Posted by: @scalextrix

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Posted by: @jamespa

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Are there 2*22mm (eg upstairs and downstairs) or only 1*22mm. 

The current boiler has a single 22mm pipe, but as you suggest that splits into a 22mm feed for upstairs and a 22mm feed for downstairs.  That split occurs inside the airing cupboard so a new installation could have a larger diameter to an ASHP to feed them both if necessary.

If DT5 @22mm can carry 6kW as the chart suggested, that should be enough for each leg. Also the largest room has 2kW heat loss, but has two emitters which could be roughly equally sized, so they would easily be below the 2.75kW cap for 15mm.

After checking some utility bills, annual gas consumption is roughly 16000kWh, will be interesting to see if my insulation works reduce that at all.

This is where my experiment comes unstuck, because the current boiler has a DT11 and so that means the single 22mm feed is enough now, but not enough at DT5. As such I can't really tell if it would work.

Though if I balance my rads now, and create a DT5 across them all, that would be a way to test it?  But I would be limited to 6 kW down the single 22mm boiler feed...

Afaik there are two considerations namely pump head and water velocity (causes noise and erosion).  A short piece of straight 22mm isn't going to increase the head needed much.  If it is in a noise insensitive area then the noise doesn't matter.  From an erosion standpoint up to 1.5,m/s is sometimes considered acceptable for copper pipes, so about one third more capacity than the heat geek guidance.  In summary you may well get away with it!

16000kWh is roughly 7-8kW, so an 8kW pump is reasonable unless you are doing something odd.

Posted by: @cathoderay

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I assume my heat pump is 300% efficient

Be careful with that assumption - why

The CoP includes many things a gas boiler efficiency doesn't. A gas boiler doesn't include any electric inputs.

Example when running in heating mode I am getting a CoP of 5 to 6. However when not running, you are still running the circulation pump, controller, any crankcase heaters and they are all added to the CoP figure. On mild day, my daily CoP can drop to 3.5, while the heat pump will run 4 to 6 time for 20 to 30min over 24 hrs. No much heat added compared to standby losses. As it gets colder the standby losses reduce as heat pump runs longer. Also the better insulated your house the more pronounced this effect. But the real point is below.

Posted by: @cathoderay

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then using the chart above, I can estimate my total heat loss at a design OAT of -1 degrees C to be 12kW

Moving to chart you could well be a mile out. By a factor of three. CoP has nothing to do with heat pump sizing.

The CoP is just a measure of energy in to energy out. So if you need 4kW of gas (I assume your vertical part of the chart is kW not kWh) you need 4kW for any heat source. So at zero you need approx 4kW not 12kW. So allowing for DHW I would be looking at a 6kW heat pump based on your facts presented.

Design temp really needs the lowest temp you are likely see over 99.6% of the year. Most places are well under -1 or -2.

Posted by: @jamespa

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16000kWh is roughly 7-8kW, so an 8kW pump is reasonable unless you are doing something odd.

I am not sure the divide fossil fuel kWh by 2000 rule of thumb works. I was going to remark 16000kWh per annum is rather on the high side, suggesting a high heat loss, and a correspondingly large output heat pump, very likely more than 8kW. Take my own very approximate historic figures, 1000L of oil say 10000kWh per annum = I need a 5kW heat pump when actually I need a 12kW heat pump.

The discrepancy may arise from different usage patterns, steady Eddie vs timed fast and furious.

Posted by: @cathoderay

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10000kWh per annum = I need a 5kW heat pump when actually I need a 12kW heat pump.

See my post above - you don't need 12kW.

Posted by: @scalextrix

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Though if I balance my rads now, and create a DT5 across them all, that would be a way to test it? 

I really would not do that, if your heating is working leave it. 

1. You boiler may not like it or cope with dT5.

2. If you mess up and you need heat, you may well be out of heating properly mode for a couple of days, while you reverse everything.

If you know the approx heat loss and you can measure your rads do the radiator calculation, to see your output at different flow rates.

Posted by: @cathoderay

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Take my own very approximate historic figures, 1000L of oil say 10000kWh per annum = I need a 5kW heat pump when actually I need a 12kW heat pump.

The discrepancy may arise from different usage patterns, steady Eddie vs timed fast and furious.

If you want to run a heat pump inefficiently then of course you will need a higher capacity.

How did you come to the conclusion you need 12kW?

If you live in an uninsulated barn and heat only for a small proportion of the day then the divide by 2000 rule of thumb will be well out.  If your house takes many hours to cool when you turn the heating off, lead a reasonably normal life, and  live in the UK but not in some far flung area of the UK, then it's unlikely to be so far off that it can't serve as a sanity check, which is all that is claimed.

Posted by: @Anonymous

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Be careful with that assumption

I should have explained more. The 300% efficiency is actually based on my monitored energy in/energy out (energy out divided by energy in), and is a long term or SCOP type of efficiency figure. The real figure is just under 300%, but I rounded it as i was just showing an example of my workings.

Posted by: @Anonymous

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CoP has nothing to do with heat pump sizing.

It does, a bit, indirectly (if you choose a heat pump with a dreadful COP you will need a bigger one, or rather one that uses more energy) but the point here is I am not using the COP, or efficiency as defined above, to determine heat pump size, but to estimate energy out, that is, energy delivered to the house. Assuming a steady IAT, then that heat supplied is the heat loss.

Posted by: @Anonymous

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I assume your vertical part of the chart is kW not kWh

It is kWh (I am measuring energy in and energy out, not instantaneous power) but as it is over an hour, it could also be viewed as power. Each point is the last hour's energy use (because that's what I monitor), in kWh, so a point at 2kWh is also 2lW for an hour... 

My logic is this: if the heat pump used 2kWh in the last hour, (ie ran at an average of 2kW for the hour) then, given a measured efficiency 300%, the house had 6kWh of heat delivered. If the IAT is constant, then it follows that the heat loss must also be 6kW.

I did mention design OAT does vary on location, I just used -1 or -2 degrees as it happens to be the figure for around here.   

Using the logic again: if at an OAT of -1 the heat pump uses 4kWh over the hour (as in the chart), then the heat delivered to the house is 12kWh, and that, given a steady IAT, is the heat loss.

Posted by: @Anonymous

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See my post above - you don't need 12kW.

I do - see my previous post! Plus it also happens to be the figure decent heat loss calculations come up with.

Posted by: @jamespa

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If you want to run a heat pump inefficiently then of course you will need a higher capacity.

I'm not sure that answers my question (is the divide by 2000 rule of thumb unreliable?). You divided 16000kWh pa (current/historic fossil fuel use) by 2000 to get 8kW heat pump output; I divided 10000kWh historic fossil fuel use pa by 2000 and got 5kW heat pump, when in fact I actually need 12kW. Might the 8kW figure be unreliable? 

Posted by: @cathoderay

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not sure that answers my question (is the divide by 2000 rule of thumb unreliable?). You divided 16000kWh pa (current/historic fossil fuel use) by 2000 to get 8kW heat pump output; I divided 10000kWh historic fossil fuel use pa by 2000 and got 5kW heat pump, when in fact I actually need 12kW.

How did you come to the conclusion you need 12kW (what calculation did you do?).  I'm genuinely trying to understand.  It doesn't prima face make much sense so there must be some factors that aren't obvious to me (or @johnmo it seems).

Posted by: @jamespa

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How did you come to the conclusion you need 12kW

He multiple gas kW by CoP (off 3) 4x3, so 12kW.

Posted by: @ianmk13

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Posted by: @jamespa

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@ianmk13 

Where are you located?  With this info maybe someone here can provide.a recommendation 

I'm in Milton Keynes.

I have performed my own quite thorough heat loss analysis. taking into account my detailed knowledge of the construction of my unconventional home. I also paid a modest sum for a remote, desk-based report as a 'sanity check'. When calculating my heat loss through windows, I contemplated replacing my old (35 year old) DG units with new argon-filled ones but this didn't give a sufficiently marked improvement to the heat loss.  By far the biggest unknown was the loss due to air change.  I don't consider my home to be draughty, but short of commissioning a pressure test, my estimation of the heat loss is no better than a finger in the air. 

During last November to January, I set my boiler to its lowest setting of 12kW (I didn't notice any modulation of the fan while the burner was on) with no overnight setback and recorded daily gas use, flow and return temperatures, IAT and OAT.  I have a lot of data if I can find a proper ASHP heating engineer who will make use of it. I also have a detailed plan of my existing plumbing which includes pipe gauges but I need to add lengths.

 

I strongly recommend a door pressure test. We had one done for ~£300 and it showed our air leakage was less than one air change per hour. Since we have an MVHR our loss is ~2% of total but with default values it would have been +30% more heat loss.

The guy who did the measurement left it running and encouraged us to go around the house finding which rooms had most leakage and where. It’s so easy to find the leaks when the flow is forced.