I too moved from LPG and log burner to a heat pump but back in the heady days when electricity was 15p/kWh. I certainly made good savings from the efficiency of the HP. Now that electricity prices are much higher and volatile the picture is less clear. Probably still paying less but it is a much finer calculation now. The other benefits of a HP are difficult to discount, a much better evenly heated house and less fluctuation in temperature and no logs to cut stack and dry.

The coldest winter months can see a large bill,  but over the year I still see a benefit.  Installation of a HP in an old draughty building requires careful consideration and good installation. Asking a HP to belt out hot water at fossil fuel boiler temps will bring large a bill and disappointment.

@goflying Dont worry, now we can get down to looking at practical answers to your problems.

Some more information on your house would be helpful, such as construction, level of insulation now and after grant.

The issue of domestic hot water temperature could be addressed in a variety of ways. I would be interested to know what the temperature at your tap is? The NHS limits it to 41C but you might want it a little hotter if you like a hot bath.

Worst case scenario is that a modern high temperature heat pump can heat to 65C if necessary. There will be a bit of a hit in terms of cost, but not as much as an immersion heater. Bog standard heat pumps can get to 55C which can be topped up to 60C with an immersion. Im assuming that the cooling effect is worse in winter.

A better answer would be to insulate the hot water pipes. Its likely that some pipes will be exposed when they do the heating so perhaps insulation could be slid on the hot water pipes at the same time? Might it be possible to reposition the hot water cylinder in the center of the house so that the pipe runs are shorter?

Its also worth factoring in using an off peak tariff to heat your hot water, will you also get solar panels fitted?

I hope that helps. DHW production is a small percentage of the energy needs of your house so it shouldnt be an insurmountable problem.

Hello @goflying I know exactly how you feel - I'm on a low income and had a heat pump installed about 6 weeks ago. I was also worried about running costs and unlike many can't possibly afford to run solar or batteries (which really do cut running costs a lot).

Most of the last few weeks of my heat pump ownership the weather has been relatively mild and I have been tweaking my system to get it running efficiently and cheaply and I hate being cold. But temperatures are dropping now and so far it is not costing me any more than I was paying last year for gas and this year the entire house is warm. 

BUT.... If you want to get your installation set up as best you can within the limits of your ECO grant, the best thing you can do is share more details of your home - size, levels of insulation etc., and what your installers said is the heat loss of the property.  Do you have a list of the radiators they want to change? 

You are in a good place to get help here - there's lots of experts and people with experience. Best to ignore the legal nonsense because although it is correct, it's not addressing your worries which are perfectly understandable.

If your installers want to put in a 12kw heat pump is your property on the biggish size or is it uninsulated or very old? If you can share more details people can advise.

What I can say is provided your installation is done fairly well with a good heat loss survey, good system design and the right sized radiators and water tank, there is no reason for your running costs to be higher than your gas. Or for you to be cold or not have enough hot water. 

It pretty simple to hybridise any heat pump. I did a self funded ASHP install and had a 2 year old boiler, which I didn't see the point in selling for peanuts. So have kept in the system. Have used the ASHP hybrid controls but didn't like the lack of control, so went stand alone.

I use a plate heat exchanger in the system to separate ASHP and boiler circuits.

You have two modes to consider, they are very different, DHW and CH. So you need to configure the boiler as PDHW, which will give you two flow temps. I have set mine up three temps, using WC, a setback WC and WC off, with max flow temp pegged at 60 degs.

So for DHW, I use a relay, driven from the same power supply for the diverter valve. So when diverter valve is in DHW mode, I use it to break the WC signal, this fires the boiler to high temp mode, and runs in unison with heat pump to heat cylinder. 

In CH mode I use Home Assistant and a couple of shelly relays, to do what I want and switch between two flow temperatures. I use the boiler when my battery is running low to stop using high priced electric (E7) and when really cold,when CoP drops due defrosts.

DHW recovery time has halved compared to heat pump only.

I have done purely because I could, had all the materials sat there, so no cost and was interested in getting it all working. But generally with a decent ASHP set up, it's not worth the effort. If you do install, you want the ASHP to be doing 80 to 90% of the work. As mentioned 60 Deg water is waste of time. You don't need it, you have to cool it down. We heat to 50 and it's hot.