Benjseb

New ASHP incoming... Running tips pls!

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So after months of deliberation between an ASHP and a GSHP we’ve decided that paying £15000 for boreholes doesn’t really herald a decent enough increase in COP to warrant the extra outlay. Also it seems quite a risk that if they aren’t sized correctly you’re rather stuck, vs worst case with an ASHP is upgrading the unit. 

 

Our next toss up was between a northern European brand of ASHP and one of the more mainstream/air con manufacturers

 

We’ve decided to go with a Mitsubishi Ecodan, for 2 reasons. 1. Maintainability - we don’t have to find a service partner that’s familiar with a unit primarily used overseas and 2. Advances in refrigerant technology seem to be better with the likes of Mitsubishi. Ecodan was also about £8k cheaper with same RHI return. 

 

We have a fairly big old house, 250sqm, probably 1800s built but it’s fairly well insulated and draft free for its age. We did ponder about waiting and doing more insulation first but we’d already done all the internal walls, some floors and recently all the ceilings so were at a point where additional changes were going to start getting expensive and take more time. 

 

Our peak heat loss  is currently about 13kW (from survey) and this was before doing some floor insulation and we’re going to be replacing some doors and windows at some point in the next few years. 

 

So, a 14kW Ecodan incoming (would have gone smaller but... MCS!)

 

Id love to hear people’s tips on running it effectively. I’m already planning on keeping the water temp as low as possible and using our 7kw wood burner to topup on the colder days. 

 

Have people found running it at night on E7 works well for an older house or just gradually 24/7 better?

 

What sort of set back temp works best?

 

Do people time the hot water (as we currently do) or set the DHW to keep a constant tank temp (seems the norm on ASHP, we have one at work and quite hard to time it)

 

we've just installed UFH in half of the downstairs but left the rads on the walls for now. Undecided if we’ll keep them running and then hopefully be able to run a lower water temp overall or decommission them. 

 

Any tips much appreciated. 

 

Ben

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The Ecodan has a good reputation, I don't think you will go far wrong with that.

 

I think the key is to read and re read the manual for the controller that comes with it and understand how it works.  Some of the controllers supplied with them are far from intuitive.

 

Talk to the installer and understand how he has set it up, and discuss the setup with him so you get it as you want it.  e.g  don't set the HW temperature too high, most of us find 48 degrees is plenty.

 

I would time the hot water if you can for just daytime operation.  Heating hot water is the time it will most likely need to defrost, and it's obviously colder at night, so if the tank will keep enough hot water for the morning routine then you can wait until the day has warmed up a bit for heating DHW.  That is how I run ours at the moment (but mainly to get maximum self use of the solar PV)

 

Ours essentially stays on heating all day with the room thermostats controlling it. but I do have it set to turn off over night as I like a completely silent house, and even the very faint hum of a circulating pump is more than I want at night.

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I agree with @ProDave, can't really go far wrong with the Mitsubishi, they have a good solid reputation.  I've found that fine tuning the ASHP settings has given a significant performance improvement over the default settings our unit shipped with.  As above, defrost frequency seems to be a fairly big hit on efficiency, so anything that reduces the risk of icing tends to make a useful improvement in performance.

 

We can get away with running our ASHP only at night, on E7, really because our house is really slow to lose heat, so with the concrete slab warmed up overnight there's almost always enough residual heat to keep the house warm during the day, especially as our heating requirement is very low, and a fair bit of that is met just from us being in the house with the TV on, cooking, other appliances running etc.  I'm not sure that the same would be true for an older house, that's probably a fair bit less well sealed and insulated.  My guess is that you will find you need to run the heating during peak rate periods as well, just because the house thermal time constant may be fairly short, especially in cold weather.  Even so, the running cost of the ASHP should be significantly lower than pretty much any other heating fuel.  With the electricity price set to drop slightly this autumn I suspect that an ASHP may well be cheaper than mains gas.

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Our previous house was built to 2003 standards. Quite well insulated but nowhere near the air tightness of our present house.  We only ran the heating in the day with it off at night and you could notice a drop in temperature over night.  But I never felt the need to mess around with set back temperatures. Ours was either on or off.

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Thanks @ProDave @JSHarris Very useful insights

 

I think I’ll start off running it when we’re in and need it and then extend the running time as/when needed if the low flow temp can’t reach heat during that period. 

 

The house does does tend to keep its temp quite well due to the insulation and some exposed stone and brick so hopefully we won’t need to keep it running 24/7. Especially as I can’t stand it hot, so would rather it be slightly chilly than too warm. 

 

18c is toasty in our house!

 

Do you think PIV would have a negative effect on energy consumption? Or be balanced out by the reduction in humidity and therefore less wasted latent heat?

 

Cant wait to rip the dirty old oil boiler out!

 

 

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On 13/08/2019 at 20:30, Benjseb said:

Do you think PIV would have a negative effect on energy consumption?

 

What is PIV?

 

On 13/08/2019 at 20:30, Benjseb said:

Cant wait to rip the dirty old oil boiler out!

 

When’s the big day?

 

We retained our boiler a back up, but it’s been awesome not having those fumes around the house any more.

 

Do you have PV panels?

 

good luck with your installation.

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what fumes?

fumes mean a fault and danger. Are you sure you want to rely on a faulty device as your backup?

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42 minutes ago, Home Farm said:

What is PIV?

 

Positive Input Ventilation.

 

They are a great quick fix for older poorly insulated properties that suffer from condensation.

 

On 13/08/2019 at 20:30, Benjseb said:

Do you think PIV would have a negative effect on energy consumption? Or be balanced out by the reduction in humidity and therefore less wasted latent heat?

I think it will have a negative effect the majority of the time. Warm air is pushed out of the building and is continually replaced with ambient cold air. 

Although the incoming cool air is drier and therefore requires less energy to heat I don't think it would balance.

Of course it depends on the difference in humidity between inside and outside air and the difference in temperature. 

 

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50 minutes ago, dpmiller said:

what fumes?

fumes mean a fault and danger. Are you sure you want to rely on a faulty device as your backup?

 

Maybe I made that sound worse than it is - there's an exhaust outside our utility room where the boiler is located, and when you're outside, and the boiler kicked in, you were hit with all these fumes which was not pleasant. 

 

Inside the utility room, there was a dieselly whiff, which we assumed was normal as the diesel was running through the boiler - we experienced similar smells with the gas boiler in our old house. They don't smell toxic, just not fresh. Nt sure if that's normal.

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Sounds like an ordinary old oil boiler - the old floor standing utility models used to give a faint aroma most of the time as they draw air from inside the room for the burner. 

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Just now, PeterW said:

Sounds like an ordinary old oil boiler - the old floor standing utility models used to give a faint aroma most of the time as they draw air from inside the room for the burner. 

 

The guys we purchased the house from said it was a couple of years old... and the way you've described it is fitting of our eau de diesel.

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3 minutes ago, Home Farm said:

 

Maybe I made that sound worse than it is - there's an exhaust outside our utility room where the boiler is located, and when you're outside, and the boiler kicked in, you were hit with all these fumes which was not pleasant. 

 

Inside the utility room, there was a dieselly whiff, which we assumed was normal as the diesel was running through the boiler - we experienced similar smells with the gas boiler in our old house. They don't smell toxic, just not fresh. Nt sure if that's normal.

You should only smell Kerosene inside if you have an oil leak, or the last plumber was careless and spilled some.  Once it gets into floorboards the smell is there for good.

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Thanks Dave - the smell has gone since the ASHP has gone in, but I assume it'll be back if the oil boiler has to kick in for a prolonged period of time, which I hope will not be the case.

 

The oil boiler occasionally kicks in for 2-3 minutes after a cool evening and when the heat pump has not been called to heat anything overnight and we take a shower, so it 'spikes' our DHW. There is no smell in these cases.

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47 minutes ago, willbish said:

Although the incoming cool air is drier and therefore requires less energy to heat I don't think it would balance.

 

Why do you think drier air takes less energy to heat?

 

Normally wet things are harder to heat because you first have to put in quite a bit of energy to evaporate a lot of the water but with humid air the water's already evaporated so that's not an issue. The specific heat capacity of water vapour (at constant pressure) is a bit higher than that of dry air (about 1.8 kJ/kg·K vs 1 kJ/kg·K) but with the very small quantities present I'd think that would only make a tiny difference, not worth considering. Is there another relevant consideration?

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27 minutes ago, Ed Davies said:

Is there another relevant consideration?

Unlikely

I think I probably fell for the sales pitch from the PIV, which claimed lower heating bills because the humidity would be lower.

I hadn't considered any difference between heating water or heating water vapour.

 

The specific heat capacity figures you mention show nearly twice as much energy required to heat water vapour as completely dry air?

 

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1 hour ago, willbish said:

They are a great quick fix for older poorly insulated properties that suffer from condensation.

surely this should be your prime goal -- to  address the reasons  for condensation first -

 

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Potentially ignorant question on my part, but why would/does humidity have an impact on interior house temperature? Are we talking about ambient humidity in rooms? 

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The humidity does not affect the temperature, just how you perceive it.

Temperature is really just how fast the molecules are moving when unhindered.

There is often a confusion when using the term 'heat'.

To some it means temperature, but really it means energy.

We now use energy as the standard term.

Edited by SteamyTea

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7 hours ago, willbish said:

The specific heat capacity figures you mention show nearly twice as much energy required to heat water vapour as completely dry air?

 

Yes, but even in quite humid air there's only a few grams, or tens of grams of water vapour for each kg of dry air so the extra heat capacity will have little effect.

 

E.g., air at 25°C, 100% relative humidity (a moderately extreme case for a house in the UK) would only have 20 grams of water vapour for each kg of dry air. I think (but I'm not completely sure) that you can just add the heat capacities in the obvious ratio so, using the approximate heat capacities of the gasses given above, its specific heat capacity would be (1 + 0.02 * 1.8)/1.02 ~= 1.0157 kJ/kg·K. I.e., not much more than the approximation of 1 kJ/kg·K of dry air.

 

https://upload.wikimedia.org/wikipedia/commons/9/9d/PsychrometricChart.SeaLevel.SI.svg

Edited by Ed Davies
SVG not displayed properly by forum software so just link used

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We don’t actually have any condensation issues at all,  not even in the bedroom if the windows are closed overnight

 

the thinking behind PIV was that our humidity is over 70% currently (usually about 60-65%) and I just want to ensure the air is as fresh as possible for health reasons. With an old stone house there’s always going to be moisture in the air so it’s more of a way of proactively managing this I guess. 

 

 But as usual it’s a balancing game as don’t want to be pushing all the warm air out. 

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9 hours ago, Home Farm said:

 

 

When’s the big day?

 

We retained our boiler a back up, but it’s been awesome not having those fumes around the house any more.

 

Do you have PV panels?

 

good luck with your installation.

 

Sept 9th the install starts. If they can get the old 500l hot water tank out of the room! 😂

 

no PV yet but we’re converting a garage into a holiday let later this year and it’s got a decent East/West roof so planning on adding some to that. 

 

Weve asked for a buffer tank with an immersion so we can divert to DHW or buffer tank if we’re lucky enough to have excess at the right time of year. 

 

I think we can either fit a 6kw system on single phase or 9kw on 3 phase. But trying to avoid using 3 phase as it will mean we get to consume less of the electric we generate. 

Edited by Benjseb

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6 hours ago, Benjseb said:

But trying to avoid using 3 phase as it will mean we get to consume less of the electric we generate. 

Unless you can divert to batteries/cars.  Though in summer you will still loose a lot with a 9 kWp system.

 

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