Heat Pump Ready

[JamesPa]

4 hours ago, Bramco said:

 

That sounds like fun!!

 

What was maddening about our situation was that I had to set out the logic of what it was doing before they'd row back from the sucking teeth and 'you need a new cylinder'!    A lot of people that hadn't informed themselves would have simply gone along with the replacement cylinder at another 3 or 4 grand.

 

Simon

 

You were very lucky your installer didn't insist on replacing the cylinder.  It seems to be essentially a given in the industry that this is part of the job.  That's a throwback to older refrigerants that can't reach a sufficiently high temp.

 

I'm currently looking through hp manufacturers specs with this in mind.  Daikin specify, even for their HT model which can run at ft70, that the dhw coil must be at least 1.1 sq m in area, effectively ruling out re-using any (UK) cylinder not specifically designed for heat pumps.  Oddly enough they specify exactly the same minimum area for their lt models, which makes zero sense.

 

I have challenged them but they refuse to engage because I'm not an installer.

 

The Daikin nominated installer I am speaking with described their HT models as 'pointless' and, based on the specs, I agree.  It makes good sense in a retrofit to run dhw at a high flow temp, because the additional energy consumed is small in comparison to the upgrade cost.  The reverse is true for emitters.  So if you can't use the HT models with existing dhw cylinders, they are indeed pointless (except that they are generally black and quieter).

 

I have yet to check whether the other manufacturers of HT heat pumps do something similar, I hope they don't.

 

We don't need the gas boiler industry to obstruct heat pump deployment, the heat pump industry is doing a fine job of that on its own!

 

[sharpener]

1 hour ago, JamesPa said:

I have yet to check whether the other manufacturers of HT heat pumps do something similar, I hope they don't.

 

I have had no luck on this score with Vaillant tech support or their recommended installers despite the following claim in their publicity material:

 

I am assuming that if you can run it up to 75C the heat transfer to water at 55 will have no difficulty in getting rid of the minimum o/p, maybe you can check for me if you have got the formulae handy for a 0.8 m2 coil @JamesPa.

 

According to this chart the 12kW unit will defo turn down to under 6kW and that is for the stated conditions "Topný faktor (COP) a topný výkon při A/W 65-57"

(Heating factor (COP) and heating output at A/W 65-57)

 

They don't publish figures for 75C but they can hardly be any greater.

 

 

 

Running off E7 as I propose to do even a CoP of unity would be fine and it achieves this OK (at 65C) but I get met with "the CoP will be awful" to which I reply "E7 is so cheap in comparison (1/3) that I do not care in the least". Though heating up from cold it will not be as low as that for most of the journey anyway:

 

 

If it turns out to be less than 1.0 in practice I can always use the immersion. Thinking about it maybe I should tell them that is the plan, no that won't work, MCS says it has to be the HP even if less efficient.

 

HTH

[Bramco]

11 hours ago, ProDave said:

Do you mean the tank is making a noise when heating DHW, perhaps a rattling sort of sound coming from inside the tank?

 

Yes, that's exactly it.

 

11 hours ago, ProDave said:

I had this with my Telford tank.  By trial and error I cured it by opening up the bypass valve so it was bypassing water all the time.

Righto, off in search of a bypass valve.....   😄

 

Thanks for a great tip.

 

11 hours ago, ProDave said:

I think the issue was the heat pump demands such a high water flow rate, that flow rate was more than the tank input coil was happy with.  I couldn't just turn down the pump or the HP would trip on low water flow rate, so allowing some water to bypass the tank seemed to "fix" it.

I think you're right, and reading some information on the web, it can happen in smooth coils as well - something to do with non laminar flow kicking it off.

 

Even though we're using PV and the cheap rate Octopus Go for the DHW boost, it would be good to make sure this is fettled for the future, just in case.

 

Simon

[Bramco]

11 hours ago, JamesPa said:

You were very lucky your installer didn't insist on replacing the cylinder.  It seems to be essentially a given in the industry that this is part of the job.  That's a throwback to older refrigerants that can't reach a sufficiently high temp.

 

I'm currently looking through hp manufacturers specs with this in mind.  Daikin specify, even for their HT model which can run at ft70, that the dhw coil must be at least 1.1 sq m in area, effectively ruling out re-using any (UK) cylinder not specifically designed for heat pumps.  Oddly enough they specify exactly the same minimum area for their lt models, which makes zero sense.

 

We could show that the cylinders were man enough through the Newark specs, plenty of spare square meterage.  As I said, it did take me a while and quite a lot of emails before they came to the same conclusion as us - the reason that the HP hit it's internal maximum working temperature and threw the error was because the sensor was telling it the tank wasn't up to temperature, so it just carried on pumping heat in, raising the temperature until the tank was at the HP internal maximum temperature.

 

Fortunately I had some sensors monitoring things, so could use that as evidence as well.

 

Simon

 

[Bramco]

9 hours ago, sharpener said:

Thinking about it maybe I should tell them that is the plan, no that won't work, MCS says it has to be the HP even if less efficient.

Don't tell them.  If you want to use your cheap rate for heating, have the DHW immersion on in parallel.  It's up to you how you use the system once installed.

 

Afaik, all HPs have priority for DHW over heating, so if you are running yours on E7, then if the DHW needs topping up, this will happen first and it could mean you get to the end of the E7 period with the HP not having serviced the heating demand (or not enough of the heating demand).   And if you run the DHW outside the E7 window, even with the COP, it will cost more than using E7.  In our case on Octopus Go, the cheap rate is 9.5p and the normal rate of just under 30p. So the HP needs to be at at least a COP of 3 which on a cold winter's night will be hard to acheive.

 

Simon

[sharpener]

The thought experiment was about adding the TS and DHW systems myself after the MCS install but that wouldn't work. Certainly I could reprogram it how I want once installed.

 

8 minutes ago, Bramco said:

Afaik, all HPs have priority for DHW over heating,

 

Yes but only I think within the timing window you programme for the DHW. Also they typically have a programmable max duration and lockout times to prevent it hogging the heat supply.

 

Anyway a 210 litre cylinder takes about 10kW to heat from cold so with a 300 TS as well the total will be about 25kWh. Worst case a 12kW HP would take 2 hrs, leaving 5 hrs for space heating. So it would probably be split it into two sessions either by choice or by operation of the default settings.

 

I envisage the HW/TS getting up to temp by 0600 so the UFH can come out of setback before we go downstairs. In the depths of winter there will also be the AGA to warm the kitchen.

[JamesPa]

11 hours ago, sharpener said:

I am assuming that if you can run it up to 75C the heat transfer to water at 55 will have no difficulty in getting rid of the minimum o/p, maybe you can check for me if you have got the formulae handy for a 0.8 m2 coil @JamesPa.

Here you go.  This is a time sequence in 1 minute intervals.  Min is just the minute number, Temp the water temp, Power is how much power is delivered from the coil to the water (capped at 12kW corresponding to the HP capacity), Energy how much energy delivered from the coil to the water, temp rise the amount by which the water temp rises.  This is a 200l cylinder with FT75 and a 0.8sqm coul.  The coil is assumed to be copper so a coil with scale may do somewhat worse!.  Its also assumed that the tank is all at the same temperature.  

 

 

 

Min	Temp	Power (kW)	Energy (kJ)	Temp rise
1	10.00	12.00	720.00	0.86
2	10.86	12.00	720.00	0.86
3	11.71	12.00	720.00	0.86
4	12.57	12.00	720.00	0.86
5	13.43	12.00	720.00	0.86
6	14.29	12.00	720.00	0.86
7	15.14	12.00	720.00	0.86
8	16.00	12.00	720.00	0.86
9	16.86	12.00	720.00	0.86
10	17.71	12.00	720.00	0.86
11	18.57	12.00	720.00	0.86
12	19.43	12.00	720.00	0.86
13	20.29	12.00	720.00	0.86
14	21.14	12.00	720.00	0.86
15	22.00	12.00	720.00	0.86
16	22.86	12.00	720.00	0.86
17	23.71	12.00	720.00	0.86
18	24.57	12.00	720.00	0.86
19	25.43	12.00	720.00	0.86
20	26.29	12.00	720.00	0.86
21	27.14	12.00	720.00	0.86
22	28.00	12.00	720.00	0.86
23	28.86	12.00	720.00	0.86
24	29.71	12.00	720.00	0.86
25	30.57	12.00	720.00	0.86
26	31.43	12.00	720.00	0.86
27	32.29	12.00	720.00	0.86
28	33.14	12.00	720.00	0.86
29	34.00	12.00	720.00	0.86
30	34.86	12.00	720.00	0.86
31	35.71	12.00	720.00	0.86
32	36.57	12.00	720.00	0.86
33	37.43	12.00	720.00	0.86
34	38.29	11.75	704.91	0.84
35	39.12	11.48	688.80	0.82
36	39.94	11.22	673.06	0.80
37	40.75	10.96	657.67	0.78
38	41.53	10.71	642.64	0.77
39	42.29	10.47	627.95	0.75
40	43.04	10.23	613.60	0.73
41	43.77	9.99	599.57	0.71
42	44.49	9.76	585.87	0.70
43	45.18	9.54	572.48	0.68
44	45.86	9.32	559.39	0.67
45	46.53	9.11	546.61	0.65
46	47.18	8.90	534.11	0.64
47	47.82	8.70	521.90	0.62
48	48.44	8.50	509.98	0.61
49	49.05	8.31	498.32	0.59
50	49.64	8.12	486.93	0.58
51	50.22	7.93	475.80	0.57
52	50.79	7.75	464.92	0.55

[sharpener]

Wow, thanks for the full model.

 

It seems not to be quite linear in delta T but by a bit of extrapolation it looks to transfer about 7.18 kW at W = 55 and falls to 6kW at W = ~61C, would that be about right? It seems plausible at least and so a bit of margin for scaling (not that I will get any with the rainwater, on the outside of the coil at least).

 

Separately I calculate 34 litres/min required to shift 12kW at delta T of 5 deg, which in 28 mm pipe is 1.1 m/s so all good.

 

I will tackle them with this but am not overly hopeful, I fear they will not countenance running at 75 any more than they will run the rads at >55 or the UFH at >45.

 

Edited July 28, 2023 by sharpener

[JamesPa]

Just now, sharpener said:

Separately I calculate 34 litres/min required to shift 12kW at delta T of 5 deg, which in 28 mm pipe is 1.1 m/s so all good

Delta t will probably be higher initially and will reduce as the water heats up, unless your hp modulates the pump.

[JohnMo]

3 hours ago, JamesPa said:

Delta t will probably be higher initially and will reduce as the water heats up, unless your hp modulates the pump.

Mine starts to expand out as the temperature increases - only a degree or so, as it goes from 35 to 60.

 

Plus DHW flow ramps up to manage dT, it isn't a set temperature, as dT decreases, the heat pump controller increases flow temp to a managed dT and provides variable flow temp (slowly increasing as it moves through the heat cycle). Once flow hits max temperature the heating cycle is stopped, it doesn't stay there it stops, because it's ran out of control of dT.