Many ways to set up an ASHP

[joe90]

After so much help here about the farce getting my ASHP to work, I thought I would start a new thread to discuss the ways it can interact with UFH and DHW.  Our house is in a windy location and its been mentioned in other threads that this leads to a greater house heating requirement even at passiv levels of insulation. I have a 90litre buffer tank heated by the ASHP, the UFH loops are fed from this buffer tank via pump and manifold. If I get the ASHP to heat the buffer every time the room stat calls for heat then the UFH will take a long time to heat the house (no idea how long). If however the buffer (in winter) is permenantly heated to say 35’ and the room stat controls the UFH manifold pump the reaction time will be shorter. I have found the tanks I have are very well insulated and loose very little heat, plus any losses heat the house anyway as background heat. 

 

With regard DHW, I don’t want the ASHP to defrost so could limit the temp from the ASHP (by tank stat) to say 40’. As we want DHW at about 47’ the changeover contact on the tank stat when it turns the ASHP off could switch the immersion on which will turn itself off when 47’ is reached.

(just read the stat info and it appears to switch 10A! a 3Kw immersion draws 12.5 A so I will need a relay I guess?).

 

I would welcome any thoughts on this plan.

Edited November 12, 2018 by joe90

[ProDave]

I would treat the buffer tank as a thermal store where the "heat in" and "heat out" are usually separate control loops.

 

So the heat out is simple, whenever any room calls for heat, the UFH controller will have a "call for heat" output that will turn on a pump and / or open a valve and draw heat from the buffer.

 

The heat input control will be in it's simplest form:  whenever the buffer tank cools down, the tank thermostat will call for heat from the ASHP.  That on it's own would be too simple as the buffer would be maintained 24/7/365 which you don't want. So add in a timer so the buffer tank is only being kept heated when the heating is on.

 

In my house I have chosen only to have the UFH working only in the daytime, mainly because I want the house silent at night (the noisiest component of my entire heating system is the UFH circulating pumps) so that is controlled by a conventional central heating controller.  That timed output could power both the heat in and heat out control loops so the buffer would only be heated when the heating is on.

[ProDave]

Regarding DHW, be careful of over complicating it.

 

I am heating HW to 48 degrees with the heat pump.  Most of the time the HP does not need to defrost.  However we had a 1 week cold spell recently with an average temperature of 0 and about -6 overnight.  For the first time during that cold snap I did see the HP defrost when heating HW (I can confirm even at -6 it did not need to defrost when heating 37 degree water for UFH)  the consequence of that is the HW for that week consumed 30KWh compared to a normal week of 22KWh.  So it was using roughly 1KWh more per day either to defrost, or through a lower COP (probably a bit of both)

 

I am shortly to be implementing a "boost" system using a modulating instant in line hot water heater.  This is being installed mainly to cover those "oops ran out of hot water" moments, but it will allow the HW tank to run at a lower temperature with the electric heater boosting it as it is used.  If you are going to use resistance heating then doing so real time heating just what you use will be more economical than heating the whole tank that last bit with the immersion heater.   Once this is installed I will experiment with reducing the tank temperature during very cold weather to reduce or eradicate HP defrosting.

[Jeremy Harris]

1 hour ago, ProDave said:

I am heating HW to 48 degrees with the heat pump.  Most of the time the HP does not need to defrost.  However we had a 1 week cold spell recently with an average temperature of 0 and about -6 overnight.  For the first time during that cold snap I did see the HP defrost when heating HW (I can confirm even at -6 it did not need to defrost when heating 37 degree water for UFH)  the consequence of that is the HW for that week consumed 30KWh compared to a normal week of 22KWh.  So it was using roughly 1KWh more per day either to defrost, or through a lower COP (probably a bit of both)

 

 

I reckon you've found much the same as I did when experimenting with our ASHP, that defrost cycling really does start to hit the efficiency damned hard.  Very low air temperatures aren't usually a problem, I found, as the air will be pretty dry by the time the temperature has dropped to around -6 deg C.  The worst case was if the ASHP was running at a fairly high output as the air temperature dropped towards zero, following damp weather. 

[Ed Davies]

Maybe it's carb icing ?

[Jeremy Harris]

6 minutes ago, Ed Davies said:

Maybe it's carb icing ?

 

Happens under much the same conditions!

[joe90]

1 hour ago, ProDave said:

I am shortly to be implementing a "boost" system using a modulating instant in line hot water heater.

 

I too have one of those as a plan B as I thought it would use less lecky than an immersion, plus the lower the DHW temp that’s stored the less the losses (although unlike @JSHarris I have found our tank looses very little heat, in fact er indoors complained that the airing cupboard was the same temp as the rest of the house?)

[ragg987]

17 hours ago, joe90 said:

I don’t want the ASHP to defrost so could limit the temp from the ASHP (by tank stat) to say 40’. As we want DHW at about 47’ the changeover contact on the tank stat when it turns the ASHP off could switch the immersion on

Makes little sense to me. Defrost is part of the design, so I can not see why the need avoid it.

 

Direct electric heating gives COP of 1. As long as you stay above this even with defrosting you are saving money.

[Jeremy Harris]

6 minutes ago, ragg987 said:

Makes little sense to me. Defrost is party of the design, so I can not see why the need avoid it.

 

Direct electric heating gives COP of 1. As long as you stay above this even with defrosting you are saving money.

 

Because defrost works by operating the 4 way reversing valve, which then puts the heat pump into reverse, which then pumps heat out of the house (which you've already paid for) to warm up the heat exchanger and melt the ice.  Every 10 minutes spent defrosting effectively negates 10 minutes heating, so the effect is 20 minutes of the heat pump consuming electricity for near-zero heat input to the house.  In other words, a defrost cycle just throws energy away for no gain, so anything you can do to prevent it gives a really worthwhile saving in energy use. 

[ProDave]

What I observed when mine defrosted is the compressor and fan slowed to a stop, the 4 way valve operated then the fan and compressor started up again.  It only took 2-3 minutes to melt all the ice then it stopped the compressor, swapped the valve back and re started.  During the defrost it energises the inbuilt willis heater I assume to reduce the amount of heat it sucks out of the house.

[Jeremy Harris]

Sounds like it, as that's a much faster defrost period than mine.  I haven't timed it accurately, as I have it set now so that it never defrosts, but when I was playing around trying to correct the daft factory settings I think it took around ten minutes for every defrost cycle to complete, but then our ASHP doesn't have a resistance heater and relies on drawing heat back out of the house to melt the ice.

[MikeSharp01]

2 hours ago, Ed Davies said:

carb icing

Are not all icings carbs - well C₆H₁₂O₆ like anyway, feel like a right onoff I do.

[Onoff]

1 hour ago, MikeSharp01 said:

Are not all icings carbs - well C₆H₁₂O₆ like anyway, feel like a right onoff I do.

 

You having a dig Sugar?

[joe90]

11 hours ago, JSHarris said:

Sounds like it, as that's a much faster defrost period than mine.  I haven't timed it accurately, as I have it set now so that it never defrosts, but when I was playing around trying to correct the daft factory settings I think it took around ten minutes for every defrost cycle to complete, but then our ASHP doesn't have a resistance heater and relies on drawing heat back out of the house to melt the ice.

 

I did wonder if a resistance heater near the ice would be a cheaper way to defrost the ASHP (I am sure I saw parameter 106, external heat source or defrost).

[Jeremy Harris]

3 minutes ago, joe90 said:

 

I did wonder if a resistance heater near the ice would be a cheaper way to defrost the ASHP (I am sure I saw parameter 106, external heat source or defrost).

 

There is a factory option for some Carrier ASHPs to have external heaters, I believe.  Not sure whether they can be fitted after installation, or to the models we have, though.  Not sure whether it's more efficient overall to use an external heater for defrost or to put the heat pump into reverse, I suspect it may be slightly more economical to use direct electric heating inside the unit, due to the faster warm up, perhaps.  As ours doesn't defrost now I've not looked at it closely enough to be able to say one way or the other.

[joe90]

I little fan heater pointed at the fins????.

[joe90]

17 hours ago, ProDave said:

During the defrost it energises the inbuilt willis heater I assume to reduce the amount of heat it sucks out of the house.

 

As @ProDave says about the Willis heater, it occurred to me that my buffer tank that is right next to the ASHP might act the same, as long as the temp in the tank does not go below the UFH temp then the house temp will not suffer!!

Edited November 13, 2018 by joe90

[joe90]

@JSHarris as your ASHP is very similar to mine, I have been going through all the settings again now I am beginning to understand how it works. Parameter 106 is (1) external heat source or (2) defrost, and the default setting is external heat source (1). Surely with no external heat source installed this should be set as 2?. 

 

I have said before (not wishing to go on about it ?) that I would have thought that the heat pump could recognise when it’s about to defrost and simply stop itself (max water temp achieved without defrosting). I wonder if when the external heat source is activated the pump stops till the icing has stopped, if so then if i select external heat source but don’t instal one I will achieve my goal?

Edited November 13, 2018 by joe90

[ProDave]

I did think that myself, but having observed what happens in a defrost cycle:

 

The temperature had dropped below 0 having been (just) above 0 in the day. As soon as the HW demand came on and the compressor ramped up to full speed,  you could see the ice forming on the air heat exchanger.  After about 20 minutes the defrost cycle kicked in and melted the ice in a few minutes.

 

I observed at the end of the HW demand, there was still ice there, but not enough to trigger a defrost,  When it ramped back down to a low power for the UFH no more ice formed, but neither did any of the ice there melt. It wouldn't because it was below 0.

 

So I don't think "switch off and wait" would work in any but a very few situations.

 

Mine has a "silent mode" that you can select that limits noise by limiting maximum power. Next time we get a cold snap I will try that and see if it manages to heat the HW more slowly and thus avoid defrosting.

  

[joe90]

Ah, makes sense. Back to my little fan heater then?. Whilst thinking this through I was also thinking of E7 or E10 to reduce running costs but as these are mostly night times, temps will be lower so defrosting may be more of a problem, so will the savings be worth the defrosting (when). Gosh I wish I had mains gas! (Actually apart from the programming I am enjoying the challenge to get it right). Using the little grey cells (what’s left ).

[Jeremy Harris]

I think I'd just set it up for what you want, including delivering DHW at the temperature you need, run it through the winter until the end of the heating season and see what the running costs look like.  You can do as I've done and fit a small energy meter in the ASHP supply if you wish (no more than £20 for a meter and small enclosure to fit it, and a DIY wiring job if you fit it downstream of the ASHP isolating switch), as that then tells you exactly how much electricity the thing has used.  Tracking this will give you a feel for the running cost and you can then make a decision as to whether to make things more complex or not, based on how acceptable those costs are.

[ProDave]

Or do as I have and fit a dual rate meter that switches rate as the HP switches from heating to DHW (so switched by the feed to your 2 way valve) and then you have heating and hot water energy usage metered individually.

 

 

[joe90]

Just spoke to my  neighbour who has an ASHP (he’s a plumber) and he runs his DHW at 48’ and it very rarely defrosts so that looks good for our location!?. Yes @JSHarris, I will run it and see how it settles down, measure consumption and decide in the future if changes are needed. How do people measure their water temp, I tried a mouth thermometer from when the kids were young but when it gets above 40 it has a fit!!!

Edited November 13, 2018 by joe90

[Stones]

As part of the process in selecting our preferred DHW / Heating solution, I considered heating DHW to a set temp using the ASHP then topping up with immersion to get the desired target temperature, but on costing it out, found that any savings accrued on the ASHP side in terms of improved CoP were offset by the cost of running the immersion.  In simple terms, (without PV) it cost the same either way.  Why complicate things when you don't have to?

[joe90]

39 minutes ago, Stones said:

As part of the process in selecting our preferred DHW / Heating solution, I considered heating DHW to a set temp using the ASHP then topping up with immersion to get the desired target temperature, but on costing it out, found that any savings accrued on the ASHP side in terms of improved CoP were offset by the cost of running the immersion.  In simple terms, (without PV) it cost the same either way.  Why complicate things when you don't have to?

 

@Stones, What temp do you heat your DHW.

Edited November 13, 2018 by joe90