T50 on new install?

[Ric2013]

Re rad output a 30 deg rad does not emit 30/50 or 60% of a 50 deg rad, it emits (30/50)^1.3 or "only" 51.5% of that of a 50 deg rad.

Ah, right. So 30/50 or 60% but then hat 1.3. What does hat 1.3 mean? You're losing me I've already stated that Stelrad uses 52% (rounded from a correction factor of 0.515), but I don't understand how they get that figure. Not saying they are wrong, but what I'm saying is I fail to understand how they calculate that figure as it conflicts with the logic of heat loss calculations used in U values. Perhaps U values are only a guideline are are known to be imprecise?

To be pedantic, W or Wh/h (flow of energy) is power, whereas Wh (quantity) is energy and I think this might be the source of the confusion. So obviously it will take more than twice the energy to heat a room/house by 10 degrees that it would take to heat it by 5 as it's losing heat the whole time. Obviously, if you could stick the house in a Thermos flask and heat it, 10° would only take a little over double as there would be almost no heat lost. But the energy required to maintain the higher temperature (Wh/h) is only double.

So, yes, I agree with you, basically.

Obviously we also overlook the fact that the heating system will struggle as the delta T of emitters will fall as the rooms get warmer and that since even an unheated house will be a few degrees above ambient due to solar gain and incidental gains from the body heat of the occupants and any electrical appliances in use, the first few degrees of warming are essentially already compensated for heat loss.

 

[John.g]

A long time ago I couldn't figure out why a 30 deg rad etc wasn't emitting 60% of that of a 50 deg rad and so on, so I found a table giving the deg rad vs output and put into a spreadsheet and by clicking on the trendline excel will show the formula that determines the trend and I saw that the calc is the deg rad/50 to the power (^) 1.3. Now I don't know if all manufacturers use this or not but I would think that its some number very close to 1.3. I will post a screen shot of the above shortly.

 

[John.g]

Here it is, both methods give the same result, its just easier not to have to remember 0.0062.

 

[hometech]

I have a cup of tea sitting outside in the winter. The tea is 60 degrees, the air temp is zero
My tea goes cold quicker than if it were 20 degrees outside...agree.
Now insulate the cup...does it take longer to cool down even in the same outside temperature? Yes.
Now add a insulated lid to the cup..does it extend even further the time it takes to cool down...Yes.
I let the tea cool to 15 degrees and need it reheated to 60. It takes the same energy input to raise the tea temperature at whatever the outside temperature it is heated in. Slow down the rate and sources of heat loss and you have totally different needs in a radiator. Like a combi and the ratio it can modulate at. Higher modulation, greater efficiency. A number of principles of physics are being rolled into one here...

 

[John.g]

I'm afraid I can't raise my cup to that, it will only take the same energy to heat it from 15C to 60c if its fully insulated, remove the insulation and it will take more energy as the cup is losing heat to the outside but must still be heated to 60C, exactly as you say in reverse, the more the insulation the longer it takes to cool down so less energy needed to heat up, if it was 100% insulated it wouldn't take any energy to heat it up as it would never cool down in the first place.

I can only see it your way if you fully insulated the cup and then heat it from 15C to 60C, remove the insulation, let it cooldown, replace the insulation, same exact energy required, do not replace the insulation more energy required because you are heating up the cup + making up for the losses due to no insulation, maybe we are just going around in circles here.

 

[hometech]

yes, you use your laws of physics, I'll use mine lol

 

[John.g]

One last go at it, if you had a 1 kg cup of water at 15C and you heated it in a fully insulated cup to 60C it will require 1X(60-15)/860, 0.0523kwh or 52.3 wh. If you heated it in uninskulated cup it will require 52.3 wh to heat the water plus whatever wh are lost to outside, if you put only 52.3 wh into the uninsulated cup then it will not reach 60C.

 

[siricosm]

I turned the tank down to 55C and it seems to make hot water fine, but that is 55C at the bottom of the tank, so I am guessing the top is warmer. It is really a shame those tanks don't have pockets for thermometers.

I'll have to see if it needs to be turned up when the weather gets colder, and the incoming water temperature drops.

I wonder if anyone makes a weather compensating cylinder stat.

 

[John.g]

What is the boiler setpoint temperature?

You have a temperature gauges on the hot water inlet (from boiler) to the tank so that will tell you the temperature at the tank top (and to the Hx)., depending on the boiler output and circ pump flow rate and boiler set point the water to the tank top could be 20C higher than the temperature at the stat, the water is being heated from the top down (vs from the bottom up with a immersion heater or tank coil) so the positioning of the stat is very important, but if the boiler setpoint is set to say 5C above the stat temperature setpoint then the top of the tank can't be more than (in your case), 60C). IMO the boiler setpoint should always be linked to the tank stat setpoint, say tankstat SP+5C?. So if you want a tank temp of 55C, set the tankstat to 50C and the boiler SP to 55C.

 

[siricosm]

What is the boiler setpoint temperature?

You have a temperature gauges on the hot water inlet (from boiler) to the tank so that will tell you the temperature at the tank top (and to the Hx)., depending on the boiler output and circ pump flow rate and boiler set point the water to the tank top could be 20C higher than the temperature at the stat, the water is being heated from the top down (vs from the bottom up with a immersion heater or tank coil) so the positioning of the stat is very important, but if the boiler setpoint is set to say 5C above the stat temperature setpoint then the top of the tank can't be more than (in your case), 60C). IMO the boiler setpoint should always be linked to the tank stat setpoint, say tankstat SP+5C?. So if you want a tank temp of 55C, set the tankstat to 50C and the boiler SP to 55C.

I have an old Ideal FF360, which has no gauges. The manual says the max temperature is 82C when it is turned all the way up. With a thermal store I assumed that you would always just run the boiler on the maximum setting. I don't think there is any advantage to turning it down, is there?

I have some thermometers in the pipework, but I get the feeling they report a lower temperature than reality, they say the water coming from the boiler never gets much over 70C or so, and the return temperature reads about 5C lower than the thermostat setting when it cuts out. I am going to insulate the pipework soon, so maybe that will change.

 

[SJB060685]

When calculating heat loss you are determining the output required to reach and maintain a target temperature at design outside temperature. U values, area, air changes etc are all taken into account. If you have a load of 20 KW at -2 outside temperature and a target temperature of 21 then you need 869.565 watts per degrees c difference between inside and out. If you can't supply that power it will never reach target temperature. Say your output is now 15KW at the conditions mentioned above, you will only raise the final temperature to 17.25°c above outside temperature, in my example above that was -2, your final temperature would be 15.25°c.

 

[John.g]

So, once the heat loss co efficient (watts/M2/degC) is calculated then the house /outside deltaT (and house area) determines the energy required and is completely, a linear relationship?.

 

[John.g]

I have an old Ideal FF360, which has no gauges. The manual says the max temperature is 82C when it is turned all the way up. With a thermal store I assumed that you would always just run the boiler on the maximum setting. I don't think there is any advantage to turning it down, is there?

I have some thermometers in the pipework, but I get the feeling they report a lower temperature than reality, they say the water coming from the boiler never gets much over 70C or so, and the return temperature reads about 5C lower than the thermostat setting when it cuts out. I am going to insulate the pipework soon, so maybe that will change.

Since you have a SE boiler then no big gains to be had by running it at a lower boiler flow temperature but the tank radiation losses will be higher, the advantage in running at a higher temperature is that it effectively increases the stored volume of hot water, if you have effectively say 275 litres at 60C, then by increasing it to say 75C, will give a effective stored vol (if used at 60C) of 275*(75-10)/(60-10) or 360 litres (assuming mains at 10C). this is very useful if you were heating the tank with night rate immersion (if electric heating was the only option).
The reason that you are only seeing 70C is that the boiler will increase the temperature based on the boiler output and the (boiler) circ pump flow, a deltaT of 20/25c would be reasonable, you would/will have to increase the cylinder stat temperature to ~ 60C to get around 80C tank temperature.
You can easily see what the boiler deltaT is as you also have a gauge on the boiler return just before the circ pump, just subtract this from the boiler flow temperature.

 

[SJB060685]

So, once the heat loss co efficient (watts/M2/degC) is calculated then the house /outside deltaT (and house area) determines the energy required and is completely, a linear relationship?.

Yes mate.

 

[SJB060685]

Shows the linear relationship.

 

[John.g]

I have no problem with air changes, what I do find hard to reconcile though is the U values through walls floor roof etc in that a one degree change at say a room temp of 25C and that at 10C. results in the same loss.

 

[siricosm]

You can easily see what the boiler deltaT is as you also have a gauge on the boiler return just before the circ pump, just subtract this from the boiler flow temperature.

It looks to only be about 12C. 60C on return and 72C on flow when the stat cuts out if it is set to 65. I estimate they read about 5C lower than reality. Also the boiler is some distance away from the tank.

I suppose the delta across the boiler depends on the flow. The boiler pump is a Grunfos Alpha 2 on its maximum setting (III).

 

[SJB060685]

You're overthinking it mate. The U value is as you say the rate of heat transfer through a structure, measured in w/m².k. The lower the U value the better it's insulated, similarly the the R value is a measure of a materials resistance to heat transfer. If you have a low U value you'll have a high R value and visa versa.
Once calculated these values are fixed (providing there's no break down of material etc).

 

[Ric2013]

You're overthinking it mate. The U value is as you say the rate of heat transfer through a structure, measured in w/m².k. The lower the U value the better it's insulated, similarly the the R value is a measure of a materials resistance to heat transfer. If you have a low U value you'll have a high R value and visa versa.
Once calculated these values are fixed (providing there's no break down of material etc).

What I'm finding hard to reconcile is that the same does not apply to the heat loss from a radiator to the room. Because all being equal, the same relationship should apply.

All I can imagine is that all is not equal because the house has plenty of natural air movement from the external wind whereas a radiator creates a convection current, so the availability of cold surrounding air for a radiator to heat is partly driven by the temperature difference, whereas in the case of a house, this factor is hardly worth taking into consideration?

 

[Ric2013]

It looks to only be about 12C. 60C on return and 72C on flow when the stat cuts out if it is set to 65. I estimate they read about 5C lower than reality. Also the boiler is some distance away from the tank.

One you've got the pipes warmed up, it's very hard to have a 10°C loss between the boiler and the cylinder (boiler flow 82 and reaching cylinder at 72), unless your pipe is 100m long? If you can beg steal or borrow a clip-on or infra-red thermometer, that will give you a way of comparing the two temperatures. If your boiler stat is that inaccurate, it may be worth seeing if you can still find a replacement.

I suppose the delta across the boiler depends on the flow. The boiler pump is a Grunfos Alpha 2 on its maximum setting (III).

Yes, it does depend on the flow. But if your boiler is flowing at 72 and returning at 60, that sounds about where a non-condensing boiler would work happily so I wouldn't worry.

 

[SJB060685]

What I'm finding hard to reconcile is that the same does not apply to the heat loss from a radiator to the room. Because all being equal, the same relationship should apply.

All I can imagine is that all is not equal because the house has plenty of natural air movement from the external wind whereas a radiator creates a convection current, so the availability of cold surrounding air for a radiator to heat is partly driven by the temperature difference, whereas in the case of a house, this factor is hardly worth taking into consideration?

I'm not sure of what you're asking mate. Sorry, I'm absolutely shattered. Only got a couple hours sleep last night 😕

 

[Ric2013]

I'm not sure of what you're asking mate. Sorry, I'm absolutely shattered. Only got a couple hours sleep last night 😕

Not asking anything, really, just sympathising with John.g. Really I think we should continue this as a private chat, if you both want, as I feel we're massively OT and it's not helping the OP.

 

[siricosm]

One you've got the pipes warmed up, it's very hard to have a 10°C loss between the boiler and the cylinder (boiler flow 82 and reaching cylinder at 72), unless your pipe is 100m long?

Sorry, maybe I wasn't clear. I meant the two pipe thermometers gave readings of 72 for flow, and 60 for the return, but my guess is that it was more like 77 for the flow and 65 for the return, so a 12C delta across the boiler.

 

[John.g]

Not asking anything, really, just sympathising with John.g. Really I think we should continue this as a private chat, if you both want, as I feel we're massively OT and it's not helping the OP.

Still keeping Siricosm happy I think but won't dwell too long more now on the above.

A 150 litre insulated cylinder has 90W loss stamped on it which is a loss of 0.6C/hr, AFAIK, this is obtained by maintaining the cylinder at 60C with a surrounding air temp of 20C, deltaT of 40C, a 150 litre cylinder will roughly have a surface area of 1.33 M2 so the "U factor" is 90/1.33/40, 1.692w/m2/degC......check 1.692*1.33*40 = 90watts. If this loss is linear then the cylinder, in falling from 60C to 55c will lose 1.692*1.33*5,11.25 watts and will lose the same in falling from 25C to 20C, both obviously equate to a loss of 90 watts/hr. or a loss of 0.6C/hr.

Now, its patently obvious that the cylinder will not lose heat at a rate of 0.6C/hr, 11.25watts, in falling from 25C to 20C but it will be close to losing it at a rate of 0.6C/hr, 11.25 watts in falling from 60C to 65C.
If these losses are/were constant why do they keep the cylinder at a set temperature and why not just monitor the heat loss in falling from 60C to 20C and convert this to the loss, a good bit less than equating to 0.6C/hour I would guess.

 

[siricosm]

If this loss is linear then the cylinder, in falling from 60C to 55c will lose 1.692*1.33*5,11.25 watts and will lose the same in falling from 25C to 20C, ...

OK, I'll chime in. I think there is confusion about the terms "linear" vs "constant". It is linear with respect to the gradient of temperature. So will lose heat at a rate of 8x more at 60C vs 25C. The hotter it is, the faster it loses heat, hence the desire to design for as cool as practical.

The same applies to the output from the plate exchanger. It appears that most commercial heat banks have undersized DHW plate exchangers to run at lower temperatures. With condensing boilers and heat pumps so popular, I am a bit surprised this is still the case.