Notes payable consisted of the following at December 31, 2018 and 2017 (in thousands):

	December 31,
	2018			2017
Class B promissory notes	$	—		$	100
Class C promissory notes		—			333
10% promissory note due on demand		—			500
8% convertible promissory note due March 2019		—			250
8% secured promissory notes due July/August 2018		250			1,350
8% promissory notes due March 2019		—			715
0% promissory notes due May 2018		—			261
8% promissory note due March 2019		—			200
8% promissory notes due March 2019		—			1,776
8% secured promissory notes due March 2019		—			3,455
0% convertible notes due January 2021		275			—
4% promissory note due April 2021		847			—
0% promissory note due April 2019		418			—
8% promissory note due March 2021		2,107			—
12% convertible notes due October 2019		4,000			—
12% convertible notes due November 2019		1,220			—
Total face amount		9,117			8,940
Unamortized discount		(1,780	)		(26	)
Total carrying value		7,337			8,914
Amount classified as current		4,464			3,374
Amount classified as long-term	$	2,873		$	5,540

The Class B and C promissory notes were issued by the Company in December 2016 in connection with the acquisition of Cooltech Distribution prior to the Merger. The notes were non-interest bearing and were due upon the closing of an initial public offering by the Company. The Class B notes were paid in June 2018, and the Class C notes were retired on August 15, 2018 as part of the debt exchange described in Note 13.

In May 2015, the Company issued a promissory note for $500,000, with interest payable on the first of each month at a rate of 10% per annum, and a maturity date of April 30, 2016. This loan was amended on May 1, 2016 to be due on demand. It was retired in the August 15, 2018 debt exchange.

In July and August 2017, the Company issued secured promissory notes with 1-year terms bearing interest at 8% in the aggregate amount of $1,350,000. Of that amount, $350,000 was paid in August 2018 and $750,000 was retired in the August 15, 2018 debt exchange. The remaining $250,000 note payable was not paid upon maturity, and went into default. Upon agreement with the holder, the note was paid in full, including additional default interest on February 14, 2019.

In January and March 2017, the Company issued an aggregate of $715,000 0% promissory notes with 1-year terms that were later modified to 8% notes and with maturities extended to March 2019. All of these notes were retired in the August 15, 2018 debt exchange.

The $261,000 of 0% promissory notes payable in May 2018 were ultimately paid in November 2018.

The $200,000 8% promissory note payable in March 2019 was refinanced in September 2018 as part of a note consolidation.

The $1,776,000 of 8% promissory notes payable in March 2019 were all retired in the August 15, 2018 debt exchange.

Of the $3,455,000 of 8% secured promissory notes payable in March 2019, $2,780,000 were retired in the August 15, 2018 debt exchange and the remaining $675,000 was refinanced in September 2018 as part of a note consolidation.

In January 2018, the Company issued an aggregate of $1,000,000 of 3-year 0% convertible notes and warrants. The notes are convertible into an aggregate of 570,287 shares of common stock of the Company and the warrants are exercisable for 570,287 shares of common stock of the Company at an exercise price of $9.15 per share. The Company valued the debt and the warrants in accordance with ASC 470-20-25-2 using a binomial option pricing model for the warrants, and the conversion feature, which was determined to be a Beneficial Conversion Feature, was recorded at fair value based on the difference between the closing market price of the Company’s stock on the date of the transaction and the implied conversion price in the fair value of the debt. The valuation assumed a 105% volatility rate of the Company’s common stock, a risk-free interest rate of 2.20% and a credit spread of 7.70%. The warrants were assigned a value of $127,000 and the conversion feature was assigned a value of $144,000. The remaining value of $729,000 was assigned to the debt. The aggregate discount of $271,000 is being amortized to interest expense over the 3-year life of the notes on a straight-line basis. In connection with the debt exchange on August 15, 2018 (see Note 13), holders of an aggregate principal amount of $725,000 of the notes converted their notes to common stock, leaving a principal balance of $275,000 outstanding. The unamortized discount related to the converted notes amounted to $163,000, which amount was recorded as accretion expense in the period. Excluding this amount, accretion of the discount for the year ended December 31, 2018 amounted to $45,000.

In April 2018, the Company issued a $1,000,000 installment note bearing interest at 4.02% per annum due April 30, 2021. The note specifies varying monthly payments of principal and interest through 2021.

In August 2018, in connection with the Unitron acquisition described in Note 20, the Company assumed the remaining balance of the $868,000, 0% promissory notes with $450,000 paid in October 2018 and $418,000 due in April 2019.

In September 2018, the Company entered into a Note Consolidation Agreement with a lender in which 12 promissory notes and associated accrued interest were consolidated into a single unsecured 8% promissory note in the principal amount of $2,107,000. The note is due in a lump sum on March 31, 2021. Interest compounds annually. Because the present value of the cash flows under the terms of the new debt instrument was less than 10% different from the present value of the aggregate remaining cash flows under the terms of the original instruments, the debt instruments were not considered to be substantially different and the transaction was not considered a debt extinguishment.

In October 2018, the Company issued an aggregate of $4,000,000 of 1-year 12% convertible notes and warrants. The notes are convertible at the option of the holder after six months from the date of issuance into an aggregate of 941,181 shares of common stock of the Company and the warrants are exercisable for 470,592 shares of common stock of the Company at an exercise price of $4.25 per share. The Company valued the debt and the warrants in accordance with ASC 470-20-25-2 using a binomial option pricing model for the warrants, and the conversion feature, which was determined to be a Beneficial Conversion Feature, was recorded at fair value based on the difference between the closing market price of the Company’s stock on the date of the transaction and the implied conversion price in the fair value of the debt. The valuation assumed a 90% volatility rate of the Company’s common stock, a 25% discount on the value of the underlying stock due to trading restrictions, and a risk-free interest rate of 2.47%. The warrants were assigned a value of $769,000 and the conversion feature was assigned a value of $1,173,000. The remaining value of $2,058,000 was assigned to the debt. The aggregate discount of $1,942,000 is being amortized to interest expense over the 1-year life of the notes on a straight-line basis, which approximates the effective interest method. Accretion of the discount for the year ended December 31, 2018 amounted to $324,000.

In November 2018, the Company issued an aggregate of $1,220,000 of 1-year 12% convertible notes and warrants. The notes are convertible at the option of the holder after six months from the date of issuance into an aggregate of 277,274 shares of common stock of the Company and the warrants are exercisable for 138,638 shares of common stock of the Company at an exercise price of $4.40 per share. The Company valued the debt and the warrants in accordance with ASC 470-20-25-2 using a binomial option pricing model for the warrants. The conversion feature was not assigned any value as the implied conversion price in the fair value of the debt was higher than the closing market price of the Company’s stock on the date of the transaction. The valuation assumed a 90% volatility rate of the Company’s common stock, a 25% discount on the value of the underlying stock due to trading restrictions, and a risk-free interest rate of 2.52%. The warrants were assigned a value of $118,000 and the remaining value of $1,102,000 was assigned to the debt. The discount of $118,000 is being amortized to interest expense over the 1‑year life of the notes on a straight-line basis. Accretion of the discount for the year ended December 31, 2018 amounted to $10,000.

Interest expense for notes payable for the years ended December 31, 2018 and 2017 amounted to $557,000 and $279,000, respectively.