mean that to get10 liters per min hot water at 40c (from 20c to 40c) rise of 20 c in temperature I will need 21Kw of heat energy and even more if the ambient water temperature was say 10c.

I know of someone who had a 10.5Kw electric shower installed, he was not happy with the overall flow rate and temperature of the shower, he was probably comparing it to gas powered shower via a boiler rates at around 24 - 28Kw.

Excellent advice. I have just been out boiling the kettle. I have no idea what you are talking about regarding the boiler sizing but I successfully used the kettle to fill a bucket of water. I took this upstairs and tipped it over the head of my other half who was standing in the shower. She can confirm that the temperature was great but the flow rate was a little high! - all said in jest.

 

[Dan Robinson]

A simple calculation would of taken all of 5 seconds

Less in fact .