T50 on new install?

[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.

 

[John.g]

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.

The Heat store has potential to give very high boiler efficiencies by increasing the condensing effect with reduced return temperatures as you have good control over that, just assume you set the cylinder stat to 40C, it will then have a cut in somewhere around 35C, the stat should be located a bit above the return to the boiler, as you start using hot water; cold water will start replacing the hotter water above it and you could have water as low as 35C or even lower to the boiler return which with a HE (condensing) boiler will give very high efficiencies, you can still get a deltaT of say 25C/28C by reducing the pump speed, this will still give a minimum store temp of 60C/65C.

 

[John.g]

The Heat store has potential to give very high boiler efficiencies by increasing the condensing effect with reduced return temperatures as you have good control over that, just assume you set the cylinder stat to 40C, it will then have a cut in somewhere around 35C, the stat should be located a bit above the return to the boiler, as you start using hot water; cold water will start replacing the hotter water above it and you could have water as low as 35C or even lower to the boiler return which with a HE (condensing) boiler will give very high efficiencies, you can still get a deltaT of say 25C/28C by reducing the pump speed, this will still give a minimum store temp of 60C/65C.

Apologies, that's incorrect as you are not drawing any water off the store as you have a HX for that!!

 

[John.g]

The Heat store has potential to give very high boiler efficiencies by increasing the condensing effect with reduced return temperatures as you have good control over that, just assume you set the cylinder stat to 40C, it will then have a cut in somewhere around 35C, the stat should be located a bit above the return to the boiler, as you start using hot water; cold water will start replacing the hotter water above it and you could have water as low as 35C or even lower to the boiler return which with a HE (condensing) boiler will give very high efficiencies, you can still get a deltaT of say 25C/28C by reducing the pump speed, this will still give a minimum store temp of 60C/65C.

You wondered what flow/temp you might get at a lower cold water temp, I found my old calcs from years ago and should give a fair indication of what you might expect, the heat input shows practically the same (at your primary flow rate of 33 LPM estimatd) and shows ~ HW flow rates of 7.4LPM at 72C & 7.23LPM at 57.4C.
You may be interested in the calcs I used.

 

[siricosm]

You wondered what flow/temp you might get at a lower cold water temp, I found my old calcs from years ago and should give a fair indication of what you might expect, the heat input shows practically the same (at your primary flow rate of 33 LPM estimatd) and shows ~ HW flow rates of 7.4LPM at 72C & 7.23LPM at 57.4C.
You may be interested in the calcs I used.

Thanks. I would target an output temp of 48C on the hot side of the DHW.

The main question is that if the plate exchanger has a rating of 165Kw at a delta of 40C, then what is the minimum tank flow temperature required to get a flow rate of 10l/m at 48C if the cold water feed is 5C?

The delta of 40C is for rating the exchanger, I would expect the delta to be considerably lower in operation. The Grundfos Alpha is rated at a max of 50l/m (I think), so 33l/m is probably a good estimate for the max primary flow because there will be some resistance in the plate exchanger.

 

[John.g]

IF only using HW, then, theoretically, a store temp of 48.6C will give ~ 10 LPM at 48C with a return of 25C (with a reduced pump flow of 18.6LPM) see below, but its not as simple as that because the rad return temperatures will influence the return since it will be mixed with that 25C and also the total energy required will increase due to the rad requirements, so realistically you might require a store temp of 60C which should give a output of 50kw, 30kw for HW + say 20kw for rads.
If installing new rads, I would aim for T30 rads, a factor of X1.9 of a T50, and I would suggest calculate each room energy requirement separately in watts, multiply this by a factor of say 2, so if you require say a 2500 watt rad for a room, you look up your brochure for 5000 watt rad (T50) and you can then pick the appropriate rad, single, double, or triple and see how it might look in each room.

HOT WATER ONLY